Multiple choice general knowledge A rift valley is formed mainly due to .... The forces of tension in the earth crust The subsidence of the floor of a river valley The valley formed after the formation of fold mountains The deepening of a valley by ice action Reveal answer Fill a bubble to check yourself A Correct answer Explanation Rift valleys form when tectonic plates move apart, creating tension forces that stretch and thin the Earth's crust until it drops between parallel faults. This is different from valleys formed by erosion (river or ice action) or folding processes.