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Chapter 6 Plate Tectonics Preview Section 1 Earth's Structure Section 2 The Theory of Plate Tectonics Section 3 Deforming Earth's Crust Section 4 California Geology Concept Map < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Bellringer Many fossils of the same ancient plants and animals are found on different continents separated by oceans. Write a few sentences to explain how this could happen and what it suggests about the continents. Write your answers in your science journal. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure What You Will Learn • Earth’s interior can be divided into layers based on chemical composition and physical properties. • Scientists use seismic waves to study Earth’s interior. • Continents are drifting apart from each other now and have done so in the past. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure The Layers of the Earth • Earth is made of several layers. • The materials in each layer have distinct properties. • Earth’s layers can be described in terms of their chemical composition or physical properties. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure The Layers of the Earth, continued The Compositional Layers of Earth • Earth is divided into three compositional layers. 1. At Earth’s center, the dense metallic core is made mainly of the metal iron. 2. The dense, thick middle layer is the mantle. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure The Layers of the Earth, continued • The mantle is made up largely of silicon, oxygen, and magnesium. 3. The surface layer, or crust, is composed mostly of silicon, oxygen, and aluminum. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure The Layers of the Earth, continued < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure The Layers of the Earth, continued Continental and Oceanic Crust • There are two types of crust. • Continental crust is thicker than oceanic crust. • Both types are made mainly of the elements oxygen, silicon, and aluminum. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure The Layers of the Earth, continued • But oceanic crust has almost twice as much iron, calcium, and magnesium as continental crust does. • These three elements form minerals that are denser than the minerals in continental crust. • These dense minerals make the thin oceanic crust heavier than the thicker continental crust. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure The Layers of the Earth, continued The Physical Structure of Earth • Earth is divided into five layers based on physical properties. 1. Earth’s outer layer is the lithosphere, which is a cool, rigid layer that includes the crust and the upper part of the mantle. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure The Layers of the Earth, continued • The lithosphere is divided into pieces called tectonic plates. 2. Below the lithosphere is the asthenosphere, which is a layer of the mantle that is made of very slowflowing solid rock. • Tectonic plates move on top of the asthenosphere. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure The Layers of the Earth, continued 3. Below the asthenosphere is the mesosphere, which is the lower part of the mantle. • The mesosphere flows even more slowly than the asthenosphere. • The Earth’s core has two layers. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure The Layers of the Earth, continued 4. The outer core is a layer of liquid iron and nickel. 5. At Earth’s center is the solid inner core. This layer is made mostly of nickel and iron. • The inner core is very hot, but it is solid because it is under enormous pressure. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Mapping Earth’s Interior • Scientists have learned about Earth’s interior by studying earthquakes. • An earthquake produces vibrations called seismic waves. • Seismic waves travel through Earth at various speeds. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Mapping Earth’s Interior, continued • Machines called seismometers measure the time seismic waves take to travel various distances from an earthquake’s center. • Scientists use these distances and times to calculate the density and thickness of Earth’s layers. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Mapping Earth’s Interior, continued • The speed of seismic waves is affected by the type of material that the waves are traveling through. • For example, some types of waves can travel through rock but not through liquids. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Mapping Earth’s Interior, continued • These waves never reach the seismometers on the side of Earth opposite the earthquake. • Therefore, part of Earth’s interior must be liquid. • This liquid layer is the outer core. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Mapping Earth’s Interior, continued < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Continental Drift • Continental drift is the idea that a single large landmass broke up into smaller landmasses to form the continents, which then drifted to their present locations. (OLD Theory, no longer relevant) • However, some parts of this theory helped to create the current theory of plate tectonics. • First, it explains how the continents seem to fit together like puzzle pieces. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Continental Drift, continued • Second, Continental Drift explained why fossils of the same plant and animal species were found on continents that are far away from each other. • Many of these ancient species could not have crossed an ocean, so, the continents must have been connected at one point. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Continental Drift, continued • The locations of mountain ranges and similar types of rock also support continental drift. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Continental Drift, continued • Scientists have used rock and fossil evidence to reconstruct past patterns of climate regions. • The distribution of these ancient climatic zones supports the idea of continental drift, too. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure The Breakup of Pangaea • Alfred Wegener, the scientist who proposed the theory of continental drift, proposed that the large continent of Pangaea gave rise to today’s continents. • Scientists have determined that Pangaea existed about 245 million years ago. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure The Breakup of Pangaea, continued • Pangaea split into two continents—Laurasia and Gondwana—about 135 million years ago. • These two continents then split into the continents we know today. • These continents slowly drifted to their present positions. (centimeters per year) < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure The Breakup of Pangaea, continued < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Sea-Floor Spreading • Evidence for continental drift lies on the sea floor. • A chain of submerged mountains runs through the center of the Atlantic Ocean (Mid-Atlantic Ridge). • This mountain chain is part of a worldwide system of mid-ocean ridges. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Sea-Floor Spreading, continued • Mid-ocean ridges show patterns of magnetism. • The pattern on one side of a ridge is the mirror image of the pattern on the other. • The magnetism of rocks aligns with Earth’s magnetic field as it was when the rocks formed. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Sea-Floor Spreading, continued < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Sea-Floor Spreading, continued • Throughout Earth’s history, the north and south magnetic poles have changed place many times. • This is called magnetic reversal. • As rock forms from magma, minerals that contain iron form. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Sea-Floor Spreading, continued • Some of these minerals are magnetic and act like compasses. • They form so that their magnetic fields align with the magnetic fields on Earth. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Sea-Floor Spreading, continued • When the molten rock cools, these tiny compasses are locked into position in the rock. • After Earth’s magnetic field reverses, new magnetic minerals that align in the opposite direction form. < Back Next > Preview Main Chapter 6 Plate Tectonics Magnetic Reversals and Sea-Floor Spreading < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Sea-Floor Spreading, continued • At a mid-ocean ridge, magma rises through fractures in the sea floor. • As magma cools, it forms new rock. • As the new rock forms, the older rock gets pulled away from the mid-ocean ridge. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Sea-Floor Spreading, continued • The process by which new sea floor forms as old sea floor is pulled away is called sea-floor spreading. • The record of magnetic reversals on the sea floor provides evidence that the continents are moving. • Sea-floor spreading is one process that moves continents. < Back Next > Preview Main Chapter 6 Section 1 Earth's Structure Sea-Floor Spreading, continued < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Bellringer When water is heated in a pot over a flame, the flame touches only the bottom of the pot. How does the water become heated? Why does all of the air in a room become warm even if heat enters the room only through one furnace vent? Write your answers in your science journal. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics What You Will Learn • Earth’s lithosphere is broken into pieces called tectonic plates. • Heat from Earth’s interior causes convection in the mantle. • Tectonic plates move at an average rate of a few centimeters per year. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tectonic Plates • Plate tectonics is the theory that Earth’s lithosphere is divided into tectonic plates that move around on top of the asthenosphere. • Pieces of the lithosphere that move around on top of the asthenosphere are called tectonic plates. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tectonic Plates, continued • Earth’s tectonic plates differ in size. • Some plates contain both continental and oceanic crust. • Others contain mostly oceanic crust, or mostly continental crust. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tectonic Plates, continued < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tectonic Plates, continued < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tectonic Plates, continued • Tectonic plates float on the asthenosphere. • The plates cover the surface of the asthenosphere, and they touch one another and move around. • Thick plates made of continental lithosphere displace more asthenosphere than do thin plates made of oceanic lithosphere. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tectonic Plate Boundaries • A boundary is a place where tectonic plates meet. • Tectonic plate boundaries are located by studying the locations of earthquakes, volcanoes, and landforms such as mid-ocean ridges and ocean trenches. • A plate boundary can be a convergent, divergent, or transform boundary. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tectonic Plate Boundaries, continued < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tectonic Plate Boundaries, continued Convergent Boundaries • The boundary at which two tectonic plates collide is a convergent boundary. • At a convergent boundary, three types of collisions may happen. 1. Continental/Continental: Two plates made of continental lithosphere collide, forming a high mountain range. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tectonic Plate Boundaries, continued 2. Oceanic/Continental: A plate of oceanic lithosphere collides with a plate of continental lithosphere. • The denser oceanic lithosphere will sink beneath the less-dense continental crust, in a process called subduction. • Subduction can cause a chain of volcanoes to form parallel to the plate boundary. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tectonic Plate Boundaries, continued 3. Oceanic/Oceanic: Two plates of oceanic lithosphere may collide. • The denser of the two plates will subduct. • A series of volcanic islands, called an island arc, may form parallel to the plate boundary. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tectonic Plate Boundaries, continued Divergent Boundaries • The boundary at which two tectonic plates separate is a divergent boundary. • Most divergent boundaries happen on the sea floor. • These boundaries are characterized by mid-ocean ridges. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tectonic Plate Boundaries, continued • As the plates pull away from each other, fractures form in the oceanic lithosphere. • Magma rises through these fractures to the ocean floor. • There, the magma solidifies to form new lithosphere. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tectonic Plate Boundaries, continued Transform Boundaries • The boundary at which two tectonic plates slide past each other is a transform boundary. • Most transform boundaries occur in the sea floor at mid-ocean ridges. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tectonic Plate Boundaries, continued • At these locations, transform boundaries run perpendicular to the ridge where plates are pulling apart. • The transform boundaries cause offsets between shorter segments of the ridge. • These offsets give mid-ocean ridges a zigzag pattern. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tectonic Plate Boundaries, continued • The San Andreas fault system is a well-known example of a transform boundary. • This system occurs both on the sea floor and on land. • The fault system is located where the Pacific and North American plates are sliding past each other. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Causes of Tectonic Plate Motion • Tectonic plate motion is the result of density differences that are caused by the flow of heat within Earth. • Earth’s core and mantle are very hot, due to minerals that have radioactive atoms. • These atoms release heat as they decay. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Causes of Tectonic Plate Motion, continued • Heat from Earth’s center flows toward the surface. • However, rock is a poor conductor of heat. • Therefore, most heat transfer happens through convection. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Causes of Tectonic Plate Motion, continued • When rock is heated, it expands, becomes less dense, and rises toward Earth’s surface. • At the surface, cold, dense rock tends to sink during subduction. • This process causes convection currents in the mantle. < Back Next > Preview Main Chapter 6 Plate Tectonics Causes of Tectonic Plate Motion •There are Three Forces that drive Tectonic Plate Motion: 1.Convection 2. Ridge Push 3.Slab Pull < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tracking Tectonic Plate Motion • Tectonic plate motion is so gradual that it is measured in centimeters per year. • The average rate of movement for different plates ranges between 2.5 cm/year and 15 cm/year. < Back Next > Preview Main Chapter 6 Section 2 The Theory of Plate Tectonics Tracking Tectonic Plate Motion, continued • Scientists use GPS to measure the movement of tectonic plates. • GPS continuously records the exact distance between satellites and ground stations. • Scientists use changes in distances to calculate rates of tectonic plate motion. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Bellringer Look at the photographs of the mountains displayed by your teacher. Explain how each mountain might have formed. (Pg. 208-209 Figures 4,5, and 6) Write your answers in your science journal. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust What You Will Learn • Stress is placed on rock as plates move. The stress causes rocks to fold and break. • The formation of mountains results from the motion of tectonic plates. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Deformation • Stress is the amount of force per unit area on a given material. • Rock may bend or break when different amounts of stress are applied. • The process by which the shape of a rock changes in response to stress is called deformation. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Folding • The bending of rock layers in response to stress is called folding. • Scientists assume that all rock layers start as horizontal layers. • When scientists see a fold, they know that deformation has taken place. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Folding, continued • All folds have a hinge and two limbs. • Limbs are the sloping sides of a fold. • A hinge is the bend where the two limbs meet. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Folding, continued < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Folding, continued • Anticlines and synclines are the two most common types of folds. • An anticline is a fold in which the oldest rock layers are in the center of the fold, like an arch. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Folding, continued • A syncline is a fold in which the youngest rock layers are in the center of the fold, like a trough. • Folds can range from centimeters wide to hundreds of kilometers wide. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Folding, continued • Rock layers can bend into symmetrical or asymmetrical folds. • In a symmetrical fold, each limb dips in the same way. • In an asymmetrical fold, one limb may dip more steeply than the other does. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Folding, continued • An overturned fold is a fold in which one limb is tilted beyond 90°. • Rock layers may also be bent so much that a rock appears to be lying on its side. • Geologists call this lying-down fold a recumbent fold. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Faulting • Rock may break if placed under too much stress. • The surface along which rocks break and slide past each other is called a fault. • The blocks of crust on either side of the fault are called fault blocks. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Faulting, continued • When a fault is not vertical, it forms two types of fault blocks. • The footwall is the block of rock that lies below the plane of the fault. • The hanging wall is the block that lies above the plane of the fault. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Faulting, continued < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Faulting, continued • Faults are classified into three categories according to how the fault blocks move relative to each other. • The type of fault that formed can be used to determine the type of stress that caused the fault. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Faulting, continued Normal Faults • Along a normal fault, the hanging wall moves down relative to the footwall. • Normal faults usually form where tectonic plate motions cause tension. -> • Tension is stress that pulls rocks apart. • Therefore, normal faults are common at mid-ocean ridges. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Faulting, continued Reverse Faults • Along a reverse fault, the hanging wall moves up relative to the footwall. • Reverse faults usually form where tectonic plate motions cause compression. -> <• Compression is stress that causes rocks to push together. • Therefore, reverse faults are common in subduction zones. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Faulting, continued Strike-Slip Faults • Along a strike-slip fault, the two fault blocks move past eachother horizontally. • Strike-slip faults usually form where tectonic plate motions cause shear stress parallel to Earth’s surface. • Therefore, strike-slip faults are common along transform boundaries. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Faulting, continued < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Faulting, continued Recognizing Faults • The position of rock layers can help scientists to recognize a fault and determine offset. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Faulting, continued • Features such as grooves, striations, or polished surfaces called slickensides also show where rocks have moved. • Fault offset is often obvious for faults that have lengths of many kilometers. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Faulting, continued • Streams commonly change their direction of flow at a fault. • A scarp, or row of cliffs formed by faulting, can also identify a fault. • Scarps form when rock on one side is raised vertically relative to rock on the other side of the fault. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Plate Tectonics and Mountain Building • As tectonic plates move around Earth’s surface, their edges grind against each other. • Over time, this process may crumple and push up the margins of the plates. • When this happens, mountain-building may occur. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Plate Tectonics and Mountain Building, continued • When rock layers are squeezed together and pushed upward, folded mountains form. • These mountain ranges form at convergent boundaries where continents have collided. • When continents collide, compression folds and uplifts the rock. • Examples include the Appalachians and the Himalayas < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Plate Tectonics and Mountain Building, continued • When tension in Earth’s crust causes the crust to break into a large number of normal faults, fault-block mountains form. • These mountains form when tension causes large blocks of Earth’s crust to drop down relative to other blocks. • The Tetons in Idaho and Wyoming are an example of Fault-Block Mountains < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Plate Tectonics and Mountain Building, continued • When molten rock erupts onto Earth’s surface, volcanic mountains form. • Most of the world’s major volcanic mountains are located at convergent boundaries. • At these boundaries, the motion of the plates causes hot mantle rocks to rise beneath the plate. < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Plate Tectonics and Mountain Building, continued • The molten rock rises to the surface and erupts. • Volcanic mountains form both on land and on the ocean floor. • Sometimes, these mountains can rise above the ocean surface to become islands (like Hawaii). < Back Next > Preview Main Chapter 6 Section 3 Deforming Earth's Crust Plate Tectonics and Mountain Building, continued • Most of the active volcanic mountains on Earth have formed around the tectonically active rim of the Pacific Ocean, this area is known as the Ring of Fire. • Subduction zones are what allow volcanoes to form at these convergent boundaries. • Mount Shasta in northern California is in the Ring of Fire. < Back Next > Preview Main Chapter 6 Plate Tectonics Types of Mountains < Back Next > Preview Main Chapter 6 Section 4 California Geology Bellringer How many geologic features from California can you name? Brainstorm a list of specific features of California geology, such as faults and mountains. Write your list in your science journal. < Back Next > Preview Main Chapter 6 Section 4 California Geology What You Will Learn • Plate tectonics has been the most important force in shaping California’s geologic history. • The San Andreas fault marks a transform boundary between the North American plate and the Pacific plate. < Back Next > Preview Main Chapter 6 Section 4 California Geology Building California by Plate Tectonics • The region that we know as California has been at an active plate boundary for the past 225 million years. • As a result, plate tectonics has been the most important force shaping California’s geologic history. < Back Next > Preview Main Chapter 6 Section 4 California Geology Building California by Plate Tectonics, continued < Back Next > Preview Main Chapter 6 Section 4 California Geology Building California by Plate Tectonics, continued • Before about 225 million years ago, North America’s western edge was much farther east than it is now. • The area where Nevada and the eastern deserts of California are today was the west coast of North America. • Most of what is now California was either part of a distant oceanic plate or did not exist. < Back Next > Preview Main Chapter 6 Section 4 California Geology Building California by Plate Tectonics, continued • When Pangaea began to break up, the North American plate moved west. • The continent’s western edge became an active convergent plate boundary. • A long period of subduction began, which was an important period of geologic “building” in California. < Back Next > Preview Main Chapter 6 Section 4 California Geology Building California by Plate Tectonics, continued • Three major tectonic plates influenced California’s geologic history: the North American plate, the Farallon plate, and the Pacific plate. • A convergent boundary existed between the North American and Farallon plates. • The Farallon plate subducted beneath the North American plate. < Back Next > Preview Main Chapter 6 Section 4 California Geology Building California by Plate Tectonics, continued • The ancient Farallon plate lay between the North American and Pacific plates. • About 25 million years ago, the entire Farallon plate was subducted at one part of the boundary. • The Pacific Plate touched North America for the first time, forming a transform boundary. < Back Next > Preview Main Chapter 6 Section 4 California Geology Building California by Plate Tectonics, continued • As the Farallon plate continues to subduct, the transform boundary continues to grow longer. • Today, it is about 2,600 km long. • The Juan de Fuca plate off northern California is part of what remains of the ancient Farallon plate. < Back Next > Preview Main Chapter 6 Section 4 California Geology Building California by Plate Tectonics, continued < Back Next > Preview Main Chapter 6 Section 4 California Geology Subduction and Volcanism • The subduction of the Farallon plate caused rocks to melt and caused chunks of rock to collide with the North American continent. • Subduction caused a great deal of magma, or molten rock, to form in the lithosphere. < Back Next > Preview Main Chapter 6 Section 4 California Geology Subduction and Volcanism, continued • This magma solidified to form a huge mass of granite called the Sierra Nevada batholith. • A batholith is a large mass of igneous rock that forms deep below the surface. • Batholiths are the “roots” of subduction zone volcanoes. < Back Next > Preview Main Chapter 6 Section 4 California Geology Subduction and Volcanism, continued • A chain of huge volcanoes must have formed above the giant magma chamber. • These volcanoes probably stood twice as tall as today’s Sierra Nevadas. • The granite batholith is exposed in the Sierra Nevada mountain range. < Back Next > Preview Main Chapter 6 Section 4 California Geology Subduction and Volcanism, continued • Today, the Juan de Fuca plate is subducting beneath the North American plate. • This area is known as the Cascadia subduction zone. • A chain of active volcanoes is present in this zone, in the Cascade Mountains of California, Oregon, and Washington. < Back Next > Preview Main Chapter 6 Section 4 California Geology Subduction and Accretion • During subduction, pieces of the plate that subducts may be scraped off and attached to the overriding plate. • This process, called accretion, forms mountain chains. • These mountain chains are parallel to the plate boundary. < Back Next > Preview Main Chapter 6 Section 4 California Geology Subduction and Accretion, continued • The rocks in California’s Coast Ranges and Transverse Ranges are thought to have been formed by accretion. • The Central Valley, Los Angeles Basin, and Ventura Basin separate some of these mountain ranges. < Back Next > Preview Main Chapter 6 Section 4 California Geology Subduction and Accretion, continued Accreted Terranes • The chunks of lithosphere that are scraped off of subduction plates and added to the edge of a continent are called accreted terranes. • The rocks of a terrane differ from the surrounding rocks by age or composition. < Back Next > Preview Main Chapter 6 Section 4 California Geology Subduction and Accretion, continued California Gold • The foothills along the western side of the Sierra Nevadas contain rocks filled with gold. • These rocks are thought to be accreted terranes. • This rock formed near submarine volcanic vents. < Back Next > Preview Main Chapter 6 Section 4 California Geology Subduction and Accretion, continued • After the terranes were accreted, the gold became concentrated in the quartz veins of the Mother Lode. • Gold is an important part of California’s history. < Back Next > Preview Main Chapter 6 Section 4 California Geology The San Andreas Fault System • California is home to the most famous transform plate boundary in the world, the San Andreas fault system. • The San Andreas fault system extends for about 1,000 km. • The San Andreas fault forms the boundary between the Pacific and North American plates. < Back Next > Preview Main Chapter 6 Section 4 California Geology The San Andreas Fault System, continued • Most of California is on the North American plate. • A small part of California, west of the San Andreas fault, lies on the Pacific plate. • The Pacific plate is moving to the northwest relative to the North American plate. < Back Next > Preview Main Chapter 6 Section 4 California Geology The San Andreas Fault System, continued • Not all plate movement takes place on the San Andreas fault itself. • In the San Francisco area and southern California, motion takes place on other faults of the system. • These faults lie west and east of the San Andreas fault. < Back Next > Preview Main Chapter 6 Section 4 California Geology The San Andreas Fault System, continued • In these areas, it is best to think of the boundary between the North American and Pacific plates as a zone, not a line. < Back Next > Preview Main Chapter 6 Section 4 California Geology The San Andreas Fault System, continued • The Pacific and North American plates have been moving along the San Andreas fault system for about 25 million years. • During the last 16 million years, the separation, or offset, along the fault has been 315 km. • Geologists use rocks to estimate the amount and rate of movement along the fault. < Back Next > Preview Main Chapter 6 Section 4 California Geology The San Andreas Fault System, continued • Geologists determine offset by matching unusual rocks that have been separated by the fault. • They date these rocks to determine when they formed. • Then geologists use that date to determine when the areas were not separated. < Back Next > Preview Main Chapter 6 Section 4 California Geology The San Andreas Fault System, continued • In southern California, the San Andreas fault makes a huge bend as it passes east of Los Angeles. • Because of this bend, the Pacific and North American plates collide as they move past each other. • As a result, the motion along this boundary is partly convergent. < Back Next > Preview Main Chapter 6 Section 4 California Geology The San Andreas Fault System, continued • Because southern California is being compressed, areas near the bend are being uplifted or dropped down by active faults. • The San Bernardino Mountains and the San Gabriel Mountains are tectonically created mountain ranges. • The Los Angeles basin is a large depression bordered by active faults. < Back Next > Preview Main Chapter 6 Section 4 California Geology Plate Tectonics and the California Landscape • Much of California’s landscape has been formed by plate tectonics. • Compression has recently uplifted California’s rugged mountains. • The steep, rocky coastlines have been formed by uplift along the plate boundary. < Back Next > Preview Main Chapter 6 Section 4 California Geology Plate Tectonics and the California Landscape, continued • Major river valleys, mountain ranges, and the coastline are oriented in a northwesterly direction. • A northwesterly orientation is parallel to the faults of the plate boundary. • The Transverse Ranges are oriented east-west, due to motion from the San Andreas fault system. < Back Next > Preview Main Chapter 6 Section 4 California Geology < Back Next > Preview Main Chapter 6 Plate Tectonics Concept Map Use the terms below to complete the concept map on the next slide. folds synclines strike-slip faults reverse < Back Next > Preview Main Chapter 6 Plate Tectonics Concept Map < Back Next > Preview Main Chapter 6 Plate Tectonics Concept Map < Back Next > Preview Main