Composition of earth

The crust is composed of two basic rock types granite and basalt. The continental crust is composed mostly of granite. The oceanic crust consists of a volcanic lava rock called basalt. Basaltic rocks of the ocean plates are much denser and heavier than the granitic rock of the continental plates. Because of this the continents ride on the denser oceanic plates. The crust and the upper layer of the mantle together make up a zone of rigid, brittle rock called the Lithosphere. The layer below the rigid lithosphere is a zone of asphalt-like consistency called the Asthenosphere. The asthenosphere is the part of the mantle that flows and moves the plates of the Earth.

The crust

• The mantle is approximately 2900 kilometers thick, making it Earth's largest layer. The mantle has a property called "plasticity" (where a solid has the ability to flow like a liquid). You might call the mantle "partially molten". Remember that the temperature of the mantle increases the deeper you go. This difference in temperature causes CONVECTION CURRENTS to form. This type of current forms when hot things rise and cooler things sink. These convection currents tumble throughout the mantle. They cause the Lithospheric plates floating on the mantle to move around. These currents cause our continents and oceans to change location slightly each year. The currents are the driving force for Plate Tectonics or Continental Drift, which we will discuss in more detail in a later section. The forces which drive continental drift seem to come from the mantle. The hot rock, which boils up at mid-ocean ridges, comes from the upper mantle. This rock spreads out forming new oceanic plates. When these meet the continents they plunge back down into the mantle, sometimes going down as far as the outer core.

• In addition there are hot spots, which start at the outer core and rise up through the mantle to form islands such as Hawaii or Iceland.

Convection Currents - Large convection systems in the mantle may carry along the plates of the lithosphere like a conveyor belt

Outer Core & Inner core

Inner core

The inner core of the Earth has temperatures and pressures so great that the metals are squeezed together and are not able to move about like a liquid, but are forced to vibrate in place as a solid. The inner core begins about 4000 miles beneath the crust and is about 800 miles thick. The temperatures may reach 9000 degrees F. and the pressures are 45,000,000 pounds per square inch. This is 3,000,000 times the air pressure on you at sea level!!!