Erosion

• 1. Introduction

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  Erosion can move a particle of soil just a few millimetres, or it can carry it for thousands of kilometres before setting it down again. This is the engine that quietly rewrites the surface of the planet, taking soil, rock, and dissolved material from one place on the Earth's crust and depositing it somewhere else. It works through water flow, through wind, through ice, and through the simple downhill pull of gravity. Left to nature, it grinds along at its own steady pace. But human activity has sped it up. Globally, people have increased the rate of soil erosion by 10 to 40 times. At farming sites in the Appalachian Mountains, intensive practices have pushed it to as much as 100 times the natural rate. So how does a falling raindrop begin to dismantle a hillside? Why do glaciers cap the height of the world's tallest mountains? And how can the loss of rock in one place lift the crust beneath another? The answers run from the impact crater of a single drop of rain to the channeled scablands of eastern Washington.

• 2. How Water Carves

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  A single raindrop, when it strikes bare soil, punches a small crater and flings particles outward. Those ejected grains can travel as far as 0.6 metres straight up and 1.5 metres sideways on level ground. This is splash erosion, the first and gentlest stage of a four-part sequence. When soil becomes saturated, or rain falls faster than it can soak in, surface runoff begins to move loosened grains down the slope. That overland flow is called sheet erosion. As the water concentrates, it cuts small, short-lived channels a few centimetres deep, known as rills, which both supply sediment and deliver it down the hillside. The most severe stage is gully erosion, where runoff gathers in narrow channels and strips soil to a considerable depth during heavy rain or melting snow. A gully is set apart from a rill by a critical cross-sectional area of at least one square foot, large enough that ordinary tillage can no longer erase it. Pushed to its extreme on easily eroded bedrock where vegetation cannot take hold, gully erosion can build the stark terrain of badlands.

• 3. Rivers And The Sea

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  Valley erosion begins as a downward cut, deepening a channel into a sharp V-shaped cross-section while the stream gradient stays steep. Once a stream reaches some base level, its energy turns sideways. It widens the valley floor, builds a narrow floodplain, and meanders across nearly flat ground while laying down sediment. By far the most erosion happens during floods, when more water moving faster can carry a heavier load. The water rarely acts alone. Suspended particles, pebbles, and boulders scrape the surface as they travel, a process called traction. Where moving water meets frozen ground, thermal erosion takes over. Along the Lena River in Siberia, banks made of permafrost-cemented non-cohesive material fail in large slumps, driving rapid channel migration. The same melting attacks the Arctic coast. Along a 100 kilometre segment of the Beaufort Sea shoreline, erosion averaged 5.6 metres per year between 1955 and 2002. On open coasts, waves do the work through several mechanisms. Hydraulic action compresses air trapped in a joint until the rock cracks. Abrasion, which hurls sea load against a cliff, is the most effective and rapid form of shoreline erosion. On carbonate coastlines, living organisms bore, scrape, and grind the rock in a process called bioerosion.

• 4. Ice As Sculptor

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  Debris frozen into the base of a glacier scrapes along the bedrock like sandpaper on wood, polishing and gouging the rock beneath. This abrasion, along with plucking and ice thrusting, makes glaciers among the most powerful shapers of the land. Over roughly 100,000 years, a glacier grinding through homogeneous bedrock settles into the U-shaped parabolic cross-section seen in glaciated valleys today. In ice thrusting, a glacier freezes to its bed, then drags large sheets of frozen sediment forward as it surges, a method that gouged out many of the thousands of lake basins dotting the edge of the Canadian Shield. Glaciers also set a ceiling on how tall mountains can grow. Global analysis of topography shows that the relief between mountain peaks and the snow line is generally confined to less than 1500 metres, an effect so reliable it earned the name glacial buzzsaw. As mountains rise higher, they invite more glacial activity, which strips mass faster than isostatic rebound can rebuild it, a clean example of a negative feedback loop. Yet ice does not only cut. In some places it acts as a glacial armor. Sampling eight summits of northwestern Svalbard using Be10 and Al26, scientists found the region shifted from a glacier-erosion state under mild glacial maxima to a glacier-armor state under colder ones, where cold-based protective ice preserved even steep alpine land as the Quaternary ice age deepened.

• 5. Wind And Sudden Floods

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  In the Great Plains, soil loss from wind in a drought year can run as much as 6100 times greater than in a wet one. Wind erosion dominates arid and semi-arid regions, and it comes in two forms. Deflation lifts and carries away loose particles, while abrasion wears down surfaces struck by airborne grains. Deflation itself splits three ways: surface creep, where heavy particles roll along the ground; saltation, where grains bounce across the surface; and suspension, where the smallest particles ride the wind for long distances. Saltation does most of the work, accounting for 50 to 70 percent of wind erosion, followed by suspension at 30 to 40 percent and surface creep at 5 to 25 percent. Water can also strike with sudden violence. At extremely high flows, vortices called kolks form from large volumes of rushing water. They pluck bedrock and carve pothole-type features known as rock-cut basins. The floods released by glacial Lake Missoula produced exactly this, creating the channeled scablands across the Columbia Basin region of eastern Washington.

• 6. Gravity And The Deep

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  Mass wasting is the downward and outward movement of rock and sediment on a slope, driven mainly by gravity. It is often the first stage in breaking down weathered material in mountainous areas, carrying it to lower ground where streams and glaciers can take over. Some of these movements creep along imperceptibly. Others arrive suddenly and with disastrous results. Rapid rockfall leaves behind a scree slope, a pile of loose debris gathered at the base of a cliff. Slumping strikes steep hillsides along distinct fracture zones, often in clay that can move quickly once released, leaving a spoon-shaped depression where the ground has begun to slide. Far below the waterline, the same force operates on the continental slope. Turbidity currents, bodies of sediment-laden water, race downslope and carve channels and submarine canyons into substrates ranging from loose sediment to hard crystalline bedrock. These currents leave deposits called turbidites, which form some of the thickest and largest sedimentary sequences on Earth.

• 7. When The Land Pushes Back

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  Erosion does not only respond to the shape of the land. It can reshape the forces beneath it. When erosion strips large amounts of rock from a region and deposits it elsewhere, it lightens the load on the lower crust and mantle. That unloading can drive tectonic or isostatic uplift in return. In rare cases these twin feedbacks concentrate, localizing extremely rapid exhumation of deep crustal rock. Beneath the steep terrain of Nanga Parbat in the western Himalayas, this effect has been called a tectonic aneurysm. Tectonics works the other way too, raising fresh unweathered rock toward the surface and steepening slopes so that erosion accelerates. The pace of this contest is almost unimaginably slow. Scholars Pitman and Golovchenko estimate it takes probably more than 450 million years to wear a mountain mass like the Himalaya down to an almost-flat peneplain, assuming no major sea-level changes. The Timanides of Northern Russia show the aftermath. Their erosion fed sediments now found in the East European Platform, including the Cambrian Sablya Formation near Lake Ladoga, and the studies of those deposits suggest the wearing-down began in the Cambrian and intensified in the Ordovician.

• 8. The Human Acceleration

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  Water and wind erosion together cause about 84 percent of the global extent of degraded land, making excessive erosion one of the most significant environmental problems worldwide. The damage falls into two categories. On-site, the loss of nutrient-rich upper soil layers cuts agricultural productivity and, on natural landscapes, can bring ecological collapse, sometimes ending in desertification. Off-site, eroded material clogs waterways, triggers eutrophication of water bodies, and damages roads and houses. Intensive agriculture, deforestation, roads, climate change, and urban sprawl rank among the human activities that stimulate erosion most. The projections sharpen the warning. Under climate change, erosivity is expected to rise significantly in Europe, where soil erosion may increase by 13 to 22.5 percent by 2050. In Taiwan, rising typhoon frequency in the 21st century has been tied to heavier sediment loads in rivers and reservoirs. Yet the same source notes that many prevention and remediation practices can curtail erosion of vulnerable soils. In the United States, farmers working highly erodible land must comply with a conservation plan to remain eligible for agricultural assistance, a small bureaucratic line standing against a force that levels mountains.