Weathering and Erosion

The weathering process breaks down the rocks into soluble materials and solid particles that form soils. These weathered products are then swept away by the various erosional agents, such as rivers and glaciers, that shape the planet’s rocky surface.

Destructive Forces

Although the landscape appears to rarely change, constructive and destructive forces are at work on Earth, building the crust up, breaking the rocks down, and carrying the resulting debris away. The destructive forces are known as weathering and erosion. Weathering refers to the mechanical disintegration and chemical decomposition of rocks and minerals at or near the ground surface. No movement of these materials is implied. Exposure to weather causes rocks to change their character and either crumble into soil or become transformed into even smaller particles readily available for removal. Erosion refers to the processes by which particles already loosened by weathering are removed by the action of moving air or flowing water. This process involves two steps. First, the loose materials must be picked up, or entrained. Second, the materials must be physically carried, or transported to new locations. The major ways Earth materials are eroded are through rivers, underground water, moving ice, waves, wind, and landslides.

Weathering is a near-surface phenomenon because it involves the response of Earth materials to the elements of sunlight, rain, snow, and the like. It does not affect rocks that are buried within the crust. Only after these rocks are exposed at the surface, after a long period of uplift and removal of overlying material, does weather begin to affect them. In the changed environment, they are subject to the comparatively hostile actions of acid rain, subfreezing temperatures, and high humidity. The resulting transformations that take place in the rock are what is termed weathering.

Mechanical and Chemical Weathering

Scientists recognize two types of weathering—mechanical and chemical. Although the two types are generally discussed separately, it is important to remember that they generally work hand in hand. Mechanical weathering (also known as disintegration) involves the physical breakdown of the rock into smaller and smaller grains, usually because of the application of some kind of pressure, such as the expansion of water during freezing or the growth of plant roots in rock crevices. The chemical composition of the rock, however, remains unchanged. Mechanical weathering results in smaller pieces of rock that are identical in composition and appearance to the original larger rock mass.

Chemical weathering (also known as decomposition) involves a complex alteration in the materials that compose the original rock. These materials are chemically changed into different substances by the addition or removal of certain elements, usually through the action of water. The familiar rusting of iron is an example of chemical weathering, characterized by a total change in the composition and appearance of the original material. At first, there is a hard, silvery metal; afterward, all that remains is a soft, reddish-brown powder.

Consider the effects of mechanical and chemical weathering on a cube of rock that measures six centimeters on a side, thus having a total surface area of 216 square centimeters. Assume that by means of mechanical weathering, the cube is broken down into 216 cubes measuring one centimeter on a side, having a combined total surface area of 1,296 square centimeters. Now, much more surface area is available for chemical attack. For this reason, chemical weathering proceeds much faster when a rock is first broken into smaller pieces by mechanical weathering.

In nature, mechanical weathering can proceed in a variety of ways. The best-known example involves the action of freezing water. Because water increases about 9 percent in volume as it freezes, enormously large outward-directed pressures develop within a rock when water freezes in its pore spaces and cracks. This is sufficient to force pieces of the rock apart. One example of the result of this action is the angular rock fragments found scattered about most mountain tops and sides. Soil also contains water, and horizontal lenses of ice may form within the soil when water freezes in it. These create bumps in lawns and the familiar “frost heaves” of mountain roads. When heavy trucks rumble over these heaved pavements during thaws, the pavement gives way to create potholes.

In deserts, soil water is drawn upward through the rock and evaporates at the hot upper surface, leaving its dissolved salts behind as crystals growing in the pore spaces of the rock. These growing salt crystals also exert powerful pressures within the rock, so that porous rocks, such as sandstone, undergo continuous grain-by-grain disintegration in desert climates. Mechanical weathering can also be produced when the extreme heat from a forest fire or a lightning strike causes flakes to chip off a rock or when a growing plant or tree extends its root system into cracks and splits a rock apart. Another type of rock splitting is known as exfoliation, caused by the spontaneous expansion of rock masses when they are freed from the confining pressures of overlying and surrounding rock. This process produces large, dome bedrock knobs with an onionlike structure. Stone Mountain, Georgia, and Half Dome, in Yosemite National Park, are examples.

Chemical weathering is a more complex process than mechanical weathering because the original rock material is actually transformed into different substances. The rusting of iron has already been mentioned as an example of chemical weathering. Many common rocks and minerals contain iron. During chemical weathering, the iron in these substances combines with oxygen from the air to form various iron oxides.

Another way in which chemical weathering attacks rocks is by dissolving them. Large areas of Earth’s surface are underlain by a rock type known as limestone. Limestone is readily dissolved by water containing small quantities of acid. All rainfall is weakly acidic as a result of its dissolving carbon dioxide from the air to produce dilute carbonic acid. Rains that originate in areas of high air pollution are even more acidic, a condition known as acid rain. When rainfall that contains carbonic acid comes in contact with limestone bedrock, the acid reacts with the calcium carbonate in the rock to produce calcium bicarbonate, a soluble substance that is readily carried off in solution.

A final example of chemical weathering involves the weathering of granite, a hard igneous rock composed primarily of feldspar and quartz. When granite undergoes chemical weathering, each mineral is affected differently. The feldspar is gradually transformed into a new mineral, clay, which is soft and easily molded when wet. Clay offers very little resistance to erosion. Quartz, by comparison, is highly resistant to chemical attack and is left behind when the clay is removed. Some of the quartz grains remain in the soil, but most will be carried off by rivers, becoming rounded as they tumble along. Eventually, they form the sands of beaches and, in time, the sedimentary rock known as sandstone.

Erosion

The term erosion refers to those processes by which the loose particles formed by weathering are picked up and carried to new locations. Erosion is a highly significant geomorphological phenomenon of Earth’s surface. Examples of erosion range from small gullies in a farmer’s field to a catastrophic landslide in a high mountain valley. Nevertheless, the general principle involved in all types of erosion is the same: Weathered Earth materials move downslope from their place of formation to a new location, with gravity as the driving force. The materials may simply slide downhill as in a landslide, or they may be carried down the hill by an erosional agent, such as running water. Worldwide, running water in the form of streams and rivers is probably the single most important erosional agent. Locally, other erosional agents may be highly significant, including underground water flow, glaciers, waves, and wind action.

The downhill movement of weathered materials under the influence of gravity alone results in landslides if the downslope movement is rapid but in creep if the movement is imperceptibly slow. When large quantities of water are present in the weathered material, the downslope movement is called a mudslide. Running water can erode material from its channel banks in four ways. Soluble material can be dissolved by weakly acidic river water, bedrock can be worn smooth due to abrasion by sand and gravel carried along the streambed, unconsolidated bank and bed materials can be swept away by a strong current (resulting in bank caving), and upwardly directed turbulent eddies in the water may lift small particles from the bottom and entrain them in this fashion. Underground water erodes bedrock primarily by dissolving it, whereas glaciers act more like rivers, abrading the underlying rock by means of rock fragments frozen in the ice. Glaciers can also pluck rock masses from their channel walls when these rock masses have frozen to the main ice mass. The rocks are torn loose as the ice moves on.

Waves erode shorelines, wearing rock surfaces smooth by means of the sand and gravel they carry. Waves can also dislodge particles from a cliff face. Cracks quickly open in cliffs, seawalls, and breakwaters, and when water is forced into these cracks, the air in the cracks becomes highly compressed, exerting still further pressure on the rock. Wind erosion, in contrast, relies on the abrasive action of sand grains transported by the wind and on the lifting power of eddies, which are able to entrain finer-grained soil particles.

Study of Weathering and Erosion

Scientists have analyzed the rate at which rocks weather and have found that the important factors are rock type, mineral content, amount of moisture present, temperature conditions, topographic conditions, and amount of plant and animal activity. A rock type may be highly resistant to weathering in one climate and quite unresistant in another. Limestone, for example, which forms El Capitan, the highest peak in the desert region of southwest Texas, underlies the lowest valleys in the humid climate of the Appalachian Mountains.

Using field observations and laboratory experiments, scientists have studied the rate at which different minerals are attacked by chemical decomposition. Among the minerals formed by igneous activity, quartz is least susceptible to chemical attack, whereas olivine, a greenish-colored mineral rich in iron and magnesium, is one of the most susceptible. The reason is that olivine forms at high temperatures and pressures when melted rock first begins to cool and is consequently unstable at the lower temperatures and pressures that prevail at Earth’s surface. Quartz, by comparison, forms late in the cooling process, when the temperatures and pressures are more similar to those encountered at Earth’s surface. Therefore, quartz more readily resists attack by chemical weathering. Scientists have concluded that the more the conditions under which a mineral forms are akin to those at Earth’s surface, the more resistant to chemical attack the mineral will be.

Numerous observations have been made about the rapidity with which weathering occurs. The eruption of Mount St. Helens in Washington State on May 18, 1980, has provided a natural laboratory for such study. During the eruption, vast quantities of volcanic ash were hurled into the air and deposited to depths of several meters near the volcano. Scientists have carefully analyzed the changes that are taking place in the ash because of mechanical and chemical weathering and the rate at which this ash is being converted into productive soil for the growth of vegetation. Scientists also study the rate at which tombstones and historic monuments of known age are attacked by weathering. For marble tombstones in humid climates, the weathering within a single lifetime may amount to several millimeters.

The rate at which earthen materials are moved from place to place on Earth’s surface by the various agents of erosion is also of interest. One way to approach this problem is to measure the quantity of sediment being carried by a river each year and then to calculate how much of a loss this amount represents for the entire area drained by the river. Data from various locations in the United States suggest that the overall rate of erosion amounts to approximately 6 centimeters per 1,000 years. Corroborating evidence comes from another source: Photographs made in scenic areas and compared with photographs made from the same vantage point one hundred years ago or more show surprisingly little erosional modification of the surface. However, once humans occupy an area intensively, erosion rates increase significantly.

Significance

Weathering affects not only bedrock outcrops but also human-made structures. Unless they are continually repaired and restored, all structures become weather-beaten and, in time, weaken and fall into ruin. Beginning in the early 1970s, people also became aware of the harmful consequences of acid rain. Concrete, limestone building blocks, and the marble used for statuary are all susceptible to the dissolving action of acid rain. In fact, many well-known statues adorning public buildings in Europe have become essentially unrecognizable because of this process.

The earthen materials produced by weathering are of great significance. The larger grains are known as regolith and, by the addition of partly decomposed organic matter, are turned into soil, the basis for agriculture. Other grains are carried by rivers into the sea to become the raw materials from which beaches are made. Residues of weathered materials are sometimes left behind, such as clay and ores of iron, aluminum, and manganese, which may form valuable mineral deposits.

Erosional processes also affect human life. Gravity’s influence may bury small villages in catastrophic landslides, or it may trigger the imperceptible downslope movements, known as creep, that cause structures on hillsides to collapse. When the Alaska Pipeline was being built, construction of all kinds was hampered by problems caused by soil flow when the permafrost in the ground thawed. The erosional activity of rivers shapes the landscape, cutting gorges and supplying sediment to alluvial fans, floodplains, levees, and deltas. Sometimes, the erosional activity of a river gets out of hand, causing devastating floods. Even where no stream channel is present, farmlands can be seriously damaged by soil erosion.

The erosional power of moving ice sculpts some of the world’s most spectacular mountains. Millions of years ago, North America's Appalachian Mountains were 30,000 feet (9,000 meters) tall, but in the twenty-first century, they measure just over 6,500 feet (2,000 meters). Similarly, along coastlines, wave erosion creates cliffs or threatens human-made structures such as lighthouses, seawalls, and breakwaters. Coastal currents may carry sand away, causing severe beach erosion. The wind contributes to erosion by moving sand grains to create a sandstorm. This blinding cloud can sandblast the paint off a car or break a telephone pole in half. Even dust-sized material, when lifted from the ground as a dust storm, can have a devastating effect. During the 1930s, an area known as the Dust Bowl developed in the Great Plains region of the United States. A prolonged drought and unwise agricultural practices resulted in severe dust storms that blew away valuable topsoil, lowering the ground level by nearly one meter in some places.

Principal Terms

abrasion: the wearing away of rock by frictional contact with solid particles moved by gravity, water, ice, or wind

acid rain: rain with higher levels of acidity than normal, formed by contact of atmospheric moisture with airborne pollutants

chemical weathering: the decomposition of rock by chemical rather than mechanical processes

erosion: the general term for the various processes by which particles already loosened by weathering are transported from one location to another by the action of moving air or water

granite: a coarse-grained, igneous rock composed primarily of the minerals quartz and feldspar

limestone: a sedimentary rock composed of calcium carbonate

mechanical weathering: the physical disintegration of rock into smaller particles having the same chemical composition as the parent material

mineral: a naturally occurring, inorganic, crystalline material with a unique chemical composition

sandstone: sedimentary rock composed of grains of sand that have become cemented together under the influences of pressure and time

weathering: the general term for the group of processes that break down rocks at or near the ground surface

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