Final answer:
A slump is the type of mass movement involving a downward slide along a curved surface.
Step-by-step explanation:
The type of mass movement that involves a downward sliding mass of rock or unconsolidated material moving along a curved surface is known as a slump. A slump occurs when material on a slope becomes too weak to support its own weight, which can be due to various factors such as erosion, earthquakes, volcanic activity, and over-saturation from water. During a slump, the material moves downward and outward along a concave-upward or curved surface, and the slippage manifests as a block of land that tilts backward, toward the slope. This contrasts with other types of mass wasting like creep, which is a very slow movement of soil down a slope, rockfall, which involves the free fall of rock from a steep cliff, and landslide, which refers to the rapid slide of a large mass of dirt and rock down a mountain or cliff.