The correct option is Slump.
Explanation
Mass movements involve the detachment and downslope transport of soil and rock material under the influence of gravity. These movements are classified based on the mechanism of movement (slide, flow, fall) and the nature of the material involved. A slump is a specific type of slide characterized by rotational movement along a curved failure surface.
Option Analysis
- Debris slide Debris slide: This involves the rapid sliding of rock debris or soil down a slope. However, debris slides typically occur on a planar (flat) surface, resulting in translational movement without backward rotation. Therefore, this option is incorrect.
- Rock fall Rock fall: This refers to the free fall of individual rock blocks from a steep cliff or slope. It involves vertical or near-vertical movement through the air, not sliding with rotation. Therefore, this option is incorrect.
- Slump Slump: A slump is a type of mass movement where a coherent mass of rock or soil moves a short distance down a slope. The movement occurs along a concave-upward or curved slip plane. As the mass slides, the upper surface tilts backward toward the slope, exhibiting backward rotation. Therefore, this option is correct.
- Debris avalanche Debris avalanche: This is a very rapid and turbulent flow of debris, often mixed with water or air. It behaves more like a fluid flow than a coherent slide and does not involve the specific backward rotation characteristic of a slump. Therefore, this option is incorrect.
Key Takeaway
Slump is distinguished from other mass movements by its movement along a curved slip plane, which causes the sliding block to undergo backward rotation relative to the slope.