What Is Weathering And Mass Wasting

[Manric Hock]

Masswasting is the movement of rock and soil down slope under the influence of gravity. Rock falls, slumps, and debris flows are all examples of mass wasting. Often lubricated by rainfall or agitated by seismic activity, these events may occur very rapidly and move as a flow. Landslide triggers may include:

A Debris Flow is a type of landslide made up of a mixture of water-saturated rock debris and soil with a consistency similar to wet cement. Debris flows move rapidly downslope under the influence of gravity. Sometimes referred to as earth flows or mud flows.

Erosion is the geological process in which earthen materials are worn away and transported by natural forces such as wind or water. A similar process, weathering, breaks down or dissolves rock, but does not involve movement. Erosion is the opposite of deposition, the geological process in which earthen materials are deposited, or built up, on a landform. Most erosion is performed by liquid water, wind, or ice (usually in the form of a glacier). If the wind is dusty, or water or glacial ice is muddy, erosion is taking place. The brown color indicates that bits of rock and soil are suspended in the fluid (air or water) and being transported from one place to another. This transported material is called sediment.

Physical erosion describes the process of rocks changing their physical properties without changing their basic chemical composition. Physical erosion often causes rocks to get smaller or smoother. Rocks eroded through physical erosion often form clastic sediments. Clastic sediments are composed of fragments of older rocks that have been transported from their place of origin. Landslides and other forms of mass wasting are associated with physical weathering. These processes cause rocks to dislodge from hillsides and crumble as they tumble down a slope.

Plant growth can also contribute to physical erosion in a process called bioerosion. Plants break up earthen materials as they take root, and can create cracks and crevices in rocks they encounter. Ice and liquid water can also contribute to physical erosion as their movement forces rocks to crash together or crack apart. Some rocks shatter and crumble, while others are worn away. River rocks are often much smoother than rocks found elsewhere, for instance, because they have been eroded by constant contact with other river rocks.

Liquid water is the major agent of erosion on Earth. Rain, rivers, floods, lakes, and the ocean carry away bits of soil and sand and slowly wash away the sediment. Rainfall produces four types of soil erosion: splash erosion, sheet erosion, rill erosion, and gully erosion.

Gullies carry water for brief periods of time during rainfall or snowmelt but appear as small valleys or crevasses during dry seasons. Valley erosion is the process in which rushing streams and rivers wear away their banks, creating larger and larger valleys.

The Cape Hatteras Lighthouse was built on the Outer Banks, a series of barrier islands off the coast of the U.S. state of North Carolina, in 1870. At the time, the lighthouse was nearly 457 meters (1,500 feet) from the ocean. Over time, the ocean eroded most of the beach near the lighthouse. By 1970, the pounding surf was just 37 meters (120 feet) away and endangered the structure. Many people thought the lighthouse would collapse during a strong storm. Instead, thanks to a significant engineering feat completed in 1999, it was moved 880 meters (2,900 feet) inland.

The battering force of ocean waves also erodes seaside cliffs. The action of erosion can create an array of coastal landscape features. For example, erosion can bore holes that form caves. When water breaks through the back of the cave, it can create an arch. The continual pounding of waves can cause the top of the arch to fall, leaving nothing but rock columns called sea stacks. The seven remaining sea stacks of Twelve Apostles Marine National Park, in Victoria, Australia, are among the most dramatic and well-known of these features of coastal erosion.

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.

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.

Eroded sediments have profoundly influenced the development of civilizations around the world. Agricultural development is often reliant on the nutrient-rich soils created by the accumulation of eroded earth. When the velocity of wind or water slows, eroded sediment is deposited in a new location. The sediment builds up in a process called sedimentation and creates fertile land. River deltas are made almost entirely of sediment that has eroded from the banks and bed of a river.

The rich delta soils of the San Joaquin and Sacramento rivers in northern California, for example, have created one of the most agriculturally productive areas in the world. Loess is an agriculturally rich sediment made almost entirely of wind-blown, eroded sediment.

Erosion is a natural process, but human activity can make it happen more quickly. Human activity altering the vegetation of an area is perhaps the biggest human factor contributing to erosion. Trees and plants hold soil in place. When people cut down forests or plow up grasses for agriculture and development, the soil is more vulnerable to washing or blowing away. Landslides become more common. Water rushes over exposed soil rather than soaking into it, causing flooding.

Global warming, the current period of climate change, is speeding erosion. The change in climate has been linked to more frequent and severe storms. Storm surges following hurricanes and typhoons can erode kilometers of coastline and coastal habitat. These coastal areas are home to residences, businesses, and economically important industries, such as fisheries.

The rise in temperature is also quickly melting glaciers. The slower, more massive form of glacial erosion is being supplanted by the cumulative impact of rill, gully, and valley erosion. In areas downstream from glacial snouts, rapidly melting glaciers are contributing to sea level rise. The rising sea erodes beaches more quickly.

Erosion control is the process of reducing erosion by wind and water. Farmers and engineers must regularly practice erosion control. Sometimes, engineers simply install structures to physically prevent soil from being transported. Gabions are huge wireframes that hold boulders in place, for instance. Gabions are often placed near cliffs. These cliffs, often near the coast, have homes, businesses, and highways near them. When erosion by water or wind threatens to tumble the boulders toward buildings and cars, gabions protect landowners and drivers by holding the rocks in place.

Erosion control also includes physically changing the landscape. Communities often invest in windbreaks and riparian buffers to protect valuable agricultural land. Windbreaks, also called hedgerows or shelterbelts, are lines of trees and shrubs planted to protect cropland from wind erosion. Riparian buffers describe plants such as trees, shrubs, grasses, and sedges that line the banks of a river. Riparian buffers help contain the river in times of increased stream flow and flooding.

Living shorelines are another form of erosion control in wetland areas. Living shorelines are constructed by placing native plants, stone, sand, and even living organisms such as oysters along wetland coasts. These plants help anchor the soil to the area, preventing erosion. By securing the land, living shorelines establish a natural habitat. They protect coastlines from powerful storm surges as well as erosion.

Far-Flung Dust

Wind is a powerful force. It can carry huge amounts of dust over long distances. In the winter and spring of 2004, winds eroded 45 million tons of dust from a spot called the Bodele Depression in the desert of northern Chad all the way across the Atlantic Ocean to Brazil.

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