1. What causes mountains, volcanoes, and ridges to form? Why do earthquakes happen? All of these things happen because of the movement of the earth’s tectonic plates. These “plates” form the top part of the earth, called the earth’s crust. They are constantly moving.
2. The crust can be thought of as a mixture of solid and liquid, like crackers floating on a bowl of hot soup. The soup (the lava) is gently stirred by currents below. The crackers (the plates) move slowly and gently, but there’s friction where two plates push against each other. When one suddenly gives way, a big change occurs.
3. Scientists who have studied the movement of these plates have come to these conclusions:
4. One plate may slide under the other. This may cause a volcanic eruption. An example of this can be found in the Northwestern U.S., where a line of volcanoes stretches from California to Canada.
5. Plates can pull away from each other. Then magma wells up in between. Magma is lava that’s still under the ground. The built-up magma creates a ridge. An example of this is occurring under the Atlantic Ocean, where the seafloor is expanding as molten rock pushes up, and it’s widening the distance between America and Europe by about 3 inches each year!
6. Plates can push against each other, going in opposite directions. One can move north and one south, or one east and one west. A split occurs between the two, called a fault. This allows the two plates to move in opposite directions. An example of this is the San Andreas Fault in California.
7. Two plates may crash directly into each other. This causes each to buckle up and creates mountain ranges. An example of this is the Himalaya mountain range, where the plate carrying India crashed into the plate carrying Asia.
8. So, although we cannot feel or see the movement of tectonic plates, we can see the results. And, we can conclude that these tiny movements cause many great changes.
What Makes Up the Earth’s Crust
by Agnes Giberne
1. People tend to think of land and water on the earth as if they were fixed in one changeless form. As if every continent and every island were of exactly the same shape and size now that it always has been and always will be.
2. Yet, nothing can be further from the truth. The earth’s crust is a scene of constant change, struggle, building up, and wearing away.
3. The work may go on slowly, but it does go on. The sea is always fighting against the land, beating down her cliffs, eating into her shores, swallowing bit by bit of solid earth. Rain and frost and streams are always busily at work. They help the ocean in her work of destruction. Year by year and century by century, it continues. Not one country in the world, which is bordered by the sea, has the same coastline that it had one hundred years ago. Every land loses a part of its material every century. It is washed away, bit by bit, into the ocean.
4. Is this hard to believe? Look at the crumbling cliffs around old England's shores. See the effect upon the beach of one night's fierce storm. Mark the pathway on the cliff, how it seems to have crept so near the edge that here and there it is scarcely safe to walk upon. Very soon, such a path will become impassable. Just from a mere accident, of course,—the breaking away of some of the earth, loosened by rain and frost and wind. But, this is an accident that happens daily in hundreds of places around the world.
5. Leaving the ocean, look now at the river near your own neighborhood. See the slight muddiness which seems to color its waters. Why is that? Only a little earth and sand were carried off from the banks as it flowed. It is very unimportant and small in quantity, doubtless, just at this moment, and just at this spot. But what of that little going on week after week, and century after century, throughout the whole course of the river, and throughout the whole course of every river in our whole country and in every other country. A vast amount of material must every year be torn from the land and given to the ocean. And the land's loss here is the ocean's gain.