Active Figure 18.2: Three stages in the origin of granitic continental crust. Andesitic island arcs formed by the partial melting of basaltic oceanic crust.

Active Figure 18.2: Three stages in the origin of granitic continental crust. Andesitic island arcs formed by the partial melting of basaltic oceanic crust are intruded by granitic magmas. As a result of plate movements, island arcs collide and form larger units or cratons. (a) Two island arcs on separate plates move toward each other. (b) The island arcs shown in (a) collide, forming a small craton, and another island arc approaches this craton. (c) The island arc shown in (b) collides with the craton. Fig. 18-2, p. 419

Active Figure 18.2: Three stages in the origin of granitic continental crust. Andesitic island arcs formed by the partial melting of basaltic oceanic crust are intruded by granitic magmas. As a result of plate movements, island arcs collide and form larger units or cratons. (a) Two island arcs on separate plates move toward each other. Fig. 18-2a, p. 419

Active Figure 18.2: Three stages in the origin of granitic continental crust. Andesitic island arcs formed by the partial melting of basaltic oceanic crust are intruded by granitic magmas. As a result of plate movements, island arcs collide and form larger units or cratons. (b) The island arcs shown in (a) collide, forming a small craton, and another island arc approaches this craton. Fig. 18-2b, p. 419

Active Figure 18.2: Three stages in the origin of granitic continental crust. Andesitic island arcs formed by the partial melting of basaltic oceanic crust are intruded by granitic magmas. As a result of plate movements, island arcs collide and form larger units or cratons. (c) The island arc shown in (b) collides with the craton. Fig. 18-2c, p. 419

PLATE TECTONICS The unifying theory of geology
Plate Tectonics explains many geologic phenomena, such as volcanism, earthquakes, mountain building, changes in climate, flora and fauna distributions, and the distributions of natural resources.

Basics of Plate Tectonics
Earth’s surface is broken into numerous tectonic plates (8 larger plates and several smaller pieces) Tectonic plates are moving (variable rates from 1 cm to 18 cm per year) Tectonic plates interact at their boundaries Moving tectonic plates cause the simultaneous renewal and destruction of the Earth’s surface. Heat-transfer mechanism known as convection in the mantle is thought to be the mechanism for the movement of tectonic plates.

Plate Tectonics Continental Drift + Sea Floor Spreading = Plate Tectonics The continents are carried with the moving sea floor as the lithospheric plates travel.

The Lithospheric Plates

Tectonic Plates Figure 2.14: A map of the world showing the plates, their boundaries, relative motion and rates of movement in centimeters per year, and hot spots. Fig. 2-14, p. 38

The Earth’s Layers The layers of the Earth result from density differences between the layers caused by variations in composition, temperature, and pressure. Continental Crust: SiAl (rock) Oceanic Crust: SiMa (ferromagnesian rock—basalt--mafic) Mantle: FeMg (Peridotite—ultramafic) (rock) Core: Fe and a small amount of Ni (metal)

Figure 1.10c: In this way, a differentiated Earth formed, consisting of a dense iron-nickel core, an iron-rich silicate mantle, and a silicate crust with continents and ocean basins. Fig. 1-10c, p. 14

Lithosphere and Asthenosphere
Lithosphere is the solid, brittle outer layer of the Earth composed of: Oceanic and continental crust Top part of the mantle Asthenosphere is the plastic layer of the mantle directly below the lithosphere over which the lithospheric plates move. The lithosphere is broken into many pieces called plates.

Figure 1.11: A cross section of Earth, illustrating the core, mantle, and crust. The enlarged portion shows the relationship between the lithosphere (composed of the continental crust, oceanic crust, and solid upper mantle) and the underlying asthenosphere and lower mantle. Fig. 1-11, p. 15

Plate Tectonics and Surface Features
Trench Ridge Fault Volcanoes Mountain Ranges Islands or Island Arcs

Mantle Plumes and Hot Spots
The Hawaiian Islands

The Mechanism for Plate Motion is Convection in the Mantle
Figure 1.12: Earth’s plates are thought to move as a result of underlying mantle convection cells in which warm material from deep within Earth rises toward the surface, cools, and then, on losing heat, descends back into the interior. The movement of these convection cells is thought to be the mechanism responsible for the movement of Earth’s plates, as shown in this diagrammatic cross section. The Mechanism for Plate Motion is Convection in the Mantle Fig. 1-12, p. 15

Figure 1. 17: Plate tectonics and the rock cycle
Figure 1.17: Plate tectonics and the rock cycle. The cross section shows how the three major rock groups—igneous, metamorphic, and sedimentary—are recycled through both the continental and oceanic regions. Fig. 1-17, p. 20

Three types of plate boundaries
Figure 1.14: An idealized cross section illustrating the relationship between the lithosphere and the underlying asthenosphere and the three principal types of plate boundaries: divergent, convergent, and transform. Three types of plate boundaries Divergent plate boundary 2. Convergent Plate Boundary 3. Transform Plate boundary Fig. 1-14, p. 18

Table 2-2, p. 39

Table 1-3, p. 16

Active Figure 2. 15: History of a divergent plate boundary
Active Figure 2.15: History of a divergent plate boundary. (a) Heat from rising magma beneath a continent causes it to bulge, producing numerous cracks and fractures. (b) As the crust is stretched and thinned, rift valleys develop and lava flows onto the valley floors. (c) Continued spreading further separates the continent until a narrow seaway develops. (d) As spreading continues, an oceanic ridge system forms and an ocean basin develops and grows. Fig. 2-15, p. 40

Plate Boundary Structures
Oceanic Ridges (MOR and Iceland) Rift zones (East African Rift Zone) Trenches (Marianas Trench) Island Arcs (Japan, Indonesia) Mountain Ranges (Alps, Andes, Himalayans) Faults (San Andreas Fault) Volcanoes (Mount St. Helens, Mont Pelee, Vesuvius)

Mid-Oceanic Ridge: Divergent Plate Boundary

Mid-Oceanic Ridge: Divergent Plate Boundary: formation of an ocean

Mid-Oceanic Ridge: worldwide distribution

Iceland

Iceland and the Mid-Oceanic Ridge (MOR)

Iceland and the MOR

Spreading center activity leading to separation of continental crust
East African Rift Zone Spreading center activity leading to separation of continental crust Figure 2.16: The East African Rift Valley is being formed by the separation of East Africa from the rest of the continent along a divergent plate boundary. The Red Sea represents a more advanced stage of rifting in which two continental blocks are separated by a narrow sea. Fig. 2-16, p. 41

Convergent Plate Boundaries
Active Figure 2.17: Oceanic–oceanic plate boundary. (a) An oceanic trench forms where one oceanic plate is subducted beneath another. On the nonsubducted plate, a volcanic island arc forms from the rising magma generated from the subducting plate. (b) Satellite image of Japan. The Japanese Islands are a volcanic island arc resulting from the subduction of one oceanic plate beneath another oceanic plate. Subduction zone Volcanic Island Arc Convergent Zone: oceanic crust melts and volcanoes develop Fig. 2-17, p. 42

Active Figure 2. 19: Continental–continental plate boundary
Active Figure 2.19: Continental–continental plate boundary. (a) When two continental plates converge, neither is subducted because of their great thickness and low and equal densities. As the two continental plates collide, a mountain range is formed in the interior of a new and larger continent. (b) Vertical view of the Himalayas, the youngest and highest mountain system in the world. The Himalayas began forming when India collided with Asia 40 to 50 million years ago. Convergent Plate Boundary: Himalayan Mountains or Alps form as a result of the collision of two continental masses Fig. 2-19, p. 43

The Alps

The Marianas Trench

San Andreas Fault, California

Figure 2.20: Horizontal movement between plates occurs along a transform fault. (a) The majority of transform faults connect two oceanic ridge segments. Note that relative motion between the plates occurs only between the two ridges. (b) A transform fault connecting two trenches. (c) A transform fault connecting a ridge and a trench. Fig. 2-20, p. 44

Vesuvius and Pompeii

Active Figure 2.23: The Emperor Seamount–Hawaiian Island chain formed as a result of movement of the Pacific plate over a hot spot. The line of the volcanic islands traces the direction of plate movement. The numbers indicate the ages of the islands in millions of years. Fig. 2-23, p. 46

Active Figure 2. 24: Two models to explain plate movement
Active Figure 2.24: Two models to explain plate movement. (a) Thermal convection cells are restricted to the asthenosphere. (b) Thermal convection cells involve the entire mantle. Fig. 2-24, p. 47

Catch all Explains how rocks now present in one climatic region of the Earth actually formed in another. Explains that the continents and sea floor are in motion.

Table 2-1, p. 28

Plate tectonics is evidenced by continental drift and by seafloor spreading

CONTINENTAL DRIFT Based upon geologic, paleontologic and climatologic evidence. The continents were one united in to a single supercontinent called Pangaea.

Early Ideas For Continental Drift
Edward Suess (1885) Recognized the presence of Glossopteris flora on five southern “continents” (Antarctica, Asia, Africa, India and South America) Called this collection Gondwanaland. Explains this similarity among fossils by the appearance and disappearance of land bridges

Figure 2.3: Alfred Wegener proposed the continental drift hypothesis in 1912 based on a tremendous amount of geologic, paleontologic, and climatologic evidence. He is shown here waiting out the Arctic winter in an expedition hut. Alfred Wegener (A German meteorologist developed the hypothesis of continental drift in 1915) Fig. 2-3, p. 29

Alfred Wegener (1915) Credited with the development of the hypothesis of Continental Drift (1915) Proposed that all land masses were originally united into a single supercontinent called Pangaea. Pangaea subsequently separated in a series of breakups. Based his conclusions upon geologic, paleontologic and climatologic evidence. He did not put forth a convincing mechanism for the drift of the continents

Alexander du Toit (1937) Placed the five southern “continents”—Gondwanaland--at the south pole to explain the presence of glacial deposits Placed the northern continents—Laurasia—near the equator to explain the presence of coal deposits.

Evidence for Continental Drift
Continental Fit Similarity of Rock Sequences and Mountain Ranges Rocks of the Appalachians end abruptly and appear again in Greenland, Ireland, Great Britain and Norway (figure 2.5) Glacial Evidence Glacial till Glacial striations in exposed rock (figure 2.6)

Pangea (the supercontinent)
Active Figure 2.4: The best fit among continents occurs along the continental slope, where erosion would be minimal. Pangea (the supercontinent) Fig. 2-4, p. 30

Figure 2.5: When continents are brought together, their mountain ranges form a single continuous range of the same age and style of deformation throughout. Such evidence indicates the continents were at one time joined together and were subsequently separated. Appalachian and Caledonian Mountains are same age, but have been separated by moving plates. Fig. 2-5, p. 31

Figure 2.6: (a) If the Gondwana continents are brought together so that South Africa is located at the South Pole, then the glacial movements indicated by the striations make sense. In this situation, the glacier, located in a polar climate, moved radially outward from a thick central area toward its periphery. (b) Permian-aged glacial striations in bedrock exposed at Hallet’s Cove, Australia, indicate the direction of glacial movement more than 200 million years ago. Glacial Striations formed when great thickness of ice moved over the rock. Direction of striations associated with this rock formation agrees with the configuration of these continents. Fig. 2-6, p. 31

Fossil Evidence Glossopteris flora of Late Paleozoic Mesosaurus of Late Permian Lystrosaurus and Cynognathus of Early Triassic (figure 2.7)

Figure 2.2: Glossopteris leaves from the Upper Permian Dunedoo Formation, Australia. Fossils of the Glossopteris flora are found on all five Gondwana landmasses, thus providing evidence that these landmasses were at one time connected. Glossopeteris leaves from upper Permian Period. Found on all five Gondwanaland landmasses. Fig. 2-2, p. 29

Paleontological and Paleobiological Evidence for Pangea
Figure 2.7: Some of the animals and plants whose fossils are found today on the widely separated continents of South America, Africa, India, Australia, and Antarctica. These continents were joined together during the Late Paleozoic to form Gondwana, the southern landmass of Pangaea. Glossopteris and similar plants are found in Pennsylvanian- and Permian-aged deposits on all five continents. Mesosaurus is a freshwater reptile whose fossils are found in Permian-aged rocks in Brazil and South Africa. Cynognathus and Lystrosaurus are land reptiles that lived during the Early Triassic Period. Fossils of Cynognathus are found in South America and Africa, and fossils of Lystrosaurus have been recovered from Africa, India, and Antarctica. Paleontological and Paleobiological Evidence for Pangea Fig. 2-7, p. 32

--Paleomagnetism and Polar Wandering Remnant magnetism in ancient rocks Iron-bearing minerals atoms in magma or lava aligns with the Earth’s magnetic pole Record both direction and strength of the magnetic field The iron-bearing minerals are “frozen” into place at the Curie point, when the magma or lava cools. The movement of the crust away from the ridge or spreading zone shows a pattern of magnetic pole reversals (figure 2.10) Arrangement of continents in Pangaea based upon paleomagnetic mapping is supportive of Continental Drift

Earth’s Magnetic Field:
Today Figure 2.8a: Earth’s magnetic field has lines of force just like those of a bar magnet. Fig. 2-8a, p. 34

Figure 2.8b: The strength of the magnetic field changes uniformly from the magnetic equator to the magnetic poles. This change in strength causes a dip needle to parallel Earth’s surface only at the magnetic equator, whereas its inclination with respect to the surface increases to 90 degrees at the magnetic poles. Notice the 11½-degree angle between the geographic and magnetic poles. Magnetic field lines show the position of the magnetic north and south poles. Fig. 2-8b, p. 34

Figure 2.10: Magnetic reversals recorded in a succession of lava flows are shown diagrammatically by red arrows, and the record of normal polarity events is shown by black arrows. Fig. 2-10, p. 35

The iron in the lava is aligned with the current magnetic configuration and is frozen in place when the lava cools. Active Figure 2.11: The sequence of magnetic anomalies preserved in the oceanic crust on both sides of an oceanic ridge is identical to the sequence of magnetic reversals already known from continental lava flows. Magnetic anomalies are formed when magma intrudes into oceanic ridges; when the magma cools below the Curie point, it records Earth’s magnetic polarity at the time. Seafloor spreading splits the previously formed crust in half, so that it moves laterally away from the oceanic ridge. Repeated intrusions record a symmetric series of magnetic anomalies that reflect periods of normal and reversed polarity. The magnetic anomalies are recorded by a magnetometer, which measures the strength of the magnetic field. Fig. 2-11, p. 36

Pole paths actually indicate the motion of the plates leading up to Pangea when both continents were adjacent to each other Figure 2.9: The apparent paths of polar wandering for North America and Europe. The apparent location of the north magnetic pole is shown for different periods on each continent’s polar wandering path. Fig. 2-9, p. 35

Figure 2.13: The age of the world’s ocean basins established from magnetic anomalies demonstrates that the youngest oceanic crust is adjacent to the spreading ridges and that its age increases away from the ridge axis. Relative ages of sea floor rocks are oldest away from the ridge. This supports the idea that the sea floor is spreading away from the ridge. Fig. 2-13, p. 37

Figure 2.6a: If the Gondwana continents are brought together so that South Africa is located at the South Pole, then the glacial movements indicated by the striations make sense. In this situation, the glacier, located in a polar climate, moved radially outward from a thick central area toward its periphery. Fig. 2-6a, p. 31

Figure 2.6b: Permian-aged glacial striations in bedrock exposed at Hallet’s Cove, Australia, indicate the direction of glacial movement more than 200 million years ago. Fig. 2-6b, p. 31

Figure 2.12: Artist’s view of what the Atlantic Ocean basin would look like without water. The major feature is the Mid-Atlantic Ridge. Fig. 2-12, p. 36

Figure 2.14: A map of the world showing the plates, their boundaries, relative motion and rates of movement in centimeters per year, and hot spots. Fig. 2-14, p. 38

Plate Tectonic Theory Provides a framework for interpreting the composition, structure and internal processes of the Earth (see Table 1.3). Explains that Earth is broken into lithospheric plates that move over the asthenosphere. Shows relationship between plate boundaries and zones of major geologic activity. Establishes a basis for explaining cyclical and systematic destruction and renewal of the crust. Based upon Alfred Wegener’s Continental Drift hypothesis, which was later revised and improved upon by Harry Hess’s theory of sea-floor spreading.

Active Figure 2.15: History of a divergent plate boundary. (a) Heat from rising magma beneath a continent causes it to bulge, producing numerous cracks and fractures. Fig. 2-15a, p. 40

Active Figure 2.15: History of a divergent plate boundary. (b) As the crust is stretched and thinned, rift valleys develop and lava flows onto the valley floors. Fig. 2-15b, p. 40

Active Figure 2.15: History of a divergent plate boundary. (c) Continued spreading further separates the continent until a narrow seaway develops. Fig. 2-15c, p. 40

Active Figure 2.15: History of a divergent plate boundary. (d) As spreading continues, an oceanic ridge system forms and an ocean basin develops and grows. Fig. 2-15d, p. 40

Active Figure 2. 17: Oceanic–oceanic plate boundary
Active Figure 2.17: Oceanic–oceanic plate boundary. (a) An oceanic trench forms where one oceanic plate is subducted beneath another. On the nonsubducted plate, a volcanic island arc forms from the rising magma generated from the subducting plate. Fig. 2-17a, p. 42

Plate Tectonic Theory Provides a framework for interpreting the composition, structure and internal processes of the Earth (see Table 1.3). Explains that Earth is broken into lithospheric plates that move over the asthenosphere. Shows relationship between plate boundaries and zones of major geologic activity. Establishes a basis for explaining cyclical and systematic destruction and renewal of the crust. Based upon Alfred Wegener’s Continental Drift hypothesis, which was later revised and improved upon by Harry Hess’s theory of sea-floor spreading.

Active Figure 2. 17: Oceanic–oceanic plate boundary
Active Figure 2.17: Oceanic–oceanic plate boundary. (b) Satellite image of Japan. The Japanese Islands are a volcanic island arc resulting from the subduction of one oceanic plate beneath another oceanic plate. Fig. 2-17b, p. 42

Active Figure 2. 18: Oceanic–continental plate boundary
Active Figure 2.18: Oceanic–continental plate boundary. (a) When an oceanic plate is subducted beneath a continental plate, an andesitic volcanic mountain range is formed on the continental plate as a result of rising magma. (b) Aerial view of the Andes Mountains in Peru. The Andes are one of the best examples of continuing mountain building at an oceanic–continental plate boundary. Fig. 2-18, p. 42

Active Figure 2.18: Oceanic–continental plate boundary. (a) When an oceanic plate is subducted beneath a continental plate, an andesitic volcanic mountain range is formed on the continental plate as a result of rising magma. Fig. 2-18a, p. 42

Active Figure 2.18: Oceanic–continental plate boundary. (b) Aerial view of the Andes Mountains in Peru. The Andes are one of the best examples of continuing mountain building at an oceanic–continental plate boundary. Fig. 2-18b, p. 42

Active Figure 2.19: Continental–continental plate boundary. (a) When two continental plates converge, neither is subducted because of their great thickness and low and equal densities. As the two continental plates collide, a mountain range is formed in the interior of a new and larger continent. Fig. 2-19a, p. 43

Active Figure 2.19: Continental–continental plate boundary. (b) Vertical view of the Himalayas, the youngest and highest mountain system in the world. The Himalayas began forming when India collided with Asia 40 to 50 million years ago. Fig. 2-19b, p. 43

Figure 2.20: Horizontal movement between plates occurs along a transform fault. (a) The majority of transform faults connect two oceanic ridge segments. Note that relative motion between the plates occurs only between the two ridges. Fig. 2-20a, p. 44

Figure 2.20: Horizontal movement between plates occurs along a transform fault. (b) A transform fault connecting two trenches. Fig. 2-20b, p. 44

Figure 2.20: Horizontal movement between plates occurs along a transform fault. (c) A transform fault connecting a ridge and a trench. Fig. 2-20c, p. 44

Figure 2. 21: Transform plate boundary
Figure 2.21: Transform plate boundary. The San Andreas fault is a transform fault separating the Pacific plate from the North American plate. Movement along this fault has caused numerous earthquakes. The inset photograph shows a segment of the San Andreas fault as it cuts through the Carrizo Plain, California. Fig. 2-21, p. 44

Figure 2. 22: Reconstructing plate positions using magnetic anomalies
Figure 2.22: Reconstructing plate positions using magnetic anomalies. (a) The present North Atlantic, showing the present ridge and magnetic anomaly 31, which formed 67 million years ago. (b) The Atlantic 67 million years ago. Anomaly 31 marks the plate boundary 67 million years ago. By moving the anomalies back together along with the plates they are on, geologists can reconstruct the former positions of the continents. Fig. 2-22, p. 45

Figure 2.22: Reconstructing plate positions using magnetic anomalies. (a) The present North Atlantic, showing the present ridge and magnetic anomaly 31, which formed 67 million years ago. Fig. 2-22a, p. 45

Figure 2.22: Reconstructing plate positions using magnetic anomalies. (b) The Atlantic 67 million years ago. Anomaly 31 marks the plate boundary 67 million years ago. By moving the anomalies back together along with the plates they are on, geologists can reconstruct the former positions of the continents. Fig. 2-22b, p. 45

Active Figure 2.24: Two models to explain plate movement. (a) Thermal convection cells are restricted to the asthenosphere. Fig. 2-24a, p. 47

Active Figure 2.24: Two models to explain plate movement. (b) Thermal convection cells involve the entire mantle. Fig. 2-24b, p. 47

Figure 2.25: Plate movement is thought to be aided by “ridge-push” or “slab–pull” mechanisms. In ridge-push, the intrusion of magma into a spreading ridge provides an additional force that pushes the plates apart. In slab-pull, the edge of the colder and denser subducting plate descends into the interior and pulls the rest of the plate downward. Fig. 2-25, p. 47

CHAPTER OBJECTIVES 1 Plate tectonics is the unifying theory of geology and has revolutionized geology. 2 The hypothesis of continental drift is based on considerable geologic, paleontologic, and climatologic evidence. 3 The hypothesis of seafloor spreading accounts for continental movement, and thermal convection cells provide a mechanism for plate movement. 4 The three types of plate boundaries are divergent, convergent, and transform, and along these boundaries new plates are formed, consumed, or slide past one another. 5 The interaction of plates along their boundaries accounts for most of Earth’s earthquake and volcanic activity. 6 The rate of movement and motion of plates can be calculated in several ways. 7 Some type of convective heat system is involved in plate movement. 8 Plate movement affects the distribution of natural resources.

CHAPTER OUTLINE Introduction Early Ideas about Continental Drift Evidence for Continental Drift CULTURAL CONNECTIONS: The Struggle toward Scientific Progress Magnetic Reversals Related to Seafloor Spreading Plate Tectonic Theory Three Types of Plate Boundaries Plate Movement and Motion The Driving Mechanism of Plate Tectonics Tectonics and Natural Resources GEO-FOCUS 2.1: Oil, Plate Tectonics, and Politics GEO-RECAP

The ice-carved mountains of Glacier National Park, Montana, were uplifted by the forces of plate tectonics. Fig. 2-CO, p. 26

Figure 2.1: The Beartooth Mountains in Montana show how geologic forces have produced dynamic change through time. Tectonic uplift helped to form the rugged mountains, but the soil and plants in the foreground show that the rocks of the mountains are being broken down by other forces that could level these mountains over time. Fig. 2-1, p. 28

CHAPTER SUMMARY The concept of continental movement is not new. The earliest maps showing the similarity between the east coast of South America and the west coast of Africa provided the first evidence that continents might once have been united and subsequently separated. Alfred Wegener is generally credited with developing the hypothesis of continental drift. He provided abundant geologic and paleontologic evidence to show that the continents were once united into one supercontinent he named Pangaea. Unfortunately, Wegener could not explain how the continents moved, and most geologists ignored his ideas. The hypothesis of continental drift was revived during the 1950s when paleomagnetic studies indicated the presence of multiple magnetic north poles instead of just one as there is today. This paradox was resolved by constructing a hypothetical map and moving the continents into different positions, making the paleomagnetic data consistent with a single magnetic north pole. Magnetic surveys of the oceanic crust revealed magnetic anomalies in the rocks, indicating that Earth’s magnetic field has reversed itself in the past. Because the anomalies are parallel and form symmetric belts adjacent to the oceanic ridges, new oceanic crust must have formed as the seafloor was spreading.

CHAPTER SUMMARY Seafloor spreading has been confirmed by radiometric dating of rocks on oceanic islands. Such dating reveals that the oceanic crust becomes older with distance from spreading ridges. Plate tectonic theory became widely accepted by the 1970s because of the overwhelming evidence supporting it and because it provides geologists with a powerful theory for explaining such phenomena as volcanism, earthquake activity, mountain building, global climatic changes, the distribution of the world’s biota, and the distribution of some mineral resources. The supercontinent cycle indicates that all or most of Earth’s landmasses form, break up, and re-form in a cycle spanning about 500 million years. The three types of plate boundaries are divergent boundaries, where plates move away from each other; convergent boundaries, where two plates collide; and transform boundaries, where two plates slide past each other. The average rate of movement and relative motion of plates can be calculated in several ways. The results of these different methods all agree and indicate that the plates move at different average velocities.

CHAPTER SUMMARY The absolute motion of plates can be determined by the movement of plates over mantle plumes. A mantle plume is an apparently stationary column of magma that rises to the surface where it becomes a hot spot and forms a volcano. Although a comprehensive theory of plate movement has yet to be developed, geologists think that some type of convective heat system is the major driving force. A close relationship exists between the formation of some mineral deposits and petroleum and plate boundaries. Furthermore, the formation and distribution of some natural resources are related to plate movements.

Active Figure 18.2: Three stages in the origin of granitic continental crust. Andesitic island arcs formed by the partial melting of basaltic oceanic crust.

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Active Figure 18.2: Three stages in the origin of granitic continental crust. Andesitic island arcs formed by the partial melting of basaltic oceanic crust.

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Presentation on theme: "Active Figure 18.2: Three stages in the origin of granitic continental crust. Andesitic island arcs formed by the partial melting of basaltic oceanic crust."— Presentation transcript:

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