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Home , Cascade Range, Cascade Volcanoes, Castle Crags, Farallon Plate, Gorda Plate, Juan de Fuca Plate

Northern Standing more than 10,000 feet (3,000 m) above the surrounding terrain, Mt. Shasta is the largest in GeologyGeolo of Northern and symbolizes the dynamic geologic processes that have shaped a spectacular California landscape. Frank DeCourten Department of Earth Science Sierra College ESSENTIAL QUESTIONS TO ASK Northern California.1 Introduction What are northern California’s physiographic provinces? What is the Farallon zone? What two types of plate boundaries exist in northern Californiaal today? What are , how do they originate, and why are theyth important in northern California? Northern California.2 The Sierra : California’salifornia’s GeologicGe Backbone What is the ? What kinds of rocks surround the Sierrarra Nevada batholith?batholi When and how was the modern Sierrara Nevada uplifted? What types of gold deposits occur in thee Sierra Nevada? What is the Mother Lode?e? Northern California.3 The Klamath MMountains In what ways are thet Klamathath Mountains andan the Sierra NevadaNeevada similar? What kindsds of rocksro comprise the ophioliteso in the Klamathath MountainsM and what tectonic events do they signify? What mineralineral resources occuroccu in theth ?ntai Northern California.4Ca rnia Thee GrGreat What factorsfa s have led to theth formation of fertile soils ini thehe Great Valley?Valle Whatat was thet origin of the sedimentaryime y rocks in the GreatGrea Valley Sequence?Seq What is the originori of the naturall gas producedp duced in the GreatGr Valley? Whyy is the Great Gre Valley so prone to chronicchron flooding?ooding Northernrn California.5Ca The NortheNorthern CoastCoa Ranges What is the and how did it form? How did the San AndreasAndr s system originate?or What is the Salinianinian block? How old are the northernnorther Coast RangesRa and what tectonic forces elevated them? Northern California.6ornia.6 Volcanoesolcan of the and Modoc PlPlateau What Californiafornia volcanoesvolca are part of the Cascade Range? What is the CascadiaCascad subduction zone? What kind ofo volcanic activity typifies the Cascade Range? Why is MountMou Lassen an especially interesting volcano in the Cascade Range? Northern California.7Ca The Basin and Range of Northeast California WhatWh tectonic forces are responsible for the pattern of alternating mountains and valleys in the ? What mountains and basins in northern California belong to the Basin and Range province? Northern California.8 The Northern California Ice Ages During what time periods did northern California experience Ice Age conditions? What landscape features resulted from the Epoch glaciations in northern California? How did the Pleistocene landscape of northern California differ from the modern setting? Northern California.9 Northern California What plate tectonic settings are associated with northern California earthquakes? What is the likelihood of another major in northern California? What might be the effects of a large northern California earthquake? Given the severity of the potential hazards, how can the effects of earthquakes be mini- mized in northern California? Northern California.10 Living on the Edge: Coastal Hazards in Northern California In comparison to the coast of , why is the northern California shoreline so rugged and scenic? What coastal hazards exist in northern California? How do activities affect coastal hazards? 2 of Northern California

Northern California.1 none can be regarded as simple or monotonous. Each of the physiographic provinces in northern California is a varied Introduction and fascinating geological realm with endless opportunities for applying the knowledge you have gaineddin in your physical California’s Varied Landscape: California is arguably the best geology course. Collectively, they representsent a region of such place in the world to study geology. Few areas of comparable complex origin that scientists have yet too develop completelyco size are as geologically varied, physiographically diverse, or so satisfactory explanations of all of the geologicogic featuresfeatu in this spectacularly scenic as the Golden State. California’s abun- varied terrain. The common attributeribute of all thehe physiographicphysiograp dant natural resources reflect in large measure its rich geolog- provinces of northern Californiania is that, oneon wayay or another,an ical history, and earthquakes, , and mass wasting events each reflects the consequenceses of plate tectonictecton interactionsnt underscore the importance of ongoing geologic processes for along the western margin off over the past residents and visitors. With the boundary between two of the 500 million . largest lithospheric plates on the planet running for more Geologic Map off California:Calif Geologic mapsm are indispen- than 1,300 kilometers along the western side of the state, sable assets in exploringoring the naturalnatu history and geologic set- California is an outstanding natural laboratory for studying ting of any region. Suchuch maps show the distributiontion of Earth processes and plate interactions. types andd agesage on the surface,surfac along with informationmatima about During the 1960s and 1970s, our understanding of earth the orientationentation of thet variousario rock bodies, thee naturena of their dynamics shifted dramatically as the modern theory of plate contactct with adjacent rockck masses,m and thee trends and extent was developed and refined. Many of the new con- of geologicologic structures suchsu as faults and folds.olds. This informa- cepts of that era either were developed in California or were tiontio is essential in unravelingunra the geologiceolog history of a regionregi specifically formulated to explain and/or reinterpret its becausebecau it reveals spatialsp patterns in the distributiondis bution of rocksro incredibly varied geological features. Today, with a new the- of variousvariou types, ages, andnd degreeee of deformationdeformat n that reflectre oretical framework and vastly improved tools for explorationration the tectonictectoni and geologic eventsven of the past. In addition,ii geo- and analysis, scientists continue to consider Californiania a logicogic maps are of critical importancemporta e in locatingl surface and geologic paradise. subsurface deposits of earth resourcessourc and in identifying the The rocks in California tell an amazing storyry of the evolu- areasare most susceptibleible to variousvariou geologicgeol hazards. tion of land and life at the edge of North America.merica. AlthoughAltho it would require many years to explore all the geologic won- ders in California, a first course in physicalphysi geologylogy is an excellent start to a lifetime of fascinatinginat adventureeinthe in the Figure NC.11 The Physiographic Provinces of California Golden State. The purpose of this chaptercha is threefold: (1)1 to introduce the broader aspects of California’slifornia’s geologic setting, (2) to outline the major tectonic eventsnts that have shapeds its landscape over geologic time, and (3) to examine the impacts on of the state’s ongoing geologic evolution. I also hope you will considerconsid this chapterchap an invitationnto to begin your own lifelong explorationexplo nofon of one of the mostt magnifi-magn cent landscapespes in the world.wo California’s PhysiographicPhysio hic Provinces:Pro The Californiarnia landscape can be divided intoo a dozendoz regions of distinctive and characteristicaracterist geology, landforms, climate,cl geomorphic trends, soils andan vegetation, andd drainage.draina These natural areas are known as physiographicphic provincesprovinc . After the geo- logic alignmentent ofo the state’s majorajor mountainmou systems, most of the geomorphicorphi provincesince are oriented in a northwest-to- southeast trendend (Figure(F e NC.1). HenceH some of the provinces, such as the Sierra Nevadaada and the Coast Ranges, extend from the northern part of the statete to the southern portion. For the purposes of the present discussion, we will arbitrarily define northern California as the region between Monterey Bay (latitude approximately 36.5° N) and the California– border. In this portion of California, seven natural physio- graphic provinces comprise the landscape: the northern Sierra Nevada, the northern Coast Ranges, the northern Central Valley, the Klamath Mountains, the Cascade Range, the Modoc Plateau, and portions of the Basin and Range. Though there are consistent geologic patterns within each of

these regions that make them distinctive and identifiable, Map courtesy of the U.S. Geological Survey Northern California.1 Introduction 3

Figure NC.2 Geological Map of California eologica Conservatio Californ Reproduced with permission, California Department of Conservation, Geological Survey

The geologic map of California (Figure NC.2) has been Nonetheless, you will notice at a glance the strong similarity compiled by the California Geological Survey over many between the physiographic provinces in California and the decades of geologic mapping. This map is scaled to show distribution of various rock types. For example, notice the the entire state and, as such, it portrays only the broader similarity in location, trend, and extent of the Sierra distribution of various California rocks and structures. Nevada province and the large area of red, blue, and green 4 Geology of Northern California

Figure NC.3 Interactions between The Farallon, Pacific, and North American plates over the past 40 million years.

40 M.Y.A 20 M.Y.A 0 M.Y.A

Queen Pacific Farallon Charlotte plate plate Pacific- remnant Farallon Ridge Juan de FucaFuc platee Farallon North plate North North American American American plate Incipient plate Incipientent plate San Andreas SanSa Andreas Present day Pacific- transform transformtransfo volcanoes Farallon fault fault P P Ridge a a c c i Farallon if f Gulf of i i c c Gulf of Mexicoplate Mexico p p Mexico l la remnant a t t e e

colors on the geologic map. These colors representnt the southwestthwes of North America.Ame On the otherothe sideide of o that Mesozoic-age plutonic igneous rocks that comprisee the ancient spreadingspr center, thehe PacificPa fic plateplat was sliding to the core of the Sierra Nevada (red) and the olderr rocksro thathat west, while the Farallonlon platep e movedmov in the opposite direc- were metamorphosed by the emplacement of theth tiontio toward the advancingancing marginma in of North America. A little (blue and green). A very similar pattern is observed in the less than 30 millionion years ago, the western edge of North Klamath Mountains physiographic provinceovi northwest of America collidedded with,ith, and eventuallyeve overran, the Pacific- the Sierra Nevada, suggesting that the twotw regionsions share Farallon ridgedge nearn thee latitudelatit of modern . As a some common geologic traits. Thee similaritysi betweeneen theth consequence of thisth collision,li the physiographic provinces of Californiarni and the distribution camee into contactcon ct withwit the (Figure NC.3). of various types and ages of rock in the state underscoresund The collision between North America and the Pacific- the importance of the geologic foundationndation in shapingsha the Farallonn ridge endeden plate convergence in the California character of the landscape. regionregio and establishede the modern transform plate The Westwardard Migration: For the past 500 million years boundarybounda . After the initial collision of the ridge and the convergent plate tectonictect interactionsterac have prevailedailed along continent,continen the transform plate boundary expanded north the westernern margin ofo North America.A Earlyy in the and south so as North America continued moving west, over- Mesozoic Era,ra, about 20020 millionllion yearsye ago, the rate of plateplat runningru more of the ridge in the process (Figure NC.4). convergence increasedreased significantlyficantly as the ancient supercon-con- The relatively small Cocos, Rivera, and Juan de Fuca plates tinent of Pangaeaang began to break up.u The North Americann represent modern remnants of the ancient . plate separatedeparated fromf the northern part of the supercontinent The famous system developed as a conse- via thee openingopenin of the Atlanticc OceanicOcean basin. Seafloor quence of the transform plate boundary between the North spreading in this basin propelledled the NorthNor American plate America and the Pacific plates. to the west while the Eurasia and AfricanAfric plates moved in The geologic setting of modern northern California is the oppositee direction.direc SeveralSeve differentff oceanic plates were thus influenced by two different kinds of boundaries subducted under the leading edgeed of the North American between the North American and oceanic plates to the plate as it slowly movedved west. TheT last and largest of the west: a transform boundary from Monterey Bay to Cape oceanic plates to descend beneathbene North America was the Mendocino and a remnant to the Farallon plate, remnants of which still exist along the north. We will explore the consequences of these interac- western margin of North and . This plate tions in more detail in the sections that follow. tectonic interaction along the western edge of North California and Accreted Terranes: One of the conse- America produced the Farallon subduction zone, which quences of the long history of plate convergence along the was established in mid-Mesozoic time and persisted from western margin of North America was the of some 160 million years ago until about 30 million years ago. numerous blocks of rock to stable crust of the continent. Many of the major geologic trends in California are the Such blocks added to the edge of a continent by plate con- result of this long history of plate convergence. vergence are known as accretionary terranes, or simply Origin of the Modern Transform Plate Boundary: The terranes. Five hundred million years ago, there was no land Farallon plate originated at an oceanic ridge to the west and where the Pacific Coast now stands. The crust of California Northern California.2 The Sierra Nevada: California’s Geologic Backbone 5

Figure NC.4 Major terranes of western North America. In 200 million years ago. Several others were added in earlier addition to the Franciscan Complex and the Salinian block accretionary events in the Paleozoic Era, before North illustrated here, dozens of smaller terranes have been identified in America separated from . It was not until Cenozoic northern California. time that all of the terranes werewer in place along the Pacific Coast and the modern Californiafornia landscape began to emerge. Seward California, as a whole,, therefore represents a geologic Peninsula collage, an amalgam of piecesces assembledassem through the con- North vergence of platess along the west edge of North America EndicottSlope over the past 5000 million years. Northernorthern California is espe- Ruby cially intriguing because here bothbo a remnantr of an ancient Nixon Fork convergent boundaryndary and the modernm transform boundary to the south continuetinue to shape thet landscape. It is not sur- Alaska Yukon prisingg that geologistsists find northernno California such a fasci- Wrangellia Yukon– nating region. It is a place where the geologic past meets the Tanana dynamicc present, and there is no place in the world better Alexander suiteds forr geologicgeolo exploration. Let’set’sslo look a little closer.

Section Northern California.1alifornia.1 SummaryS Wrangellia Stikine ● The geology andd landscapelandsca of northern California is extremelyy varied,v d, with seven different physiographic provinces, each with distinctivedistincti rockk assemblages and geologic histories.ories. Wrangellia ● Convergentnvergent plateate boundariesb have existed for 500 mil- lionn years in the northernn California region. After North CanadaC U. S. A. AmAmericaerica separated from Pangaea early in the Mesozoic Wrangelliagellia A. EEra, thee rate of convergence increased as several oceanic platplates werees subducted under the west-moving continent. Franciscann Complex Klamath Moutainss ● The subduction of the Farallon plate in the Mesozoic Era resulted in many of the geologic trends that can be obobserved in modern California. About 30 million years ago, plate convergence ended as the transform boundary Salina between the Pacific and North American plates began to develop. The unique geologic setting of modern north- ern California is shaped by both a remnant of the Mesozoic subduction zone and the continuing evolution of the more recent transform plate boundary. U. S. A. Mexico

300 km Northern California.2 The Sierra Nevada: California’s has since beenn assembled asse in piecemeal fashion as seamounts, Geologic Backbone island arcs, coral reefs, and small continental blocks that were The Sierra Nevada is California’s best known mountain carried on subducting oceanic plates collided with the west- system. Stretching for more than 700 kilometers from ern edge of the continent and were embedded into the exist- in the north to Tehachapi in the south, this ing margin. The rocks in the various terranes were northwest-trending mountain system is home to three metamorphosed and deformed as each was sutured into national parks and the highest peak in the coterminous North America like pieces of a mosaic. In this manner, North at (14,495 feet/4,418 meters America grew incrementally westward with the addition of above sea level). Winter storms passing east from the Pacific each fragment. are in general agreement that Ocean over the high Sierra Nevada produce heavy snowfall about 100 such terranes were accreted to the western mar- that is an essential supply of water to the entire state. gin of North America since the breakup of Pangaea about Materials released from the weathering of Sierra Nevada 6 Geology of Northern California bedrock help sustain the fertility of California’s rich agricul- beneath the surface. Most of the plutons comprising the tural soils. It was in the Sierra Nevada foothills that gold Sierra Nevada batholith consist of felsic rock such as gran- was discovered in the 1800s, and the course of California ite, rich in light-colored quartz, potassium feldspar, and history was forever changed. Without the Sierra Nevada, sodium-rich plagioclase (Figure NC.6). SomeSom of the plu- California would simply not be California. tons are richer in the darker ferromagnesiannesian minerals and The bedrock of the Sierra Nevada is dominated by the consist of rock more similar to diorite or gabbro. Mesozoic Sierra Nevada batholith, one of the largest and The Sierra Nevada batholith representsnts the deep roots most complex masses of granitic rock in the world. Adjacent of a Mesozoic thatat developeddeve along thehe west-w to the batholith, and sometimes as xenoliths and roof ern margin of North Americaca above the Farallonallon subduc-su pendants within it, are older Mesozoic and Paleozoic meta- tion zone. In this ancient subduction zone,zon magmamag was morphic rocks that were invaded by the subterranean produced by partial melting and migrated upwardup through magma. More recent episodes in the evolution of the Sierra older crustal material to build a chain of andesiticande volcanoes Nevada are documented by Cenozoic volcanic and sedimen- that rivaled the modernmod Andesndes MountainsMoun of South tary rocks that rest on the granite-metamorphic basement. America. Recent geologiceologic studiesstudi have revealed evidence of The Sierra Nevada Batholith: The geologic map of large formeded by explosive eruptions in this ancient California clearly indicates that the Mesozoic granite of the volcanic chain.chain Late in the Cenozoic Era, the upliftupupli of the Sierra Nevada batholith comprises the core of the Sierra Sierra Nevada resultedres d in the nearly completeplete of Nevada. Vast exposures of such light-colored plutonic rock the volcanicolcanic edifice thatt originallyor concealedealed the batholitic in the high country of Yosemite, Kings Canyon, and rocks.. However, isolatedisolate remnants of thee ancient volcanoes Sequoia National Parks, is in part what led John Muir to canca stillill be seen in places alongg the crestrest of the rangeran refer to the Sierra as “the range of light” (Figure NC.5). (Figure(Figu NC.7). The Sierra Nevada batholith is a complex assemblage of Paleozoic-MesozoicPaleo AccretionaryA tionary TerranesTerr nes of the perhaps as many as 200 individual plutons representingenting Northern Sierra Nevada:d : TheT e magma thatth formed the magma bodies emplaced mostly between 140 million and Sierra Nevada Nev batholith wasas emplacedemp ced intoin older rocks that 80 million years ago at depths of 10 to 300 kilometerski rs either accumulated on the seafloorafloor west of the margin of

Figure NC.5 Vast exposures of granitic rock characterizeerize the Sierra Nevada, John Muir’s “RangeRang off Light”.Light Frank DeCourten Northern California.2 The Sierra Nevada: California’s Geologic Backbone 7

Figure NC.6 Close up of granite from the Sierra Nevada Figure NC.8 Folds in these Paleozoic sedimentary rocks along the Batholith. Dark crystals are biotite and hornblende, white crytals are Yuba River were developed during the collision between a block of plagioclase and potassium feldspar, and the gray translucent crystals oceanic rocks and western edge of North America in the Mesozoic Era. are quartz. Frank DeCourten

Figure NC.7 Dark-coloredd rocksrock of the Ritter Range in thet east- ern Sierra Nevada are remnantsants ofo the Mesozoiczoic Sierra volcanicvolc arc. Frank De Frank DeCourten

these rocks have been affected by multiple periods of meta- morphism. In addition, these rocks are commonly folded and faulted by the compressional stress generated along ancient convergent plate boundaries (Figure NC.8). It has taken geologists many decades to unscramble the complicated history recorded in the metamorphic rocks sur- rounding the Sierra Nevada batholith. Within the granitic core of the Sierra Nevada, exposures of these rocks are found as roof pendants and xenoliths (Figure NC.9) pre- served in the plutonic rocks. In the foothills north and west of the batholithic core of the Sierra Nevada, extensive expo- sures of metamorphic rocks have revealed a complex history

Dick Hilton of metamorphism and accretion. Because there has been much greater Cenozoic uplift in the southern Sierra Nevada than in the north, erosion there has progressed to a deeper Paleozoic North America or were accreted to the edge of level in the crust and much of the older rock that formerly the continent in convergent plate interactions that pre- surrounded the granitic plutons has been removed. ceded the Farallon subduction zone. These old rocks were Within the western foothills metamorphic belt, geolo- deformed and metamorphosed during several different gists have identified three primary units of metamorphosed accretion events and altered by the heat and fluids associated rocks that are separated from each other by major faults. with magma rising from the Farallon subduction zone in These faults probably were produced when seamounts, later Mesozoic time. Metamorphic rocks such as marble, undersea volcanic arcs, , or other fragments of slate, schist, serpentinite, and greenstone are common in lithosphere collided with the edge of North America. The the pre-batholithic terranes of the Sierra Nevada. Some of metamorphosed and deformed rocks comprising these 8 Geology of Northern California

Figure NC.9 Dark-colored xenoliths in Sierra granite represent presence of mélanges, chaotic mixtures of deformed and fragments of older rock assimilated into the magma prior to metamorphosed rocks that are formed in subduction zone crystallization. settings. We will learn more about ophiolites and mélanges when we explore the northern Coast Range,ang where they formed within the much younger Farallonon subductionsu zone. Cenozoic Rocks of the Sierra Nevada:da: At the ende of the Mesozoic Era about 65 million years ago,, the SierraSier Nevada was an elevated volcanic terrainn perchedperch on a complex base-b ment comprised of older rockscks intruded by the concealedconc Sierra Nevada batholith. Ass Cenozoic time (thee Tertiary Period) began, igneous activityity appears to havehav temporarily subsided in the Sierra region, and erosion waswa beginning to attack the summitss of theth dormantant volcanoes.volcano The volcanic highland extended into what is nown western Nevada and the ancient shore of thee ancestral PacificPac Ocean was located along thehe foothillsfoo off the dormantd volcanic arccin in central California.rnia. Rivers drainingning the volcanic highlandghlan ran west acrosss the site of the modernode Sierra Nevada.ada. TheseThe ancient rivers steadily wore awayaw the volcanic bedrock, anda trans- portedpo great quantities of sedimentt to theth ocean basin to thet Frank DeCourten west. Thishis long periodperi of erosion in the EarlyEa Cenozoic EraE producedoduce a pronounced plain,pl which has since beeneen elevatedelev terranes are almost entirely of oceanic origin, and generallynerally hundreds ofo feet above thehe floorsflo rs of modern stream valleys older than the tectonic events that sutured them to the inn the Sierra Sierr Nevada regionn (Figure(Figu NC.11).NC. North American continent. Some represent mafic magmama During the Eocenene Epoch,Epo , aboutabou 505 million years ago, erupted by undersea volcanoes, and the characteristicacteris pillow river-depositedriv sedimentsediments beganbe an to accumulate in the structures can still be observed (Figure NC.10). OtherO ancientanc river systemssys ems of the SierraSie Nevada. In the stream terranes include oceanic sedimentary rocksroc suchuch as chert, channels, coarseoarse gravelravel deposits formed. On the - mudstone, and limestone that havee been metamorphosedmorphosed plains, fine sand,sand silt, and mud accumulated. These river- to varying degrees. There is strongg evidencee that someome ofo depositedited sedimentss iment are still preserved in many places in the accreted fragments traveled thousands of kilometersk the foothills ofo thehe SierraS Nevada as colorful exposures of before the now-vanished plates carryingarrying them sanksa into conglomerate,glomerate, sandstone,sa and mudstone. Geologic map- ancient subduction zones. Ophioliteses, sequences of rocks ping in the early 1900s demonstrated that exposures of the representing oceanic lithosphere, are presentresent in i some ter- ancientancien river gravels g were aligned as elongated ribbons, indi- ranes, and probablyobab originated as fragments splintered from cating theth location and drainage pattern of the 50-million- the descendingding oceanicocean plateses suturedsutu into the variousvario ter- year-old river channels. Soon after the California Gold ranes. Addingding complexitycomplexi too some of the terraneses is the RushRh began, prospectors discovered that some of the -ageE river gravels contained rich concentrations of Figure NC.10 The roundedro pillowillow structuresstru in this outcrop of gold in the form of nuggets, flakes, and fine particles. The mildly metmorposedmorp in the western metmorphic belt indicate richest gravel deposits became known as the “auriferous eruptionon of ono the seafloor. gravels.” Such gold-bearing sediments were intensely mined and processed in the mid-1800s, primarily by wash- ing away the weakly-cemented material with powerful steams of water and separating the small gold particles from the loosened sand, pebbles, and cobbles (Figure NC.12). Known as hydraulic mining, this process was phased out after 1884 because so much sediment had been washed from the Sierra slopes that agriculture and river navigation downstream in the Central Valley was being adversely affected. The best exposures of the auriferous gravels today are in the scars and excavations left from the hydraulic min- ing activities more than century ago. During the and Epochs, 35 million to 5 million years ago, volcanic activity resumed along what is today the crest of the Sierra Nevada and areas to the east. The initial eruptions were violent -forming explo-

Frank DeCourten sions that sent great flows of ash and fragments of volcanic Northern California.2 The Sierra Nevada: California’s Geologic Backbone 9

Figure NC.11 Beyond the deep canyon of the Yuba River on the Sierra Nevada west slope, the flat surface reflects a lengthy interval of erosion in the early Tertiary Period. Frank DeCourten

Figure NC.12 Hydraulic mining in the Sierra Nevada involved Figure NC.13 Oligocene tuff near Soda Springs, California washwashing Eocene sediments with powerful jets of water. Photohoto takentake consists of ash partcles and white fragments welded into a in 1905 at Junctionon City,City California. coherent rock. Bettmann/Corbis Frank DeCourten rock surging west through the canyons carved by the Eocene-age rivers. These pyroclastic deposits hardened into tuffs and welded tuffs that partially filled the ancient the ancestral Sierra Nevada, forcing the ancient streams canyons (Figure NC.13). The later eruptions produced from their channels. In the past few million years, as the basalt flows, volcanic breccias, and volcanic modern Sierra Nevada rose, vigorous down-cutting has left deposits known as . The Tertiary-age volcanic rocks the erosion-resistant valley-filling Tertiary volcanic rocks completely filled some of the valleys of the western slope of perched high above modern river beds. Sinuous ridges 10 Geology of Northern California

capped by Tertiary volcanic rocks demonstrate the phe- Figure NC.14 Inverted topography in the Sierra Nevada foothills nomena of inverted topography in many places in the is a common consequence of Miocene volcanic activity. Sierra foothills (Figure NC.14). Recent Uplift of the Sierra Nevada: Though the rocks of the Sierra Nevada document a long history of geologic unrest, good evidence suggests that the modern range reflects a relatively recent pulse of uplift. Although the Sierra Nevada was probably an elevated tract of land since the mid-Mesozoic Era, recent geologic studies suggest that 5 million to 10 million years ago, the mountain system rose to its current elevation, primarily by westward tilting along normal faults located along the eastern escarpment. These faults, many of which remain active today, have uplifted and tilted the range to the west, producing a spec- tacularly rugged eastern escarpment and a gently inclined western slope. The steep eastern escarpment of the Sierra Nevada was a formidable barrier to the migration of peo- ple during the Gold Rush. Even today, only a few highways cross the crest of the northern Sierra Nevada through (a) passes that range in elevation from more than 3,000 meters (9,945 feet) to just over 2,200 meters (7,259 feet) above sea level. Geologists have not yet determined the precise causeuse for the recent uplift of the Sierra Nevada. The forces affectingting the Sierra region are complicated by its proximityprox too regions of differing plate tectonic interactions.ns. In the BasinB and Range province to the east, tensionalal stresses are stretching the crust and producing thehe numerousn rous normal faults, such as those along the easterntern escarpmentnt of the Sierra Nevada. To the west, the modernmod transform bound-bound ary between the Pacific and North American plate producesp shear stresses that tend to move thee Sierra Nevada block to the northwest. Recent studies also suggestuggest that the dense lower crust of the southern Sierra Nevadad may have been removed in late Cenozoic time by heat associated with material upwelling fromfro thee mantle.mantl The removaloval ofo the (b) dense roott beneath the SierraS a NevadaNevad would have increasedncrea the buoyancy of the SierraSie Nevada, causing it to rise. All of these factors may have been involvedinvolve in the recent ascentt of the Sierraa Nevada.Nev Whateverr the precise pr cause of uplift may be, therere is no doubtd that it is still continuing.ntinu The northern portionn of theth Sierra Nevada is currently risingr at a rate of 2 to 3 millimeters per year, estimatedmated on theth basis of stream incision ratess and the increased tiltilt of ancienta streambeds. The Motherther Lode:L Gold and Geology of the Sierra Foothills: CaliforCalifornia iss known asa the “Golden State” for a reason: since the initiall discoverydiscover in the mid-1800s, more than 115 million ounces of gold have been produced in the state, an amount equivalent to a volume of more than 190 cubic meters! At current prices, the cumulative value of California gold is more than $110 billion. Though gold has been found in many places in California, more than 75% of the amount recovered historically has come from the west- ern foothills of the Sierra Nevada. It was also in this region

that James Marshall first noticed the glittering nuggets in James S. Monroe the American River in 1849 and ignited one of the most dra- (c) matic human migrations in history. Northern California.2 The Sierra Nevada: California’s Geologic Backbone 11

Gold was initially discovered in California as flakes and Mesozoic era. During metamorphism related to either bur- nuggets resting in modern river gravels. Such deposits are ial, tectonic accretion, or the emplacement of magma, hot known as placers, and early miners invented several tech- fluids were introduced into the rocks of the Mother Lode niques to wash the gold particles from the loose river sedi- belt and migrated through themhem along fractures, faults, or ments. Soon, miners began to exhaust the placer deposits, shear zones. Interactions betweenbet the circulating hot and looked upstream from the gravel bars to find the source (hydrothermal) fluids and the metamorphicmetamo rock concentrat- of the gold particles. They eventually found two sources for ed gold in the quartz-rich fluids. TheT hydrothermal fluids the placer gold. First, it was discovered that the Eocene-age eventually cooled as theythe circulatedulated throughthroug the fractured river deposits were also rich in gold, particularly the lower metamorphic rock,ck, leaving veinsv of quartzquar laced with pure channel-filling conglomerates. The “auriferous gravels” gold. When the gold-rich metamorphicmeta ph rocks were eroded were more difficult to work than modern placers because by Cenozoic-agee streams, the heavyhe gold particles accumu- they were more consolidated and generally exposed high lated in nearby streamtream sediments and the less dense compo- above the modern streams. The invention of the hydraulic nents of theth bedrockk were washedwash farther downstream. mining methods mentioned earlier helped to solve these In the MotherMothe Lode belt, all three types of gold difficulties, but the disastrous environmental consequences deposits—placers,s—placers, auriferousauri gravels,s, andan lodes—were pres- ended this practice in the 1880s. At the same time, prospec- ent.e It is not surprisingsu that this regionegioion was the primary tar- tors discovered a second source of the placer gold: quartzrtz get ofo thee GoldG Rush prospectorsector and miners. Though veins in the bedrock in the western foothills metamorphichic productionroducti of gold in Californiaornia fell dramatically after the belt were found to contain large masses of native gold. Suchuch 1860s, peoplepeo occasionally still find gold in the streams of gold-bearing quartz veins are called lodes, and the largestest the Sierraerra Nevada. concentration of such ores in the Sierra foothillss becamebec known as the Mother Lode (Figure NC.15). Geologists are still not certain about thehe source of all the gold in California, but the variety of ore typespes suggests mul- tiple origins. Much of the Mother LodeLod gold may haveh been Sectionion NorthernNo thern California.2C Summary originally disseminated in the oceanicceanic metamorphicmeta hic rocksroc or in the younger igneous bodies. Some of theth gold may have ● The Sierra NevadaNev is California’s geological backbone originated in the volatile gasesgase associatedociated with thet granitic anand thehe largestlar mountain system in the state. It is a magma that intruded thehe SierraSie Nevadada basementbasemen during the regregion off majesticma scenery with three national parks, and the hhighest peaks in the coterminous United States. ● Thehe core of the Sierra Nevada is comprised of the SieSierra Nevada batholith, a large mass of granite emplaced Figure NC.15 Specimen of gold from GrassG Valley, California.ifornia. Inset map shows location of the MotherMothe Lode region in thee western iinto older rocks during the Mesozoic Era. Magma rising foothills of the Sierra Nevada. from the Farallon subduction zone constructed the batholith beneath a volcanic arc similar in origin to the modern Mountains of . Surrounding the Sierra Nevada batholith are older metamorphic rocks in which numerous accreted terranes can be recognized. The metamorphic rocks form roof pendants and xenoliths in the batholithic rocks and extensive bedrock exposures in the western foothills. Cenozoic sedimentary and volcanic rocks record more recent periods of erosion and pyroclas- tic volcanic eruptions in the Sierra Nevada region. ● Though the Sierra Nevada was an elevated region

oby during the Mesozoic Era, the uplift of the modern mountains appears to have occurred primarily during the past 5 million to 10 million years, when normal faulting along the eastern side was initiated. The Sierra Nevada was lifted and tilted to west during this time, creating the gentle western slope and the abrupt eastern escarpment. ● Gold in the Sierra Nevada was concentrated in plac- ers, Eocene stream gravels, and in lode deposits of the western foothills. In the famous Mother Lode belt of the Sierra Nevada foothills, all three types of gold deposits are present. National Museum of Natural History, Specimen #R12197, Photo by D. Penland. National Museum of Natural History, 12 Geology of Northern California

Northern California.3 this province have a crude north-to-south alignment or a weakly curved trend. The Klamath Mountains Even a quick glance at the geologic map of California reveals a strong similarity in outcrop colorsrs anda patterns of The Klamath Mountain region is a rugged highland with a the Klamath Mountains region and the SierraSier Nevada (see general elevation of 1,500 to 2,150 meters (5,000 to 7,000 Figure NC.2). These similarities arisese from comparable,com feet) above sea level. This elevated terrain is deeply incised though not identical, geologic histories in the twotw regions. by the and its major tributaries, the Trinity In a general sense, the Klamathth MountainsMo s can be consid-con and Salmon Rivers. These rivers carry water to the Pacific ered a northwest extension of the geologicgeolo trends ofo the Coast through winding and spectacularly rugged canyons Sierra Nevada. However, thehe continuity betweenbe n the two (Figure NC.16). The deep river gorges separate the regions is broken by the youngng volcanic rocks of the south- Klamath highland into several distinct mountain ranges, ern Cascade Range and by thehe sediments ofo the northern including the and the Siskiyou, Marble, and part of the Centralal Valley.Valle The Klamath MountainsM shares Salmon Mountains, some of which extend northward into with the Sierra Nevadavada a long historyh of subduction-related . These rugged mountains are generally accretion, beginningg in the PaleozoicPaleo Era duringuring which lower than the Sierra Nevada to the south, with the maxi- numerousus oceanicoce terranesrranes collided with the westernwesest edge mum elevations reaching only about 2,750 meters (9,000 of Northrth America.America Thee dozendo or so terraneses recognizedre by feet). The Klamath Mountains separate the northern Coast geologistsgists in the KlamathKla ath region are separatedparated fromf each Range on the west from the volcanoes of the Cascade Range other by major east-dippingeast-dip fault zones. Granitic plutonsp of to the east. Unlike most other geologic provinces in north- MesozoicM zoic age were emplacedem intoo the accretedcreted terranes in ern California, the Klamath Mountains lack a prominent severalsever areas of the Klamath region,ion, thoughthou such rocks are northwest orientation. Instead, the individual mountains in notot as widespreadw in thee KlamathK math region as theyt ey are in the Sierra Nevada.Ne Both theh SierraSie Nevada and the KlamathK Figure NC.16 The rugged canyon of the Salmon River in the Mountains experienced glaciationaciati duringduri the Pleistocene Klamath Mountains privince. ice ages, though the lower elevationse vation in the Klamath region resultedres in smallerr and less extensiveext nsive . The Klamath MountainsMo regiongion also experiencedexperi gold mineralization during the lateate MesozoicM ozoic Era, though none of the Klamath gold districtsts werewer as richch asa the Mother Lode of the Sierra Nevada.da. DespiteDe ite theseth similarities, the geologic story of the Klamathmath MountainsMou ains is not exactly the same as the Sierra Nevada.vada. In the followingf sections, we focus on the unique attributesattri tes of the Klamath Mountains with respect to the similarsimila Sierra Nevada. KlamathKlam Mountain Terranes and Ophiolites: In gener- al, the Klamath Mountains region consists of numerous oceanic terranes representing fragments of crustal material thatth were embedded into the western margin of North America since Early Paleozoic time. The fragments include metamorphosed volcanic and sedimentary rocks that represent volcanic island arcs, submarine plateaus, reeflike bodies of limestone, and deep ocean sediments that were intensely deformed during accretion. It has taken geologists many years to unravel the complicated tectonic pattern in the Klamath Mountains, but it now appears that as many as a dozen different accreted terranes exist in the region. Some of these terranes no doubt represent the same fragments recognized in the western metamorphic belt of the Sierra Nevada. In general, the Klamath terranes become younger from east to west, a pattern that probably reflects the westward growth of North America through a series of plate collisions along the western edge of the con- tinent. Some of the Klamath terranes may have originated thousands of kilometers from North America, whereas others were probably of local origin. Fossils discovered recently in the eastern Klamath terranes suggest that rocks as old as Late Precambrian, nearly 600 million years old,

Dick Hilton are included in the oldest accreted fragments. Northern California.3 The Klamath Mountains 13

Figure NC.17 Ophiolites are sequences of rock that represent younger, about 162 million years old, and is exposed in the oceanic lithosphere emplaced on land at convergent plate boundaries. northwestern part of the Klamath region. The Josephine ophiolite may be in part equivalent to similar rocks in the western metamorphic belt of the Sierra Nevada. Neither the Josephine nor the Trinity ophioliteophio sequences are preserved in their entirety in the Klamathlamath Mountains;Mou they were dis- rupted and metamorphosed during accretion. Nonetheless, Deep-sea there is little doubtt that ophioliteso tes of the KlamathKla Mountains sediments represent fragmentsents of accretedaccrete oceaniceanic lithosphere.lit Pillow Mesozoic Plutonictonic Rocks of theth Klamatham Mountains: The Mesozoic Farallonon subduction zonezon that generated great vol- umes of felsic magmaagma in the SierraSie Nevada region extends Sheeted Oceanic dikes north to theth Klamathmath region.region However, the magma that crust intrudeded the KlamathKlam basement was emplaced mostly as iso- Massive gabbro lated plutonsutons of relativelyrelativ limited sizeize and did not coalesce intoin largerer massesma similar to the SierraSierSi Nevada batholith. Layered Thee relativelyr vely small exposures of light-coloredlig granitic rocks gabbro Upper inn the KlamathK mat Mountains contrast with the darker meta- mantle Peridotite morphic rockro of the terraness to create spectacular landforms such as CastleC Crags (Figure(Fig re NC.18). TheTh Mesozoic-age plutons of the Klamathath region rangeange in age from about 136 million to 174 millionlion years, and tendend to be somewhat more mafic (containingnin more ferromagnesianferrom i silicate minerals) than the granitictic rocksroc comprisingcomp the Sierra Nevada. Gold and Chromitehrom Mineralization: The geologic Of particular interest in the KlamathKlam regionn are exten- similarityarity betweenbetwe theth Klamath Mountains and the sive exposures of mafic and ultramafic rocks knownknow as MotherMo her Lode regionregio of the Sierra Nevada did not escape ophiolites, sequences of igneousigne rocks thought by geolo- the noticetice ofo prospectors and miners in the mid-1800s. gists to represent disruptedpted oceanico lithosphere.ithosphere. Ophiolites GoldGol was discovereddisc in placer gravels along the Klamath consist, in ascending order,or of upper mantlemant peridotitete andnd TrinityT rivers soon after the Gold Rush began. overlain by layered and massive gabbro,ga sheeted basaltasalt AdditionalA ition gold deposits were eventually located in dikes, and basaltic pillowlow lavas (Figure(Figur NC.17). Ophioliteiolite ancientanci river sediments and in the metamorphic basement sequences can be emplacedaced on land during platete conver-con r- rocks.rock Ultimately, the Klamath Mountains became the gence simultaneously with the accretion ac process.ess. As rela-re seconds largest gold producing region in California. tivelyively thin and dense oceanic plates sink into subduction During and just after World War II, deposits of chromite zones, fragmentsfr ts of theth descendingg lithospherelith e are some- were extensively mined in the Klamath region. Chromite is times incorporatedincor ated intoint the accretionarynary mass.m The heat, he an iron chromium oxide mineral (FeCr2O4) that is the only pressure, anda chemicallyhemical active fluids thatat accompanyaccompan plate source of chromium, a valuable metal used in the manufacture convergencevergen usuallysually resultre in significant deformation,eformat dis- ruption,ru and metamorphismetamor of the ophiolite sequences. In manyma exposures of the KlamathKlam Mountains ophiolites, the Figure NC.18 is an exposure of 167-million-year old mafic igneous rockss have alteredalte into the metamorphic rock granite in the Klamath Mountains. serpentinite, but careful studiesstud can still reveal their origin as componentsco of oceanic lithosphere. We will learn more aboutabou sepentinitentinit in our examination of the northern Coast Ranges,Rang where they are also abundant. In the Klamath Mountains region, the Trinity and Josephine ophiolitesioli are two of the largest and best known rock sequences of their type in the world. These two rock assemblages represent slivers of oceanic lithosphere that were incorporated into the accreted terranes at different times and different places in the Klamath region. The Trinity ophiolite is part of the Paleozoic–Early Mesozoic Eastern Klamath-Yreka superterrane, which is very similar to the Northern Sierra of roughly the same age to the south. Most geologists feel that the Trinity and the Northern Sierra terranes were originally part of the same accreted

oceanic plate. The mid-Mesozoic Josephine ophiolite is Frank DeCourten 14 Geology of Northern California of specialized steel alloys and in corrosion-resistant steel (50 miles) wide. The northern portion of the Great Valley is plating. Chromite is very rare in crustal rocks, but compris- known as the , and the southern two es a significant fraction of the upper mantle material. In the thirds is designated as the . These names Klamath Mountains region, the mantle slices preserved in originate from the two large rivers systemsms that drain the the Trinity and Josephine ophiolites contained numerous interior basin from the north and south,, respectively.resp These pods of chromite-bearing rock that were rich enough to be rivers meet southwest of Sacramento in the DeltaDel region mined. Much of the ore was depleted after the 1960s, and and eventually drain into the San Franciscoco Bay. production of chromite from the Klamath Mountains has The Sacramento Valley is a remarkablyremark flatlat interior basinb since declined dramatically. For a time, however, the surrounded by elevated terrainerrain to theth west (Coastal(C Klamath Mountains was the primary source for this valu- Ranges), north (Klamath Mountainsountains and CascadeC de Range), able strategic metal. and east (the Sierra Nevada). The system receives runoff from these adjacent highlandshighlan via several major tributaries includinginclud thee Feather, Yuba, American, Section Northern California.3 Summary Pit, and Bear Riversers (Figure NC.19). Because northern California receives significantly morem rain thanan southerns ● The Klamath Mountains province is a mountainous California,ia, theth Sacramentoamento River is California’srninia’s largest upland deeply incised by the Klamath River and its trib- river, carrying aboutabo 188 millionm acre-feet off water,wat six times utaries. Several distinct mountain ranges comprise the more than the San JoaquinJo uin River, towardd the sea annually. region, with elevations up to 2,750 meters (9,000 feet). This water is a criticallycriticall important resourcerce for agriculture,ag In contrast to the strong northwest orientation of industry,in ry, commerce, anda domestic use statewide.s tewide. California’s major landforms, the ranges in the Klamath SoilsSo and AgricultureAgricu in the Great Valley:Va y: The surfacesurfa Mountains province have a weak north-to-south align- off the Sacramento ValleyVall is covered by recent and ment or a curved trend. Pleistocene-agePleistocen alluvium washedwa ed into the bottomlandsb l by ● The bedrock of the Klamath Mountains province has streams draining dra the adjacentent highlands.hig ands These stream sed- strong similarities with the Sierra Nevada too the south, iments consist of a heterogeneoushetero neous assemblage of channel and can be considered as a northwest continuationnuati of the gravels,gra river banknk sands, silt,silt and deposited on the geologic patterns of the later. In both areas,eas, metamor-metamo broadbro floodplain,ain and in some places,p peat deposits repre- phic bedrock comprising multiple terranesrrane is intrudedntrude by senting plantnt litterlitt thathat accumulatedac in lakes and wetlands. Mesozoic granite plutons and overlainrlain by Cenozoic-agezoic-age The rivers drainingdrain thee SacramentoSa Valley typically follow sedimentary and volcanic rocks. TheTh rocks comprisingrising meanderingdering coursescourse that shift continuously across the some of the Klamath Mountainsns terranes are almost identical in type, age, and origin too those of the northernno Figureure NC.19 TheT drainage basin of the Sacramento River. Sierra Nevada. At least a dozen terranesranes have beenbe iden- tified in the Klamath Mountains, some off which can be assembled intonto largerla superterranes. ● Ophiolites,olites, representingrepresen fragmentsfragm of oceanicnic litho-lit sphere, arere prominent in the terranester of the Klamathamath Mountains region.gion. TheTh Josephinesephine and Trinity ophiolitesites are dominated by maficm igneousneous rocksro commonly altered to greenstoneeenstone and serpentinite. ● Goldld and chromite have been minedm from the Klamath Mountains region. The gold probablypr originat- ed in a manneranner similar to the MotherMothe Lode deposits of the Sierra Nevada.Neva The chromitech deposits are concen- trated in the mantlem rocks presentpre within the Klamath Mountains ophioliteophi sequences.equences.

Northern California.4 The Great Valley A Great Depression: Nestled between the Sierra Nevada on the east and coastal mountains to the west, the Great Valley of California is a vast elongated basin extending nearly 700

kilometers (430 miles) and averaging about 80 kilometers Map courtesy of the U.S. Geological Survey. Northern California.4 The Great Valley 15

nearly flat floodplain. Over time, the migrating rivers have thickness. Because they are folded downward to depths of deposited thick sequences of interlayered river sediments. 15,000 feet or more, the strata of the Great Valley These alluvial deposits are generally several hundred Sequence are only exposed at the surface in the foothills of meters thick, but can be much thicker in the lower areas on the highlands on either side off theth valley. To the east, along the west side of the valley. the Sierra Nevada foothills,ills, thet rock layers dip gently The soils developed on the alluvium of the Great Valley toward the center of the valley, althoughalth they are steeply are among the most fertile in the world. These soils, coupled upturned to the west, wherehere they are exposed in the with abundant water, mild climate, and a lengthy growing foothills of the Coastoast RangesRa (FigureFigure NC.21).NC.2 season have made the Great Valley one of the most agricul- The Great Valleyalley SequenceSequenc is comprisedcompris mostly of sand- turally productive regions in North America. The annual stone and shale representing sedimentsed t that accumulated in a value of California agricultural products exceeds $32 billion deep forearc basinsin that developed in the Mesozoic Era. This and most of the products are grown in the Great Valley. In basin was located between the offshoreof trench and the ances- the Sacramento Valley, the principal crops are rice, almonds, tral Sierraierra volcanic arc above the Farallon subduction zone walnuts, orchard fruits, grapes, and feed grains. Even before (Figuree NC. 22). MuchM of the sediment eroded from the vol- cultivation, the fertility of the Great Valley was noted by canic andnd metamorphicmetamorph rocks in the Klamath and Sierra early explorers who reported that the Sacramento River NevadaN regionsregion was transported too the deep seafloor by flowed through an incredibly lush grassland, with largege turbiditybid currentscurr . These greateat underseaun proba- stands of oaks trees and numerous wetlands. bly were triggeredrigg by volcanicanic eruptions,erup earthquakes, or Soil fertility is the consequence of several inter-relatedted storm eventseven along the ancientnt . Repeated factors, including the texture, chemistry, micro floraflo andnd depositionosition of sedimentnt by turbidityurbidity currentscurren leads to the fauna, moisture, and organic content of the soil. However,Howe developmentdevelop of submarinemarine fansfa on the seafloor,seaf and geolo- some vital nutrients can be obtained by plants onlyy from gists recognizeniz sedimentdiment of this typet einman in many portions of the chemicals released during the weatheringg of mineral grains Great Valley Sequence.Sequ nce. Some of the sediments in the Great in the soil. These mineral nutrients includeclude potassium, Valley Sequencece are richich ini organic matter, primarily the magnesium, and calcium. The alluviumuviu of thee SacramentoSac remainssofmic of microorganismsorgan that continually rained down to Valley is naturally rich in mineralsals such as feldsparar and mica the seaflooreafloor from theesh shallow waters above. Over time, this that release these nutrients as they undergoun chemicalche organicrganic matter has become transformed in the subsurface to weathering. These mineralral grains,g in turn, originatedorig pri- the combustiblembusti hydrocarbons that comprise natural gas. marily in the plutonicc rocksrock and associatedsociated metamorphicm WhereWh thesese gasesg are trapped in the porous subsurface sand- terranes of the Sierra NevadaNe and the KlamathKlama Mountains.s. stones,ones, productive gas wells can be developed. From 1977 to The agricultural productivityuctivity of the SacramentoSa Valley soils 1998,19 8, theth annual production of natural gas in the Sacramento reflects, in part, the vast exposures ofo these rocks inn the ValleyValle region averaged about 100 trillion cubic feet. The upper reaches of the Sacramentoramento RiverRiv watershed.d. majoritymaj of this gas originates in the Great Valley Sequence. Deeper Structure of the GreatG Valley: Beneathath the allu-al Above and Below the Great Valley Sequence: The sedi- vium and soils at the surface, the bedrock off the Great ments of the Great Valley Sequence were deposited on top of Valley is ccomprisedised of a thick sequencence ofo mostlyy Mesozoic oceanic lithosphere that can still be recognized by geophysi- and CenozoCenozoic sedimentaryedimen rocks that are downfoldeddownfoldddi in a cal techniques. The deeply buried oceanic crust in the Great great asymasymmetricalical synsyncline (Figure NC.20).C.20). These strata Valley is 7 to 8 kilometers thick and the moho beneath is 11 are calledalled the Great ValleyVa Sequence, an enormousormou mass of to 16 kilometers below the surface. Surprisingly, this relative- ococeanic sedimentsents exceeding exc 6,000 meters in maximum ly dense oceanic basement appears to be perched above the western extension of the Sierra Nevada batholith. The pres- ence of a thick slab of dense oceanic rock deep beneath the Figure NC.20 Generalized diagramdia of the subsurface structure of Great Valley may act like a gravitational anchor, helping to the GGreat Valley, Coastt Ranges, anda Sierra Nevada. explain why this region has remained so low with respect to the surrounding terrain. The forearc basin in which the Great Valley Sequence accumulated persisted into the early part of the Cenozoic Era. By about 25 million years ago, the rate of subsidence decreased and the basin began to fill in the north. Starting in Late Cenozoic time, the oceans gradually drained away from the Great Valley, and all of the sediments washed into the lowland accumulated in terrestrial environments such as river beds, lakes, and swamps. As we will soon see, volcanoes in the Sierra Nevada and Cascade Range became active at roughly this same time, and some of the erupted material flowed or fell into the Great Valley. Consequently, along the

Diagram courtesy of the U.S. Geological Survey. modern periphery of the Great Valley, the Late Cenozoic 16 Geology of Northern California

Figure NC.21 Submarine fan deposits of the Great Valley Sequence exposed in the foothills of the Coast Ranges dipping steeply to the east (left). Frank DeCourten

Figure NC.22 The Mesozoic forearc basin in which sediments of FigureFi re NC.23NC.2 The are the only prominent moun- the Great Valley Sequence accumulated was located between the taintains in the Great Valley of northern California. ancestral Sierraa NevadaNeva volcanic arc and relatedate to the Farallon subduction xone.x

Volcanic chainn Franciscan Great Valleyalley sequenceseque subductionn ccomplex in forearcorearc basin

Oceanicc crust Descending Farallon plate of North American plate Diagram courtesy of the U.S. Geological Survey, Frank DeCourten rocks are characterized by interlayered volcanic and terres- monotonously flat interior of the Great Valley that they trial sedimentary rocks. have attracted attention ever since humans first arrived in The Sutter Buttes: About 12 kilometers northwest of northern California (Figure NC.23). The core of the Sutter Marysville stand the Sutter Buttes, a circular cluster of Buttes consists of several domes emplaced as shal- ragged volcanic peaks, about 17 kilometers (10 miles) in low intrusions from 1.6 million to 1.3 million years ago. diameter, that rise to an elevation of more than 650 meters Surrounding this core is a circular zone of slightly older (2,100 feet). These peaks are such a striking feature in the pyroclastic material of andesitic composition. Geologists Northern California.4 The Great Valley 17

Figure NC.24 Flowing on a low gradient with an extensive flood plain, the Sacramento River is subject to seasonal flooding that places millions of people at risk. James S. Monroe

generally consider the Sutter Buttes to be the southernmostst Figure NC.25 Flooding in the Sacramento Valley, 2005–2006. expression of volcanic activityivity relatedrelate to the Cascadescade RangeRange Photo on the left was taken from space on Dec. 10, 2005 immedi- to the north. However, development of the Sutterutter ButtesBu ately prior to heavy late winter rains. Photo on the left is from Jan. 4, may alalso have been influenced byy theth migrationation of the 2006. Note that the Sutter Buttes were entirely circled by flood Mendocino tripleple junction.junc We willl reviewrevi bothh of thesethes waters from the Sacramento River. aspects of northernn hern CaliforniaCal geology in later sections.section FloodinFlooding inn the SacramentoS Valley: Because it is a nearly flatfl surfaceface draineddr by California’s largestlar river (Figure(F NC.24), the Sacramento Valley is prone to recur- ringrin floods (Figure NC.25). The annual flood cycle gener- ally peaks in latee winter and early spring as heavy winter rains give way to the melting of the snow pack in the bor- deringderin mountains.unt BeforeBef people settled there, the flood- plain adjacentcent to the Sacramento River and its tributaries was repeatedlyatedly inundatedinun in a cyclic pattern. The annual floods reneweded the floodplain soils and scoured the channel of accumulated sediment and . Today, the intense agricultural and residential develop- ment in the Sacramento Valley has resulted in a serious flood hazard for millions of people. Billions of dollars have been invested in flood control measures that include large in the foothills, miles of levees, and river bypass channels. Even with these measures, the threat of seasonal floods has not been eliminated from the Sacramento Valley. Attempts to reduce flooding along the Sacramento River have also dimin-

ished the beneficial effects of this natural process. Photos from NASA Earth Observatory. 18 Geology of Northern California

Section Northern California.4 Summary Northern California.5 ● The Great Valley is an elongated lowland situated The Northern Coast Ranges between the Sierra Nevada on the east and the Coast The rugged of California is one ofo the most sce- Ranges on the west. The northern portion of the Great nic seashores in the world. In most placesaces along this rocky Valley is referred to as the Sacramento Valley, named for coast, beaches are narrow and sea cliffs rise from thet water’s the large south-flowing river system that drains it. The edge to elevations of several thousandousan metersters in just a few watershed for the Sacramento River includes the high- kilometers. Spectacular examplesmples of coastaltal landformslandf lands of the northern Sierra Nevada, Cascade Range, such as sea stacks, sea archesrch , andnd marinem et terraces Klamath Mountains, and the northern Coast Ranges. (Figure NC.26) provide evidenceence of recent emergenceem of the Because these mountainous regions receive more precip- northern California Coast. Thehe vigorous erosionerosio that accom- itation than other regions of the state, the Sacramento is panies such an activelytivel rising coast is responsibleresp for the California’s largest river. spectacular ruggednessness of the Northern California seaside. ● Great quantities of alluvium have been deposited The relatively youngoung coastal mountainsm of CaliforniaC across the floor of the Great Valley by rivers draining define thee CoastCo Rangesges physiographicphysiograp provincee thatth extends the adjacent highlands. The fertility of the soils of the more thanhan 6006 kilometersters (960 miles) from theth TransverseT Great Valley reflects, in part, the concentration of vital Rangeses in the south too theth Oregon border,rder, andan beyond. mineral nutrients derived from the weathering of rocks Withinin this region, coastalcoas l mountains suchh as the SantaS Cruz in the adjacent watersheds. The natural fertility of the Mountains,M ntains, the MendocinoMendo Range, the Gabilan Range, anda Great Valley soils, coupled with the prevailing climate the Diabloblo Ranges of northern Californiaalifornia are all aligned ini a and abundance of water, has made the Great Valley consistentonsist northwestth trend.end Thehe Coast RangesRang are generallygener one of the most productive agricultural regions inn the oriented parallel to theeG GreatGr Valley and Sierrarra Nevada N world. provinces tot the east, and have a similarmilar extent.e This suggests that the geologic evolutionlution off these regionsgio may be linked to ● In the subsurface of the Great Valley, MesozoicMesoz and Cenozoic sedimentary rocks of the Great ValleyVall Sequence are downfolded in the form of a largerge syncline.syncli Figure NC.266 Steepteep cliffs, sea stacks,sta and sea arches charac- terize northern California’sCali a’s emergentem coast. The Great Valley Sequence sedimentsnts accumulateda ulated in a deep forearc basin associated withh the Farallon subduc-ubduc- tion zone. Most of the sandstone and shale in these rocksock was transported to the deep oceann floor by turbidityturbidi cur- rents and accumulated as submarinene fans. OrganicOrgan mat- ter in parts of the Great Valley Sequenceuence has produced significant quantities of natural gas. ● The Great ValleyValle Sequenceuence accumulated on densed oceanic lithosphere thatth lies deepde under thee GreatGre Valley. Thiss dense slab of rockock is thoughtth to be relateded to the low elevationon of the region ini relation to the sur-ur- roundingg highlands.hig ● Thee Sutter Buttes are a distinctiveive clusterc of volcanic domess that tha erupted in the Sacramentoacramento ValleyV 1.6 million to 1.3 million years ago. The magma thattha surfaced in this location may be relatedd tot the CascadeCasc Range volcanoes to the northth or tot the migrationmigrat of the Mendocino alonglong thet coast.oast. ● Because it is a nearly flatat valley va drained by California’s largest river, the Sacramento Valley is naturally prone to cyclic flooding. Over geologic time, the recurring floods that inundated the floodplain have added alluvium to the valley surface and renewed soil components. Flood con- trol measures, such as levees and upstream dams, are necessary to protect floodplain communities and indus- tries, but they also limit the beneficial effects of the annual flood cycles. Sue Monroe Northern California.5 The Northern Coast Ranges 19

some of the same tectonic events. However, the recent emer- Figure NC.27 Chert of the Franciscan complex exposed in gence of the Coast Ranges was strongly affected by the evo- Marin County. Most of the layers are 5 cm thick. lution of the San Andreas fault system and the development of the modern transform boundary between the Pacific and North American plates. Thus, though the older bedrock of the Coast Ranges can be related to broader tectonic patterns, the story of their more recent emergence involves plate tec- tonic events that affected only the coastal region. The have traditionally been divided into northern and southern portions, with Bay arbitrarily dividing the two. However, for sound geologic reasons, it is more meaningful to divide the Coast Ranges into northern and southern portions separated by the San Andreas fault system. This great fault zone slices diagonally across the Coast Range, and the areas on opposite sides have unique and distinctive geologic histories. By this e Mon definition, the northern Coast Ranges extend northwardard Sue Monroe from approximately Monterey Bay and the Gabilan Rangee to the Oregon border. It is this part of the larger Coast Rangesges province that we will refer to as the North Coast Ranges,Rang andnd rockss such as pillow basalt, gabbro, and peridotiteperi are pres- on which we will focus in the following sections. ent. TheTh igneous materialsaterials probablypr ably comprisedcompr portions of The Chaotic Franciscan Complex: In most of theth the Farallonnp plate that originated at ancient spreading ridges, Northern Coast Ranges, the basement rock consist of an as submarine lava flows,ows, or in theth upper part of the mantle. incredibly complex assemblage of intenselyly deformed and As the Farallonn plateplat collidedollide with North America, and was metamorphosed sandstone, shale, chert,cher basalt,t, and plutonic forced downward,downwa , the igneous rocks were subjected to high rocks. In many exposures of theseese rocks, layeringl g and other temperatures,peratures, extremeextr me pressures,p and chemically active fluids internal structures have been obliteratedbliterated byb internalal move- m includingncl ding hot seawater.seaw Alteration of the iron and magne- ments such that some geologistsgeolo have describeddescrib it as a sium-bearingsium earing silicates in the igneous components of the “churned” rock assemblage.blage In the early 1900s,1900s the name FarallonFara platelate produced such minerals as chlorite, pumpel- Franciscan was applieded to these mangledled rocksr for theireir lyite,yite, antigorite,a chrysotile (asbestos), and lizardite. These prevalence in the San Francisco Bay region.re Since then,, the greenish-coloredg nish minerals are abundant in greenstone and Franciscan rocks have become one of the most famouss and serpentiniteserp (Figure NC.28), both of which are common in well studied rock units inn the world. For decadess the com-m- the Franciscan complex. Serpentinite plays an important role plexities of the Franciscan assemblage defieded geologicgeolo in creating the “churned” appearance of the Franciscan com- explaexplanation. With the advent of the plate tectonicnic paradigm plex. It is significantly less dense than the igneous rocks from in the 191970s, geologistseologist began to unscramblenscra thehe origin ofo which it develops. Under the temperature and pressure con- this chaotic rockck sequence.seque Today, thereere is generalgeneral agree- ditions that prevail in subduction zones, serpentinite can ment that the FranciscanFranci rocks representsent a subductionsubd become mobile and plastic, squeezing upward into the over- complex,plex, a mixturexture of rocks that form in associationassociat with lying rocks to disrupt the original layering. susubducting oceanicanic lithosphere. lith The internal structure of the Franciscan rocks is so com- The age of rocks comprisingomp the Franciscan complex plex that it bewildered geologists for decades. The original ranges from aboutt 200 millionmilli to 80 million years. Recall layers of the sedimentary rocks are highly deformed (Fig- that about 140 millionillion years ago,a the Farallon plate began to ure NC.29) or even completely obliterated by serpentinite subdsubduct beneath western North America, generating the intrusion and tectonic deformation. The igneous rocks are magmmagma thatat ultimatelyulti formed the granite batholith of the usually metamorphosed, fragmented, and broken into iso- SierrSierra Nevadaevada (see FigureF NC.22).Offshore of that ancient lated blocks. The entire rock assemblage is further compli- contcontinentall margin, a deep trench developed on the oceanic cated my numerous faults and shear zones that separate the floor. In this trench, sand and rock fragments were washed sequences into discontinuous blocks. Such a mixture of from the continent and accumulated in great thickness. Such deformed, jumbled, and altered rocks is known as a sediments comprise graywacke, an impure sandstone that is mélange, a French term for a mixture of different compo- the most common rock type in the Franciscan assemblage. nents. The origin of these rocks was a mystery to geologists The Franciscan graywacke deposits commonly exhibit graded until the plate tectonic theory provided a new model for bedding, suggesting that turbidity currents transported interpreting the complexity of the Franciscan assemblage. them into the deep basin. Other deep ocean sedimentary The Franciscan rocks, as a whole, are now thought to be rocks in the Franciscan complex also include fine-grained a subduction complex, a heterogeneous mass of deformed shale and chert (Figure NC.27). In addition to the sedimen- and altered rocks that forms in and near subduction zones. tary components of the Franciscan rocks, mafic igneous The sedimentary components of the Franciscan complex 20 Geology of Northern California

Figure NC.28 Serpentinite, like this sample from the northern Coast Ranges, is California’s official State Rock. Frank DeCourten represent sand, silt, and ooze that accumulatedmulated in the trench TheTh subductionsubduc of the Farallon plate persisted under the or on the Farallon plate as it approached North America northernnorth CoastC Ranges for more than 130 million years, during the MesozoicMesozo Era. Some of these sedimentsmen were extendingextendin well into the Cenozoic Era. During this time, a scraped offff the plate as it descendeddescend beneath thehe leadinglea thick wedgew of Franciscan rocks developed along the lead- edge of Northrth America anda deformeddeforme by the immensese pres-pre ingin edge of North America, but the coastal mountain sure generated in the zonezo off plate convergence.c Some sedi- rangesr had not been elevated from the seafloor. About mentary rocks were alsoal pulleded downwarddown by the descendingng 30 million years ago, the plate boundary along the west Farallonn plate tot depths of 15 kilometers or so beneath the coast began to shift from a convergent type to a transform surface.. In this high-pressureh and low-temperaturelow-tem environ- boundary. The emergence of the modern Coast Ranges is ment, blueschistblue was produced,d, a metamorphicmetamo rock rich in linked to development of the modern transform boundary the bluish mineral glaucophane.ne. The igneousign components between the Pacific and North American plates. of the Farallonlon platepl descendedsce to even e greater depths and Evolution of the San Andreas System: Recall that the encounteredd higherhigh temperaturemperatu conditions where the ser- Farallon plate originated at an offshore spreading ridge that pentintite and greesnstoneg nstone formed.form Slices of rock were separated it from the Pacific plate moving to the west (see sometimes broken from thehe descendingdes slab and thrust over Figure NC.3). Segments of this spreading ridge were offset or crushed against the edge of the continent. In addition, the by transform faults to create an irregular divergent bound- Farallon plate was evidently carrying isolated seamounts, ary. Through the Mesozoic and Early Cenozoic Eras, the lava plateaus, and coral reefs that became detached and North American plate was moving faster to the west than embedded into the Franciscan mélange. Geologists currently was the Farallon plate to the east. Consequently, the west recognize no fewer than nine such micro-terranes in the edge of North America eventually collided with the spread- Franciscan Complex in the San Francisco Bay region. The ing ridge as the intervening Farallon plate was consumed by chaotic nature of the Franciscan complex thus reflects the subduction. This collision first occurred at the approximate diversity of rocks types, the varying degrees of metamor- latitude of modern Los Angeles about 30 million years ago phism, the accretion of exotic terranes, and the intensity of (Figure NC.30). One consequence of this collision was the deformation that result from plate subduction. creation of a point where three plates—the Farallon, Pacific, Northern California.5 The Northern Coast Ranges 21

Figure NC.29 Deformed chert in the Marin Headlands terrane Figure NC.30 Evolution of the San Andreas fault system as a of the Franciscan complex. transform boundary between the Pacific and North America Plates. The north of the Mendocine Triple Junction is a remnant of the Farallon plate. tesy o Diagram courtesy of the U.S. Geological Survey.

FigureFigur NC.31 Majorr Fault of California,rnia, including the San AndreAndreas and related stike-slip faults that represent thet boundary between thee PacificPa and North AmericanAmerica plates. James S. Monroe

and North American—met.erican— Such pointsoints are calledlled tripletripl junctions, and they signifysig places wherere different kindsk of plateate boundariesboun s exist in proximity to each other. Afterr the initialnitial tripletri junction developed,ed, thet North AmAmerican plate continued to overrun more of the spreading ridge.rid This caused thee originalorigi triple junction between the Farallon, Pacific, and North American plates to separate into two (see Figuregure NC.30).NC.3 The continued westward motimotion of the NorthhA American plate caused the two triple junctjunctions to migratemigra in opposite directions along the conti- nentalnent margin. TheTh northern triple junction migrated through the San FranciscoF Bay area about 10 million years ago. Today, the Mendocino triple junction is located near , more than 200 kilometers (130 miles) north of San Francisco. The corresponding point to the Map courtesy of the U.S. Geological Survey. south is the Rivera triple junction, located near the south- ern tip of Baja California. and directions of plate motion gave rise to a transform plate The collision between the spreading ridge and the North boundary, in which the two plates slide laterally past each American continent brought the Pacific plate into contact other. The development of the transform boundary between with the North America plate. Because both plates had west- the North America and Pacific plate initiated the famous erly motions, this new boundary ended the long era of plate San Andreas fault system (Figure NC.31). The San Andreas convergence and subduction. Instead, differences in the rates fault is perhaps the best known fault in the world, but it is 22 Geology of Northern California

only one of hundreds of faults in the zone that bears the Figure NC.33 Major faults in the . The same name. Collectively, these faults accommodate the rel- blue arrows represent their average displacement rates. ative motion between the North American and Pacific plates through right-lateral strike-slip displacement (Fig- 123° 122° ure NC.32). It is important to understand that the San A 0 30 Kilometers

Rodgers Creek fault Figure NC.32 The right-lateral displacement on the San Andreas Fault is illustrated by the offet of this stream channel in the southern Coast Ranges. View is to the west from the North GreGreen Valley fault American plate (foreground) to the Pacific plate (distance).

38°

PACIFIC OCEAN San Gregorio fault

Francisco

Hayward fault

CalaverasCala fault San Andreas fault aala

ault Displacement-ratement-rate vector—vec Showingng direction and amountnt of relative movementmoveme 0200 Vector scale—In millimetersimete per year Active fault—Dotted wwhere concealed 37° John S. Shelton Map courtesy of the U.S. Geological Survey

Figure NC.34 Granite of the Salinian Block with dark-coloredark-co xenolithss at PointPo Reyes National Seashore.ashore. This granitenite is very similar to the plu- tonic core of the Sierra Nevada, Dick Hilton Northern California.5 The Northern Coast Ranges 23

Andreas fault system is actually a zone of dominantly strike- The granites of the Salinian block are 78 million to 110 slip faults that ranges from 100 kilometers to nearly 300 kilo- million years old, and are remarkably similar in age and meters wide. The collective motion of these faults allows the composition to the granites of the southern Sierra Nevada. Pacific plate to slip to the northwest relative to the North In addition, metamorphic rockscks associated with the granite American plate at an average rate of about 5 centimeters basement of the Salinian block are almost identical to the (2 inches) per year. This may sound very small, but it is enough roof pendants of the Sierrara Nevada. TheseT similarities sug- to displace older geologic features by more than 300 kilome- gest that the Salinian block was part of theth Sierra Nevada ters (185 miles) since the transform boundary originated. batholith before thehe developmentdev ent of the SanS Andreas fault In the San Francisco Bay area, the San Andreas fault is system. The Salinianlinian block has since beenb transported a one of several large right-lateral faults that comprise the minimum of 5500 kilometers (340 miles) to the northwest by plate boundary (Figure NC.33). Other major faults in this right-lateral displacementplacement along thist fault. region include the Calaveras, Hayward, and San Gregorio The Rise of thehe Northern CoastC Ranges: Overlying the faults that are nearly, but not exactly, parallel to the plate basementment rocks of both thet Salinian block and the boundary. The San Andreas fault extends offshore from Franciscanscan complexcomple are as much as 6,000 meters (nearly Point Arena to the Mendocino triple junction. The San 20,000 feet) of Cenozoic-ageCen sedimentarysedi rock that Andreas fault zone produces continuous earthquakes, some recordsr thehe emergenceem of the modernodeern Coast Ranges. This of which pose serious threats to California residents. We sedimentaryim ry recordr is complicatedicate by motion along the will examine the pattern of seismic activity along the San San AndreasAnd as system,s but it is clear thatt there were several Andreas in a later section. different basins where oceanicanic and terrestrial sediments The Salinian Block—An Immigrant from thee South:Sou In accumulated.umula The olderder depositsde osits in these basinsba are mostly the northern Coast Ranges, rocks of the Franciscann complexcomp mud, ooze,o and sandnd that settledse ed out on the continental are absent west of the San Andreas fault. In places suchuch asa shelf. Someme sequences,ences, such as thehe MontereyMon Formation Point Reyes National Seashore, the basementment rocks consist (Miocene Epoch;och 111 million to 22 million years old), accu- of Mesozoic-age plutonic and metamorphicic rocks overlain mulated in deepeep water,w er, wherew organic material became by Cenozoic-age sediments (Figureure NC.34).N TheseThe rocks concentratedtrated ini the mudm and ooze (Figure NC.36). The comprise a block that can be tracedaced south from Point ArenaA Montereyterey FormationForma ni is believed to be the source rock of to the southern Coast Ranges. Because suchsu materialsria are muchmuch of California’sCalifornia petroleum. especially prominent aroundund the Salinas Valley, geologists BBy Lateate PlioceneP time, 3 million to 4 million years ago, refer to this mass as thee SalinianSalin blockk (Figure NC.35).N the coastall basinsba were receiving mostly terrestrial sedi- ments, indicating that the northern Coast Ranges were ris- Figure NC.35 Right lateralateral displacement alonga the San Andreasas ingin from the seafloor during this time. However, the Fault has moved the Salinianan Block more than 500 kilometers to theh complexcom association of Late Cenozoic river, estuary, and northwest. beachbea deposits in the northern Coast Ranges indicates that thet precise timing and rate of uplift was somewhat variable. Fault–Dotted where concealed: arrows indicate direction of Accompanying the uplift of the modern northern Coast relative movementent C. CaCalaveras; H. Hayward;H S. Sargent Ranges was the eruption of and lava in the

Great ValleyVal sequenceence and areas around Sonoma (3 million to 8 million years ago) and Crust rangera ophiolitelite at base Parkfield Clear Lake (2 million to 10,000 years ago). Ash from these SalinianSalin block window Cholamc eruptions is commonly found within the layers of Late New IDRIA Franciscan assemblage window Cenozoic sedimentary rocks. This relatively recent volcanic

as fault activity in the northern Coast Range is probably related to Upper Cretaceous and PACHECOHEHECECC Tertiary strata pass windowndonddo Andre the northward migration of the Mendocino triple junction.

San In the Clear Lake volcanic region, shallow residual magma S is probably responsible for the steam that has produced as ALLEY much as 2,000 megawatts of electricity annually at The MountM Diablo C window H Geysers geothermal field. The forces that raised the north Coast Ranges in Late San Francisco REAT GREAT VALLEY Cenozoic time are continuing to elevate the region. The general northwest alignment of mountain ranges and drainages is attributable to the orientation of the strike-slip PACIFIC OCEAN faults that influenced the Late Cenozoic uplift. Several of the mountain ranges are still being squeezed upward by compression generated by small differences in the align- ment of active faults. The coastal mountains of northern 0 100 ? N California have only recently emerged as land, and they will Kilometers (approximate) grow even higher as the Pacific plate continues to slide past the North American plate. Diagram courtesy of the U.S. Geological Survey. 24 Geology of Northern California

Figure NC.36 Folded layers of the Monterey Formation in the Monterey Peninsula area of the northern Coast Ranges. Dick Hilton

Sectionn NorthernN California.5liforn Summary plates. This boundary evolved when North America col- lided with and overran the Farallon-Pacific plate spread- ● Thee northernnorthe Coast Ranges include mountains such ing center, beginning about 30 million years ago. as the MendocinoM Range, the Santa CruzC Mountains, Right-lateral strike slip motion on the San Andreas and and the Diabloiablo Range that are alignedaligne parallel to the other faults helps to accommodate the northwest motion Great Valleyley andan Sierrarra Nevada.d These are relatively of the Pacific plate relative to the North American young mountainountain ranges,nges, and their continuing uplift is plate.The Salinian block is a large fragment of crust in related to the emergenceem ence of the northern coast. the northern Coast Ranges that has been transported at ● The bedrock of the northernth Coast Ranges is domi- least 550 kilometers to the northwest by right-lateral dis- nated by the Mesozoic-age Franciscan complex, an placement along the San Andreas fault zone. incredibly varied mixture of sedimentary and metamor- ● The mountains of the northern Coast Ranges are all rel- phic rocks known as a mélange. The Franciscan complex atively young, with uplift beginning only 3 million to 4 mil- accumulated in or adjacent to the Farallon subduction, lion years ago. The rise of the Coast Ranges is documented and includes several exotic terranes that originated far by the shift from marine to terrestrial sediments in Late from northern California. time. The forces that lifted the Coast Ranges ● The San Andreas fault zone cuts diagonally across the appear to be related to the interaction of blocks separated northern Coast Ranges and is part of the modern trans- by various faults within the San Andreas fault system and to form boundary between the Pacific and North American the northward migration of the Mendocino triple junction. Northern California.6 Volcanoes of the Cascade Range and the Modoc Plateau 25

Northern California.6 Both the Modoc Plateau and the Cascade Range in northern California represent only a portion of larger geo- Volcanoes of the Cascade Range logic provinces that extend into adjacent states. The Cascade Range extends northwardhw from the southern ter- and the Modoc Plateau minus at Mount Lassen forr moremor than 800 kilometers (500 The landscape of northeastern California is dominated by miles) into southern Britishtish Columbia.Columb Other prominent the Cascade Range and the adjacent Modoc Plateau (see include Oregon’sOregon , Mount Figure NC.1), which together create a scenic volcanic won- Baker, and Mountt Hood,Hoo alongng with MountMo Rainier and derland in this part of the state. The Cascade Range Mount St. Helensens of .Washi n. The Modoc Plateau is includes majestic (4,319 meters/14,161 feet likewise continuousuous with the ColumbiaCo bi Plateau of Oregon, high; Figure NC.37), Mount Lassen (3,188 meters/10,457 Washington, andd . The ModocM Plateau encompasses feet high), and scores of other smaller volcanoes of all types. nearly 26,000 squarequare kilometers;kilometer the is The Modoc Plateau is a high lava plain east of the border- roughlyhly 20 timest larger.arger. ing averaging about 1,350 meters (4,500 feet) in Thee principle volcanicv features of the Cascade Range elevation (Figure NC.38). Many volcanoes are perched on and Modocodoc Plateau are all relativelytive young, generally this arid plateau, but the largest of them is the Medicinee resultingr from eruptions that occurredoccucur during the past Lake volcano (Figure NC.39) near the eastern margin of 3 millionill yearsyear (Figure NC.40).0). EarlierE Cenozoic-age vol- Cascade Range. This immense rises to an canicanic activityact ty did occur in the region,regi but the structures elevation of 2,375 meters (7,795 feet) and covers more thanhan built duringdurin this time have either beenbee eroded or obscured 2,000 square kilometers (770 square miles). beneathath thet younger lava flows.ows. The most recent activity

Figure NC.37 Mt. Shasta is the second highest composite in the CascadeCasc Range and measuresures abouta 20 kmk in diameter at the base. Diagram courtesy of the U.S. Geological Survey. 26 Geology of Northern California

Figure NC.38 in Lassen County is one of several young shield volcanoes that erupted the basaltic lava in the Modoc Plateau region. James S. Monroe

Figure NC.39 Map showing location of Mt.M Shasta and the Figure NC.4040 RecentRe eruptions of the volcanoes of the Medicine Lake shield volcano, 60 kilometersers to the east. Cascadesades Range.Ra Note that Mt. Shasta and Mt. Lassen in California haveve both been activeve in the past several centuries. Map courtesy of the U.S. Geological Survey Cascade Volcano Observatory. Map courtesy of the U.S. Geological Survey Cascade Volcano

that exists along the margin of North America, north of the Mendocino triple junction. Here, oceanic plates are descending beneath the continent in the Cascadia subduc- tion zone, in the process generating magma that continues

Map courtesy of the U.S. Geological Survey. to rise into the Cascade region. The : The Cascadia subduc- occurred in the years 1914 to 1917, when Mount Lassen tion zone extends for 1,200 kilometers (750 miles) along the experienced a series of small summit eruptions (Fig- west edge of North America from southern British ure NC.41). The recent volcanic activity in the Cascade Columbia to northern California (Figure NC.42). Along Range and Modoc Plateau is related to the plate convergence this convergent plate boundary, the Juan de Fuca plate (and Northern California.6 Volcanoes of the Cascade Range and the Modoc Plateau 27

Figure NC.41 A summit eruption from Mt. Lassen in 1915. Figure NC.42 Plate tectonic setting of the . Subduction of Juan De Fuca plate beneath North America accounts for the volcanism of the Cascade Range.

Cascade Range

A

Juan de Fuca Juann Ridge de Fucauca Mount St. Helens Plate B

North American Plate PacificPac PlatePla LaLassen Peak Spreadingg Ridgege Subductionuction Zone

Juan dee FucaFu Ridgedge Mountun St. Helens Mount Adams AB

North PacificP Juan de Fuca American PlaPlate Plate Plate

thatth separates the Juan de Fuca (Gorda) plate from the Pacific plate (see Figure NC.42). Because there is no plate convergence south of the Mendocino , no subduction-related volcanic activity occurs in that part of Geolog California. Mount Lassen, at the southern end of the Cascade Range, is located just slightly north of the land- ward projection of the . California’s Cascade Volcanoes: Among the hundreds of volcanoes in the California portion of the Cascade Range, iagram cour Diagram courtesy of the U.S. Geological Survey. three merit special attention because of their size and potential for future activity. Though future eruptions could occur almost anywhere in the region, Mount Shasta, its southernmostso rnmost segment,s the ) is moving under Medicine Lake, and Mount Lassen volcanoes appear to the aadvancingncing continentcontin at a rate of 3 to 4 centimeters per have been the most active over the past several centuries. year. Water releasedrelease from the sinking slab causes subsurface Each of these volcanic centers has a unique history, and rock to melt, forming magma bodies beneath the Cascade poses distinctive threats to residents and visitors. Range. Magma produced in this way is dominantly andesitic Mount Shasta (see Figure NC.37) is one of the largest in composition and commonly contains large amounts of composite volcanoes in the Cascade chain. This volcano dissolved gases such as water vapor, carbon dioxide, and sul- was constructed over the past 100,000 years by hundreds of furous vapors. The larger volcanoes in the Cascade chain are eruptions, including at least 13 in the past 10,000 years. composite cones built through a series of explosive erup- These eruptions produced andesitic lava flows, pyroclastic tions, such as the 1980 eruption of Mount St. Helens, alter- flows, and debris flows, some of which extended well beyond nating with less violent effusions of viscous magma. the base of the volcano. , a small volcanic cone on The Cascadia subduction zone is terminated to the the western flank of Mount Shasta (Figure NC.43), resulted south by the Mendocino fracture zone, a transform fault from eruptions that occurred between about 9,300 and 28 Geology of Northern California

Figure NC.43 Mt. Shasta and Shastina, seen from the north Figure NC.45 Mt. Lassen, the southern most peak in the Cascade Range. Mount Shasta Shastina Sue Monroe Lyn Topinka/USGS Lyn

9,700 years ago. The most recent activity at Mount Shasta Mountount Lassen (Figure(Figu eN NC.45) is unusualsual with respect to was about 200 years ago, when several small debris flows, the otherher volcanoes of the Cascade Rangege in that it is notno a possibly triggered by steam eruptions, originated from the compositecom site volcano. Rather,R it is a lava domedo volcano, a bul-b summit region. bous steep-sideds p-sided structure st formedmed by viscousviscou magma aboveab At Medicine Lake, eruptions began about 700,000 years a volcanicolcani conduit. Thee rockr comprising the lavaava domedom at ago, when great volumes of basalt lava flowed onto thee sur- Mount LassenLas is mostly dacitedacit , representingrepres sticky silica- face, building the base of this large shield volcanoano (Figureure rich magma that oozed up the conduitc duit a little more than NC.44). The eruption of voluminous basalt flows is gener- 11,0001 years ago. Before theth developmentdeve of the Mount ally associated with divergent plate boundariesndaries or hot LassenLas ,e, a much largerlarge composite volcano existed spots, and is unusual in subduction-related volcanic areas.a to the west. GeologistsGe ogists have namedn this ancient volcano The basalt eruption at Medicine Lakee is probablypr y related to Mount Tehamahama (Figureure NC.46),N and it was the site of the proximity of the region to the Basin and Range numerous violentiolent volcanicani blasts over the past 600,000 province immediately to the east.. InI this area, the earlyea years.s. MountMoun Lassen is the only California volcano to be stages of continental rifting may be in progress asa a new observederved eruptingerupti in historic time. Between 1914 and 1921, divergent boundary develops in Nevada.evada. Perched atop or a series of small eruptions,e mostly producing ash and steam, along the gently sloping flanks of thee Medicine Lake vol- occurredoccur ed near the summit of the lava dome. In May of cano are numerous cinder cones that provide sources for 1915,1915 however, somewhat larger eruptions sent glowing decorative rock,ock, concretecon aggregate,g and roadbedd material.m avalanchesavalanch of debris down the flanks of Mount Lassen, and The moree recent eruptionseruptio at MedicineMedi Lake, someme proba-pr producedproduc a small lava flow on the. Today, visitors to bly less thanhan 100 yearsyea old, havehav produced silica-richica-ric LassenL Volcanic National Park can explore hot springs, magma that resultedsulted in obsidianidian flowsflo and rhyolite domesomes boilingb mud pots, and gas-discharging fumeroles that pro- on the higher slopes. vide hints that the volcanic system is still active. Volcanic Hazards in Northern California: All of the volcanoes of the Cascade Range and Modoc Plateau region Figurere NC.44NC Basalt Flow and Cindernder Cone in the Medicine Lake Highland of Northern California.a. are geologically young, and it is almost certain that there will be future eruptions in the region. In addition, the pres- ence of magma at shallow depths may be responsible for the geothermal activity around Clear Lake in the northern Coast Ranges. Though these areas are distant from large population centers, future eruptions could produce wide- spread impacts, adversely affecting distant communities as well as local residents and visitors. A major volcanic erup- tion in the Cascade Range or elsewhere in northern California could also affect agriculture, transportation, communications, water quality, timber resources, and recreation over a broad area. Volcanic eruptions can threaten human welfare by gener- ating destructive flows of lava or debris, by filling the skies with clouds of ash and other fine rock particles, and by emit-

Rick Hazlett ting noxious gases such as carbon dioxide, chlorine, or acidic Northern California.6 Volcanoes of the Cascade Range and the Modoc Plateau 29

Figure NC.46 The original profile of Mt. Tehama near the lava dome of .

Eagle Peak 2810 m Brokeoff Mountain 2815 m Lassen Peak 3187 m 3397 m riginal for d o m of Ski Heil 2713 m me Mou 3154 resu nt Te P hama 2790 Sulphur Works 2164 m 2426

SW 01 NE km 1915 dacite from Lassen Peak Breccia (vent filling) of Talus DecomposedDecompo Dacite domes Andesites of Mount Tehamaehama Pre-Lassen Peak from the Lassenen Peak vent Diagram courtesy of the U.S. Geological Survey.

compounds. Of these multiple potential hazards, largege flowsflo MindfulMi of thee serious implicationsimp ations a largela eruption in of pyroclastic material from Cascade volcanoesanoes would be the northern CaliforniaCal ia would have,have geologistsgeologi have prepared greatest threat in northern California. Thee geologic evidence volcanic hazardrd mapsm s of theth most sensitive areas and have indicates that between 300,000 andd 360,0003 0 yearsyear ago, an helped develop evacuationevacu on plans p for disaster response agen- at Mount Shastahasta producedd a volcanicv cies. Inn addition,additio both Mount Shasta and Mount Lassen that traveled 45 kilometersmeters from the volcanolcan and (alongng with other areas in central and southern California) buried an area exceeding 450 squarequare kilometerskilo in rubble arere continuouslyontinuously monitoredm by scientists from the U.S. (Figure NC.47). Even largerarger volcanicnic ,mudflows known as GeologicalGe cal SurveySu for signs of renewed activity. In the lahars, resulted from prehistoricpreh eruptionsions at both Mount absenceabsen of anyan means to accurately predict future volcanic Lassen and Mount Shasta.has And during thehe relatively smallmall eruptions,uption the combination of hazard assessments before- eruptions of 1915, ash particles from MountM Lassen fell as farf hand,han strategicst preparedness, and monitoring will help limit away as Elko, Nevada, 50000 kilometers (300( miles) to the east.t. the harmfulh consequences of future eruptions.

FigureFigu NC.47 TheTh hummocky terrain inn the foregroundd was producedproduce by an avalanche of volcanic debris from Mt. Shasta more than 300,000 yeyears ago.go. Dick Hilton 30 Geology of Northern California

immediately to the east. The displacement between the Section Northern California.6 Summary Warner Range and Surprise Valley is a result of the motion ● The Cascade Range of northern California is part of a along normal faults along the base of the steep eastern chain of mostly composite volcanoes that extends north escarpment of the mountain block. Thesee faultsf comprise into . This chain includes the famous the Surprise Valley fault zone, which hasas a totalto cumulative volcanoes of the Pacific Northwest including Mount of more than 14,000 feet. Normal faulting in this area Rainier, Mount St. Helens, , and Crater began as early as 14 million years ago, butut much of the dis- Lake. In northern California, Mount Lassen and Mount placement has occurred in morere recentrec times.mes. The HoneyHo Shasta are the most prominent volcanoes of the Cascade Lake basin to the south of Surpriseurprise Valley is also a depres-de Range. The Modoc Plateau to the east is a high volcanic sion bounded by normal faults.ts. table land with numerous shield volcanoes, such as that The pattern of alternatingg high mountains and interven- in the Medicine Lake highland, and cinder cones. ing basins in these small areasas of northern CaliforniaC con- tinues eastward forr hundredshun of kilometers into Nevada and ● The magma that sustains the volcanic activity in the portions of the adjacentacent states. This corrugated landscape, Cascade Range arises from the Cascadia subduction covering more thann 770,000 squaresqua kilometersrs (300,000( zone, in which the Juan de Fuca plate descends beneath square miles) of westerntern NorthN America is knownknoow as the North America. Water released from the subducting Basin and Range province.p nce. In this immense region,regi tension- plate induces the formation of magma bodies more than al forcesces are stretchingstretchi the Earth’s crustt and fracturingfra it 100 kilometers beneath the surface. This magma is gen- into blocks bounded by normal faults. Thehe hanginghangin walls of erally intermediate to felsic in composition, relatively theth normalormal faults movemo downwardd to formorm the basins or viscous, and laden with magmatic gases. These factors grabensgrab , while the mountain blocksocks remainrem high to formfo commonly lead to explosive eruptions and the construc- horstsorsts. Surprise Valley andan thee Honey Lake basinasin are only tion of composite volcanoes in the Cascade Range. two of hundredshun of depressionsssio formed as theth crust in the ● Mount Shasta is a large composite volcano builtbu in a Basin and Range R province yields to tensionaltensi stresses. series of eruptions over the past 100,000 years.ears. Mount Geology and Geothermaleotherm l ResourcesRes of the Warner Lassen is a lava dome volcano that was lastt active in the RangeRa region: Thehe bedrock ofo the Warner Range consists early 1900s. The Medicine Lake highlandd is a largela mostlymo of volcaniccanic rocks eruptederupt since about 36 million shield volcano, with smaller cinderr conescon andd obsidianobsi years ago. IncludedInclu in thist thick accumulation are flows on its flanks and summit. The most recentt erup- deposits, tuffsuffs of variousus types,t and felsic to intermediate tions in the Medicine Lake regionon may have occurredrred lava flows.ows. Thesehese oldero volcanic rocks appear to be related only 100 years ago. All of these volcanoes are potentialpo to subductionubduction of the Farallon plate. Younger basaltic (more sites of future eruptions in northernrn California. mafic)fic) lava flows in the region are about 3 million to 8 mil- lion yearsars old andan probably signify a shift from subduction- ● Because they all have the potential for futurefutu activity, relatedrelate volcanicvolcan activity to the rifting-related eruptions. the Cascade RangeRan volcanic centers pose serious volcanic The prominentpro normal faults are relatively young, displac- hazards foror people ini northernhern California.C The geologicgeo ing the entire succession of volcanic rocks. record of previous eruptionse ions demonstratesd thathat thet NumerousN hot springs and “mud volcanoes” occur along Cascade volcanoeslcanoes can produceduce pyroclaticpy flows, volcaniccanic a north-to-south zone near the base of the Warner Range, mudflows knownwn as lahars,l and ashas falls that could affectect within the Surprise Valley fault zone. Exploratory drilling communitiesities hundreds of kilometerskilomete away. These threats along the fault zone has encountered water as hot as 170° F can bee mitigatedmitigate by continuous monitoringtor of the volcanic at depths from 500 to 1,500 meters (1,600 to 4,900 feet). areas, assessmentassessm of the potentialal hazards beforehand, and This geothermal activity probably reflects the interaction preparations for responses to the future eruptions.e between water moving down from the surface and hot young in the subsurface. Since 1984, the hot water from the Surprise Valley springs has been used to heat schools and hospitals in the small communities of the valley. Northernern California.7lifornia In 1993 alone, this saved roughly $50,000 in fossil fuel costs. The Graben: Though it rests along the east- The Basin and Range of ern edge of the Sierra Nevada physiographic province, the geological structure of the Lake Tahoe basin is similar to Northeast California the grabens of the Basin and Range province. This magnifi- East of the Modoc Plateau in extreme northeastern cent lake fills a deep depression created by normal faulting California, the volcanic table land is broken by numerous to a depth of nearly 500 meters (more than1,600 feet). large normal faults into a series of elevated blocks and Volcanic activity 3 million to 4 million years ago created a lower basins. Principal among these mountains and valleys natural of basalt and at the north end of the are the Warner Range, rising to 3,017 meters (9,892 feet), graben that blocked the drainage of the upper Truckee and Surprise Valley, lying 1,600 meters (5,200 feet) lower River. The basin-forming normal faults are still active in Northern California.8 The Northern California Ice Ages 31

Figure NC.48 Several scarps mark the trace of active faults on the floor of the Lake Tahoe Basin. Note the large blocks of rock on volcanoes along the fault zone at the base of the eastern the lake bottom that document an ancient catastophic . escarpment indicate the presence of young and hot vol- canic rock in the shallow subsurface.bs These hot springs have been used locally ass a sourceso of energy for space heating. ● The Lake Tahoee basinba lieses in a graben that developed through the downwardwnwar displacementcement of rocksro along nor- mal faults to the east and west.w The elongated graben slopes gently too the north, and was blocked by lava flows that erupted 3 million to 4 millionmill years ago. The natu- ral volcanicvol damm impounded the waters of the ancestral Truckeeckee River,Riv creating the beautiful alpine lake. Ongoingoing seismic activityac in the Lake Tahoe region sug- gests thathat the normanormal faults are stillsttill activea and the basin is still evolving.volvin

NorthernNort e California.8Californ a.8 The NorthernNorthern CaliforniaCa f Ice Agess Duringing the PleistocenePleisto Epoch, 1.8 million to about 10,000 yearsear ago, all of theth regions of northern California were Image courtesy of the U.S. Geological Survey affectedaffe by multiplemu climatic oscillations. In the higher ele- vatiovations of thehe Sierra Nevada, Klamath Mountains, and the the Lake Tahoe region,n, producingp many smallsm to moderaterate Cascadeascade Range, numerous valley glaciers formed, and in earthquakes. Ancient landslide rubble on the floor of LakeLak the basinsasi of the Great Valley and Basin and Range, lakes Tahoe is thought to haveave resulted fromfro a major landslidedslide grew as water accumulated in the lower terrain (Figure triggered by a powerful Ice Age earthquake.ear Recentecent imag-im g- NC.49).N Even the coastal regions were affected, because sea ing of the lake (Figure NC.48) reveals numerousmerous faultfa level fell when the glaciers were expanding and rose when scarps resulting from recent displacement,em a furtherrther indica- they were in retreat. Based on evidence from the Sierra tion of actiactive faulting.ulting. Nevada, where the effects of the Pleistocene glaciers are most striking, geologists have identified four major glacia- tions corresponding to periods of cool climatic conditions. Sectionction Northernrthern California.7 Summaryumma Some of these glaciations appear to be comprised of multi- ple cycles of glacial growth and decline. The glacial periods ● The Basin and Rangege provincep of extreme northeast were separated by warm interglacial intervals when glaciers California is a landscapeandscape ofo alternating mountains and melted back and the lowland lakes diminished. The multi- valleys that extendsends eastwardeastwa into Nevada and adjacent ple ice ages of the Pleistocene Epoch in northern California statstates. The corrugatedugated terrain is the result of intense signify a period of erratic oscillation in climate, not a long nornormal faultingfaultin caused by the tensional stresses that interval of enduring cold. affeaffect thehe entire BasinB and Range region of western Because of their oscillatory nature, establishing a detailed NoNorth America. The mountain blocks such as the and accurate history of glacial events from the geological Warner Range represent the footwall (or horsts) of the evidence is difficult in most places. Each glacial advance normal faults, whereas basins such as Surprise Valley tends to obliterate or obscure the evidence for the previous represent the downthrown grabens. The Honey Lake one. Geologists are in general agreement, however, that the and the Lake Tahoe basins also developed as grabens dis- two most recent glacial epochs in northern California placed downward by normal faults along the eastern peaked at about 160,000 years ago (the Tahoe glaciation) edge of the Sierra Nevada province. and 20,000 years ago (the Tioga glacation). Of these two ● The Warner Range is the most prominent mountain glaciations, the earlier Tahoe appears to be longer and more range of northeast California, and is composed primarily severe than the later Tioga. There is also geologic evidence of Cenozoic-age volcanic rocks. Hot springs and mud for earlier glaciations, but it is more fragmentary and local- ized than for either the Tahoe or the Tioga event. 32 Geology of Northern California

Figure NC.49 Donner Lake in the northern Sierra Nevada, rests in a U-shaped glacial trough carved by Pleistocene glaciers. Frank DeCourten

Effects of Valley Glaciers: The overallverall effect of glacial FigureFi re NC.50NC.5 Glacial striations on bedrock the northern Sierra erosion in mountainous regions was to increase the relief NevaNevada. and sharpnessss of theth landscape through the developmentop of U-shaped glacial troughs,trou horns, arêtes, andd cirquescirq . Although these landformslandform arere displayeddispla in spectacularlar fash-fas ion around Yosemiteosemite NationalN nal ParkPar in the central Sierra Nevada, they are alsoals presentsent in many other mountainain regions of northernnor California.iW Wherever the Pleistocene glacierss existed,existed glacial polish and striationsstria on bedrock surfacess are common (Figure NC.50). SuchSu erosional fea- tures are widespread in the Trinitynity Alps, in the higher peaks of the Cascadeade Range,R and in thehe alpine a terrain west and south of Lakeake Tahoe.Ta The Pleistocene-ageP valley glaciers also left extensivetensive depositsposits of morianem of several types in these same mountain areas,reas, alongal with glacial erratics (Figure NC.51).

Lowland Lakes and Wetlands: During the cool climatic Frank DeCourten intervals of the Pleistocene Ice Ages, increased precipita- tion and reduced evaporation led to the development of large lakes and wetlands in the lower areas of northern large lakes formed repeatedly during the Pleistocene California. Ice Age lakes that form in response to cool cli- Epoch. One of the largest Ice Age lakes in the Great Valley mate cycles are known as pluvial lakes and there were sev- existed between 800,000 and 600,000 years ago and covered eral of them at various times in the Honey Lake basin, in the entire valley from its north end to the southern extrem- Surprise Valley, and in the Great Valley. In the Great Valley, ity. At other times, there were several smaller lakes with Northern California.8 The Northern California Ice Ages 33

Figure NC.51 Erratics of light-colored granite perched on dark-colored volcanic bedrock in the northern Sierra Nevada. Dick Hilton

extensive wetland habitats betweeneen them. The lushh native glacierslac ers melted, andan sea level rose. By about 8,000 years vegetation provided forageage forfo a remarkableemarkable menagerieme of ago,ago sea water had once again invaded San Francisco Bay to Ice Age creatures, includingudin giant bison,n, , groundgr sloths, re-establishre-es hth the estuary of modern times (Figure NC.52). mammoths, large cats,s, and bears. The remainsrem of theseese creatures have been found in excavationexcavat sites throughoutghou the Great Valley. FigureF NC.52 Sea level rise over the past 20,000 years has con- The Ice Age Landscapeape of San Francisco Bay: EarlierEa er vverted the San Francisco Bay area from a river valley to an estuary. than about 600,000 years ago, the Sacramentonto and SanS JoaquiJoaquin Rivers did not flow to the PacificP Oceancean via San N Francisco Bay as they now do. Insteadtead thet drainagenage from the surrounsurroundingg highlandshighl accumulateded in one or more lakeskes in theth Centralntral Valley.Val Tectonic activityty in the northernno 20 km Coast RangesRa had alreadyalre produced a valley wherewher the bay nonow is, but onlyll local streams flowed into and through it. SomSometime after 600,000000 yearsy ago, water from central California began to flow into the San Francisco Bay, where it mingled with oceancean water to create the large estuary that exists today. However,ver, cycles c of climate change during the

8.3ka

Late Pleistocenetocene caused sea level to fluctuate several times. 10.5ka

DuriDuring coolool intervalsinterval leading to glacial advances, sea level –18m felll by as much as 120 meters (390 feet), causing water to 11.5ka 14ka withdraw from San Francisco Bay to the west. About 14.2ka –37m 18ka –55m 20,000 years ago, for example, the shoreline of the Pacific –73m Ocean was located beyond the , more than –110m –91m 30 kilometers (18 miles) west of the modern Golden Gate Bridge. At that time, San Francisco Bay was a broad river valley where streams from the Great Valley met with local tributaries before turning west through a valley that then existed in the Golden Gate area. Since about 20,000 years

ago, the climate has become warmer, the Late Pleistocene Map courtesy of the U.S. Geological Survey. 34 Geology of Northern California

includes some devastating events that provide a chronic Section Northern California.8 Summary reminder of the natural hazards facing those who live near a ● During the Pleistocene Epoch, California was affected plate boundary. The infamous 1906 San Francisco earth- by multiple cycles of climatic oscillations. Cool intervals quake (estimated magnitude 7.8 on thee Richter scale) resulted in the growth of valley glaciers in the higher occurred along a segment of the San AndreasAndr fault more elevations of the Sierra Nevada, Klamath Mountains, than 400 kilometers long, ignited fiercece building fires, and and the Cascade Range, and pluvial lakes developed in claimed more than 2,000 lives. In 1989, the magnitudemag 6.9 the lowlands. Loma Prieta earthquake in thee SantaSant Cruzz Mountains left 62 people dead in the San Francisco Bay areaa and causedc ● The two best documented glacial intervals are the nearly $10 billion in damages.ges. In the dynamicdyn i tectonic Tioga glaciation about 20,000 years ago and the preced- environment of northern California,lifornia, where millionsm of resi- ing Tahoe glaciation that peaked about 160,000 years dents live in close proximity to active faults, future seismic ago. Both of these glacial intervals involved multiple disasters are as certaintain as those of the past haveh been tragic. advances and retreats that reflect the climatic oscillations Earthquakes in northern CaliforniaCa are associated with of the Pleistocene. There is evidence of earlier glacia- three different platee tectonic settings.sett The Sanan AndreasA tions, but the precise chronology and extent for them fault system,tem, includingi g scoresscore of subsidiary faults,ults,sg generates has yet to be determined with certainty. many tremors in theth Sanan FranciscoF Bay areaa and the north- ● Valley glaciers in the mountainous regions carved U- ern Coast Ranges. TheseT se earthquakese releaselease stressstre associ- shaped valleys and sculpted arêtes, horns, and cirques in ated with the northwardnorthwar motion of the Pacific platep along bedrock exposures. Moraines and glacial erratics can also theth edgedge of the NorthNort Americann plate.plat The earthquakesearthquak be identified in most mountains above 2,000 meters in generatedgene ed along this plate boundaryary tend to have very shal-sh elevation. loww foci,foci less than 15 kilometerslom rs deep (Figure NC. 53), and can be veryver powerful. In the denselyensely populatedpopul urbanized ● In the lower elevations of the Basin and Range and area around aroun San Francisco Bay, manyany buildingsb have been Great Valley, pluvial lakes expanded and declineddecli in constructed on soils or bedrockbed ck thatth are not very resistant rhythm with the Pleistocene climatic cycles.es. TheTh largest to seismic vibrations.ons. In both thehe 1906 1 San Francisco and of these Ice Age lakes submerged the floorr of the entireent the 1989 Lomaa PrietaPrieta earthquakes,earthquak intense ground shaking, Great Valley between 600,000 and 800,00000,00 yearsars ago.ago liquefaction,, and firee all contributedc to scope of the disaster ● Sea level rose and fell multipleple times duringng the and were relatedelated to thee poorpo response of weak soil or rock Pleistocene Ice Ages. During timesmes of glacial advances,nces to earthquakethqua vibrations.vibra sea level fell as much as 120 meters.ers. Under such condi- The second arearea ofo notable seismic activity is associated tions, San Francisco Bay was a river valley throught withh the MendocinoMendo triple junction along the northern which rivers carried water to the Pacific coastcoas located coast,coas where eacheac year 80 to 100 earthquakes greater than about 30 kilometers west of the modern shore. When magnitudemagn ude 3 occur. Here, forces generated as the Juan de the Californiarnia glaciersgla melted during warm interglacialter Fuca (Gorda)(G plate descends into the Cascadia subduction intervals,, sea level rose.ro The modernmo estuaryy in San zone canca produce powerful earthquakes in a zone that dips Franciscoo Bay formed aboutut 8,0008,00 years ago as glacierslacier eastwardt beneath the continent. In this plate tectonic set- in the interiorr mountainsmount were meltingm under the influ-flu- ting,ti earthquakes occur in both the oceanic plate and on ence of a warminggc climate. land, including some that can be powerful. In July and August of 1991, four major earthquakes, ranging from mag- nitude 6.3 to 7.1, occurred near the Mendocino triple junc- tion, and there is good geologic evidence for even larger prehistoric events, possibly as great as magnitude 9. In Northernrn California.9C ia.9 addition to the impacts of local communities, large coastal earthquakes in northern California have the potential to Northernern Californiaaliforn Earthquakes produce that could affect distant regions border- Earthquakes in California C nia are an inevitable consequence of ing the Pacific Ocean. the interactions between tectonict plates along the forward Along the eastern side of the Sierra Nevada, active nor- edge of a west-moving continent. The forces generated mal faulting associated with the continuing uplift of the along the transform boundary between the Pacific and mountain block produces a third zone of seismic activity North American plates, and at the convergent boundary from the Lake Tahoe area south to . The east- between the Juan de Fuca and North American plates, ern Sierra earthquakes are mostly generated along normal result in thousands of earthquakes every year. The ground faults, but several prominent strike-slip faults in the region literally shakes continuously in California, but most of indicate that both tension and shear stresses exist in this these tremors go unfelt by people and rarely cause damage. area. Although the majority of eastern Sierra earthquakes Nonetheless, the seismic history of northern California are small, several greater than magnitude 6 have occurred Northern California.9 Northern California Earthquakes 35

Figure NC.53 Distribution of earthquake foci along the San Andreas Fault from San Francisco to Parkfield. Note that most earthquakes occur within 15 km of the surface. The seismic gap filled by the 1989 Loma Prieta earthquake is represented on the bottom panel.

Epicenter of Loma Prieta earthquake

San Francisco

fault San And reas

San Juan Bautista Parkfield Los AnAngeles Southern San Francisco Santa Cruz Parkfieldeld Peninsula gap Mountains gapap gap eld ie fie fi San Francisco Portola Valley Lom Loma P Prieta Sa San Juan Bautista Bautista Parkfield 0 5 10

0100 2200 3000 400 0

Depth (km) 5 10

0 100 200300 400 Distanceance (km)

in the past 40 years. Inn Lone , alongalo the southern por- time,time the probability of a major earthquake in California is tion of the Sierra Nevadada fault system,system an 1872 earthquakeearthq ke 100%,100 but the likelihood of such an event tomorrow is very destroyed all but seven houses and killed no fewerwer than 277 low.lo Geologists generally try to forecast earthquakes over peoppeople. This earthquake occurred before seismometerseismometers periods of 20 to 50 years to coincide with the time frames were developed,dev , but it may have been California’srnia’s mostmos most widely used by planning commissions, disaster powerful historichi ic temblor.temb A recurrencence of such an earth- response agencies, and the insurance industry. In 1989, the quake alonalong thehe SierraSierr Nevada fault systemystem wouldwoul have U.S. Geological Survey developed the first widely accepted majorr impactsim on the resort communitiess in theth Reno- earthquake forecast for the San Andreas fault system. This TTahoe area, andd regions region farther south. forecast estimated the probabilities for major earthquakes Earthquake Forecaststs ini Northern California: Even between 1988 and 2018 within the entire fault zone. The though scientists have been intensely studying earthquakes probabilities varied from 10% to 90% at various locations in California for more than a century, no reliable means to along the fault system. Interestingly, the Loma Prieta earth- predpredict their occurrencerence withw precision has been developed. quake occurred in an area where a seismic gap had been InsteInstead, geologistseologi can forecast the probabilities of future identified in this study (see Figure NC.53). earthearthquakeke occurrencesoccurren based on measurements of ground In 2007, an interdisciplinary team of scientists known as motmotion andd strain, historich patterns of seismic activity, and the Working Group on California Earthquake Probabilities geologic evidence of fault activity. In recent years, the developed a new updated earthquake forecast called the analysis of seismic gaps, seismically “quiet” segments Uniform California Earthquake Rupture Forecast, Version along active faults, has helped to identify places most prone 2 (UCERF). The new forecasts involve a broader array of to future earthquakes. Because no two faults are exactly the data and techniques for estimating probabilities than did same, the forecasts for major earthquake potential vary earlier assessments. Statewide, the UCREF estimates the across the state, from very unlikely in some places to almost probability of a magnitude 6.7 earthquake in the next 30 certain in others. years at more than 99%. Californians also face a 46% The time frame is a key consideration in making and chance of experiencing a magnitude 7.5 or larger earth- using earthquakes forecasts. Over the long span of geologic quake in the same time frame. 36 Geology of Northern California

Figure NC.54 Major faults of the San Andreas system in the San Francisco Bay Area. duration of such an induced earthquake, This map from the United States Geological Survey also identifies features resulting from and even a moderate temblor could have recent offset along these faults by the circled numbers. disastrous consequences in densely popu- lated areas. In the absenceenc of any safe mechanism to control seismicseis events in California, careful planninganning and prepara- tion for them makes goodod sense.sense The Californiaornia GeologicalG gical Survey, the United Stateses GeologicalGeologica Survey,rvey, andan the California Office of EmergencyEm Managementnt have collaborativelycollabor devel- oped forecastst models and seismic hazard maps thatth are used to planp communities andnd make preparationspr for coordinated responsesponses to earthquakes.earth In addition,addi in the most sensitives areas, stringentgenent building codesco havehav been developeded thatth result in moremor earthquake-resistantarth nt structures.structu Local zoningoning ordinances in manyany areas limit resi- dentialdenti and commercialmmerc l development in locationsloc where strong groundg nd shaking or liquefactionacti are anticipated.anticipat d. PersonalPers preparationion is also important,importa andd check- lists are availableavaila at thet aforementioned agenciesncies thatth provideprovi guidelines for prepar- ingng homes andan apartmentsapa for a potential earthquake. TheTh combination of continu- ing earthquakeearth research, careful planning, stringentent buildingb codes, wise land use, and sy of the U.S.G personalpe preparation is the best way to limit the property damage and loss of life in

Map courtesy of the U.S.Geological Survey. Ma future northern California earthquakes.

SectionSectio Northern California.9 Summary In northernhern California,Califor thehe probabilitiesprob are lower than ● NorthernNo California earthquakes occur as a conse- for the stateate as a whole.whole UCERFCERF estimatese that therehere is a quence of convergent plate interactions in the Cascadia 93% chance of a magnitudemag de 6.7 or greater earthquakequake subduction zone, in relation to the transform boundary in the northern partpar of thehe state,stat with a 63% chancence between the Pacific and North American plates to the of the tembloremblo occurring inn the SanS Francisco area (Fig- south, and along the normal fault zone at the base of the ure NC.54).C.54). TheT faults most likelyyto to produce this large eastern escarpment of the Sierra Nevada. The San earthquakeuake area the Hayward-Rogers-Rogers CreekCr (32% proba- Andreas fault zone, part of the transform plate boundary, bility), San Andreas (21% chance),hance), and Calaveras faults has historically produced the most large earthquakes, but (18% probability).ability The UCERFRF estimatese only a 2% destructive earthquakes could occur in the other plate probability for an earthquakehquake of magnitude 8 in northern tectonic settings in northern California as well. California, illustratingillustr g that very largel earthquakes are rare, even along thehe SanS Andreasdreas faultfau system. This may seem ● Earthquakes in populated regions of northern comforting, but we shouldd not forget how disastrous California have resulted in intense ground shaking lead- earthquakes in the magnitude 6 to 7 range have been in ing to collapse of structures, liquefaction, landslides, and California. Earthquakes in this range of magnitude have fires that have claimed many lives and caused billions of resulted in more than $65 billion in property losses in dollars of property damage. The 1906 San Francisco and California since 1971. the 1989 Loma Prieta earthquakes were the two most Living with Earthquakes in Northern California: destructive earthquakes in northern California history. Although human activities such as underground fluid injec- ● Geologists can formulate earthquake forecasts along tion can initiate earthquakes, such practices to reduce the active faults on the basis of ground motion and strain earthquake threat in northern California would be extremely measurements, historic seismicity including “quiet” risky. There is no way to guarantee the size, location, or Northern California.10 Living on the Edge: Coastal Hazards in Northern California 37

tectonically active Pacific Ocean basin. Winter storms segments known as seismic gaps, and geological evidence approach the exposed northern coast from the open ocean, of prehistoric fault activity. The most recent forecasts bringing with them powerful wind-generated waves. In estimate the probability of a magnitude 6.7 or greater addition, normal wind swells, longshorelon currents, and occa- earthquake in northern California at 67%. Faults in the sional tsunamis also strike thehe northno coast with full force. In San Francisco Bay area within the San Andreas fault contrast, the east-to-west alignment of much of the south- zone are most likely to produce this future earthquake. ern California coast partiallyy protects the shoreline from the ● Because it is currently impossible for humans to pre- vigorous erosion experiencedexperi in the north.north Consequently, vent or weaken earthquakes, preparation for them is the northern Californiaalifornia coastcoas is rugged and rocky rather essential in reducing their impacts on people. In north- than broad and sandy (Figure NC.55).N ) Some locations along ern California, continued monitoring of active faults the north coast, such as and Mendocino, are world coupled with wise land use regulations, stringent build- famous for their spectacular coastalcoas scenery. ing codes, and effective response plans will substantially Thehe CaliforniaCali coast also is unique in its recent emer- reduce the impacts of future earthquakes. gence and in the variety of rock types exposed along the steep seashore.eashore. RecallRecal from our earlierearl discussions that mountainsm ns of the northern Coastst RangesRaR have been ele- vatedd overr justju the past severalal millionm years and, in fact, arere still rising.ing The coast off northernnorthe California is there- Northern California.10 fore an emergentem coast, wherehere the land is being lifted by tectonicnic forcesf relativee to seaea level. As bedrockbed rises from Living on the Edge: Coastalal the seafloor,sea it is exposedxposed to thee various agentsag of coastal erosion. Inn mostm places, the upliftu ift exposesexpo rocks of the Hazards in Northern California heterogeneousus FranciscanF nciscan complexcom or the Salinian ter- With 1,800 kilometers (1,120 miles) of ocean frontage, rane. These rockock assemblagesa mbla include both easily eroded California has more coastline thann all but twoo otheroth states, materialsls suchsuc ass marinem shale or sandstone and more Florida and Alaska. The northernern portion of thee CaliforniaCali durableble igneous anddm metamorphic rocks such as granite, coast trends northwest, directlyy facing theth enormousmou and basalt,asalt, and greenstone.green The erosion of such mixed

Figure NC.55 The steepep anda rugged shore in Point Reyes Nationa Seashoreore is typicalt of theth scenic coast of northern California. Note the nearly flat marine terrace above the sea cliffs. Photo courtesy of the U.S. Geological Survey. 38 Geology of Northern California

Figure NC.56 A pocket beach developed between projecting Figure NC.58 Sea stacks along the northern Califonria coast at headlands on the Monterey Peninsula. Shell Beach. Photo courtesy of the U.S. Geological Survey, Center for Coastal Geology. Photo courtesy of the U.S. Geological Survey, Sue Monroe bedrock assemblages produces an irregular coastline, with rising for the past 20,00020,0 years. The landnd simply is risingris coves and pocket beaches developing where soft rocks fasterfas thanhan the sea. exist and headlands (Figure NC.56) projecting where Sea cliffsffs along the north coastoast are continuouslycont uously under-und harder rocks are exposed. cut by wavesw generateded duringng powerful winterw nter storms.st Coastal Landforms of Northern California: Becauseuse of Because wavesw approachinging a projectingproje headland are the pervasive coastal emergence, the shoreline landformsrms refracted around it, the wave-cute-cut notchestche in some areas can in northern California are mostly of erosionalal origin.or Seaa developd into sea cavesaves on oppositeposit sides of the bedrock cliffs and wave-cut platforms are actively formingorming in manym exposure.exp Continuednued wave erosionero can deepen the caves locations, and elevated marine terraces are commonlycommo until they connect,nnec , forming a sease arch (see Figure NC.26). observed above the coastal bluffs (Figuregure NC.57).57). Because Sea stacks (Figure(Fig NC.58),NC.58 isolated exposures of bedrock, the marine terraces originated as wave-cutwave- platformsrms near represent remnantsmnan of seaea arches that have collapsed as a sea level, geologists can estimate the rate of uplift alongalon consequenceequence offaco a continued wave erosion. The long-term the coast by establishing the age of the terraces usingus fos- effectct of the formationfo mat of wave-cut platforms, sea caves, sils preserved on their surfaces. Radiometricdiometric ages forf fossil sea arches,hes, and seas stacks along an emergent coast is the material from the terraces suggest thatat the most prominentp steady landwardlandwar retreat of the cliffs. This has placed many surfaces rose to their present elevation at rates varying coastalcoast developments at risk along the northern California from about 28 to 454 centimeters per thousand years in the shoreline.shorelin Santa Cruzuz and MontereyMonte Bayay areas.area Farther north,orth, in the In additiona to erosional features, depositional landforms Mendocinono area, the rater of emergenceeme is even higher.high haveha also developed in some areas along California’s north This helps explainplain why the coast ofo California shows such coast. In places where streams discharge into bays or coves, strong emergent features,feat even thoughtho sea level has beeneen spits (Figure NC.59) and baymouth bars (Figure NC.60) can develop from the accumulation of sediment transported Figureure NC.57NC.5 A marine terrace alongng the northernnor California by both rivers and longshore currents. Tombolos, sand coast withith remnant sea stacks. Notee the younger seas stacks forming spits that extend from the shore to a sea stack, can also be along the modern coast. seen in several places along the northern California coast (Figure NC. 61). Tombolos are generally rare, but the abundance of sea stacks along the emergent northern California coast has created many places where the condi- tions exist under which tombolos form. Coastal Hazards in Northern California: The majority of California’s population lives within 100 kilometers (60 miles) of the coast. In northern California, the rugged beauty of the seashore has resulted in extensive residential and recre- ational development along the rapidly eroding emergent coast. Geologists have determined that coastal erosion is causing some sea cliffs in northern California to retreat at rates as high as 1.1 meters (3 feet) per year. In spite of recent

James S. Monroe regulatory action to restrict coastal development, there are Northern California.10 Living on the Edge: Coastal Hazards in Northern California 39

Figure NC.59 A spit at the mouth of the Russian River. Figure NC.61 A small tombolo connecting the shore to a sea stack remnant in northern California

Spit

Tombolo James S. Monroe

Figure NC.60 A baymouth bar in Marin County, California.nia.

Baymouth bar Sue Monroe

Figuregur NC.62 Mass wasting in Bodega Bay resulted from under- cuttcutting of the sea by waves generated during winter storms of 191997–1998. James S. Monroe

stilll hundrhundreds of miles of coastline in northernrthern CaliforniaCa where peoplep and propertypro are threatened by retreating cliffscli and unstable ground. The principal hazardard alongalon the northern California coast is mass wastingg triggered by wave undercutting, heavy rainfall, or earthquakes.uakes. Rock slides and slumps have repeat- edly causedd extensiveex ve damage to structures perched above sea cliffsc (Figure NC.62).N These problems are most severe wherewher thee bedrock is soft, sheared, or water saturated. Unfortunatelyly thesethes conditions are common along the tec- tonically active north coast, which is underlain by the highly James S. Monroe deformed rocks such as those comprising the Franciscan complex. Mass wasting activity along the northern coast also Because the northern California coast faces the Pacific extends offshore and down the continental slope. Turbidity Ocean basin where strong earthquakes occur frequently, currents initiated by earthquakes or storm events have tsunamis pose an additional hazard. Tsunamis can travel scoured several impressive submarine. Monterey Canyon, thousands of miles from their source, so even earthquakes the largest submarine canyon on the Pacific Coast, is deeper in Japan or South America could affect people in northern than the Grand Canyon and extends offshore for more than California. In fact, the principal threat to northern 400 kilometers (250 miles). California may be distant, not local, earthquakes. This is 40 Geology of Northern California because the dominant strike-slip motion along the San damage associated with mass wasting along this scenic Andreas fault system, much of which is on land, would not highway has been significant in such coastal communities likely produce large vertical displacements on the seafloor. as Bolinas, Stinson Beach, and Pacifica (Figure NC.63). Subduction-related earthquakes generated in convergent Farther south along the same highway at HalfHa Moon Bay, a plate boundaries more commonly result in the type of dis- breakwater completed in 1961 to protectct the harbor result- placement that initiates tsunamis. ed in a fourfold increase in the rate of coastal erosione to In April 1964, a 7-meter (23-foot) tsunami generated by the south. This was the natural responsese to the interrup- the magnitude 9.2 Alaskan Earthquake struck Crescent tion of the southward flow of sand alongal thehe beach and the City on the north coast, resulting in 11 deaths and an esti- refocusing of wave energy produced by thee breakwater.breakw mated $15 million in property damage. In the same town, a Many cities and planning commissions carefullyca ll review boat harbor suffered $10 million in damages from a tsuna- proposals for coastal developmentopment in an efforteff to reduce mi that struck in November 2006. Other coastal towns in the negative impacts that result from alteringal natural northern California to experience tsunami damages include slopes and shores.s. However,How much of thet development Eureka, Bay, and Santa Cruz. A major earth- along the north coastast was approvedapp before the impacts of quake in the nearby Cascadia subduction zone is a primary human activity were fully evaluated or understood.ood. Coastal concern in these coastal areas because it could potentially erosion will continuee to beb a concern for residentsreesi of produce a much more disastrous tsunami than any experi- California’srnia’s scenic northth coast.c enced thus far. There is good geologic evidence of a major earthquake, perhaps as large as magnitude 9, in this area 300 years ago that may have caused a tsunami in Miho, Japan. Fortunately, most populated areas along the north- ern California coast have tsunami warning systems and SectionSecti NorthernN California.10C fornia.10 SummaryS mmary evacuation plans in place. ● Human Activities and Coastal Hazards: Although mass The northernn part of California’sC fornia long coastline wasting and wave erosion are natural processescesse alongg directly faces the Pacificacific Oceancean anda is vigorously eroded California’s north coast, they have been intensified and by wind waves, stormtorm events,even , longshorelon currents, and triggered in places by human activity. Whenevernever an ocean-oce occasionalo tsunamis.amis. In addition,additio geologic forces are still facing slope or shore is modified for constructioncons ion of roads lifting the CoastCoas Ranges fasterfaste than sea level is rising so or buildings, there is potential for otherwiseothe stableble slopes that new bedrockbedr continuouslyntinu appears along the emer- to fail. Highway 1, along the northh coast,c has experiencedience gent coast. In manym coastal locations, the emerging repeated closures due to mass wasting events. PropertyP bedrockdrock is a mixedixed assemblagea of hard and soft rock such as the FranciscanFrancis complex that does not respond uni- formlyy to the agents of erosion. The combination of thesethes factors produces an irregular and spectacularly Figure NC.63 Coastal erosion in northern California threatens thousands of homesome such as this one near Bolinas. rugged coast with many projecting headlands and small coves anda pocket beaches. ● Erosional landforms such as wave-cut platforms, sea cliffs, sea caves, sea arches, sea stacks, and marine ter- races dominate the scenery along the emergent coast of northern California. In places where rivers deposit sedi- ments along the coast, or where longshore currents are active, sand can accumulate to form depositional features such as spits, baymouth bars, and tombolos. ● Many people live near the actively eroding coast in northern California. Coastal development is commonly at risk because of cliff retreat and mass wasting events that undermine support for structures. In addition, earthquakes in the Pacific Ocean basin can generate tsunamis that have affected several coastal communi- ties. Human modifications, such as breakwaters, to pro- tect portions of the coast can intensify erosion elsewhere and interrupt the transportation of sediment along the coast. Dick Hilton Review Workbook 41

Review Workbook

ESSENTIAL QUESTIONS SUMMARY stream deposits known as “auriferous”ferous gravels, and in association with quartz veins, or lodes, in Mesozoic-agezoic-age basementbase rocks. Northern California.1 Introduction What is the Mother Lode? What are northern California’s physiographic provinces? The Mother Lode is a northwest-trendingnorth nding belt of goldg mineralization Physiographic provinces are regions of distinctive geology, landforms, in the Sierra Nevadaa foothills. AlongAlo thisis trend modernm placer climate, geomorphic trends, soils and vegetation, and drainage. In deposits, ancient gold-bearingold-bearing river sediments,s ent and lode deposits asso- northern California, seven physiographic provinces are recognized: ciated with quartz veins have all producedproduc significant amounts of gold. the Sierra Nevada, the Klamath Mountains, the Great Valley, the Coast Ranges, the Cascade Range and Modoc Plateau, and the Basin Northern California.3a.3 The KlamathKlamat Mountains and Range. Collectively, these provinces endow northern California In what ways are the Klamath MountainsMo and the Sierra Nevada with extraordinary geologic diversity. Similar?? What is the Farallon subduction zone? In both thehe Sierra Nevada and the Klamathh MountainsM bodies of The Farallon subduction zone developed at the convergent plate MMesozoic-age-age granite havehav been emplacedcedd in oldero metamorphic boundary between the North American and Farallon plates about rocks representingsent numerous terraness accretedac to North America. 200 million years ago. The entire subduction zone persisted until aboutout Goldld mineralizationm lizat is common too both areasare as well, but slices of 30 million years ago, when portions of it were disrupted by the collisionion oceanic lithlithospherephe (ophiolites) arere more commonco in the Klamath between North America and the Farallon-Pacific spreading ridge. Mountains tthan in the Sierra. In general, the KlamathKlam Mountains Many of the major geologic features of northern California resultedresul representsent a northwest continuationntinuat n of the geologicalgeologica trends of the from the convergence of plates in the Farallon subduction zone.ne. Sierra NNevada. What two types of plate boundaries exist in northernern California today? What kindsds of rockss comprise the ophiolitesoph ites in the Klamath North of Cape Mendocino, the Juan de Fuca plate,ate, a remnant of the Mountains? Farallon plate, continues to be subducted under continentalntinental litho- Ophiolites representsent disrupteddi pted fragmentsfr of oceanic lithosphere con- sphere to the east. South of Cape Mendocino,ocino the transformnsform bound- sisting of mafic anda ultramaficultram igneousi rocks associated with oceanic ary between the Pacific and North Americanmerica plates shapeses the sediments.ts. During emplacementplace of ophiolite sequences on land by geologic setting of northern California.ia. The San AndreasA faultult systems plate convergence, the oceanic rocks generally became deformed and is directly related to the transform boundary,ndary, whereaswhe northern metametamorphosedorphosed to gregreenstone, serpentinite, schists, and other altered California’s volcanic centers are relatedrel to the subduction process. matmaterials. What are terranes, how do theyth originate, andd why arear they impor- NorthernNorthe California.4 The Great Valley tant in northern California?a? Terranes are large blocks of rock, typically boundedbo by faults, repre-pre- What factorsf have led to the formation of fertile soils in the Great senting seamounts, oceanicc rises, reefs, island arcs, or other oceanici ValleyValley? features accreted to continentsts at convergentconvergen plate boundaries.daries. The soils of the Great Valley have developed on alluvium derived Terranes are comprised of exotic rocksk that become detachedached from a frfrom the weathering granitic and volcanic bedrock in the Sierra subductingubdu plate and are sutured to the edge of the overridingriding conti- Nevada and adjacent ranges and deposited on a nearly flat surface by nent. In northernn California,Califo many terraness accretedacc overer the past the Sacramento and San Joaquin River systems. These factors, along 500 million yeyears have been identified by geologists.ologis with the warm climate and abundant organic matter of the Central Valley, promote the formation of nutrient-rich, water-retaining soils Northernorthern CaCalifornia.2nia.2 TheTh Sierra Nevada: California’salifornia’s GeologicGe that can support high agricultural productivity. Backbonebone What was the origin of the sedimentary rocks in the Great Valley What is the Sierrara Nevada Nevad batholith? Sequence? TheTh Sierra Nevada batholith iss a large body of plutonic rock that The Great Valley Sequence consists of sandstone, mudstone, and comprisescom the core of thee Sierra Nevada,N California’s best known shale that accumulated in a forearc basin associated with the Mesozoic mountain system. It consists of moremo than 100 individual plutons Farallon subduction zone. These rocks include sediments that were emplaced mostly betweenween 140 millionmil and 80 million years ago. transported by turbidity currents to the deep seafloor where they built WhatWh kindss of rocks surroundurr the Sierra Nevada batholith? large submarine fans. MetamorphicMetam ic rocks ofo Paleozoic and Mesozoic age surround the What is the origin of the natural gas produced in the Great Valley? Sierra Nevadaada batholith.batholith Most of these rocks are of oceanic origin and The natural gas resources of the northern Great Valley (Sacramento representpre terranesanes accretedaccr to North America before the intrusion of Valley) originate from organic matter trapped underground in the magma into the batholithh during the Mesozoic Era. oceanic sediments of Mesozoic and Cenozoic age. When and how was the modern Sierra Nevada uplifted? Why is the Great Valley so prone to chronic flooding? Though the Sierra Nevada region may have been elevated since the The Great Valley is remarkably flat, surrounded by elevated terrain late Mesozoic Era, a pulse of rapid uplift 5 million to 10 million years that receives abundant rain and snow, and has a climate that can pro- ago lifted the to its present height. The recent ascent duce both heavy winter rains and rapid spring melt of snow. The occurred via normal faulting and westward tilting along the eastern broad floodplain adjacent to the Sacramento and other rivers in the escarpment of the range. Great Valley has developed over thousands of years of recurring What types of gold deposits occur in the Sierra Nevada? floods. The natural flooding behavior of rivers in the Great Valley is a Gold occurs in the Sierra Nevada regions as particles in modern river serious threat to the populous cities, large farms, and industrial cen- sediments (placer deposits), as flakes and nuggets in Eocene-age ters that have been established in the region. 42 Geology of Northern California

Northern California.5 The Northern Coast Ranges displaced downward (the hanging wall) along these faults are repre- What is the Franciscan complex and how did it form? sented by the low valleys that separate the elevated ranges. The Franciscan complex is a complex assemblage of deformed rock What mountains and basins in northern California belong to the consisting of oceanic sediments such as graywacke, shale, and chert Basin and Range province? mixed with metamorphosed igneous rocks. Such a complex rock mix- The Warner Range, more than 3,000 meters (9,870870 feet)f high, is the ture is known as a mélange, and represents an amalgamation of rock best example of a Basin and Range mountain system in northeastnor that formed in the Farallon subduction zone. California. The adjacent Surprise Valley and thehe Lake TahoeTaho basin to How did the San Andreas fault system originate? the south both developed as a consequencence of the downward displace-displ The San Andreas fault system is the consequence of the development ment along normal faults. of a transform boundary between the North American and Pacific plates over the past 30 million years. The faults in the San Andreas Northern California.8 The Northernrthern California Ice Ages system are mostly right-lateral strike-slip faults that accommodate the During what time periods did northernthern California experienceexper Ice Age displacement between the Pacific plate (moving to the northwest) and conditions? the North American plate. Multiple cycles of climatemat change affectedffected northern California during the Pleistocene Epoch,h, but the Tahoe and Tioga glaciations, about What is the Salinian block? 160,000 and 20,000 yearsars ago respectively,respect are the two best docu- The Salinian block is a large terrane in the northern Coast Ranges that mented episodes. includes granitic and metamorphic basement rocks, similar to those of the Sierra Nevada, overlain by Cenozoic-age sediments of oceanic and What landscapelandscap featuress resultedresult from the Pleistocenene EpochEpo glacia- terrestrial origin. From its original position near the south end of the tions in northern California?Calif ? Sierra Nevada, the Salinian block has been transported more than Valley glaciers developed in the mountainous regionsgions of northernno 300 kilometers northwest along the San Andreas fault system. Californiarnia several times duringdur the Pleistocene Icece Ages. ErosionalE fefeaturess produced by these glaciers are widespreadad and include pol-pol How old are the northern Coast Ranges and what tectonic forces ele- isheished andd striated bedrockbedroc surfaces, U-shapedshaped glacialgl troughs, horns,horn vated them? arêtes,rêtes, and cirques.irques In addition, sedimentsments released fromm melting iceic The northern Coast Ranges are relatively young mountains, rising to at thehe closclose of the glacial episodesisod leftft glacial erratics andd morainemorain their present elevations in the past 3 million to 4 million years. The deposits in mmany locations. forces that lifted these coastal mountains are probably related to com- pression between fault slices in the San Andreas system and the north-th- How did the Pleistocene landscapepe of northernn hern California differ from ward migration of the Mendocino triple junction. the modern setting? DuringDu the periods of maximum glacialgla advance,ad the crest of the Northern California.6 Volcanoes of the Cascade Range and SiSierra Nevada wasas buriedburied under a thickthic ice cap. From this high mass Modoc Plateau of ice, valley glaciersacier extendedtended down the major canyons, in some cases What California volcanoes are part of the Cascade Range?R completely fillingling themt withth ice.ice Valley glaciers also existed in the Mount Shasta (elevation 4,319 meters/14.161.161 feet), Mount Lassenassen Klamath Mountainstains anda in the Cascade Range. In the lowland valleys, (elevation 3,188 meters/10,457 feet), and theth Medicine Lake highlandhla pluvialal lakes expandedex ded andan withered repeatedly in rhythm with the are the principal volcanic features of the Cascade Range in northernn Pleistocenestocene climaticclima oscillations.sci The decrease in sea level that accom- California. paniedd the glacial intervalsin exposed the floor of San Francisco Bay as a What is the Cascadia subduction zone? broad riverver valley several times. The Pacific shore was more than 30 The Cascadia subduction zone generates the magma that sustains the kilometersilome rs west of its present location. volcanic activityy in theth Cascade Range. This subduction zone is creat- Northern California.9 Northern California Earthquakes ed by the downwardwnward movementmove off the JuanJua de Fuca plate beneathbenea the What plate tectonic settings are associated with northern California northwest edge of North America.Am . The CascadiaCa subductionn zone is eaearthquakes? only partly in northernorthern California,Cali a, extendingextendi northward from near NNorthern California earthquakes occur mostly along the San Andreas Cape Mendocino too BritishBritis Columbia.mbia. fault system (part of a transform plate boundary), in the Cascadia sub- What kindd of volcanic activity typifies the th Cascade Range? duction zone (a convergent boundary), or along the eastern escarpment Cascade volcanoes are mostly composite cone (or(o stratovolcanoes) of the Sierra Nevada (a zone of developing plate divergence, or rifting). that alternaternate betweenbetw effusive eruptionss of viscousvisco andesitic lava and What is the likelihood of another major earthquake in northern more violentlen explosive events that resultesult in pyroclaticpyroc flows. The California? explosive phase of activity was most recently demonstrateddem by the Given the tectonic forces currently affecting the crust in northern 1980 eruption of MountMo St. Helens in Washington,Wash but the geologic California, another major earthquake is certain to occur eventually. record provideses evidenceevid off much more violent prehistoric activity in Modern techniques for forecasting earthquake activity, including the several Cascadede RangeRan locations.ations. analysis of seismic gaps, suggest that there is a 67% probability of an Why is Mount LassenLa an especially interestingin volcano in the Cascade earthquake exceeding magnitude 6.7 in the next 30 years. The faults Range? in the San Francisco Bay area, such as the Hayward-Rogers Creek, Mount Lassen is noteworthy among Cascade Range volcanoes because Calaveras, and San Andreas faults, are mostly likely to produce this it is the most recently active volcano in California (it last erupted in earthquake. In this densely populated and intensively developed 1914–1921) and is classified as a lava dome, not a composite volcano. region, such an earthquake could have disastrous consequences. Geothermal activity and ongoing seismic activity in the Mount Lassen What might be the effects of a large northern California earthquake? region suggest that the underground magma system is still active. Depending on the location of the epicenter and the magnitude, the Northern California.7 The Basin and Range of Northeast ground shaking of a large earthquake in northern California could California result in damaged or destroyed buildings, fires, collapsed bridges, dis- What tectonic forces are responsible for the pattern of alternating moun- rupted communications and transportation networks, contaminated tains and valleys in the Basin and Range province? drinking water, liquefaction of soils, and tsunamis along the coast. Tensional, or stretching, forces have broken the crust in the Basin and Millions of people and billions of dollars in property are vulnerable to Range into hundreds of blocks bounded by normal faults. The blocks earthquake hazards in northern California. Review Workbook 43

Given the severity of the potential hazards, how can the effects of What coastal hazards exist in northern California? earthquakes be minimized in northern California? Coastal hazards in northern California are primarily related to ero- Experience has shown that earthquakes hazards can be reduced sion, mass wasting, or seismic activity. Erosion of sea cliffs by waves through a combination of wise land use, continuous seismic monitor- causes the cliffs to retreat at rates as high as a meter per year, poten- ing, stringent building codes, and personal and public preparedness. tially affecting structures overlookingking the ocean. Mass wasting occurs Fortunately, most populated regions in northern California have plans readily along the steep slopes descendingdescen to the sea along the emer- in place for effective responses to a potential earthquake, and the reg- gent coast, especially during thehe rainy season or where the rocks are ulated development of communities has reduced the number of peo- weak and/or absorb water. Tsunamismis generatedgener by earthquakes in the ple at risk. Pacific basin have struckuck the Californiania coast many times, some from very distant epicenters.ers. Northern California. 10 Living on the Edge: Coastal Hazards in How do human activities affect coastalcoast hazards?za Northern California Whenever humans modify the natural conditions along the coast, In comparison to the coast of southern California, why is the northern dynamic responses result that can sometimessom intensify coastal hazards. California shoreline so rugged and scenic? For example,amp althoughh breakwaters canc protect one location from The northern California coast trends northwest, directly facing the wave erosion,rosion, theythe also deflecteflect the th energy of approaching waves to waves, storms, and tsunamis generated in the Pacific Ocean basin. In other placesaces where erosionerosi increases. When coastal slopes are modi- addition, the emergent coast of northern California continuously fied for thehe construction ofo roads or buildings,ings mass wasting can result exposes new bedrock to the vigorous erosion that results in such land- if the balancence betweenbetw gravity and frictionon ono a table stable slope is forms as sea cliffs, sea caves, sea arches, and sea stacks. Finally, in most compromised.pr ed. Today,T in most coastall communities,com awareness of these places, the bedrock of the northern California coast is a complex coastaloastal hazardsha ds hash led to restrictionsons on development,dev but there are assemblage of hard and soft rock that erodes to an irregular shore withith still many plplaces where people andd property are at risk. projecting headlands separated by recessed coves and pocket beaches.s.

ESSENTIAL TERMS TO KNOWNOW accretionary terranes, terranes – a large blockck of rock with emergent coastst – a coastst wherewhe land has risen with respect to sea characteristics different from surroundingg blocks.bl Terranesrranes are typi- level. cally bounded by large faults and are thoughtthoug to representnt seamounts,seam Farallonlon plate – ana ancientncie lithospheric plate that separated the oceanic rises, reefs, island arcs, or otherher oceanic featuresfe accretedcrete to Northh America plate frfrom the Pacific plate during the Mesozoic and continents at convergent plate boundaries.ries. Early Cenozoicenozoic Eras. Remnants of the Farallon plate include the alluvium – a general term for sedimentsed transportedransported and deposited momodern Juanan de Fuca, Gorda, Cocos, and Rivera plates in the eastern by running water. PacifPacific Ocean basin.b arête – a narrow jagged ridgeidg separating two glaciali valleys or cirques.ques. Farallonrallo subduction zone – the east-dipping zone of subduction basalt – a dark-colored fine-grainedne-grained that forms fromom devdevelopedped where the Farallon plate descended beneath the western magma of mafic (45% to 52%2% silica) composition.compos edge of the North American plate during the Late Mesozoic and EaEarly Cenozoic eras. batholith – a large body of plutonicutonic igneous ign rock with a surface exposure exceeding 100 square kilometers. fault – a fracture in the Earth’s crust that includes displaced rock masses. baymouthbaymou bar – a spitspi that extends acrossss the mouth of a bay, clos- ing it off from the open ocean.oce felsic – a term describing the composition of magma or igneous rock that contains more than 65% silica and is rich in sodium, potassium, blueschist – a foliatedliated metamorphicme rock that contains glaucophane,glauc and aluminum. a bluish-coloreduish-col mineral of the amphibole group. forearc basin – a basin of sediment accumulation located between a breccia – a detritaltal sedimentarysedim rock consisting of angulargul rock par- volcanic arc mountain system and an offshore oceanic trench. Forearc ticticles larger than 2 millimeters. basins are developed in association with subduction zones at conver- calderaca – a large circularlar to oval depression that develops on the gent plate boundaries. summit of a volcano in response tot the partial evacuation of the fumerole – a vent at the surface that emits volcanic gases. underlying .hamber. gabbro – a dark-colored intrusive igneous rock of mafic composition. chertcher – a fine-grainedne-gr nonclastic sedimentary rock consisting of micromicrocrystallinelline quartz,quar which may include the skeletons of silica- glacial erratic – a rock carried by a from its source to a sur- secresecreting microorganismmicroorganisms such as radiolarian. face of dissimilar bedrock. – a smallsma steep-sided volcano consisting of a loose glacial polish – a smooth and glossy bedrock surface produced by accumulation of volcanic cinder. the movement of a glacier over it. cirque – a generally circular depression that forms in the uppermost glacial striation – a linear scratch or shallow groove on a bedrock reaches of glacial troughs occupied by valley glaciers. surface produced by the movement of a sediment-laden glacier over it. composite volcano – a volcano composed of lava flows, pyroclastic graben – a term for the block of rock displaced downward as the layers, and mudflow deposits; sometimes referred to as a . hanging wall of a normal fault. dacite – an extrusive igneous rock intermediate in composition granite – a light-colored plutonic igneous rock of felsic composition. between rhyolite and andesite. greenstone – a metamorphic rock containing the greenish minerals diorite – an intrusive igneous rock with nearly equal amounts of epidote, chlorite, or amphibole that results from the metamorphism dark-colored and light-colored minerals. Diorite is intermediate in of mafic igneous materials. composition between granite (felsic) and gabbro (mafic). headland – land that projects seaward from an irregular shore line. 44 Geology of Northern California horn – a steep and jagged peak having the form of a pyramid that roof pendant – a mass of metamorphic rock preserved above the developed between glacial cirques. top of an underlying pluton. Roof pendants represent remnants of the horst – a term describing the block of rock displaced upward as the “country rock” into which subterranean magma intruded. footwall of a normal fault. sea arch – an archlike exposure of coastal bedrockk thatth results from inverted topography – a term describing any landscape feature wave erosion of sea caves along a projecting headland.dland that originated at a low elevation, but is currently elevated above the sea stack – a remnant of bedrock on a wave-cut-cut marine terrace.te surrounding terrain, or vice versa. seismic gap – a portion of an active fault that has producedprodu fewer lahar – a volcanic mudflow composed of water, ash, and particles of and smaller earthquakes than other segmentsegmen of thee same fault. volcanic rock. Sierra Nevada batholith – a largerge composite plutonn consistingconsist of limestone – a chemical sedimentary rock composed of calcite, CaCO3. more than 100 individual masses of intrusive igneousigneou rockkm making up lode – an ore deposit in which the valuable commodity is concentrated the basement of the Sierra Nevada.a. in a vein or pod within crystalline igneous or metamorphic rock. serpentinite – a greenish-black metamorphic rock that forms from mafic – a term describing the composition of magma or igneous rock mafic igneous rocks suchuch as basalt, gabbro,abbro, and peridotite.per Sepentinite that contains 45% to 52% silica and is rich in calcium, magnesium, occurs in the western metamorphicmetamorph terranes of the Sierra Nevada, in and iron. the Klamath Mountains,s, and in the northernnort Coast Ranges. It is the official California state rock.ock. mass wasting – the downslope movement of material under the influence of gravity including rockslides, mudslides, rockfalls, and spit – a sandy projection of a beachb into a body of waterwaters such as a bay debris flows. or estuary.ary. mélange – a deformed and sheared mass of metamorphic and sedi- superterranerter – a large fragmentfr m of crustal rocksks that is comprisedc mentary rocks that forms in seduction zones. of smallerller terranes amalgamamalgamated during accretionn to a continental mamargin. moraine – a ridge of mound of unsorted and nonstratified glacial sediment (till) deposited by melting glaciers. synclinesyncli – a down-arched down-ar or concave-upwardve-upward fold in a sequence ofo rockk layerlayers. (Note: the abovee definitiond ion is used to conformcon rm to thattha of – a fine-grained clastic sedimentary rock consistingg of a mudstone Monroe, WiWincander, and Hazlett)ett) mixture of sand, silt, and clay-size particles. tombolol – a spit that extends outwardutward intoto the sea or a lake that – a dip-slip fault in which the hanging wallall hash movedd normal faults connects an island or seaa stack tot land.nd. down relative to the footwall. Normal faults develop fromrom tensionalt stress and are most common in areas of divergent plateate boundaries transformtra plate boundary – a boundaryb nd between plates that slideli past one anotheroth r and whose crucrust is neither produced or – a sequence of mafic igneous rock representingp ting oceanicocea ophiolite destroyed. Thee strikstrike-sliplip faultsfau of the San Andreas fault zone lithosphere and upper mantle; ophiolites consistsist of pillowp basalt,asalt, sheeted represent the transtransform boundaryounda between the Pacific and North basalt dikes, layered and massive gabbro, andnd mantlem periodite.e America pplates. Pangaea – a supercontinent consisting off all the Earth’s landmasseses triplele junction – the pointp at which three lithosphere plates meet. that existed at the end of the Paleozoic Era,a, about 250 million yearsy ago. tsunaminami – a large seas wave that is produced when mass on the –an ultramafic igneous rock thoughthought to comprisecompri much peridotite seafseafloor is suddenly displaced by earthquakes, volcanic eruptions, or of the upper mantle. submasubmarinene landslides.landsl physiographic province – a region of unique geology, landforms, tuff – a fine-grainedfi pyroclastic igneous rock that consists of consoli- drainage, soils, climateclimate, and flora and fauna distinct from adjacentadja dated parparticles of volcanic ash. areas on the basis of these features.s. turbidityturbid currents – a dense mixture of sediment and water that – roundedround orr bulbous structures that developelop ini pillow structuresctures floflows downslope on the ocean floor. lava erupted underer water. ultramafic – a term describing the composition of magma or – a term applieddt to stream-transportedm-transpo sediment such as sandd placer igneous rock that contains 40% or less silica and abundant magne- and gravelel that containsco significant quantities of a valuable mineral sium and iron. such as gold, silver,silver or platinum. volcanic – a term describing the origin of igneous rocks from – an intrusive igneous bodyy that forms whenw magma cools pluton magma that cools rapidly after its eruption during a volcanic event. and crystallizes below the surface andd within the crust.c volcanic arc – a chain of volcanoes that develops on the earth’s sur- – a term describingd g the originin of an igneous rock that crys- plutonic face where magma rises from an underlying subduction zone. tallized from magma withinn the crust.cru welded tuffs – a hard volcanic rock composed of tiny ash articles – a smalls beacheach along an irregular coast located in a pocket beachch that are firmly welded together by the heat associated with an explo- recess between projectingroje headlands.adlands. sive volcanic eruption. pyroclastic – a term that describes the fragmental texture or charac- xenoliths – a type of inclusion in which a fragment of older rock is ter of volcanic rock such as tuff and volcanic breccia produced during incorporated into younger igneous rock. explosive eruptions. Review Workbook 45

More on Northern California Geology

California’s stunning scenery, rich geologic heritage, and abundant Roadside Geology of Northern and Central Califorina, by David Alt natural resources have stimulated considerable interest from geolo- and Donald Hyndman, 2003, Mountainou Press Publishing, gists for more than two centuries. Scientific studies in the state have Missoula, MT, 384 pages. resulted in a vast body of technical literature on California geology. The California Geological Surveyrvey is an excellentexce source of informa- Fortunately, there are many excellent summaries available that tion on earthquakes, rocks and minerals, geologicgeo hazards, and map- describe the geologic features of northern California in comprehensi- ping programs in the Goldenolde State.e. The Web site at ble terms and in the context of modern geologic concepts. Your col- http://www.conservation.ca.gov/cgstion.ca containsontains links to a vast array of lege or local library may have copies of the following books that you information on the geology of California.Cali a. In addition,ad the U.S. may find valuable in learning more about the geology of northern Geological Survey Web site (http://www.usgs.gov)(http://w offers a search fea- California: ture that allows youu to retrieve many USGSU publications on specific Assembling California, John McPhee, 2003, Farrar, Straus, and aspects of Californiaa geology, water resources,re and geologic hazards. Giroux, New York, 304 pages. Finally,y, check with thee geology departmentdep at your local college and Geology Of California, Deborah R. Harden, 2004, Pearson-Prentice universityity about otheroth sources of information and additional opportu- Hall, Upper Saddle River, NJ, 552 pages. nities to explore California geology. Most collegeco geology programs Geology of Northern California, E.H. Bailey, editor, 1966, California offer field courses and introductoryintr lectureurere coursesco that address the Division of Mines and Geology (California Geological Survey) diversed geologicologic features of the state. Bulletin 190, 508 pages. Visit http://custom.cengage.com/r any ofthemodulesfromourRegionalGeologySeriesintoyourcourse. Cengage Learningrepresentativetolearnhowyoucanincorporatethisor examples oftheconsequencesdynamicgeologicprocesses.Work withyour college studentsthroughoutnorthernCaliforniabyprovidinglocalandregional Use thismoduletohelpmakeanyintroductorygeologycoursemorerelevant how anunderstandingofthesethreatscanhelpreducetheirpotentialimpacts. volcanic eruptions,slopefailures,andfloods on-going geologicalevolutionareconsidered.We learnwhyfutureearthquakes, In addition,themultiplehazardstopeopleandpropertythatresultfromCalifornia’s are exploredinthecontextofhundredsmillionyearstectonicevolution. California. TheoriginoftherichmineralandfuelresourcesGoldenState This modulealsosurveystheaffectofgeologicalprocessesonhumansinnorthern of moderngeologytotheinterpretationscenicandvariedterrain. explored withanemphasisonapplyingthefundamentalconcepts are Ranges Coast Range region,thenorthernGreatValley, theBasinandRange,northern features ofthenorthernSierraNevada,KlamathMountains,Cascade explores therichgeologicalheritageofnorthernCalifornia.Themajor Written toaccompanyinganycollege-levelearthsciencecourse,thismodule features andgeologicalhistoryofnorthernCalifornia’s magnificentlandscape. articles onthegeologyofwesternNorthAmerica. sciences. Hehaswrittenfourbooksandnumerousresearchpaperspopular California, whereheteachescoursesingeology, , andenvironmental Frank DeCourtenisProfessorofEarthScienceatSierraCollegeinGrassValley, About theAuthor: other bonuscontentavailableandhowtoorderit. The Geology of Northern California Geology ofNorthern provides anoverviewofthephysiographic egional_geology/ merous rese will erica. y, geo affect northernCalifornians,and For yourlifelonglearningneeds: www.academic.cengage.com Custom Solutionsonlineat www.cengagecustom.com to learnmoreabout Visit Cengage Learning a CollegeinGrassV , and apers a mo rse this out ith yo egio

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