Earth's Layers by Composition

Outward to Inward Order:

- Crust - Mostly Oxygen (Least Dense)

- Mantle - Mostly Magnesium

- Core - Mostly Iron (Most Dense)

Earth's Crust

Outer Layer - 2 Types

Continental - Thicker, less dense

Oceanic - Thinner, more dense

Earth's Layers by Physical Properties

Lithosphere - Outermost shell, Rigid, will break under pressure

Asthenosphere - Below the lithosphere, able to flow

Core - Center of Earth, hot temperatures

Theory of Continental Drift

Proposed by Alfred Wegener

The continents were once joined in a super land mass called Pangaea that broke apart in to separate pieces and drifted over time to their present day locations.

Evidence for Continental Drift

1. Shape of Continents - Edges of continents look as if they could fit together like puzzle pieces. Ex. South America and Africa.
2. Fossil Evidence - Mesosaurus - reptile that swam in fresh water, found on separate continents.  Could not have swam across the ocean.
3. Mountain Chains - Identical mountains on separate continents that make a continuous mountain chain when put back together as part of Pangaea.
4. Climate - Coal in Pennsylvania is shows PA was once tropical.  Must have been closer to the equator when it was part of Pangaea.

Missing Evidence for Theory of Continental Drift

The Theory of Continental Drift was not accepted because Wegener could not explain HOW the continents moved.

New discovery to help prove the Theory of Continental Drift

Mid Atlantic Ridge

A mountain chain on the sea floor in the middle of the Atlantic Ocean

Sea-Floor Spreading

Sea Floor Spreading occurs Mid Atlantic Ridge

Heat from the CORE causes the ASTHENOSPHERE to expand, become less dense and rise.  As it moves away from the core it cools, becomes more dense and sinks.

This circulation (called convection) causes the LITHOSPHERE to pull apart at the surface.

The lithosphere (sea floor) is young and hot in temperature at the ridge and gets older and colder moving away from the ridge on both sides.

Convection

Circulation that occurs in liquids or gases when a change in temperature and density causes circulation.

Happening in asthenosphere as a result of heat from the core.  Causes the lithosphere to pull apart - Sea Floor Spreading.

Density

A measure of how tightly packed atoms and molecules are in a substance.

Substances will arrange themselves according to density.

Ex: More dense elements became the core, less dense elements became the crust.