Crustal plates are large sections of the outer layer of the Earth's surface that move along convection currents in the mantle. There are 10 major plates on the surface, and the tectonic forces behind their movement create earthquakes, volcanoes and tidal waves.
Convection currents move very slowly in the Earth's mantle and cause horizontal continental drift over tens of millions of years. The layer beneath the crust and upper mantle, called the aesthenosphere, is partially fluid because of heat and decay. This hot, thick liquid is what shifts the crustal plates.
Continental borders fall into one of three categories: convergent, divergent and transform. A convergent plate boundary is when two plates come close together. A divergent boundary is the opposite. A transform boundary occurs when two plates move side to side relative to each other.
Alfred Wegener first proposed the idea of continental drift and plate tectonics. Shortly after World War I, submarines mapped the Atlantic Ocean where scientists discovered rocks of differing ages on the ocean floor. The youngest rocks were near a plate boundary in the middle of the Atlantic, while the oldest stones were closer to shore. This discovery led to a theory that the younger ocean floor formed along a divergent plate boundary in the Earth's crust.