What is a Tectonic Plate?

Tectonic plates are the massive pieces of rock that make up the outside surface of the earth. They are also sometimes called lithospheric plates. This includes exposed landmasses, such as the continents, and unexposed land on the ocean floor. These plates are constantly moving, albeit very slowly. The places where the plates rub against one another or spread away from each other are usually high in volcanic or seismic activity, and sometimes both.

It is the movement of the plates that has formed and continues to form the size and shape of our continents. As they float along on the magma under the earth's surface the continents drift a few centimeters a year. The plates are so humongous you might wonder how they are able to float at all, but the answer lies in the composition of the plate. Continental plates, which are usually very thick, are made up of light weight stone like feldspar or quartz. Oceanic plates are made of much heavier materials and are usually thinner. The direction of the movement is determined by convection currents which are being fueled by radioactive decay deep in the center of the planet.

New plates are formed when there is a diverging plate boundary; on a continent this would be called rifting. When this occurs the ground spreads and magma is pushed through the earth's crust forming a lava flow and possibly causing earthquakes. This causes further spreading of the land as the magma creates more crust that pushes apart the plate. Eventually this rift expands creating a rift valley which can fill with water and become a sea. After time this area will become a deep rift valley as the new plate grows ever larger.

In other instances plates are destroyed during what is called plate convergence. In this case one plate will slip under another moving at a few centimeters a year. As the bottom plate moves under the top it heats up and becomes magma. After thousands of years the molten rock either solidifies slowly, becoming intrusive igneous rock like granite. Or it flows to the surface and erupts into small or large lava flows. These flows cool rapidly to form extrusive igneous rock like basalt.