Plate Tectonics
A divergent-plate boundary refers to two plates moving apart. This results in seafloor spreading or in continents breaking away from one another. A convergent-plate boundary occurs when two plates collide. This can cause the formation of mountains and volcanoes. A transform-plate boundary occurs when two plates slide horizontally, resulting in earthquakes.
Two theories exist regarding the movement of continents. Scientists believe the movement of warmer and cooler rock causes the shifting. One theory suggests that hot rock from deep inside the Earth rises to the base of the lithosphere, causing the cool rock on top to sink. Another theory states that plates move as a result of a large, vertical column of hot, flowing rock that moves to the surface and then spreads outward, causing the plates to slide downhill.
These shifting plates created the Earth's mountain ranges, such as the Mid-Atlantic Ridge.