Unit V Geology
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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 erosion by wind Cinder cones, calderas, and stratovolcanoes Area (NRA) and parts of Death Valley 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 sediment 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 streams 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 alluvial fan — 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 Maar 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 sediments 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.