You may think of the earth as being rigid, but the outermost layer of the earth actually comprises seven huge pieces, referred to as "plates," as well as several smaller pieces. The plates move very slowly, but they are massive, and tremendous stress can build up at locations where their boundaries meet. When the tension becomes too great, the huge pieces move violently, causing earthquakes. Three different scenarios can take place at the boundaries of plates. They can drift apart, collide or side-swipe each other.
The collision area is referred to as a "convergent boundary" at locations where two plates are slowly crushing toward each other. Imagine two cars colliding head-on with each other. The front of the cars are crushed, and metal hoods are folded up. Similarly, convergent plates can cause mountains (the earth "folding up") to be created. The Himalaya Mountains were created by plate collisions. At converging boundaries one plate may slide under another plate. Stress builds up and when released by the slipping movement of one plate under the other, pieces of the plates break, causing earthquakes.
There are places in the world, such as the San Andreas fault in California, where two plates are moving horizontally side by side. This is called a "transform boundary". As they side-swipe each other, pressure builds up. Earthquakes occur when the pressure is released by the sudden movement of rocks and other earth materials. More than 600 people died from the earthquake and resulting fires that occurred in 1906 in San Francisco as a result of the movement of the San Andreas fault plates.
Plates can also drift apart, traveling away from each other. As these plates separate forming a "divergent boundary," the exposed earth in between can drop down into another layer of the earth. Hot, melted rocks (magma) can seep up into the cracks, causing earthquakes and creating a new crust. Volcanoes can also be created at these boundary locations if magma reaches up to the earth's surface. The area between the two plates can be relatively large, 30 to 50 kilometers wide in the case of the East African rift in Kenya and Ethiopia.