High School Earth Science: Plate Tectonics - Theory of Plate Tectonics — Flashcards

Earth’s Tectonic Plates - The lithosphere is divided into a dozen major and several minor plates. The edges of the plates can be identified from the distribution of earthquake epicenters. That’s because most earthquakes occur at plate boundaries. A single plate may consist only of oceanic lithosphere or only of continental lithosphere, but nearly all plates are made of a combination of both types of lithosphere.    
Plates are constantly moving over Earth’s surface. Movement of the plates is termed plate tectonics. Plates move at a rate of a few centimeters a year. The reason they move is seafloor spreading, which occurs because of convection currents in the mantle. Hot material rises at a mid-ocean ridge; gradually moves horizontally away from the axis of the ridge, dragging seafloor with it; and eventually sinks back deep into the mantle at deep-sea trenches.    
Plate boundaries are the edges where two plates meet. Most geologic activities—including earthquakes, volcanoes, and mountain building—take place at plate boundaries. Because plates are always moving, they move relative to one another at plate boundaries.

Plates can move relative to one another in three different ways:    
- They can diverge, or move away from each other. This occurs at a divergent plate boundary.    
- They can converge, or move toward each other. This occurs at a convergent plate boundary.    
- They can transform, or slide past each other. This occurs at a transform plate boundary.    

The type of plate boundary and the type of crust (oceanic or continental) found on each side of the boundary determine the type of geologic activity that will occur there.