Erosion
Erosion is the removal and transport of weathered rock and soil from one place to another by water, wind, ice, or gravity.
Definition
Erosion is when water, wind, ice, or gravity picks up loose rock and soil and carries it away; it is how hills get lower and valleys get deeper over time. The agents include running water, wind, glaciers, waves, and mass wasting (landslides and creep), with running water dominant on most of Earth's surface, and the rate depends on slope, rainfall, vegetation cover, and how loose the material is. Physically, erosion occurs when the shear stress exerted by a moving fluid exceeds the critical threshold needed to entrain a particle, which scales with grain size and density. Streams carry sediment as bed load, suspended load, and dissolved load, and stream power rises steeply with discharge, so rare floods do most of the geomorphic work. Natural erosion rates on continents average a few hundredths of a millimeter per year, but agricultural land commonly erodes ten to a hundred times faster.
Example
After a heavy rain, muddy water running down a bare hillside is erosion in action, and the mud is soil being carried off. Waves and storms erode the Outer Banks, moving whole islands westward over decades; the Cape Hatteras Lighthouse had to be relocated about $880$ meters inland in 1999 as the shoreline retreated, and the Appalachians have been eroding for over $200$ million years. Glacial erosion during the Pleistocene carved U-shaped valleys and the Great Lakes basins, and the Yosemite Valley is a textbook glacially eroded trough. Headward erosion of streams eventually captures neighboring drainages, rearranging river networks.
Key Insight
Erosion is the moving truck: weathering breaks the rock into pieces, and erosion hauls the pieces away. Plants are the best erosion control, because roots hold soil and leaves break the force of raindrops, which is why bare construction sites lose soil so fast. Erosion and uplift compete. Mountain ranges reach a steady state where tectonic uplift is balanced by erosional lowering, so the height of a range depends on climate and rock strength, not just on how hard the plates push.