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Structural Geology and Plate Tectonics Physical Science II Chapter 21 in 13th edition, Chapter 22 in 12th edition Introduction - Geology Geology is the study of planet Earth – its composition, structure, processes, and history. In this chapter we will look at the basic structure of the Earth’s interior. We will see how plate tectonics is the primary mover of the Earth’s outer shell, and how it is related to mountain building, continental drift, earthquakes, volcanoes, and seafloor spreading. Intro The Earth’s Interior Structure Despite many remarkable scientific advances, most of the Earth’s interior is still unreachable. Ideas on the composition and structure of the Earth’s interior come from Earthquake body waves Meteorites – thought to be similar to composition of Earth Spacecraft measurements of gravity and magnetic variations High P & T laboratory experiments on rocks Section 21.1 Earthquake Waves Help Reveal the Earth’s Interior During an earthquake, some of the vibrations travel completely through the interior of the Earth, providing scientists with significant information. Scientists can use the speed and direction of these waves to identify the types of materials through which they move. Section 21.1 Seismic Waves Earthquakes produce seismic waves of two basic types: surface waves and body waves. Surface waves, as their name implies, travel within the upper few kilometers of the Earth’s surface. Surface waves are responsible for most surface earthquake damage. Body waves are transmitted in all directions through the interior of the Earth. Section 21.1 Body Waves Two types of body waves can be distinguished by their type of motion and speed: P (primary) waves are compressional in nature. Particles move back and forth (longitudinal compressional) in the same direction as the wave is traveling. S (secondary) waves are transverse in nature. Particles move at right angles to the direction of wave movement. Section 21.1 P and S Waves P and S waves have two other important differences: S waves will only travel through solids, P waves can travel through solids, liquids, and gases. P waves travel faster than S waves and arrive earlier at the seismic station. The difference in arrival times between the P and S waves allow geophysicists to determine the focus of the earthquake. Section 21.1 Body Waves - Refraction The velocity of body waves is dependent upon the density of the interior of the Earth. The Earth’s interior generally increases in density with depth, therefore body waves will curve and refract. Geophysicists use the travel velocity, the refraction pattern, and the travel path to interpret our modern view of the Earth’s structure. Section 21.1 Seismic Wave Travel Through the Earth’s Interior S waves do not travel through the liquid outer core. P waves are refracted at density boundaries. Section 21.1 Structure of the Earth’s Interior Scientists think that the Earth is composed of four concentric layers or zones: Inner core Outer core Mantle Crust Different compositions and/or physical properties characterize each of these layers. Section 21.1 Interior of the Earth – Four Layers Evidence suggests that the inner core is solid and is composed of iron (85%) and nickel. The outer core is interpreted to have the same composition as the inner core, but is liquid. Radius of approximately 1230 km Thickness of approximately 2240 km The mantle’s composition is distinctly different from the outer core. The thin solid outer layer where we live is called the crust. Section 21.1 Interior Structure of the Earth Section 21.1 The Earth’s Crust The Earth’s crust ranges in thickness from 5 to 11 kilometers for oceanic crust and 19 to 40 kilometers for continental crust. Oceanic crust is mainly composed of basalt. Continental crust is granitic in composition. A sharp compositional boundary exists between the base of the crust and the upper mantle. This boundary is called the Mohorovicic discontinuity, or simply Moho. Section 21.1 Crust and Upper Mantle – Physical Properties The crust and upper mantle can be divided differently if we take into account its physical properties and behavior. Lithosphere - outermost rigid, brittle layer Composed of the entire crust and uppermost mantle Most faults and earthquakes occur in the lithosphere. The asthenosphere lies beneath the crust, extending down to approximately 70 km. Due to its high temperature this layer is plastic, mobile, and is essential for tectonic plate motion. Section 21.1 Lithosphere and Asthenosphere The lithosphere is solid and slowly moves over the plastic asthenosphere. Section 21.1 Continental Drift Formulation of the Theory As one looks at a current world map, it is apparent that the coastlines of eastern South America and western Africa fit together fairly well. Is this a coincidence or were these two and the other continents once attached? Over the past several hundred years scientists have speculated as to the meaning of this observation. Have the continents drifted? Section 21.2 Continental Drift – Alfred Wegener Alfred Wegener (1880 –1930) was a German meteorologist and geophysicist. In the early 1900’s he revived/proposed the hypothesis of continental drift. Wegener proposed that about 200 million years ago all the continents were together in a supercontinent he called Pangea. During the past 200 million years Pangea broke apart and the newly formed continents slowly drifted apart. Section 21.2 Starting 200 Million Years Ago Pangea Started to Break Apart Section 21.2 Breakup of Pangea 100 Million Years Ago Section 21.2 Breakup of Pangea Present Day Continental Configuration Section 21.2 Continental Drift – Scientific Evidence Wegener brought together various pieces of geologic evidence that supported his theory. There were three prominent lines of evidence that Wegener highlighted: Biological evidence Continuity of geologic features Glacial evidence Section 21.2 Continental Drift Biologic & Paleontologic Evidence Present-day biological species on widely separated continents have similarities that suggest that these land masses were once together. Identical fossil plants and animals have been found on a number of continents, once again strongly suggesting that these continents were together when these organisms were alive. Section 21.2 Continental Drift Continuity of Geologic Features If the continents of North America, Europe, South America, and Africa were all put back together the continuity of several geologic features would become evident. In the northern hemisphere the Caledonian, Hebrides, Labrador, and Canadian Appalachians match up. In the southern hemisphere the Cape and Sierra mountains of South Africa and Brazil would line up nicely. Section 21.2 Continuity of Continental Features Illustrated If the continents had once been together and drifted apart, we would expect the continuity of geologic features when put back together Section 21.2 Continuity of Geologic Features Section 21.2 Continental Drift Glacial Evidence Geologic evidence suggests that the southern areas of South America, Africa, India, and Australia were covered with glaciers 300 million years ago (mya). There is no evidence for glaciation at this time (300 mya) in Europe and North America. This indicates that the glaciated areas were once located at very high latitudes, while North America and Europe were at low latitudes. Section 21.2 Glacial Evidence of Continental Drift Only when the continents are put back into their “Pangea” positions does this glacial episode make sense. All of these glaciated areas were located close to the south polar region 300 million years ago. Section 21.2 Continental Drift Not Generally Accepted Although a number of lines of evidence supported Wegener’s theory it was not widely accepted at the time (early 1900’s). At the time, Wegener’s theory still had one critical flaw. He nor anyone else could devise a mechanism that could explain how continental crust could “move” through the oceanic crust. Section 21.2 Seafloor Spreading In 1960, the American geologist Harry Hess suggested a viable mechanism that could explain continental drift. At the time the mid-ocean ridge system and the deep sea trenches had been mapped in fair detail throughout the world’s oceans. The mid-ocean ridge system was known to stretch throughout the world. The trenches were known to be very deep and very long and narrow. Section 21.2 Seafloor Spreading Hess proposed the theory of seafloor spreading. In this theory the seafloor slowly spreads by moving sideways away from the mid-ocean ridges. New magma wells up and cools as each side of the midocean ridge slowly splits apart. The entire ocean floor can be viewed as a giant conveyor belt where the new seafloor moves away from the ridges, and eventually descends back into the mantle at the trenches. Section 21.2 Paleomagnetics Supports Seafloor Spreading As new magma wells up and cools along the mid-ocean ridge system one of the component minerals of this new rock is magnetite. (Fe3O4) When this mineral crystallizes (at cooling) it becomes magnetized in the direction of the Earth’s prevailing magnetic field, a phenomenon called remanent magnetism. We know that the Earth’s magnetic field has abruptly and frequently reversed itself during geologic time. Section 21.2 Seafloor Spreading – Evidence Remanent magnetism of the ocean crust reveals long, narrow, symmetric bands of magnetic anomalies on either side of the Mid-Atlantic Ridge. These magnetic anomalies indicate that the Mid-Atlantic Ridge has been continuously spreading and that the Earth’s magnetic field has reversed itself many times. The mid-ocean ridge spreading rates are in the range of a few centimeters per year. Section 21.2 Magnetic Anomalies Showing Reversals in the Earth’s Magnetic Fields From Chernicoff, Stanley and Donna Whitney, Geology, T hird Edition © 2002 by Houghton Mifflin Company. Used with permission Seafloor Spreading Wegener’s original evidence (biologic, paleontologic, geologic, and glacial) supports the theory of continental drift. Hess’s theory and evidence (remanent magnetism) supports the idea of seafloor spreading. These two ideas have now been merged into the modern theory of plate tectonics. We now know that both the oceanic and continental crusts are carried as part of a thicker layer called the lithosphere. Section 21.2 Plate Tectonics We now visualize ocean basins to be in a constant cycle with new crust being created at the mid-ocean ridges and old crust descending along the ocean trenches. We also know that the lithosphere is composed of a series of solid segments called plates. These plates are constantly moving and interacting with other plates. The theory of plate tectonics encompasses all these processes. The lithosphere is divided into approximately 20 plates. Section 21.3 From From Chernicoff, Stanley and Donna Whitney, Geology,Third Edition © 2002 by Houghton Mifflin Company. Used with permission Plate Boundaries The most active areas of the Earth’s crust are along the plate boundaries. There are three types of plate boundaries: Divergent – located along mid-ocean ridges where the two plates are moving apart Convergent – zones along which two plates are driven together, one plate is consumed Transform – boundaries along which two plates slide horizontally past one another Section 21.3 The Asthenosphere Underlying the Earth’s solid lithosphere is a higher temperature layer - the asthenosphere. This layer, although basically solid, is so close to its melting temperature that it is relatively plastic and easily deforms. The asthenosphere is much more easily deformed than the lithosphere. The lithosphere may be viewed as actually “floating” on top of the asthenosphere. Section 21.3 Asthenosphere and Isostasy Isostasy – the concept that the solid lithosphere floats in gravitational equilibrium (buoyancy) on the plastic asthenosphere Continental plates float higher because they are less dense than oceanic plates. At any given time, all of the plates are in isostatic equilibrium. Mountain ranges simply represent thicker masses of continental material and therefore float higher. Section 21.3 Isostasy Similar to less dense ice that floats in water, the less dense continental crust floats on the more dense asthenosphere Section 21.3 From Chernicoff, Stanley and Donna Whitney, Geology, T hird Edition © 2002 by Houghton Mifflin Company. Used with permission Plate Movement The movement of the lithospheric is due to forces within the asthenosphere. Most geologists think that movement within the asthenosphere is caused by convection cells. Unequal temperature distribution within the asthenosphere and upper mantle results in the hot, less dense material rising, and the cooler, more dense material sinking. Section 21.3 Convection Cells in the Asthenosphere Drag from the more active asthenosphere drives the outermost solid lithosphere. Section 21.3 Divergent Boundary The mid-ocean ridge system represents a zone where two plates are moving apart – a divergent boundary. The initially molten magma is shouldered to each side of the rift and causes the lithospheric plates to slowly separate. Drag from the underlying asthenosphere keeps the plates in motion. Section 21.3 Spreading at the Mid-Ocean Ridge As the two plates move apart, new magma wells up and cools along the rift zone creating new crust. Section 21.3 Divergent Boundaries As a portion of the plate moves away from the hot, spreading center it cools, contracts, and becomes more dense. Due to the increase in density going away from the spreading center (rift), the plate gradually subsides (isostatic equilibrium) and the oceans grow progressively deeper. Section 21.3 Convergent Boundary The result of two plates converging depends on the type of plates that are interacting. Three combinations are possible: Oceanic-oceanic convergent boundary Oceanic-continental convergent boundary Continental-continental convergent boundary In two of these converging boundary-types one of the plates descends beneath the other plate, a process called subduction. A ‘subduction zone’ is where this happens. Section 21.3 Oceanic-Oceanic Convergence Two oceanic plates will have essentially the same density, about 3.0 g/cm3. When two oceanic plates collide one is eventually subducted beneath the other. Long narrow deep see trenches mark the zones where the plate is subducted. The plate subducted begins to melt as it comes in contact with the asthenosphere. Molten material begins to rise, forming a volcanic island arc on the overriding plate. Section 21.3 Oceanic-Oceanic Convergence The deep sea trench and the volcanic island arc are parallel and close to each other. Section 21.3 Oceanic-Continental Convergence Since continental crust is less dense (2.7 g/cm3), it is the oceanic crust that is always subducted. A trench will develop along the zone where the oceanic crust is subducted. As the oceanic crust descends toward the asthenosphere it begins to melt. Magma rises up through the overriding continental plate forming volcanic mountain ranges at the surface. The Andes and Cascades are volcanic mountains. Section 21.3 Oceanic-Continental Convergence The ocean trench and the volcanic mountains are parallel and close to each other. Section 21.3 The Andes Mountains were formed by oceaniccontinental convergence. Copyright © Bobby H. Bammel. All rights reserved Section 21.3 Cascade Mountains were formed by oceanic-continental convergence. Copyright © Bobby H. Bammel. All rights reserved Section 21.3 Continental-Continental Convergence Continental plates have a relatively low density. (2.7 g/cm3) Subduction of continental crust is minimal due to its low density. During convergence the plate edges are intensely deformed to construct fold-mountain ranges. Continents can increase in size during this process by suturing themselves together along fold-mountain systems. Section 21.3 Continental-Continental Convergence The Himalayas, Alps, and Appalachians are examples. Section 21.3 Transform Boundary Linear zones where adjacent plates slide past each other in opposite directions. This is a zone of shearing, or transform motion. Crust is not destroyed or created along a transform boundary since neither subduction nor magma upwelling occur. Periodic movements along these faults result in sudden energy release and repeated earthquakes. These zones are said to be seismically active. Section 21.3 Plate Motion and Volcanoes The term ‘volcano’ can refer to either a vent from which hot molten material escapes or a mountain created by solidified volcanic rock. Although the specific occurrence of volcanic activity is usually unpredictable, the locations of eruptions and potential eruptions are known. Section 21.4 Active Volcanoes of the World The vast majority of active volcanoes lies along plate boundaries Section 21.4 Most Volcanoes occur along Convergent Boundaries When two plates collide, one plate is usually subducted beneath the other During the subduction process, rock just above the subducting plate margin melts, resulting in the molten rock rising to the surface to form volcanic islands or mountains. Section 21.4 Subduction of Oceanic Lithosphere Forms a Volcanic Island Arc – such as Japan Section 21.4 Earthquakes Earthquake – sudden release of energy due to a sudden movement in the Earth’s crust or mantle, resulting from stresses Seismology – the study of earthquakes Earthquakes cause the Earth’s surface to vibrate and sometimes result in violent movements, depending on the amount of energy released Section 21.5 Earthquakes Earthquakes occur when rocks grind past each other along plate boundaries. During this process vibrations radiate out in all directions from the disturbance. The major danger from earthquakes is not the vibrations but rather the human-made structures that collapse. Section 21.5 Causes of Earthquakes Most earthquakes are caused by movements of the lithospheric plates. They can also result from explosive volcanic eruptions or by human-caused explosions. Movements of lithospheric plates generally cause faults in the crustal material. Fault – a fracture in rock along which there has been visible movement of the two sides relative to each other Section 21.5 Causes of Earthquakes Earthquakes are most likely to occur along plate boundaries. Stresses are exerted on the rock formations in adjacent plates, as movement occurs. Since rock possess elastic properties, energy is stored until the stresses can overcome the friction between the two plates. At the moment of energy release, the rocks along the fault suddenly move, the energy is released, and an earthquake occurs. Section 21.5 World Map of Recorded Earthquake Locations Earthquake occurrence closely follows the volcanic ring of fire. Section 21.5 Causes of Earthquakes Generally aftershocks will occur after a major earthquake. These are caused by the rocks continuing to adjust to their new positions. Transform plate boundaries are the locations of many of the world’s longest continuous faults (transform faults.) The San Andrea Fault in California is a transform fault that lies between the Pacific and North American plates. Section 21.5 San Andreas Fault of California The San Andreas Fault is the master fault of an intricate fault zone that runs along the coastal area of south and central California. Many earthquakes have occurred along this fault. In 1906 a major earthquake occurred in the San Francisco area, resulting in hundreds of lives lost and millions of dollars of damage. In 1989 a major earthquake in the area caused severe bridge, building, and highways damage. Little can be done to control this fault line. Section 21.5 San Andreas Fault In about 10 million years Los Angeles will move far enough north to be adjacent with San Francisco. Section 21.5 Anatomy of an Earthquake The point of the initial movement, or energy release, along the fault is called the focus. The focus is generally located underground. From a few miles to perhaps several hundred miles in depth The point on the Earth’s surface directly above the focus is designated the epicenter. This is the surface position that receives the greatest impact from the earthquake. Section 21.5 Focus and Epicenter Photo Source: From Dolgoff, Anatole Physical Geology Copyright © 1996 by Houghton Mifflin Company. Used with permission Section 21.5 Earthquake – Energy Release When an earthquake occurs the energy released from the focus propagates outward in all directions as seismic waves. A seismograph monitors and measures the seismic waves. The greater the energy released in the quake, the greater the amplitude (height) of the traces (lines) on the recorded seismogram. Section 21.5 Seismograph During a quake, the spool vibrates and the light beam is relatively still. Section 21.5 Earthquake Severity The severity and strength of earthquakes are measured on two common scales: The Richter Scale and the modified Mercalli Scale. The Richter scale measures the amount of absolute energy released during a quake by calculating the seismic wave energy at a standard distance. The modified Mercalli scale describes the results of the earthquake in terms of felt and observed effects. Section 21.5 Modified Mercalli Scale Section 21.5 Richter Scale This scale was developed in 1935 by Charles Richter of Cal Tech. This is the most common earthquake measurement. This scale correlates the largest seismogram peak during a given quake to the amount of energy released by the quake. The Richter scale gives the earthquake’s magnitude, expressed as numbers, usually between 3 and 9. Section 21.5 Richter Scale The Richter scale is logarithmic. Each whole number increment represents a 10-fold increase in amplitude tracings. Each whole number increment represents a 31-fold increase in energy release. Therefore, an earthquake of magnitude 5 releases 31 times the energy of a magnitude 4. An earthquake of magnitude 6 releases more than 900 times the energy of a magnitude 4. Section 21.5 Richter Scale One significant drawback of the Richter scale is that the magnitude of the earthquake gives no indication of the damage it may cause. Earthquake damage depends on many factors including focus location, geologic rock types, population density, and construction types. Relatively moderate quakes in areas with high populations and/or poor construction techniques can cause considerable property damage and loss of life. More severe quakes in sparsely populated areas may cause very little damage. Section 21.5 Richter Scale – Earthquake Severity Section 21.5 Related Earthquake Damage Damage from earthquakes may be directly related to the vibrational tremors or it may result from a number of secondary effects. Landslides are commonly triggered by quakes Much of the damage in the 1964 Alaskan quake was due to a series of small landslides. Fires that are initiated by the initial tremors are difficult to fight due to broken water lines. The U.S. government (FEMA) has literature available on earthquake preparedness. Section 21.5 Submarine Earthquakes When a submarine earthquake occurs, some of the energy may be release into the water to form huge waves called tsunamis. Section 21.5 Crustal Deformation and Mountain Building Along plate boundaries tremendous forces may be exerted that result in the buckling, fracturing, or shifting of rock units. These forces can rupture the plate edges into huge displace blocks and may eventually result in the formation of mountain ranges. Two basic types of structural deformation are common: Folding and Faulting. Section 21.6 Crustal Deformation - Folding Folding – buckling of the rock layers into anticlines (arches) and synclines (toughs) Folding occurs when slow compressive forces apply extreme pressures on the rock layers. The forces that cause folding may be exerted either horizontally or vertically. In general, folding occurs mainly during the early stages of mountain building. Section 21.6 Crustal Folding In an extensively folded area, it is a particular rock layer’s resistance to erosion that determines what type of topographic feature is formed. Note that anticlines do not always form high ridges. Section 21.6 Crustal Deformation - Faulting Fault – a fracture in rock along which there has been visible movement of the two sides relative to each other Stresses that form faults may be compressional. If the compression is vertical uplifts are produced. If the compression is horizontal the crust will be shortened. Tensional (pull-apart) stresses can also form faults. Tension causes the crust to lengthen. Section 21.6 Fault Terminology Fault plane – an approximately planar surface along which the actual movement takes place Hanging wall – this is the fault block that is on the uppermost side of an inclined fault plane Footwall – this is the fault block that is on the lowermost side of an inclined fault plane The fault block that has moved up relative to the other side is termed the upthrown side. Section 21.6 Fault Types There are three basic types of faults: Normal, Reverse, and Transform. Normal fault – the hanging wall (uppermost side) moves down with respect to the footwall Reverse fault – the footwall (lowermost side) moves down with respect to the hanging wall Tensional forces (pull-apart) cause normal faults. Compressive forces cause reverse faults. Strike-slip (transform) fault – stresses are parallel to the fault plane (horizontal motion) Section 21.6 Fault Terminology Illustrated Photo Source: From Dolgoff, Anatole Physical Geology Copyright © 1996 by Houghton Mifflin Company. Used with permission Section 21.6 Mountain Building The mountain building process occurs most often along and because of converging plate boundaries. Mountains can be classified into three broad categories, based on their characteristic features: Volcanic Fault-block Fold Section 21.6 Volcanic Mountains These mountains are primarily formed through a series of volcanic eruptions. Most volcanic mountains are located along convergent boundaries, since that is where most volcanoes occur. Along an oceanic-oceanic convergent boundary, chains of volcanic islands will form on the plate overlying the subduction zone. The Aleutian Islands, Japan, and the Lesser Antilles are all examples of volcanic mountains. Section 21.6 Volcanic Mountains Volcanic mountains also form along a continental-oceanic convergent boundary. The continental plate always overlies the subduction zone and this is where volcanic mountains will form. The Andes Mountains of South America were formed as the oceanic Nazca plate is subducted beneath the continental South American plate. The Cascade Mountains are also volcanic mountains. Section 21.6 Fault-Block Mountains Normal faulting can produce tilting and uplift of large crustal blocks. This will result in dramatic fault-block mountains rising abruptly above the surrounding lowlands. The Grand Tetons of Wyoming, the Sierra Nevada Mountains of California, and the Wasatch range of Utah are all examples of fault-block mountains in the U.S. Section 21.6 Grand Teton Mountains, Wyoming Copyright © Bobby H. Bammel. All rights reserved. Section 21.6 Fold Mountains Fold mountains are characterized by prolific folding of the rock strata. The Alps, Himalayas, and Appalachians are all examples of fold mountains. These mountains are also characterized by thick packages of marine sedimentary strata. This sedimentary strata was originally deposited below sea level and then uplifted and incorporated into the fold mountains. Marine fossils are regularly found high in a fold mountain range. Section 21.6 Formation of the Himalayas – Fold Mtns. During the breakup of Pangea (200 m.y.a.) the subcontinent of India broke away from Africa. As the Indian plate moved north toward Asia, oceanic lithosphere was continually subducted beneath Eurasia. During the time before continental collision, sediments were deposited in the marine waters between India and Eurasia. Section 21.6 After breaking away from Africa, India moved north and eventually collided with Eurasia. Section 21.6 Formation of the Himalayas – Fold Mtns. The sedimentary strata that was deposited between Eurasia and India was eventually uplifted and folded into the mountains. When the continental crust portions finally collided, subduction was significantly slowed. The edge of the Eurasian plate was uplifted as the continental Indian plate wedged under it. The Indian plate continues to move north today, resulting in continued uplift of the Himalayas. Section 21.6 Formation of the Himalayas The oceanic crust was subducted beneath Asia until the continental crusts collided. The collision of the Eurasian and Indian continental plates resulted in the lofty Himalayas. Section 21.6