Plate Movement

What force was powerful enough to sendgigantic plates sliding around the globe?

1. One reason that people in the 1920s doubted the continental drift theory was Wegener’s explanation of just how the continents moved. Today, most scientists believe this force is a process called convection. Convection is a circular movement caused when a material is heated, expands and rises, then cools and falls. This process is thought to be occurring in the mantle rock under the plates. The heat energy that drives convection probably comes from the slow decay of radioactive materials under the earth’s crust.

Plate Boundaries.

Where do plates form a diverging plate boundary or spreading zone?

2. As mentioned earlier, the places where plates meet are some of the most restless parts of the earth. Plates meet at their boundaries and react in one of three different ways. They pull away from each other, crash head-on, or slide past each other.

Where plates pull away from one another, they form a diverging plate boundary, or spreading zone. As the Mid- Atlantic Ridge shows, such areas are likely to have a rift valley, earthquakes, and volcanic action.

What is a subduction zone?

3. Several different things can happen when plates meet, or converge. Because continental crust is lighter than oceanic crust, continental plates "float" higher. Therefore, when an oceanic plate meets a continental plate, it slides under the lighter plate and down into the mantle. Any place where this occurs is known as a subduction zone.

When subduction occurs, volcanic mountain building and earthquakes may also occur on the continental plate. The Andes Mountains, for example, formed over millions of years as the Nazca Plate slid under the South American Plate.

Different kinds of converging zones occur where the plates of the same type collide. When both are oceanic plates, one moves, or is subducted, under the other. Often an island group forms at this boundary.

The collision of two continental platesis more dramatic. In all of the earth’s history, perhaps the most stunning collision was that of India and Asia. As the map on page 15 shows, India today is clearly part of Asia. But it is not part of the Eurasian Plate. It is the northern tip of the Indo-Australian Plate, which crashed into the Eurasian Plate millions of years ago. Earth’s highest mountain range, the Himalayas, was formedby the collision of these two plates. Even today, the Indo-Australian Plate continues to push against the Eurasian Plate at a rate of about 2 inches (5 cm) a year.

In some places the earth’s tectonic plates do not crash against each other. Rather they slip or grind past each other along faults. The Saint Andres Fault in California is an example of this type of plate boundary [11].

Diagram Skill

In a subduction zone, one plate slides or dives under another. In a spreading zone, two plates move apart from eachother creating a rift, or crack, in the earth’s crust. In a collision or convergence zone, two plates collide and push slowly against each other. At a transform fault, plates grind or slide past each other rather than colliding. Which type of plate boundary occurs along the west coast of South America.