The earth is composed of four layers that are drifting apart. These are the continental crust, the ocean crust, upper mantel and the aesthenosphere. The continental crust is the upper landmasses on the earth's surface, the ocean crust is the ocean. The upper mantel is a layer below the continental and ocean crust. The lowest layer is the aesthenosphere. The lithosphere is the name for the continental crust, ocean crust and mantel. The lithosphere is broken into large plates that float on the aesthenosphere.
Ocean currents cause plate tectonics or continental drift. Decaying radioactive elements, such as uranium, thorium and potassium, provide heat to warm the aesthenosphere and make it plastic. The heat causes circulation in the ocean currents that pushes on the lithosphere plates and moves them apart. The speed of this movement is 2 to 10 centimeters a year. The ocean floor, trenches and mountains are the site of active plate movements.
Deep-sea trenches are long, narrow and parallel to continental mountain ranges. Pressure and seismic activity are present at the crest of ocean trenches. The trenches are seismically active but have lower heat concentrations than the ridges. Island arcs are along the trenches and on the continental side of the trench. Pacific Ocean islands and along the Pacific Rim are examples of island arcs.
Three groups classify boundaries: convergent, divergent and transform boundaries. A convergent boundary is one where two plates run into each other. An example is on the coast of South America where an oceanic plate is running into the continent. It is the site of earthquakes, mountains and volcanoes. Pressure forces rock into the magma where it melts and reemerges as a volcano. Pressure forms mountains, and earthquakes are common in these regions. Divergent boundaries are plates that are coming apart. Magma seeps into whatever cracks or fissures form. Transform boundaries are those where the plates slide past each other. The San Andreas Fault in California is an example of a transform boundary. These boundaries are capable of producing large earthquakes.