Edward J. Tarbuck & Frederick K. Lutgens Earth Science, 10e Edward J. Tarbuck & Frederick K. Lutgens

Plate Tectonics Chapter 7 Earth Science, 10e Stan Hatfield and Ken Pinzke Southwestern Illinois College

Continental drift: an idea before its time Alfred Wegener First proposed hypothesis, 1915 Published The Origin of Continents and Oceans Continental drift hypothesis Supercontinent called Pangaea began breaking apart about 200 million years ago Continents "drifted" to present positions Continents "broke" through the ocean crust

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 Main objection to Wegener's proposal was its inability to provide a mechanism

Wegener’s matching of mountain ranges on different continents

Paleoclimatic evidence for Continental Drift

Plate tectonics: the new paradigm More encompassing than continental drift Associated with Earth's rigid outer shell Called the lithosphere Consists of several plates Plates are moving slowly Largest plate is the Pacific plate Plates are mostly beneath the ocean

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

Convergent: Continental to Continental Convergent plate boundaries (destructive margins) Definition: When subducting plates contain continental material, two continents collide How they move: two continental plates push each other up Landform Results: new mountain ranges Example: Himalayas Density: Same density so they both push each other up

A continental-continental convergent plate boundary

The collision of India and Asia produced the Himalayas (before)

The collision of India and Asia produced the Himalayas (after)

Convergent Plate Boundaries: Oceanic to Continental Convergent plate boundaries (destructive margins) Oceanic-continental convergence Definition: an oceanic plate and continental plate colliding How they move: oceanic plate slides underneath continental plate Denser oceanic slab sinks into the asthenosphere Land formations: Pockets of magma develop and rise, continental volcanic arcs form, chain of volcanic islands Examples include the Andes, Cascades, and the Sierra Nevadan system

An oceanic-continental convergent plate boundary

Convergent Plate Boundaries: Oceanic to Oceanic Oceanic-oceanic convergence Definition: Two oceanic slabs converge and one descends beneath the other How they move: Often forms volcanoes on the ocean floor Land formation results: Volcanic island arcs forms as volcanoes emerge from the sea Examples: include the Aleutian, Mariana, and Tonga islands, Density: same

An oceanic-oceanic convergent plate boundary

Divergent Plate Boundaries Divergent plate boundaries (constructive margins) Definition: Two plates move apart How they move: Mantle material upwells to create new seafloor Landform Results: Ocean ridges and seafloor spreading Oceanic ridges develop along well-developed boundaries Along ridges, seafloor spreading creates new seafloor Examples: Iceland, and East Africa Density: Same density

Divergent boundaries are located mainly along oceanic ridges

The East African rift – a divergent boundary on land

Transform Boundaries Definition: Plates slide past one another, horizontally (side to side) No new crust is created No crust is destroyed How they move: At the time of formation, they roughly parallel the direction of plate movement Aid the movement of oceanic crustal material Examples: San Andres fault in California Density: Plates have the same density

Plate tectonics: the new paradigm 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) Earth's magnetic field reversals Recorded in rocks as they form at oceanic ridges

Apparent polar-wandering paths for Eurasia and North America

Paleomagnetic reversals recorded by basalt flows at mid-ocean ridges

Plate tectonics: the new paradigm Evidence for the plate tectonics model Earthquake patterns Associated with plate boundaries Deep-focus earthquakes along trenches provide a method for tracking the plate's descent Ocean drilling Deep Sea Drilling Project (ship: Glomar Challenger)

Distribution of earthquake foci at plate boundaries

Earthquake foci in the vicinity of the Japan trench

Plate tectonics: the new paradigm Evidence for the plate tectonics model Hot spots Rising plumes of mantle material Volcanoes can form over them e.g., Hawaiian Island chain Chains of volcanoes mark plate movement

Plate tectonics: the new paradigm Evidence for the plate tectonics model Ocean drilling Age of deepest sediments Youngest are near the ridges Older are at a distance from the ridge Ocean basins are geologically young

The Hawaiian Islands have formed over a stationary hot spot

Plate tectonics: the new paradigm Measuring plate motion By using hot spot “tracks” like those of the Hawaiian Island - Emperor Seamount chain Using space-age technology to directly measure the relative motion of plates Very Long Baseline Interferometry (VLBI) Global Positioning System (GPS)

Directions and rates of plate motions

Plate tectonics: the new paradigm Driving mechanism of plate tectonics No one model explains all facets of plate tectonics Earth's heat is the driving force Several models have been proposed Slab-pull and slab-push model Descending oceanic crust pulls the plate Elevated ridge system pushes the plate

Plate tectonics: the new paradigm Driving mechanism of plate tectonics Several models have been proposed Plate-mantle convection Mantle plumes extend from mantle-core boundary and cause convection within the mantle Models Layering at 660 kilometers Whole-mantle convection Deep-layer model

Layering at 660 kilometers

Whole-mantle convection

Deep-layer model

End of Chapter 7