Humans now drive soil erosion 10 to 40 times faster than nature
Erosion is natural: water, wind, ice and gravity have always carried soil and rock from one place to another. But farming, deforestation, roads and sprawl have multiplied the global rate of soil loss many times over, and some intensively farmed Appalachian fields wear away a hundred times faster than normal.
Erosion differs from weathering because material actually moves: soil, rock or dissolved matter is removed from one spot and deposited somewhere else. The agents are many, from rain, rivers and waves to glaciers, floods, wind, groundwater and landslides. Human activity has raised the global pace of soil erosion by 10 to 40 times, though many prevention and repair methods can protect vulnerable ground.
Rain works in a sequence of four stages. It begins with splash erosion, when a raindrop blasts a tiny crater and flings particles as far as 1.5 metres sideways on flat ground. If soil is saturated or rain falls faster than it can soak in, runoff carries loosened grains away as sheet erosion. Flow then concentrates into rills only a few centimetres deep, and finally into gullies, channels large enough to strip soil to considerable depth. Extreme gullying on soft rock where vegetation cannot take hold can produce badlands.
Rivers cut down and back into hillsides, and bends behave unevenly: faster water on the tight outer side eats the bank while sediment settles on the gentler inner side. Rapid erosion by a large river can unload the crust enough for it to rebound upward. In Siberia, the Lena River migrates quickly because moving water thaws the permafrost that holds its banks together.
At the coast, waves compress air in rock joints until they crack, batter cliffs directly and hurl sand and stones at them, the last being especially effective. Longshore drift carries sediment along the shore, eroding where supply runs short and building banks where it piles up, and a later fall in sea level can leave a raised beach. Erosion can be chemical too, as dissolved matter; karst sinkholes are an extreme case, and Anders Rapp pioneered its study in 1960. Glaciers grind their beds by abrasion, plucking and ice thrusting.
Source: Erosion
