The hard, rigid plates that form th

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
Line aluminum and sodium, while the mantle contains some heavier elements, like iron and
(5) 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 floats 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
(10) them.
With an understanding 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
(15) 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, Australia, and Antarctica ― is called Gondwanaland. The northern one ― with
North America, Europe, and Asia ― is called Laurasia. North America tore away from
Europe about 180 million years ago, forming the northern Atlantic Ocean.
(20) Some of the lithospheric plates carry ocean floor and others carry land masses or a combination of the two types. The movement of the lithospheric plates is responsible
for earthquakes, volcanoes, and the Earth's largest mountain ranges. Current
understanding of the interaction between different plates explains why these occur
where they do. For example, the edge of the Pacific Ocean has been called the “Ring
(25) of Fire” because so many volcanic eruptions and earthquakes 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.