A rift valley is a long, narrow depression formed where tectonic plates move apart.
At a divergent boundary, the crust is stretched and broken by normal faults. Blocks of crust may drop down between the faults, creating a valley bordered by steep escarpments. Magma can rise through the thinning lithosphere, producing volcanic activity and new igneous rock.
The East African Rift is a major continental example. It includes several linked rift zones and shows how a continent can gradually split. If extension continues, the rift may develop into a new ocean basin, as occurred in the geological past when continents separated.
Rift valleys differ from ocean trenches. Trenches form mainly where one plate is forced beneath another at a convergent boundary, whereas rift valleys reflect crustal extension. They also differ from transform faults, where plates slide horizontally past each other.