Continental Drift
#4 Plate Tectonics
After 50 years of investigating, Wegener’s theory of Continental Drift became the fact of Plate Tectonics. The earth’s crust is made up of huge areas, all moving, known as plates. The edges of the plates, or plate margins, are where all the action is. Sometimes they cause earthquakes or volcanoes; sometimes they build mountains and sometimes the swallow up the earth’s crust.
The edges of the plates are known as the plate margins, or plate boundaries. Depending on the type of boundary, different things happen. There are three main types of Boundaries.
Spreading Boundaries
There are two types of spreading boundaries:
Ridges, where new rock is formed, pushing the plates apart, and Rift, where the plates pull apart, and the earth between them sinks.
On the ocean floor the plates are spreading. All along the mid ocean ridge, volcanic activity is pouring new lava out onto the seabed. As it hardens, it becomes new rock, gradually pushing the crust, on either side of it away. Another type of spreading boundary is the African Rift Valley. As the two plates are moving apart, the earth between them is becoming stretched. As it stretches it sinks, because it is thinner. It is a little bit like taking a piece of gum, and stretching it out. The pieces in your fingers stay thick, but the piece in the middle gets thinner and thinner, as it is stretched.
Converging Boundaries
Converging (coming together) Boundaries, are where two plates are banging into each other. What happens when they collide depends on the type of plate they are.
In a subduction zone one plate is being pushed beneath the other. It happens when a heavy plate hits a lighter plate. The heavier plate is pushed under the lighter one since it is more dense. This often happens where continents meet the ocean. As the heavier ocean crust is forced downwards, it takes water with it. The water causes the rock to melt at lower temperatures than normal. The pressure inside the earth is much greater than at the surface, it increases the deeper you go. The new molten rock tends to rise to the area of lower pressure, sometimes forming volcanoes. It is not uncommon to find volcanoes along a converging boundary, on the side of the heavier plate.
In a deformation zone the area where the plates meet is being crushed and pushed upwards. It happens when the two plates are almost equal in weight. Instead of one plate being pushed down under the other, they smash into each other, causing the edges to rise upwards. It is very similar to two cars crashing together, head-on. As they hit each other, the front ends crumple, and lift upwards. In this type of boundary huge mountain ranges are formed. The best example of this is the Himalayan Mountains. They were formed when the Indian Plate crashed into the European Plate. The two are still being forced into each other, and the Himalayan Mountains are slowly becoming taller.
If you think about it hard enough, you can probably see that plates with a spreading boundary on one side must have a converging or transverse boundary on the other side. The earth’s crust is moving and changing all the time, but the upward or downward movement of converging boundaries, stops the earth from growing larger. As the plates slip and slide over and under each other, they keep the size of Earth constant.
Transverse Boundaries
The last type of boundary is neither spreading nor colliding. The edges are sliding past each other, sideways. As they slide past each other they cause a huge crack in the surface of the earth. This crack, or fissure, is known as a fault line. Probably the best-known example of this is the San Andreas Fault, which is found in Western California. As they slide past each other they sometimes cause earthquakes. Areas with fault lines have more frequent earthquakes than other places.