In geology, tension is a form of tectonic stress that either lengthens rocks or breaks them apart. Magma from beneath Earth's crust pushes up against the weaker plate boundaries, pushing them apart. Tension stress is usually found at divergent plate boundaries on the ocean floor or under continental land masses.
When tension pushes against plate boundaries in the ocean, it creates a mid-ocean ridge. The magma, which is hot, liquid rock, pushes hard enough to create a fissure. The magma flows into the fissure and hardens. The process repeats, each time pushing the boundaries apart a little further. The Mid-Atlantic Ridge is one example in which the underwater tension sometimes results in volcanic activity and earthquakes.
The result of geologic tension isn't always apparent when it occurs under a continental land mass. The plates still pull apart, but instead of an obvious fissure, faults develop. As the earth moves and fractures, earthquakes are triggered along the rift. The San Andreas is one of the most famous faults formed in this manner. It sits on the eastern edge of the Pacific plate and is seismically active.
Long term geologic tension can result in the creation of a lake, or if the land drops below sea level, a shallow sea basin. The lakes in the East Africa River Valley were formed using this method, as was the Red Sea.