Earth's Layers

There are convection currents within the mantle. A convection current is when a substance gets heated, it becomes less dense, so it rises. The cooler substance sinks, gets heated at the bottom and rises. This repeats over and over again. This is important to the Plate Tectonic theory, because when the mantle moves, the crust on top is going to move as well.

The earth has four layers. The top layer is the crust. There is oceanic crust and continental crust. The next layer down is the mantle, which consists of 2 sections, the lithosphere and the asthenosphere. The lithosphere is a rocky rigid part, and the asthenosphere is made of a soft material that flows slowly through convection currents. The next layer is the outer core, the only liquid layer. The final layer is the inner core, made of iron and nickel.

The Continental Drift Theory was proposed by Alfred Wegener, who believed that all the continents were once joined together in one large land mass called Pangaea. But, many scientists doubted his theory, because although he had evidence to back it up, he could not explain why or how this was occurring.

Continental Drift

Evidence:

*Fossils

-Glossopteris: The seed fern was too big to be carried by winds over the oceans to other continents

-Mesosaurus: A fresh water reptile that could not have survived swimming across the salty Atlantic Ocean.

*Jig Saw Puzzle

-Boundaries of South America and Africa fit together like pieces of a jigsaw puzzle

*Cape Mountains

-Mountain range lines up- formed the same way (folded), same age, and same rock type

*Rock Deposits

-Coal and glacier deposits found in areas that could not be formed where continents are now.

The Sea Floor Spreading Theory was proposed by Harry Hess, and is the process that continuously adds new material to the ocean floor.

The discovery of the Sea Floor Spreading Theory is important because it was the last missing piece to Alfred Wegner's Continental Drift theory. It was the "how" he needed.

Evidence:

*Molten material: new material is erupting along the mid-ocean ridge. It hardens quickly after erupting under water looking like pillows

*Magnetic Stripes- rocks that make up the ocean floor lie in a patterned stripes. When the poles reversed themselves the iron in the rock was locked in place.

*Drilling Samples- scientists were able to determine the age of rocks- rocks at center of the mid-ocean ridge are younger and the rock farther away from the mid-ocean ridge are older.

The theory of Plate Tectonics combines what scientists know about continental drift theory and sea floor spreading theory into a single theory. The lithosphere is broken into separate sections called plates, which fit closely together along the cracks. No plate can move without affecting other plates. The plates of the lithosphere float on the top of the asthenosphere. The movement in the asthenosphere caused by convection currents, causes the plates to move.

Types Of Stress

1) Compression

Being pushed together

2) Tension

Being pulled apart

3) Shear

sliding against each other

Convergent Boundary

At a convergent boundary, the plates move towards each other.

Crustal features that form at this boundary line include mountains, trenches, island arcs, and volcanoes.