Slope processes
Slide
Mass movements happen wherever there are slopes and gravity. They are usually defined as movement of material “without a transporting medium”. On Venus this definition may need a small change: in the dense atmosphere, the hot air affects any movement.
Traces of landslides can be identified even on low-resolution radar images (Malin 1992). They appear on the slopes of steep volcanic cones and along the edges of long ridges or rift valleys. For scalloped domes, landslides that scallop their sides even control how the dome evolves. The processes are hard to identify exactly, because a mass movement may be a slump, a slide or a rock avalanche.
A mass movement leaves two main traces. One is an alcove-like hollow in the scarp, which scallops the slope. The other is the debris tongue, cone or apron, usually with a lobate margin.
Some landslide traces on the flanks of volcanic edifices may be linked to eruptions. They would then be traces of debris flows or pyroclastic surges carrying hot gases and molten rock. In the dense, hot atmosphere, however, practically every mass movement contains hot gas. So even a non-volcanic debris movement moves in a way similar to a debris flow. The ejecta blankets of craters are an example. They look as if they had been laid down as a liquid, although no (volatile) liquid takes part.
Long-runout landslides, which travel great distances, are known from Mars. On Venus, too, landslides whose debris moves far from its source are not rare. The debris does not pile up at the foot of the slope but farther away, up to 80 km (Bulmer 2012). On radar images the debris looks broken up into heaps. This suggests that the moving material was solid (Bulmer 2012).
An interesting case: between two passes of the Magellan probe in 1990, a landslide fell from a cliff. It is clearly visible on the “before” and “after” pair of images (Photojournal image PIA00248). An earthquake on a fault is thought to have triggered it. This suggests that tectonics is still active in that area today.
