Question

A rift valley is formed mainly due to _ _ _ _ _ _ _ _ _ _ _ _ _.
A. Forces of tension in the earth's crust
B. The subsidence of the floor of a river valley
C. The valley formed after the formation of fold mountains
D. The deepening of a valley by ice action

Answer 1

Hint:
- A rift valley is a linear lowland formed by the action of a geologic rift or fault between several highlands or mountain ranges.
- Rifts can form at any elevation on Earth, from the seafloor to plateaus and mountain ranges in continental or oceanic crust. They are often connected with several adjacent subsidiary or co-extensive valleys that are geologically considered to be part of the main rift valley.

Complete answer:
- A rift valley is a lowland area formed when the Earth's tectonic plates rift apart. Rift valleys can be found on land as well as at the ocean's bottom, where they are formed by seafloor spreading. Rift valleys are different from the river and glacial valleys in that they are formed by tectonic activity rather than erosion.
- The action of a geologic rift or fault produces a rift valley. A fault is a fissure in the earth's crust where the rock material on the fault plane's upper side has been pushed downward relative to the rock below the fault. To put it another way, it occurs when the earth's crust, or outermost layer, scatters or splits apart.
- Rift valleys are usually narrow and broad, with some stretching for hundreds of kilometres. Because of volcanic deposition and marine or lacustrine sedimentation, their floors are relatively smooth. Rift valleys have steeply descending sides in the form of steps and terraces. The valley walls can rise hundreds of meters at their edges.
- Thus from the above discussion we can conclude that on a divergent plate boundary, a crustal extension or spreading part of the surface forms a rift valley, which is then further deepened by erosion forces. When the tensional forces on the plate are high enough to break it apart, a centre block falls between the two flanking blocks, creating a graben.

Hence option ‘B’ is the correct answer.

Note: - The Great Rift Valley is the name given to a 6,000-kilometer (3,700-mile) long geological trench that stretches from northern Syria in Southwest Asia to central Mozambique in East Africa. A range of mountains and active volcanoes surround the rift. It is a fault and an earthquake hotspot.
- The Western Rift is bordered by some of Africa's highest mountains. It is home to the Rift Valley lakes, which have some of the world's deepest lakes (up to 1,470 meters deep at Lake Tanganyika). A large portion of this region is protected by national parks. Between the two branches is Lake Victoria, the world's second-largest freshwater lake by area.