Gravity causes erosion by pulling dirt, rocks and soil downward. For example, as water mixes with dirt to form soft mud, the mud cannot support itself against the force of gravity.
While many types of erosion have water, wind and glaciers listed as their causes, gravity is always the defining force that causes matter to move from higher elevations to lower elevations. Unsupported boulders rolling down a hill, a collapsing natural archway and a crumbling rock surface are all strongly controlled by gravity.
Because the pull of gravity is universal, large masses may collapse in a landslide. For example, if a section of soil is no longer adequately supported, it may slide down a hill; the section of soil above it, which was relying on the first section for support, may also slide with it. Any large object on top of that hill is likely to slide with it, causing damage and further erosion.
Other types of erosion occur because some things are heavier than others. As the ocean wears away at a beach, the sand, which is heavier than water, settles at the ocean floor. Any heavy and insoluble particles in that sand, such as gravel caused by the action of water on cliff faces, never get picked up by currents.