One of the Earth's layers is the mantle. The mantle is separated into 2 parts: the upper mantle and the lower mantle. The upper mantle has parts as well. The lithosphere is made up of the crust and the top of the upper mantle and the asthenosphere is located under the lithosphere in the upper mantle. The asthenosphere is the hot softer rock in the mantle that can "act" like a liquid because it can "flow". It flows slowly like tar.

The lithosphere is made up of many plates that circle the Earth. It is not continuous though, it is cracked like an eggshell. These large and small slabs of rocks are called tectonic plates.

Evidence for continental drift

Alfred Wegener proposed the continental drift theory. This is the theory that all the continents were once joined together as one big landmass which he called Pangea. Evidence can be found in climate, geology, continent shape, and fossils.

• Fit: The edge of the continental plates seem to line up or fit together like puzzle pieces.
• Climate: Greenland, which is in the cold Arctic, has fossils of tropical plants. How is this possible unless Greenland was once in the tropics.
• Geology: Rock layers from the same mountain range are found on different continents
• Fossils: Plant and animal fossils have been found to be exclusive to certain continents. For example the mesosaurus fossil is exclusive to the eastern coast of South America and southwest Africa. So it stands to reason that they were once joined.

The Theory of Sea Floor Spreading

Harry Hess is the scientist who is credited with discovering sea-floor spreading. This takes place when the Earth's crust cracks, cools, and forms new oceanic crust. The old crust is pulled away making room fro the newly formed crust. The area where the molten rock rises through the cracks is called a spreading center. This is evidence that the oceanic plates are moving.

Age of the sea floor: When scientist study the sea floor they noticed that rock samples closest to the ridge were youngest and samples taken furthest away from the ridge were the oldest. This further proves that the sea floor is spreading. As the centers of the ridges spread, the old rock is pushed further out while the newly formed rock stays close to the ridge.

Ocean Trenches: If the sea floor is spreading why doesn't the Earth grow in size? Well, there are trenches, like big canyons in the ocean floor. As the older, more dense crust moves towards the trench, it sinks between the trench into the asthenosphere where it melts into molten material again. So as the new crust is forming at the ridges, the old crust is being destroyed at the ocean trench.

Convection Currents:

Convection is heat transferred by the movement of material. The rock in the asthenosphere and the mantle just below it, moves by convection. The rock in the asthenosphere moves in a current. The hot soft rock rises (less dense), cools (more dense), and sinks. As it sinks it is heated again and rises again. This process continues (rising and sinking) and is called the convection current. This motion transfers heat in a material.