Mass wasting is the downslope movement of Earth materials (rock, soil, debris) under the influence of gravity. Landslildes, rockfalls, slump, and creep are all examples of mass wasting. Today in Indonesia's island of Java, there were a series of landslides triggered by lots of rain. Rain often acts as a catalyst for mass wasting, for several reasons. First, rain is heavy. Once soil gets waterlogged, it just plain weighs more.
Heavier masses are more likely to slide than petite ones. Second, water expands soils, pushing outwards from pore spaces. This expansion factor can cause slopes like the hillsides in this AP photograph to increase their gradient every so slightly -- sometimes beyond a critical angle called the "angle of repose". When a slope is below the angle of repose, it stays put ("reposes"). Above the angle of repose, it slides. Lastly, and possibly most importantly, water acts as a lubricant, reducing frictional inertia and allowing soil particles to slide past one another. I call this the "Slip N Slide" effect -- consider the difference between going down a waterslide with water and one that's dry. The water "greases the skids" and facilitates movement. Indonesia is particularly susceptible to landslides because of volanically-steepened slopes and heavy tropical rains. Sometimes, its landslides are triggered by seismic shaking.
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