The surface of the Earth is not a stationary solid surface. This outer layer of the Earth is the lithosphere. It is actually broken up into pieces like a giant jigsaw puzzle. These broken pieces are called plates. The Earth's surface is actually broken into about a dozen large plates and several smaller plates. These plates are constantly on the move. This is called plate tectonics or continental drift.
Plate tectonics are the mechanics behind the movement of the plates. According to the US Geological Survey, "The fastest plate races along at 15 centimeters (6 inches) per year while the slowest plates crawl at less than 2.5 centimeters (1 inch) per year." To humans this is incredibly slow, but in geological time it is movement on a massive scale. In one million years the fastest plates will have moved 6 million inches or 500,000 feet. This means that this plate could move a little over 94 miles. This would move Los Angeles north to where Santa Barbara is located. So in about 30 million years Hawaii will be part of Alaska.
The movement of the plates is a simple concept. Under the crust of the Earth is the mantle. It is a hot area with semi-solid liquid. The magma closest to the Earth's core is super hot and rises on currents of heat towards the upper levels of the mantle. As it rises, the liquid cools and becomes more solid. It also becomes heavier. These semi-solids sink and reheat on the way down towards the core. Imagine giant circles of heat driven movement. This may go on below the surface, but it affects the surface. The tectonic plates of the Earth are riding on these convection currents. These convection currents shift the plates.
With this massive movement the plates move in different ways. The surface cycle consists of plates moving, their edges being pulled under to join the material in the mantle, and the mantle welling up on the other side of the plate as a form of regeneration. The boundaries of the plates grind against each other and cause friction. This friction is released as energy in the form of earthquakes. When a divergent plate boundary is formed, the plates move away from each other. In these areas, the boundary area pushes the continents away from each other as magma moves to the surface. When a convergent plate boundary is formed, one plate rises over the top of another. One plate is pushed under the other plate to become part of the mantle. When a transformed plate boundary is formed, the plates slide against each other in opposite directions. These boundary types are fault lines like the San Andreas Fault.