Weathering is one of the forces on Earth that destroy rocks and landforms. Without weathering, geologic features would build up but would be less likely to break down. Weathering is the process that changes solid rock into sediments.
Sediments were described in the Rocks chapter. With weathering, rock is disintegrated. It breaks into pieces.
Once these sediments are separated from the rocks, erosion is the process that moves the sediments. The four forces of erosion are water, wind, glaciers, and gravity. Figure 1. A once smooth road surface has cracks and fractures, plus a large pothole. While plate tectonics forces work to build huge mountains and other landscapes, the forces of weathering gradually wear those rocks and landscapes away.
Together with erosion, tall mountains turn into hills and even plains. No human being can watch for millions of years as mountains are built, nor can anyone watch as those same mountains gradually are worn away. But imagine a new sidewalk or road. The new road is smooth and even. Over hundreds of years, it will completely disappear, but what happens over one year? What changes would you see figure 1?
What forces of weathering wear down that road, or rocks or mountains over time? Follow this link to view some animations of different types of weathering processes. Mechanical weathering also called physical weathering breaks rock into smaller pieces.
These smaller pieces are just like the bigger rock, just smaller. That means the rock has changed physically without changing its composition. The smaller pieces have the same minerals, in just the same proportions as the original rock.
There are many ways that rocks can be broken apart into smaller pieces. Ice wedging is the main form of mechanical weathering in any climate that regularly cycles above and below the freezing point figure 2. Ice wedging works quickly, breaking apart rocks in areas with temperatures that cycle above and below freezing in the day and night, and also that cycle above and below freezing with the seasons.
Ice wedging breaks apart so much rock that large piles of broken rock are seen at the base of a hillside, as rock fragments separate and tumble down. Abrasion is another form of mechanical weathering. In abrasion, one rock bumps against another rock.
Figure 3. Rocks on a beach are worn down by abrasion as passing waves cause them to strike each other. Abrasion makes rocks with sharp or jagged edges smooth and round. If you have ever collected beach glass or cobbles from a stream, you have witnessed the work of abrasion figure 3. Now that you know what mechanical weathering is, can you think of other ways it could happen?
Plants and animals can do the work of mechanical weathering figure 4. Burrowing animals can also break apart rock as they dig for food or to make living spaces for themselves. Figure 4. Mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases figure 5. With more surfaces exposed, there are more surfaces on which chemical weathering can occur. Figure 5. Mechanical weathering may increase the rate of chemical weathering.
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, shrub s, 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. It eventually formed the Grand Canyon, which is more than 1, meters 1 mile deep and as much as 29 kilometers 18 miles wide in some places. Eroded sediments have profoundly influenced the development of civilization s 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 delta s 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. The Yellow River in central China gets its name from the yellow loess blown into and suspended in its water. 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 surge s following hurricanes and typhoon s 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. Farmer s and engineer s must regularly practice erosion control.
Sometimes, engineers simply install structures to physically prevent soil from being transported. Gabion s 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 windbreak s and riparian buffer s to protect valuable agricultural land. Windbreaks, also called hedgerow s or shelterbelt s, 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 shoreline s 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.
Eroding Animals. Cave entrances can be on land or in water. Also called limestone and calcium carbonate. Dust Bowl. Also known as an ice age. Northern Hemisphere. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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Walter's Travels - Weathering and Erosion. Soil erosion is a widespread problem in rural and urban Queensland. If we want to save our soils, we need to understand the different types of erosion that can occur. Raindrops hit bare soil with enough force to break the soil aggregates. These fragments wash into soil pores and prevent water from infiltrating the soil. Water then accumulates on the surface and increases runoff which takes soil with it.
Well-structured soils are less prone to break up, and the impact of raindrops is minimised if the soil surface is protected by plant or litter cover. The vulnerability of soils to water erosion depends on:. Hill slopes are prone to sheet erosion and rill erosion. The amount of hill slope erosion largely depends on how the land is used. Sheet erosion occurs when a thin layer of topsoil is removed over a whole hillside paddock—and may not be readily noticed.
Rill erosion occurs when runoff water forms small channels as it concentrates down a slope. These rills can be up to 0. If they become any deeper than 0. Scalding can occur when wind and water erosion removes the top soil and exposes saline or sodic soils. Raindrop impact alone can result in large amounts of soil being moved. However water or wind moving over the surface will remove more soil, and contribute to sheet, rill and gully erosion.
Erosion also tends to remove the lighter, smaller soil particles first such as clay and silt , leaving fine and coarse sand behind. A combination of large amounts of fine sand and small amounts of clay at the surface means the soil tends to seal and set hard, which limits infiltration water entering the soil. Gully erosion happens when runoff concentrates and flows strongly enough to detach and move soil particles. For example, a waterfall may form, with runoff picking up energy as it plunges over the gully head.
Splashback at the base of the gully head erodes the subsoil and the gully eats its way up the slope.
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