The hard, rigid plates that form the outermost portion of the Earth are about 100 kilometers thick. These plates include both the Earth’s crust and the upper mantle. The rocks of the crust are composed mostly of minerals with light elements, like iron and magnesium. Together, the crust and upper mantle that form the surface plates are called the lithosphere. This rigid layer floats on the denser material of the lower mantle the way a wooden raft flats on a pond. The plates are supported by a weak, plastic layer of the lower mantle called the asthenosphere. Also like a raft on a pond, the lithospheric plates are carried along by slow currents in this more fluid layer beneath them.

With an understating of plate tectonics, geologists have put together a new history for the Earth’s surface. About 200 million years ago, the plates at the Earth’s surface formed a “supercontinent” called Pangaea. When this supercontinent started to tear apart because of plate movement, Pangaea first broke into two large continental masses with a newly formed sea that grew between the land areas as the depression filled with water. The southern one, which included the modern continents of South America, Africa, Autralia, and Antarctic, is called Gondwanaland. The northern one, with North America, Europe and Asia is called Laurasi. North America tore away from Europe about 180 million years ago, forming the northern Atlantic Ocean.

Some of the lithospheric plates carryocean floor and others carry land masses or a combination pf the two types. The movement of the lithospheric plates is responsible for earthquakes, volcanoes, and the Earth’s largest mountains why these occur where they do. For example, the edge of the Pacific Ocean has been called the “Ring of Fire” because so many volcanic eruptions and earth quakes happen there. Before the 1960’s geologists could not explain why active volcanoes and strong earthquakes were concentrated in that region. The theory of plate tectonics gave them an answer.