Erosion | Science for ACT PDF Download

Introduction

• Erosion is the geological phenomenon wherein earth materials are gradually worn away and transported by natural forces like wind or water. It stands in contrast to weathering, which breaks down or dissolves rock but does not involve movement. Deposition, on the other hand, is the geological process where earth materials are deposited or accumulated on a landform. 
• Liquid water, wind, and ice, typically in the form of glaciers, are the primary agents of erosion. Dusty winds, muddy water, or glacial ice indicate erosion is occurring, with the brown color suggesting suspended bits of rock and soil being transported in the fluid (air or water) from one location to another, forming sediment.

Physical Erosion

• Physical erosion involves alterations in the physical properties of rocks without changing their fundamental chemical composition. This process often leads to rocks becoming smaller or smoother. Rocks eroded through physical means often give rise to clastic sediments, comprised of fragments from older rocks transported from their original location. Events like landslides and mass wasting are associated with physical weathering, dislodging rocks from hillsides and causing them to crumble as they descend slopes.
• Plant growth can also contribute to physical erosion through bioerosion. As plants take root, they break up earth materials and create cracks and crevices in encountered rocks. Movement of ice and liquid water can further exacerbate physical erosion, as their motion compels rocks to collide or fracture. Some rocks shatter and disintegrate, while others wear away. River rocks, for instance, are often smoother than those found elsewhere, as they undergo continuous erosion from contact with other river rocks.

Erosion by Water

• Water erosion is the primary force reshaping the Earth's surface, facilitated by various water bodies such as rain, rivers, floods, lakes, and oceans. These water sources gradually carry away sediment, contributing to soil and sand erosion. Rainfall alone triggers four distinct types of soil erosion: splash erosion, sheet erosion, rill erosion, and gully erosion.
• Splash erosion results from the impact of raindrops, which can disperse tiny soil particles over short distances, up to about 0.6 meters (two feet). Sheet erosion occurs as runoff carries away soil from the surface. Rill erosion takes shape as runoff develops into discrete streams, forming small channels known as rills. Gully erosion marks the stage where soil particles are transported through larger channels, forming gullies. These gullies intermittently carry water during rainfall or snowmelt, appearing as small valleys or crevasses during dry seasons.
• Valley erosion, exemplified by the Fish River Canyon in southern Namibia, illustrates the significant impact of water erosion over millions of years. The Fish River gradually eroded hard gneiss bedrock, sculpting a canyon approximately 160 kilometers (99 miles) long, 27 kilometers (17 miles) wide, and 550 meters (1,084 feet) deep.
• Coastal erosion, driven by the relentless force of the ocean, reshapes coastlines worldwide. Waves relentlessly pound rocks into pebbles and pebbles into sand, gradually altering coastal features. Coastal erosion not only impacts human settlements but also affects coastal ecosystems profoundly. The Cape Hatteras Lighthouse, situated on North Carolina's Outer Banks, faced imminent danger due to coastal erosion. By 1970, the lighthouse stood just 37 meters (120 feet) from the ocean, prompting concerns about its collapse during storms. However, an engineering feat in 1999 successfully relocated the lighthouse 880 meters (2,900 feet) inland, mitigating the threat.
• Seaside cliffs are also susceptible to erosion by ocean waves, leading to the formation of various coastal features. Erosion can carve caves, which may eventually lead to the creation of arches. Continued wave action can cause these arches to collapse, leaving behind sea stacks. The Twelve Apostles Marine National Park in Victoria, Australia, boasts seven remaining sea stacks, exemplifying the dramatic effects of coastal erosion.

Erosion by Wind

• Wind is a powerful agent of erosion. Aeolian (wind-driven) processes constantly transport dust, sand, and ash from one place to another. Wind can sometimes blow sand into towering dunes. Some sand dunes in the Badain Jaran section of the Gobi Desert in China, for example, reach more than 400 meters (1,300 feet) high. In dry areas, windblown sand can blast against a rock with tremendous force, slowly wearing away the soft rock. It polishes rocks and cliffs until they are smooth—giving the stone a so-called “desert varnish.” Wind is responsible for the eroded features that give Arches National Park, in the U.S. state of Utah, its name.
• Wind can also erode material until little remains at all. Ventifacts are rocks that have been sculpted by wind erosion. The enormous chalk formations in the White Desert of Egypt are ventifacts carved by thousands of years of wind roaring through the flat landscape. Some of the most destructive examples of wind erosion are the dust storms that characterized the “Dust Bowl” of the 1930s in North America. Made brittle by years of drought and agricultural mismanagement, millions of tons of valuable topsoil were eroded away by strong winds in what came to be known as “black blizzards.” These dust storms devastated local economies, forcing thousands of people who depended on agriculture for their livelihoods to migrate.

Erosion by Ice

• Ice, usually in the form of glaciers, can erode the earth and create dramatic landforms. In frigid areas and on some mountaintops, glaciers move slowly downhill and across the land. As they move, they transport everything in their path, from tiny grains of sand to huge boulders. Rocks carried by glaciers scrape against the ground below, eroding both the ground and the rocks. In this way, glaciers grind up rocks and scrape away the soil. Moving glaciers gouge out basins and form steep-sided mountain valleys. Eroded sediment called moraine is often visible on and around glaciers.
• Several times in Earth’s history, vast glaciers covered parts of the Northern Hemisphere. These glacial periods are known as ice ages. Ice Age glaciers carved much of the modern northern North American and European landscape. Ice Age glaciers scoured the ground to form what are now the Finger Lakes in the U.S. state of New York, for example. They carved fjords, deep inlets along the coast of Scandinavia. The snout of a glacier eroded Cape Cod Bay, Massachusetts, U.S., and formed the recognizable fishhook shape of Cape Cod itself.

Other Forces of Erosion

• Thermal erosion describes the erosion of permafrost along a river or coastline. Warm temperatures can cause ice-rich permafrost to break off coastlines in huge chunks, often carrying valuable topsoil and vegetation with them. These eroded “floating islands” can disintegrate into the ocean, or even crash into another piece of land—helping spread new life to different landscapes. 
• Mass wasting describes the downward movement of rocks, soil, and vegetation. Mass wasting incidents include landslides, rockslides, and avalanches. Mass wasting can erode and transport millions of tons of earth, reshaping hills and mountains and, often, devastating communities in its path.

Factors Impacting Erosion

• Some of the natural factors impacting erosion in a landscape include climate, topography, vegetation, and tectonic activity. Climate is perhaps the most influential force impacting the effect of erosion on a landscape. Climate includes precipitation and wind. Climate also includes seasonal variability, which influences the likelihood of weathered sediments being transported during a weather event such as a snowmelt, breeze, or hurricane. Topography, the shape of surface features of an area, can contribute to how erosion impacts that area. The earthen floodplains of river valleys are much more prone to erosion than rocky flood channels, which may take centuries to erode. Soft rock like chalk will erode more quickly than hard rocks like granite.
• Vegetation can slow the impact of erosion. Plant roots adhere to soil and rock particles, preventing their transport during rainfall or wind events. Trees, shrubs, and other plants can even limit the impact of mass wasting events such as landslides and other natural hazards such as hurricanes. Deserts, which generally lack thick vegetation, are often the most eroded landscapes on the planet.
• Finally, tectonic activity shapes the landscape itself, and thus influences the way erosion impacts an area. Tectonic uplift, for example, causes one part of the landscape to rise higher than others. In a span of about 5 million years, tectonic uplift caused the Colorado River to cut deeper and deeper into the Colorado Plateau, land in what is now the U.S. state of Arizona. It eventually formed the Grand Canyon, which is more than 1,600 meters (one mile) deep and as much as 29 kilometers (18 miles) wide in some places.

Erosion and Human Impact

Deposition, Soil Formation, and Sedimentary Processes

• The impact of eroded sediments on the growth of civilizations globally has been profound. Agricultural progress often hinges on the nutrient-rich soils formed by the accumulation of eroded earth. As the speed of wind or water diminishes, eroded sediment settles in new areas, undergoing sedimentation and forming fertile land. River deltas, primarily composed of eroded sediment from riverbanks and beds, exemplify this process.
• For instance, the fertile deltaic soils of the San Joaquin and Sacramento rivers in northern California have fostered one of the most agriculturally productive regions worldwide. Loess, a fertile sediment predominantly composed of wind-blown eroded material, is another example.
• The Yellow River in central China derives its name from the yellow loess carried into its waters. The fertile lands surrounding the Yellow River have been among China’s most productive for millennia.

Human Intervention in Erosion Management

• While erosion is a natural occurrence, human activities can accelerate its pace. One of the significant human-induced factors contributing to erosion is the alteration of vegetation. Trees and plants play a crucial role in stabilizing soil. Deforestation or the removal of grasslands for agricultural and developmental purposes exposes soil to increased risks of erosion through water and wind. This alteration exacerbates phenomena like landslides and flooding.
• The ongoing global warming, a consequence of climate change, is intensifying erosion processes. Climate change has been associated with more frequent and severe storms, leading to extensive erosion during storm surges following hurricanes and typhoons, particularly along coastlines. These coastal regions host residential areas, commercial establishments, and vital industries such as fisheries.
• Moreover, the rising temperatures are accelerating the melting of glaciers, shifting erosion patterns from slower, massive glacial erosion to faster forms such as rill, gully, and valley erosion. Downstream regions from glaciers witness accelerated erosion contributing to sea-level rise and hastened beach erosion.

Erosion Control Measures

• Erosion control involves efforts to mitigate erosion by wind and water. Farmers and engineers regularly employ erosion control techniques. Some involve the installation of structures to physically impede soil transport, like gabions—large wireframes filled with boulders placed near cliffs prone to erosion, safeguarding nearby properties and infrastructure.
• Additionally, landscape modifications play a crucial role in erosion control. Communities often invest in windbreaks and riparian buffers to safeguard valuable agricultural land. Windbreaks, consisting of trees and shrubs planted to shield cropland from wind erosion, and riparian buffers, composed of vegetation lining riverbanks, help contain rivers during periods of increased flow and flooding.
• Living shorelines, constructed in wetland areas, represent another erosion control method. They involve the placement of native plants, stones, sand, and living organisms like oysters along coastal wetlands. These elements stabilize soil, protect against storm surges, and foster natural habitats, thus mitigating erosion effectively.