Earth’s Layered Structure (Ch. 8.4 in the Text)

Earth’s internal structure Layers based on composition Crust Thin, rocky outer layer Varies in thickness Oceanic crust: Roughly 7 km (5 miles) thick Continental crust: Averages 35–40 km (25 miles) thick

Earth’s layered structure Discovering Earth’s major layers Mohorovicic discontinuity (Moho) Separates crust from underlying mantle

Earth’s internal structure Layers based on composition Mantle Below crust to a depth of 2,890 kilometers (1,800 miles) Over 82% of Earth’s volume Common rock type in the upper mantle is Peridotite

Earth’s internal structure Layers based on composition Core Composed of an iron–nickel alloy Average density of nearly 13 g/cm3 Radius of 3,480 kilometers

Earth’s internal structure Layers based on physical properties Lithosphere Crust and uppermost mantle 5 - 100 km thick Cool, rigid, solid Asthenosphere Beneath the lithosphere Part of the Upper mantle To a depth of about 660 km Soft, weak layer that is easily deformed

Earth’s internal structure Layers based on physical properties Lower Mantle 2230 km thick More rigid layer than the Asthenosphere Rocks are very hot and capable of gradual flow Outer Core Liquid layer 2,260 km (1,410 miles) thick Convective flow of metallic iron within generates Earth’s magnetic field

Earth’s internal structure Layers based on physical properties Inner Core 1220 km thick Compressed into a solid state by immense pressure

Plate Tectonics Ch. 9 (p. 246 - 277)

Continental drift: an idea before its time Alfred Wegener First proposed hypothesis of Continental Drift in 1915 Continental drift hypothesis Supercontinent called Pangaea began breaking apart about 200 million years ago Continents “drifted” to present positions

Pangaea approximately 200 million years ago

Continental drift: an idea before its time Wegener’s continental drift hypothesis Evidence used by Wegener Fit of South America and Africa Fossils match across the seas Rock types and structures match Ancient climates Problem with Wegener’s Hypothesis: What force was moving the continents?

Similar mountain ranges on different continents

Paleoclimatic evidence for continental drift

Plate Tectonics: the new paradigm The Theory of Plate Tectonics We are now able to measure the movement of the continents due to technological advancements Associated with Earth’s rigid outer shell Called the lithosphere Consists of several plates Largest plate is the Pacific plate

Plate tectonics: the new paradigm Asthenosphere Exists beneath the lithosphere Hotter and weaker (fluid like) than lithosphere Allows for motion of lithosphere Plate boundaries All major interactions among plates occur along their boundaries

Plate tectonics: the new paradigm Plate boundaries Types of plate boundaries Divergent plate boundaries (Constructive margins) Two plates move apart Mid Ocean ridges and seafloor spreading Oceanic ridges develop along well-developed boundaries Along ridges, seafloor spreading creates new seafloor

Divergent boundaries are located along oceanic ridges

The East African rift – a divergent boundary on land

Plate tectonics: the new paradigm Plate boundaries Convergent plate boundaries (destructive margins) Plates collide, an ocean trench forms, and lithosphere is subducted into the mantle

Plate tectonics: the new paradigm Plate boundaries Types of plate boundaries Convergent plate boundaries (destructive margins) Oceanic–continental convergence Denser oceanic slab sinks into the asthenosphere Pockets of magma develop and rise Continental volcanic arc forms Examples include the Andes, Cascades, and the Sierra Nevadan system

An oceanic–continental convergent plate boundary

Plate tectonics: the new paradigm Plate boundaries Types of plate boundaries Convergent plate boundaries (destructive margins) Oceanic–oceanic convergence Two oceanic slabs converge and one descends beneath the other Often forms volcanoes on the ocean floor Volcanic island arc forms as volcanoes emerge from the sea Examples include the Aleutian, and Mariana.

An oceanic–oceanic convergent plate boundary

Plate tectonics: the new paradigm Plate boundaries Types of plate boundaries Convergent plate boundaries (destructive margins) Continental–continental convergence When subducting plates contain continental material, two continents collide Can produce new mountain ranges such as the Himalayas

A continental–continental convergent plate boundary

The collision of India and Asia produced the Himalayas

The collision of India and Asia produced the Himalayas

Plate tectonics: the new paradigm Plate boundaries Types of plate boundaries Transform fault boundaries Plates slide past one another No new crust is created or destroyed

Testing the plate tectonics model Evidence from ocean drilling Some of the most convincing evidence confirming seafloor spreading has come from drilling directly into ocean-floor sediment Age of sediments Thickness of ocean-floor sediments verifies seafloor spreading

Testing the plate tectonics model Hot spots and Mantle Plumes Caused by rising plumes of mantle material Volcanoes can form over them (Hawaiian Island chain)

The Hawaiian Islands have formed over a hot spot

Testing the plate tectonics model Evidence for the plate tectonics model Paleomagnetism Probably the most persuasive evidence Ancient magnetism preserved in rocks Paleomagnetic records show Polar wandering (evidence that continents moved)

Measuring plate motion By using hot spot “tracks” like those of the Hawaiian Island chain Using space-age technology to directly measure the relative motion of plates Global Positioning System (GPS)

Directions and rates of plate motions

What drives plate motion Driving mechanism of plate tectonics Earth’s heat is the driving force Convective Currents

Mantle Convection

Layering at 660 km

Plate Motion at the Surface

A possible view of the world 50 million years from now