The Tectonic Plates: Floating Islands of Rock
Tutoring high school science, you might be asked about plate tectonics at any time. Here is the most condensed, pragmatic explanation you’ll likely find.
To believe in plate tectonics – which, by the way, is true beyond any doubt – you need to picture this: Earth’s surface consists of large islands of solid rock floating on a sea of magma. There are about 9 large plates, plus some smaller ones. Examples are the Indian Plate, the North American Plate, the South American Plate, the Eurasian Plate, the African Plate and the Pacific Plate.
The plates more or less cover the sea of magma beneath them. At the same time, because the plates float on the magma, they move around like toy boats in a bath tub. When they move apart, you have what’s called a divergent boundary. When they crash together, you have a convergent boundary. When they rub alongside each other, going in parallel but opposite directions (perhaps like people passing each other in a crowded hallway), you have a transform boundary. Essentially, the term fault can be substituted for boundary.
At a divergent boundary, a crack eventually forms between the plates, then magma leaks up between them, so you get volcanoes. The volcanoes can eventually form high ridges as they continue to erupt. Divergent boundaries are more common under the sea than on land.
At a convergent boundary, there are two possibilities. One is a head-on collision, in which case the plates buckle as they crash (like in a car crash). The plates deform upward, resulting in mountains. The Himalayas are such a mountain range, caused by the collision of the Indian Plate and the Eurasian Plate.
The second possible outcome at a convergent boundary is subduction, which is where one plate rides over top and the other slides below. If one plate is a continental one and the other is a sea plate, the sea plate slides under and the continental one rides up, forming a mountain chain. The Andes Mountains exemplify such a situation: there, the Nazca Plate is sliding under the South American Plate.
Although each type of boundary hosts earthquakes, a transform boundary results, especially, in earthquakes. As the plates try to shove past each other, they get stuck. The pressure builds, then they break loose, resulting in sudden movement – and an earthquake. The famous San Andreas Fault at San Francisco is a transform boundary: the Pacific Plate is running NNW, while the North American Plate is running SSE, along it.
Hope this gets you started :)
Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.
I pulled this article together from several high school textbooks:
Science Probe 10, Nelson Edition, Nelson Canada: 1996.
Earth Science, Spaulding and Namowitz, McDougal Littell: 2003.
The Changing Earth, McDougal Littell Science, McDougal Littell: 2005.