The fact that the continents can be pieced back together like this gave geologists their first clue about the formation of mountains and oceans. Remember that the continents are located on plates. Oceans are, too. A plate that lies below a continent is called a continental or lithospheric plate. One that lies beneath an ocean is called an oceanic plate. As the hot melted rock in the center of the earth moves, the plates move. This is how Pangaea originally broke apart. As they move, the continental plates carry the continents along with them.
Plates can move toward, away from, or past one another.
If two plates move toward one another, they eventually collide (see diagram: convergent plate boundary). When a continental plate collides with an oceanic plate, the oceanic crust gets pushed down below the continental crust. This type of collision may cause volcanic eruptions or earthquakes.
When two continental plates collide, the earth's crust gets compressed and buckled. This is the way that many mountains, including the Appalachians and Himalayas, are formed.
Sometimes when two plates collide, weak points in the crust grow into cracks. In these cracks, molten rock, or lava, from the earth's mantle comes up to the surface, is released, and hardens, creating volcanoes. Volcanoes grow by the addition of hot molten rock that reaches the surface during eruptions. Mount Karisimbi is an example of a volcano.
If two continental plates move away from each other (see diagram: divergent plate boundary), eventually an ocean is formed.
Sometimes continental plates move alongside one another (see diagram: transform plate boundary). Mountains can be formed in this way if some of the earth's crust is displaced by this movement, but earthquakes are the more common result.
As we will observe later in this unit, the way in which mountains are formed affects the life that can be found there. Depending on the height, type of soil, orientation, and exposure of a mountain, different plants and animals will live there.
The most amazing thing about this is that the plates are still moving today! Even though we can't feel it, North America is slowly moving closer to Asia. The fastest plates move about 4 inches a year. If you were to begin in September and let your fingernails grow until June, the length your nails grew would be about the same as the distance that the plates have moved.