GEOLOGY

Far from being static, the geology of Earth is dynamic, in constant motion and change. The crust is a puzzle made up of tectonic plates, shifting against one another over a molten layer of magma. During the changing relationship of the plates, Earth’s crust has undergone com- pression and stretching, creating vast breaks in the surface known as faults. Periods of moun- tain building uplifted the great mountain chains of the world. Since the Precambrian Era, the Mojave Desert has experienced many differ- ent phases over almost incomprehensible time periods.

(See the Fun Facts chart in this unit) GEOLOGY GEOLOGY

ll three families of rock are evident still evident in the Aztec sandstone fantastic examples. Ain the Mojave Desert — igneous, found in Red Rock Canyon National Extrusive igneous rock is magma sedimentary, and metamorphic. The Conservation Area (NCA), Valley of that reaches the surface, in the form of earliest rock, related to the metamorphic Fire State Park, and Lake Mead NRA. It lava and explosive material. Ash, cinder, gneiss and schist at the bottom of the continues into Arizona and Utah where lapilli, and volcanic bombs are violently Grand Canyon, has little exposure in it is called Navajo sandstone. Red, beige, expelled by volcanoes and vents when the Mojave, but can be seen at Saddle and even white, this sandstone has a lava is very thick and filled with gas. Island in Lake Mead National Recreation propensity for fanciful by wind Cinder cones, calderas, and stratovolcanoes Area (NRA) and parts of and water. Spectacular arches can be are formed in this way. Shield volcanoes National Park (NP). Changed from pre- formed. Beehives, goblins, and castles are formed by less viscous lava. existing rock by immense temperature appear to the human imagination in Extrusive igneous rock is usually fine- and/or pressure (without melting), this other rock formations. grained from cooling quickly on the metamorphic rock is ancient — from the The beginning of the Cenozoic surface. Precambrian Era, formed about 1.6 to 1.8 Era (65 million years ago – present) The most recent period of volcanic billion years ago. heralded another tectonically active activity in the Mojave Desert occurred period. Several different types of only three thousand years ago and faulting deformed the layers of was centered at the Ubehebe Crater Once Upon and the metamorphic rock beneath. in Death Valley NP. The hot springs in Normal faults produced vertical lifting Black Canyon at Lake Mead NRA and the A Time and dropping, while strike-slip faults cinder cones and lava flows found in the shifted blocks laterally. Thrust faults Mojave National Preserve (NPres) are t is hard to imagine that during the folded older layers over younger reminders of this active period. I Paleozoic Era (570 – 225 million years deposits, producing miles of horizontal ago) much of the American Southwest displacement. This topsy-turvy was covered by shallow seas. However, construction can be seen at the Keystone Shake, Rattle, fossil impressions of marine creatures Thrust Fault at Red Rock Canyon NCA in limestone and dolomite reveal the and in the Muddy Mountains of Lake And Roll ancient beginnings of many sedimentary Mead NRA. rocks. Throughout this period, or Mojave Desert residents earth- fluctuating seas deposited thousands F quakes are active reminders of the of feet of sediment, then retreated, Earth’s dynamic nature. An earthquake exposing these layers to erosion. Magma is a shaking of the Earth caused by Different layers of marine deposits can the release of energy as rock suddenly be seen in banded mountains throughout On The Move breaks or shifts under stress. Although the desert. long with the faulting, volcanic earthquakes can happen anywhere, most During the Mesozoic Era (225 – 65 Aactivity increased across the occur along tectonic plate margins and million years ago) mountains were landscape. Igneous rock is derived from are associated with faults. uplifted, revealing the Paleozoic layers. molten layers of magma beneath the The Mojave Desert lies near the Evaporating bodies of water left behind Earth’s crust. Under great pressure, boundary between the Pacific and North salt (halite), calcium (gypsum), and the magma rises towards the surface American plates. As these plates slide many other evaporates that have been through weak points such as faults. past each other stress is created, forcing mined by humans for hundreds of years If the molten rock solidifies before rocks to break along faults. A magnitude for industrial and domestic use. Rivers reaching the surface, it is intrusive, 7.6 earthquake, centered in the western and transient carried great forming tabular sills, dikes, and large Mojave Desert, rattled a wide area on amounts of eroded material into the bodies called plutons. These rocks are June 28, 1992. It was the largest to strike lowlands. large-grained from slow cooling. Erosion California in forty years and was also The climate became even more arid of less resistant surface rock exposes the the world’s largest earthquake for 1992. than it is today, expanding great belts of plutonic formations. Plutonic rock can Aftershocks from this quake were felt for sand dunes. Jurassic winds carried the be seen all over the Mojave Desert, and many years. sand, creating the cross-bedding patterns Joshua Tree National Park has especially

UNIT V–2 UNIT V–3 GEOLOGY GEOLOGY

Whittling The Earth GLOSSARY n this arid region of little precipitation — a fan-shaped deposit rock containing water and gases, found and sparse vegetation, erosion is I formed where streams issue from the beneath Earth’s crust. a major process shaping the desert. mountains onto the lowlands. Water can seep into the smallest crack volcano — volcano formed and percolate between grains of stone. caldera — a large depression typically by a steam explosion caused when Acids, minerals, and gases are carried in caused by collapse or ejection of the groudwater meets magma summit area of a volcano. solution to react with the rock, causing metamorphic rock — a rock changed chemical alterations such as oxidation cinder cone — a small volcano built by great temperature, pressure, stress, of iron and manganese which produces primarily of rock fragments ejected and/or chemical changes, usually at many hues of red and purple. from a single vent. depth in the crust, from pre-existing The rain that does come is often in rocks (either igneous or sedimentary). torrential thunderstorms that carry cross-bedding pattern — layers large amounts of debris through inclined at a steep angle to the oxidation — the process of combining washes, forming alluvial fans below horizontal, characteristic of sand dunes chemically with oxygen, producing an the mountains and filling in the desert and river deltas. oxide. basins. Wind also distributes material, desert pavement — a surface of pluton — a structure resulting from molding sand dunes, such as those found pebbles and cobbles exposed by winds the crystallization of magma beneath at the Kelso Dunes in the Mojave NP, and sheet wash that protects the finer Earth’s surface. and abrading landforms into sculptural material beneath, a desert crust. schist metamorphic rock shapes, like the arches at Valley of Fire. — a that Gravity continually draws material dike — a long, narrow, cross-cutting readily splits into parallel layers and igneous rock downward, forming aprons of talus mass of intruded a fissure has a platy or scaly appearance. in older rock. Dikes are often oriented debris along the skirts of the mountains. sedimentary rock — a rock formed Erosion is not always a natural vertically. of deposited by wind, water process. Human activities, such as erosion — processes that fragment, (conglomerates, sandstone, shale), or overgrazing, deforestation, construction, dissolve, and remove rock and related precipitation (limestone, gypsum, salt). and water diversion, are recorded on material: wind, water, gravity. shield volcano — a broad, gently the land. Bicycles, off-road vehicles, fault — a fracture or fracture zone in sloping volcano built from fluid lavas. and indiscriminate blazing of new trails Earth’s crust along which there has desert stratovolcano — a volcano with destroy the protective layer of been movement of the sides relative to pavement steep sides, built up by different layers and promote erosion of fragile one another. desert soils. Removal of even seemingly of explosive cinder and ash, with unimportant rocks spoils the experience gneiss — a metamorphic rock where occasional lava flows. intense pressures and temperatures for others and is illegal in National Park tabular sill — an igneous body Service areas. have caused minerals to segregate intruded parallel to the layering of pre- The fascinating rock formations of the giving the rock a banded appearance. existing rock. Most sills are horizontal. Mojave Desert are inviting, but be careful hoodoo — a pillar of rock left by talus — an accumulation of rock when climbing. Many people are injured erosion. or killed in climbing accidents every debris, usually at the base of a cliff. igneous rock — a rock formed by the year. Explore and enjoy the desert safely tectonic plate — one of the geological magma and sanely. crystallization of molten . structures making up Earth’s crust. lapilli — small, stony particles ejected Related to the deformation of the crust from a volcano. by faulting and folding. lava — molten rock that makes it to volcanic bomb — a streamlined rock Earth’s surface. fragment ejected from a volcano while molten. magma — naturally occurring molten

UNIT V–2 UNIT V–3 GEOLOGY GEOLOGY

Activity 1 Activity 2 wind. A cresent with its horns pointing downwind is a barchan dune, common in Deep Time Dunes And Hoodoos all deserts. OBJECTIVE: Demonstrate the relative OBJECTIVES: List different ways rock distance of events in time. can be eroded. Describe the erosion that WATER: occurs from these processes. 1. Make a mound of sand (two inches MATERIALS: Adding machine paper tape deep in the center) on a flat plate. (at least forty feet), crayons, Span of MATERIALS: Blow dryer, empty ice tray, Time chart on page 5. flat plate, rain or a sprinkler, sand, small block the same height as the tray, three SUBJECTS: Art, math, science. quarters. SKILLS: Application, computation, SUBJECTS: Art, language arts, science. drawing. SKILLS: Analysis, comparing, description, 2. Place three quarters on top, about METHOD: evaluation, predicting, oral and written one inch apart. 1. Find a space about forty feet in communication. length. METHOD: Students can predict 2. Assign one person to represent the what they think will happen in each beginning of the Earth and have them experiment. They can write and draw pull out the paper tape. Lay the tape on what happens after each experiment and the ground. compare the results to their predictions. 3. Assign each person an event on the WIND: Span of Time chart and have them pace 1. At the end of an empty ice tray, out the distance to their assigned event. place a block that is the same height as 4. Starting with the beginning of the the tray. Earth, have each student call out their event and how long ago it occurred. 3. Place out in the rain or make your 5. Have students draw a picture to own with a sprinkler from above. represent their assigned event at the 4. Note what happens to the sand. length of tape that represents that date in time. (Note that modern events have WHAT HAPPENED AND WHY? occurred in such a tiny part of recent 2. Mound approximately 1⁄2 cup of history compared with the rest of time sand on the block. that it would be difficult to include all the 3. Use a blow dryer to blow the sand events on this tape.) across the ice tray. EXTENDING THE EXPERIENCE: Try 4. Note the shape the pile takes as the a larger format outside, using twine sand blows away. The quarters act like very hard rocks on instead of tape. Using the same format, less resistant stone. A cap rock protects make a time line for human history. 5. Note the different sizes of sand the layers beneath from erosion so that grains in the tray. What size made it the hoodoos, pinnacles, and other strange farthest? shapes are made. WHAT HAPPENED AND WHY? EXTENDING THE EXPERIENCE: Ask Wind transports and deposits sand. students to look for evidence of erosion Because of their lighter weight, small in their neighborhoods. It can involve an grains will go the farthest. The different entire mountain or just a square inch. shapes that sand dunes take depend on the speed and direction of the

UNIT V–4 UNIT V–5 GEOLOGY GEOLOGY

THE SPAN OF TIME

(INDOOR) (OUTDOOR) LENGTH FROM LENGTH FROM YEARS AGO EVENTS PRESENT PRESENT

38 feet 254 yards 4.57 billion Earth begins 29 feet 194 yards 3.5 billion Life on Earth begins 25 feet 167 yards 3 billion First fossils form; algae, fungi, and bacteria are abundant 4.5 feet 31 yards 550 million Jellyfish, sponges, and worms are abundant 3.75 feet 25 yards 450 million First primitive fish 40 inches 22 yards 400 million Earliest land plants (ferns and mosses) 35 inches 19 yards 350 million Earliest land animals (amphibians) 31 inches 17 yards 310 million First reptiles 27 inches 15 yards 270 million Reptiles abundant and well developed 24.5 inches 14 yards 245 million Age of Dinosaurs begins 18 inches 10 yards 180 million Flowering plants develop 16 inches 9 yards 160 million Birds evolve; dinosaurs abound 7 inches 4 yards 70 million Modern birds develop 6 inches 11 feet 65 million Dinosaurs extinct; Age of Mammals begins 5 inches 8 feet 50 million Mammals and birds abundant 4 inches 7 feet 40 million First elephants .5 inches 10 inches 5 million First humans .15 inches 3 inches 1.5 million Beginning of Pleistocene and Ice ages .001 inch .02 inch 10,000 End of the most recent Ice Age .0002 inch .004 inch 1,915* Mt. Vesuvius erupts in Pompeii .0001 inch .0015 inch 779* Magna Carta signed in 1215 .00002 inch .0004 inch 218* Declaration of Independence signed in 1776

Indoor Scale: .1 inch = 1 million years * Years ago calculated from 1994 Outdoor Scale: 2 inches = 1 million years

UNIT V–4 UNIT V–5 GEOLOGY GEOLOGY

Activity 3 below. The Mohs scale of hardness runs water each over three of the nails. Leave from 1 (softest) to 10 (hardest): the fourth one dry. Mystery Minerals 1 TALC 3. Sprinkle a tablespoon of salt each OBJECTIVES: List four tests used in 2 GYPSUM over two of the wet nails. geology to identify rocks and minerals. 2.5 FINGERNAIL 4. Pour a tablespoon of vinegar over Identify limestone by using a test. 3 CALCITE one of the salty, wet nails. 3.5 PENNY MATERIALS: Magnifying glasses, pennies, 4 FLUORITE 5. Have students keep a log predicting rock and mineral samples (limestone, 5 APATITE what they think will happen to each dolomite, graphite, hematite, pyrite, 6 FELDSPAR nail. Record observations in writings gypsum, quartz, etc.), steel files or 6.5 STEEL FILE OR SCISSORS and drawings. Observe and compare scissors, unglazed white porcelain tiles, 7 QUARTZ the nails every day for a week. Optional: vinegar. 8 TOPAZ Repeat 1-5, but use pennies instead of SUBJECTS: Language arts, science. 9 CORUNDUM nails. 10 DIAMOND SKILLS: Analysis, classification, WHAT HAPPENED AND WHY? When comparison, computation, observation, 2. Have students tabulate their left in contact with the air, metals can writing. results and compare the different bind with oxygen. Oxidation, or rusting, samples. Although they might not be creates the many bright colors that can METHOD: able to identify each sample exactly, be seen in desert rocks and soils. Salts, 1. Have students keep a log, recording these tests help classify various rocks. water, and acids help speed up the results and observations. For each process. Oxygen combines with iron to sample, perform these tests: EXTENDING THE EXPERIENCE: Have form a reddish-brown iron oxide called students compare their results with Test One: Observe the color of the hematite or, in more extreme cases, a those in a geology text. mystery sample. Color is important in yellowish-colored rust called limonite. identification. The salt will react with copper to produce copper chloride. Test Two: Observe how the sample reflects light. This is called its luster. Is Activity 4 EXTENDING THE EXPERIENCE: Have it dull, metallic, resinous, glassy, pearly, students look for examples of oxidation silky, or diamond-like? Oxidation in their daily environment. What causes this oxidation? Ask Test Three: Add a few drops of OBJECTIVES: Describe the changes students where they would vinegar to the sample. Observe through of a metal during oxidation. List what expect to find more oxidation a magnifying glass. Does it fizz? If it does, chemicals can speed up oxidation. — at the beach or in the it is a carbonate rock such as limestone desert. Why? or dolomite. The acidic vinegar is MATERIALS: Four iron or steel nails, reacting with the rock to produce carbon four shallow dishes, distilled water, salt, dioxide gas. vinegar, (optional: four pennies). Test Four: Streak the rock on a tile. SUBJECTS: Art, language arts, science. What color is the streak mark? Is it the SKILLS: Analysis, application, same as the rock? Example: pyrite is a comparison, drawing, evaluation, brassy yellow, but its streak is greenish observation, prediction, writing. black. METHOD: Test Five: Try to scratch the surface 1. Place an iron or steel nail in each of of the rock with your fingernail, a four shallow dishes. Students can predict penny, and a steel file or scissors. This what they think is going to happen to demonstrates the relative hardness for each nail. each sample. Compare with the scale 2. Pour two tablespoons of distilled

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UNIT V–6 UNIT V–7