Plate Tectonics BIG Idea Most geologic activity occurs

Plate Tectonics

BIG Idea: Ø Most geologic activity occurs at the boundaries between plates.

I. Drifting Continents • MAIN IDEA: The shape and geology of the continents suggests that they were once joined together.

Continental Drift ütheory proposed by Alfred Wegener (VAY guh nur) in 1912 üstated that the continents were once a single landmass üthe continents have moved. . .

-Pangaea (“all the earth”): supercontinent; single landmass -Panthalassa (“all seas”): huge ancient ocean

A. Evidence of Continental Drift 1. Puzzle-Piece Fit

2. Rock Formations Ages, types and layers of coastal rocks of widely separated regions matched…

§Western Africa and Eastern Brazil §Appalachians (along eastern U. S. ) fit Greenland Europe

3. Fossils

Mesosaurus: freshwater reptile that lived 270 million years ago • remains (fossils) found in South America and Africa • impossible for these reptiles to have crossed the Atlantic

4. Climate (a) Glossopteris: fern that grew in temperate climates § places where fossils had been found were once closer to the Equator § rocks containing these fern fossils had once been joined

(b) Coal Deposits: § Coal forms from ancient swamp plants § Coal beds found in Antarctica indicated that this frozen land once had a tropical climate § Antarctica must have been closer to the Equator

(c) Glacier Deposits: Ø found in Africa and South America Ø climates are much warmer there today

A Rejected Notion…

• Wegener’s Continental Drift Hypothesis was never accepted by the scientific community • He could not explain what forces could cause such massive movement • Wegener died in 1930, on expedition in Greenland, while collecting evidence to further support his theory

II. Seafloor Spreading • MAIN IDEA: Oceanic crust forms at ocean ridges and becomes part of the seafloor.

1. Mid-Atlantic Ridge: undersea mountain range with a steep narrow valley down the center • the middle of the ocean floor is very young

2. Renewal of the Ocean Floor: Øseafloor spreading - floor moving away from the center ØHarry Hess and Robert Dietz 3. Paleomagnetism: as magma solidifies the iron-rich minerals align with magnetic north like a compass

reverse polarity: magnetic orientation pointing south

III. Plate Boundaries • MAIN IDEA: Volcanoes, mountains, and deep-sea trenches form at the boundaries between the plates

Theory of Plate Tectonics • Plate Tectonics: theory that the lithosphere is made of rigid plates that ‘float’ on the asthenosphere • Tectonics: the study of the formation of Earth’s features

What are the inferred properties of Earth’s interior?

Zones of Earth 1. Crust: thin, solid outermost layer; surrounds Earth like a shell surrounds an egg.

Two Types of Crust:

Continental Crust: makes up the continents (land); thicker, less dense; granitic rock Oceanic Crust: under the oceans; thinner, MORE dense; basaltic rock

2. Moho: boundary between the crust and mantle.

3. Mantle: § thickest layer § sits below the crust § makes up 2/3 of the Earth’s mass

• Lithosphere (‘rocky sphere’): upper part of mantle, cool and rigid

• Asthenosphere (‘weak sphere’): lower (‘plastic’) mantle; flows like hot tar

• Plasticity: the ability of a solid to flow

4. Core: center of the Earth; two parts: • Inner Core: ball of hot, solid metals (iron and nickel)

Outer Core: only true liquid zone

If we’ve never been there, how have we learned about the interior of Earth?

By studying seismic waves… Seismic Wave: vibration that travels through the Earth; results from earthquakes or huge explosions

Types of Plate Boundaries

1. Divergent Boundaries: Boundaries • two plates that are moving AWAY from each other • as plates move apart, the asthenosphere flows up to fill the empty space

What forms when plates DIVERGE?

a. Mid-Ocean Ridges Mid-Atlantic Ridge b. Rift Valleys: (1) narrow valleys in the middle of ocean ridges (2) where continents separate East African Rift Valley

2. Convergent Boundaries • direct collision of one plate with another

Three types of converging boundaries:

• Types of Convergent Boundaries

Oceanic vs. Continental Subduction Zone: – oceanic crust moves (dives) under continental crust § forms an oceanic trench and volcanic mountain range § oceanic crust is more dense than continental, thus it dives down or subducts