Chapter 2 Plate Tectonics

Introduction 1795 1920/1930s Theory of Gradualism Continental Drift Earth was in continuous, but gradual change, constantly decaying, renewing and repairing itself Proposed by James Hutton “Father of modern geology” 1920/1930s Continental Drift Idea that the continents once fit together like pieces of a giant jig-saw puzzle, to make one vast supercontinent - Pangaea (“all the land”). Later the fragmented into separate continents that drifted apart, moving slowly to their present positions Proposed by Alfred Wegener Highly rejected because Wegener couldn't explain why

Continental Drift Evidence: Mechanisms: Geometric fit of continents Ancient glacial deposits Fossils Mechanisms: Continents plow through the ocean floor

Sea-Floor Spreading 1960s Harry Hess/Robert Dietz Sea-Floor Spreading As continents drift apart, new ocean floor forms between them New material (crust) comes up, cools and pushes old to the side As new oceanic crust is created it is pushed away from the boundary, continents move with the rest of the crust Evidence: Magnetic strips on ocean floor Mechanism: New volcanic ocean floor is being made at the ocean ridge Material on both sides of ridge is moving away

Subduction 1960s Harry Hess/Robert Dietz Subduction Production of new crust must be balanced by destruction of crust elsewhere Crust plunges into the mantle along subduction zones – coincide with zones of concentrated earthquakes Continents move toward each other when the old ocean floor between them sinks back down to the Earth’s interior

Theory of Plate Tectonics Combines the ideas of continental drift, sea-floor spreading, and subduction Lithosphere is broken into plates that are in motion The plates float on the ductile asthenosphere Explains origin and distribution of volcanoes, fault zones and mountain belts

Evidence of Plate Tectonics Ocean floors have complex topography Mountains and trenches Sea floor gave evidence that there are plates Mid Atlantic ridge

Evidence of Plate Tectonics Volcanos occur in chains along continental margins and ocean trenches

Evidence of Plate Tectonics Seismic events (earthquakes) are concentrated in lines and arcs coinciding with edges of continents, oceanic trenches, or the middle of ocean basins

Plate Driving Mechanisms Ridge – Push As material comes up from mid- ocean ridge Plate gets pushed apart Slab – Pull - Weight of plate being subducted pulls it Convection Heat rising moves material along Convection cells

Convection Heat rises to surface by convection (heat fluid becomes less dense and rises) Hot mantle rises up, cool mantle sinks down

Plate Boundaries Neighboring plates interact at diverging, converging, and sliding boundaries Continents move toward, away, and slide past each other in all directions

Convergent Plate Boundaries Crust is destroyed at convergent plate boundaries Plates are pushed towards each other, usually forming a subduction zone Different results for all three types of collisions Ocean-ocean Ocean-continent Continent-continent

Continent-continent Boundary Continental crust is too light to sink down (can’t subduct) Compression and folding uplift the plate edges to form mountains High Mountains within continents Himalayas Mts. in Asia

Ocean-continent boundary Similar to ocean-ocean Ocean crust is denser so it get subducted Volcanic arc on land with high mountains landward, coastal trench Ex. Andes Mts. in S. America

Ocean-ocean Boundary Older ocean crust is subducted under the younger Volcanic arcs and oceanic trenches Example: Japan & Aleutian Islands

Divergent Plate Boundaries Occurs where plates are moving apart, as magma pushes up from the mantle, creating new crust The farther the crust is from the rift, the older it is

Seafloor Spreading Forms mid-ocean ridges with new ocean floor Ex. Mid-Atlantic Ridge Rift Valley Forms valleys and volcanoes Ex. Red Sea (Saudi Arabia torn from Africa)

Transform Plate Boundaries Plates slide or grind past each other Fault zones with frequent earthquakes Caused by friction – no space between the plates Crust is conserved Transform boundaries link other boundaries Most widely distributed types of plate boundary Ex. San Andres Fault - CA