The great rift valley was formed by violent subterranean forces that tore apart the earth's crust. These forces caused huge chunks of the crust to sink between parallel fault lines and force up molten rock in volcanic eruptions.
Correct Answer is A. volcanic activity and tectonic movement
Explanation:
The Great Rift Valley located in central and eastern Africa is a big Geological fracture, Its formation involved the Separation of tectonic plates in a process called Divergence, which also lead to volcanic activity.
In the Divergence process, the tectonic plates are expanded by the Earth's crust, Contrary to when two tectonic plates collide and form mountains, in a divergence process the plates are being separated, creating a big ditch or trench. When the rocks sink they form a "Graben" and volcanic cones from molten lava ascending because of the tectonic movement and pressure.
A crystal face tends to be dull and rugged where a cleavage plane (most commonly seen in mica) is smooth and shiny, only being dull towards the edges it splits.
