Plate Tectonics

The Earth's lithosphere, or crust, is divided into tectonic plates. These plates are constantly in motion due to the convection currents of the Earth, which are caused by heat cycles in the mantle, the fluid layer beneath the Earth's crust. At the bottom of the mantle, the fluid gets extremely hot due to its proximity to the core. The hot liquid rises to the top of the mantle, where it then cools. Once it cools, it returns to the bottom, completing the cycle. This movement creates what is known as convection currents, which causes the tectonic plates to move.

Earthquakes occur at three types of plate boundaries, or fault lines: convergent, divergent, and transform.

• Earthquakes at Convergent Fault Lines
  
  An earthquake occurs at a convergent fault line when tectonic plates collide. Typically, at such a collision, subduction zones form as a result of a denser, older plate sliding under the other. A trench, sometimes 70 miles wide, is found at a subduction site, the result of the older plate descending into the earth.
  
  
  
  
• Earthquakes at Divergent Fault Lines
  
  During an earthquake at a divergent fault line, tectonic plates separate. In many cases, magma rises from the mantle to fill in the gap or one plate slide down to fill in the space. These types of earthquakes usually occur in the ocean.
  
  
  
  
• Earthquakes at Transform Fault Lines
  
  In an earthquake at a transform fault line, energy stored in tectonic plates is released. This release of energy causes the two plates to rub against each other.