Weathering, Soil, and Erosion+

Andisol

Young soil developed on volcanic parent material and containing abundant unweathered volcanic glass and other volcanic debris, resulting in a high cation exchange capacity and high fertility. They are also common in Hawaii. Because volcanic rocks are commonly rich in nutrients, andisols are capable of supporting intense agricultural production.

talus

An accumulation of loose, angular rocks at the base of a cliff, created as rocks broke off the cliff as a result of frost wedging.

loess

An accumulation of windblown silt derived from glacial erosion.

Earthquakes and Volcanoes

An earthquake may cause a landslide by shaking an unstable slope, causing it to move. Saturated soils are particularly prone to movement during earthquakes because seismic waves increase the pore pressure between soil particles, pushing them apart and causing the sediment to liquefy. If you have ever tapped with your foot on saturated sand at the beach, you've seen the same process: the energy from the tapping of your foot travels through the sand as small compression waves, increasing its pore pressure and causing the sand to liquefy. Mass wasting is common in earthquake-prone regions and in volcanically active areas. Steep volcanoes are prone to rock slides, particularly during earthquakes. In addition, a volcanic eruption may melt the snow and ice cap at the top of the volcano. As the meltwater flows downslope, it picks up loose sediment and ash and evolves into a debris flow. Many volcanic regions contain thick sedimentary sequences consisting almost exclusively of debris flow deposits.

Predicting and Avoiding the Effects of Mass Wasting

Every year, small rapid forms of mass wasting destroy homes and farmland. Occasionally, an enormous slide or flow buries a town or city, killing thousands of people. Rapid forms of mass wasting cause billions of dollars in damage every year. One of the most important tools in evaluating the risks associated with mass wasting, especially flows, slides, and falls, is understanding that these features commonly reoccur in the same area because the geologic conditions that cause mass wasting tend to be constant over a large area and for long periods of time. Thus, if a hillside has slumped, nearby hills may also be vulnerable to the same type of mass wasting. In addition, landslides and mudflows commonly follow the paths of previous slides and flows. If an old mudflow lies in a stream valley, future flows may follow the same valley. Awareness and avoidance are the most effective defenses against mass wasting. Geologists evaluate landslide probability by combining data on soil and bedrock stability, slope angle, climate, and history of slope failure in the area. They include evaluations of the probability of a triggering event such as a volcanic eruption or earthquake. Building codes then regulate or prohibit construction in unstable areas. For example, according to the U.S. Uniform Building Code, a building cannot be constructed on a sandy slope steeper than 27 degrees, even though the angle of repose of sand is 30 to 35 degrees. So the law leaves a safety margin of 3 to 8 degrees. Architects can obtain permission to build on more-precipitous slopes if they anchor the foundation to stable rock.

flow

Form of rapid mass wasting in which loose soil or sediment moves downslope as a slurry-like fluid, not as a consolidated mass; may occur slowly (less than 1 centimeter per year for some earthflows) or rapidly (several meters per second for some mudflows and debris flows).

slide

Form of rapid mass wasting in which the rock or soil initially moves as a consolidated unit along a fracture surface. Natural slides usually move over timeframes of a few seconds or minutes, although some slides take days or weeks to finish moving.

fall

Form of rapid mass wasting in which unconsolidated material falls freely or bounces down steep slopes or cliffs.

hardpan

General term for a soil layer that is relatively impervious to water and impenetrable to plant roots. Commonly forms from precipitation of salts in a soil B horizon by either downward or upward translocation. Hardpans can be very difficult to dig through and can be impenetrable to roots. Trees growing in soils with well-developed hardpans have shallow root systems and are easily uprooted by the wind.

Oxidation

A chemical weathering process in which a mineral decomposes when it reacts with oxygen. Iron is abundant in many minerals; if the iron in such a mineral oxidizes, the mineral decomposes. Many valuable metals such as iron, copper, lead, and zinc occur as sulfide minerals in ore deposits. When these minerals oxidize during weathering, the sulfur reacts to form sulfuric acid, a strong acid. The sulfuric acid washes into streams and groundwater, where it may harm aquatic organisms. In addition, the sulfuric acid can dissolve metals from the sulfide minerals, forming a metal-rich solution that can be toxic. These reactions are accelerated when ore is dug up and exposed by mining, and acid-mine drainage is a common problem at such sites.

Hydrolysis

A chemical weathering process in which a mineral reacts with water to form a new mineral that has water as part of its crystal structure. Most common minerals weather by hydrolysis. For example, feldspar, the most abundant mineral in Earth's crust, weathers by hydrolysis to form clay.

Dissolution

A chemical weathering process in which mineral or rock dissolves, forming a solution. Halite dissolves so rapidly and completely in water that the mineral is rare in natural, moist environments. On the other hand, if you drop a crystal of quartz into pure water, only a tiny amount will dissolve and nearly all of the crystal will remain intact. To understand how water dissolves a mineral, think of an atom on the surface of a crystal. It is held in place because it is attracted to the other atoms in the crystal by electrical forces called chemical bonds. At the same time, electrical attractions to the outside environment are pulling the atom away from the crystal. The result is like a tug-of-war. If the bonds between the atom and the crystal are stronger than the attraction of the atom to its outside environment, the crystal remains intact. If outside attractions are stronger, they pull the atom away from the crystal and the mineral dissolves. Rocks and minerals dissolve more rapidly when water is either acidic or basic. An acidic solution contains a high concentration of hydrogen ions (H+), whereas a basic solution contains a high concentration of hydroxyl ions (OH−). Acids and bases dissolve most minerals more effectively than pure water does because they provide more electrically charged hydrogen and hydroxyl ions to pull atoms out of crystals.

salt cracking

A chemical weathering process in which salts that are dissolved in water in the pores of rock crystallize, exerting an outward pressure on pore walls and pushing the mineral grains apart. Thus, salt chemically precipitates in rock, and the growing salt crystals mechanically loosen mineral grains and break the rock apart. Many sea cliffs show pits and depressions caused by salt cracking, because spray from the breaking waves brings the salt to the rock. Salt cracking is also common in deserts, where surface water and groundwater commonly contain dissolved salts

creep

A form of mass wasting in which loose material moves very slowly downslope, usually at a rate of only about 1 centimeter per year and usually on land with vegetation. Trees on a creeping block tilt downhill and grow to have a trunk shaped like a pistol butt. During creep, the shallow soil or rock layers move more rapidly than deeper material moves. As a result, anything with roots or a foundation tilts downhill

mudflow

A form of rapid mass wasting that involves the downslope movement, usually on unvegetated land, of fine-grained soil particles mixed with water; can be slow moving, as slow as 1 meter per year, or as fast as a speeding car. A slurry is a mixture of water and solid particles that flows as a liquid. Wet concrete is a familiar example of a slurry. It flows easily and is routinely poured or pumped from a truck.

landslide

A general term for mass wasting (the downslope movement of rock and regolith under the influence of gravity) and the landforms it creates. In contrast to downhill creep that takes place slowly, other forms of mass wasting can occur very quickly due to natural processes or human activities that destabilize a slope—a landslide. Rain, melting snow, or a leaking irrigation ditch all can add weight to and lubricate soil, causing it to move quickly downslope, especially on steep slopes.

calcrete

A hardpan that forms in the B soil horizon in arid and semiarid regions when calcium carbonate precipitates and cements the soil particles together. Salts from irrigation water can accelerate the development of calcrete. For example, in the Imperial Valley of Southern California, irrigation water contains high-enough concentrations of calcium carbonate to form a calcrete that farmers must rip apart with heavy machinery before continuing to farm. In addition, irrigation water applied to aridisols can concentrate so much salt in the soil that it becomes toxic to plants.

gelisol

A high-latitude soil formed over permafrost that is no deeper than two meters. Characterized by an organic-rich A horizon that usually extends to the permafrost boundary.

Soil Horizons

A layer of soil that is distinguishable from other layers because of differences in appearance and in physical and chemical properties. Five soil horizons can occur: O, A, E, B, and C. Although some soils contain all five horizons as recognizable layers, many soils contain only a subset of the five horizons.

Organic Activity

A mechanical weathering process in which a crack in a rock is expanded by tree or plant roots growing there.

Pressure-Release Fracturing

A mechanical weathering process in which tectonic forces lift deeply buried rocks upward and then erosion removes overlying rock and sediment—the net result of which is to remove the pressure from overlying material, causing the rock to expand and fracture.

cation exchange capacity (CEC)

Ability of a soil to release cations, typically by exchanging basic cations K+, Na+, Ca++, or Mg++ for H+ with plant rootlets.

Frost Wedging

A mechanical weathering process in which water freezes in a crack in rock, and the resulting expansion wedges the rock apart. During spring and autumn in a temperate climate, water freezes at night and thaws during the day. Ice formation pushes rock apart but at the same time cements it together. During the day, when the ice melts, rock fragments come loose and tumble from a steep cliff. For this reason, experienced mountaineers try to travel in the early morning before ice melts.

Abrasion

A mechanical weathering process that consists of the grinding and rounding of rock and mineral surfaces by friction and impact. Wind hurls sand and other small particles against rocks, sandblasting unusual shapes. Because the wind does not normally carry sand particles high above the ground, they tend to weather the lower portion of the rock where some develop into balanced rock formations over the course of many years. Glaciers also are powerful agents of abrasion, as they drag rock clasts, sand, and silt across bedrock.

Thermal Expansion and Contraction

A mechanical weathering process that fractures rock when temperature changes rapidly, causing the surface of the rock to heat or cool faster, and thereby to expand or contract faster, than the rock's interior. In mountains or deserts at midlatitudes, temperature may fluctuate from -5 degrees Celsius to +25 degrees Celsius during a spring day. This 30-degree difference is probably not sufficient to fracture rocks. In contrast to small daily or annual temperature changes, fire heats rock by hundreds of degrees. If you line a campfire with granite stones, the rocks commonly break as you cook your dinner. In a similar manner, forest fires or brush fires occur frequently in many ecosystems, producing cracked rock that is an important agent of mechanical weathering.

salinization

A process whereby salts accumulate in soil that is irrigated heavily, lowering soil fertility.

transported soils

A soil formed by the weathering of regolith that is transported from somewhere else and deposited. Examples of transported soils include those formed on sediment deposited on a river floodplain during a flood or on deposits of windblown silt, called loess, which is derived from glacial erosion. Also, transported soils typically are more fertile than residual soils, because they consist of a wider variety of source materials and hence supply a greater variety of minerals and nutrients.

residual soils

A soil formed from the weathering of bedrock below.

oxisols

A soil formed in a hot, humid climate and characterized by intensive leaching of soluble cations from the A horizon, little ability to retain nutrients, and very poor fertility. Very insoluble iron and aluminum oxides are concentrated.

aridisols

A soil formed in arid or semiarid environments and characterized by very low organic content, water deficiency, and precipitation of salts in the B horizon. Commonly, upward movement of water in aridisols results in the precipitation of calcium carbonate nodules or a calcium carbonate hardpan called calcrete.

alfisol

A soil formed in semiarid to humid climates, typically under hardwood cover. Characterized by accumulation of clay in the B horizon and relatively high fertility, making it productive for agriculture.

rockslide

A subcategory of slide mass wasting in which a segment of bedrock slides downslope along a fracture and the rock breaks into fragments and tumbles down the hillside; also called a rock avalanche.

slump

A type of slide in which blocks of material slide downslope as a consolidated unit over an upward-concave, curved fracture in rock or regolith; trees on the slumping blocks tilt uphill. The uphill portion of the slump usually consists of several tilted slide blocks, whereas the toe of the slump usually consists of rumpled, folded sediment.

histosols

A very organic-rich soil, typically formed in a poorly drained area where stagnant water inhibits organic decay. Typically composed of thick O and A horizons. Can be mined as peat.

Entisols

A very young soil typically lacking horizons and formed on unconsolidated parent material. All soils not classified with a different order are classified as entisols, so much diversity exists within this order.

earthflow

A viscous flow of fine-grained sediment or fine-grained sedimentary rock that is saturated with water and moves downslope as a result of gravity; usually slow moving, typically less than one to several meters per day.

exfoliation

A weathering process resulting in fracture when concentric plates or shells split away from a main rock mass like the layers of an onion; frequently explained as a form of pressure-release fracturing, but many geologists consider it could result from hydrolysis-expansion. Granite typically fractures by exfoliation. The plates may be only 10 or 20 centimeters thick near the surface, but they thicken with depth. Because exfoliation fractures are usually absent below a depth of 50 to 100 meters, they seem to be a result of exposure of the granite at Earth's surface. Although exfoliation is frequently explained as a form of pressure-release fracturing, many geologists have suggested that hydrolysis-expansion may be the main cause of exfoliation.

inceptisol

A young soil exhibiting weak horizons and developed in subhumid to humid environments. Typically retains abundant unweathered material.

Chemical and Mechanical Weathering Acting Together

Chemical and mechanical weathering can work together, often on the same rock and at the same time. After mechanical processes fracture a rock, water and air seep into the cracks to initiate chemical weathering. In environments where groundwater is salty, saltwater seeps through pores and cracks in bedrock. When the water evaporates, the dissolved salts crystallize.

Time

Chemical weathering occurs slowly in most environments, and time is therefore an important factor in determining the extent of weathering. Recall that most minerals weather to clay. In geologically young soils, weathering may be incomplete and the soils may contain many partly weathered mineral fragments. As a result, young soils are often sandy or gravelly.

Vertisol

Clay-rich, dark-colored soil characterized by periodic desiccation and deep cracking. Nearly impermeable and sticky when wet. Vertisols are common in regions where volcanic ash has weathered to clay and in the floodplains of some large river systems where much clay has been deposited.

Climate

Climate exerts a fundamental control over soil formation, mainly through average annual temperature and precipitation. Rates of chemical weathering reactions, plant growth, and plant decay are all higher in warmer climates than cold ones. Precipitation that infiltrates the soil can translocate soil particles and ions downward, leading to the development of soil horizons. Although precipitation typically seeps downward through soil, several other factors related to climate can pull the water back upward. Plant roots suck soil water toward the surface. In arid climates, subsurface evaporation can cause upward movement of water.

mollisol

Grassland soil characterized by rich A horizon, high cation exchange capacity, and B horizon rich in base cation salts; very fertile soil. Typically 60 to 80 centimeters thick.

Fall

If a rock dislodges from a steep cliff, it falls rapidly under the influence of gravity. Several processes commonly detach rocks from cliffs. Recall from our discussion of weathering that frost wedging can dislodge rocks from cliffs and cause rockfall. Rockfall also occurs when a stream or ocean waves undercut a cliff.

Type of Rock and Orientation of Rock Layers

If sedimentary rock layers dip in the same direction as a slope, a layer of weak rock can fail, causing layers over it to slide downslope. Imagine a hill underlain by shale, sandstone, and limestone oriented so that their bedding lies parallel to the slope. If the base of the hill is undercut, the upper layers of sandstone and limestone may slide over the weak shale. In contrast, if the rock layers dip into the hillside, the slope may be stable even if it is undercut. Several processes can reduce the stability of a slope. Ocean waves or stream erosion can destabilize a slope, as can road building and excavation. Therefore, a geologist or engineer must consider not only a slope's stability before construction but also how the project might alter its stability.

Slide

In some cases, a large block of rock or soil, or sometimes an entire mountainside, breaks away and slides downslope as a coherent mass or as a few intact blocks. Two types of slides occur: slumps and rockslides.

Slope Aspect and Steepness

In the semiarid regions of the Northern Hemisphere, thick soils and dense forests cover the cool, shady north slopes of hills, but thin soils and grass dominate hot, dry, southern exposures. The reason for this difference is that in the Northern Hemisphere more water evaporates from the hot, sunny, southern slopes. Therefore, fewer plants grow, weathering occurs slowly, and soil development is retarded. Plants grow more abundantly on the moister northern slopes, and more rapid weathering forms thicker soils.

Rates of Plant Growth and Decay

In the tropics, plants grow and decay rapidly and growing plants quickly absorb the nutrients released by decaying vegetation. Heavy rainfall and organic acids leach nutrients from the soil. Little humus accumulates and few nutrients are stored in the soil. Thus, even though the tropical rain forests support great populations of plants and animals, these depend on a rapid cycle of growth, death, and decay. Conversely, the Arctic tundra is so cold that organic matter in the soil decays very slowly, accumulating through time to form a significant reservoir of carbon.

litter

Leaves, twigs, and other plant or animal materials that have fallen to the surface of the soil but have not decomposed.

Steepness of the Slope

Obviously, the steepness of a slope is a factor in mass wasting. If frost wedging dislodges a rock from a steep cliff, the rock tumbles to the valley below. However, a similar rock is less likely to roll down a gentle slope.

Types of Rapid Mass Wasting

Rapid forms of mass wasting fall broadly into three categories: flows, slides, and falls. To understand these categories, think of a sand castle. Sand that is saturated with water flows down the face of the structure.

spodosols

Sandy, acidic soil developed in moist, temperate environments, commonly in coniferous or mixed coniferous-deciduous forests. Leaching has translocated base cations downward, resulting in a well-developed E horizon.

sediment-gravity flow

Sediment gravity flows occurs when gravity acts on the sediment particles and moves the fluid; this is in contrast to rivers where the fluid moves the particles.

Soil-Forming Factors

Six factors control soil characteristics: parent rock, climate, rates of plant growth and decay, slope aspect and steepness, time, and transport of soil materials.

E horizon

Soil horizon in which organic acids derived from overlying O and A horizons leach soluble cations and translocate them downward along with clays. The loss of ions and clay from E horizons usually causes them to be sandy and gray (unpigmented). E horizons are common in forested areas because forest litter is acidic and precipitation is abundant.

Components of Soil

Soil is a mixture of mineral grains and rock fragments, organic material, water, and gas. The size and interconnectedness of pore spaces in soil and therefore the rate at which water and air can infiltrate the soil depends on the soil texture—the relative proportions of sand, silt, and clay. Soils rich in sand and silt contain pore spaces big enough to allow the infiltration of water and air. In contrast, clay-rich soils are so fine grained that pore spaces between the sediment particles are very small, inhibiting the transmission of air and water. Plants rooting in such soils often suffer from lack of oxygen.

Soil Classification

Soils have been classified on the basis of their engineering properties as related to support for building foundations, their morphology, and their genesis.

ultisols

Strongly weathered soil formed in semihumid or humid environments. Intense weathering has removed most base cations, resulting in low fertility. Includes red clay soils of SE United States.

transformation

The change of soil constituents from one form to another, such as the hydrolysis of feldspar to clay.

leaching

The chemical dissolution of ions from the O and A soil horizons and their removal, usually downward into the B horizon where they accumulate.

spheroidal weathering

The combined mechanical and chemical weathering of fractured crystalline bedrock into spheroidally shaped boulders; caused by the faster weathering rate of sharp bedrock corners (where at least three faces of rock can be attacked by weathering), over edges and the faster weathering of edges over faces.

humus

The dark, organic component of soil consisting of litter that has decomposed enough so that the origin of the individual pieces cannot be determined. Humus is decay resistant and nutrient rich and is an essential component of most fertile soils. Humus-rich soils swell after a rain and shrink during dry spells. This alternate swelling and shrinking loosens the soil, allowing roots to grow into it easily. A rich layer of humus also insulates the soil from excessive heat and cold and reduces water loss from evaporation.

chemical weathering

The decomposition of rock when it chemically reacts with air, water, or other agents in the environment, altering its chemical composition and mineral content. Chemical weathering is similar to the rusting of an old truck body; the final product differs both physically and chemically from the original material. Rock is durable over a human lifetime. Over geologic time, however, air and water chemically attack rocks near Earth's surface. The most important processes of chemical weathering are dissolution, hydrolysis, and oxidation. Water, acids and bases, and oxygen in the atmosphere or in surface water or groundwater cause these processes to decompose rocks.

Mechanical weathering

The disintegration of rock into smaller pieces by physical processes without altering the chemical composition of the rock. Also called physical weathering. Think of crushing a rock with a hammer: the fragments are no different from the parent rock except that they are smaller. Five processes cause mechanical weathering: pressure-release fracturing, frost wedging, abrasion, organic activity, and thermal expansion and contraction. Two additional processes—salt cracking and hydrolysis-expansion—result from combinations of mechanical and chemical processes.

Mass wasting

The downslope movement of earth material, primarily caused by gravity.

topsoil

The fertile, dark-colored surface soil; the combined O and A soil horizons. A kilogram of average fertile topsoil contains about 30 percent organic matter, by weight, including approximately 2 trillion bacteria, 400 million fungi, 50 million algae, 30 million protozoa, and thousands of larger organisms such as insects, worms, nematodes, and mites.

downhill creep

The gradual downhill movement, under the force of gravity, of soil and loose rock material on a slope. Facilitated by the freeze-thaw cycle, in which soil particles move orthogonal to the slope surface during freezing but directly downward during thawing. For example, when soil pore water freezes or when expandable clay minerals in soil are wetted, the soil expands. As it expands, soil particles are pushed outward at a right angle to the slope surface. Later, when the soil shrinks, the pore water thaws, or the clay minerals dry out, gravity pulls the soil particles directly downward, not back toward the original slope surface. Over time, this process causes soil particles to slowly creep downslope

soil order

The highest hierarchical classification of soils by the National Resource Conservation Service. Twelve soil orders are recognized. Based on the physical and chemical character, thickness, and color of the soil horizons. Each soil order is further subdivided into suborders, great groups, groups, families, and series.

A horizon

The layer of soil below the O horizon, composed of a mixture of humus, sand, silt, and clay; combines with the O horizon to form topsoil.

soil series

The lowest hierarchical classification of soils by the National Resource Conservation Service. Over 20,000 soil series are recognized, including 50 designated "state soils." Based on the locality where that soil was first mapped and described. Fifty of these soil series—one for each U.S. state—have been selected as a state soil because the soil series has special significance to that state. Twenty of these soil series have been officially legislated as state soils and share the same level of distinction as a state bird, flower, or motto.

C horizon

The lowest soil layer, consisting of weathered bedrock. This zone contains little organic matter.

angle of repose

The maximum slope or steepness at which loose material remains stable. If the slope becomes steeper than the angle of repose, the material slides. The angle of repose varies for different types of sediment. For example, rocks commonly tumble from a cliff, to collect at the base as angular blocks of talus. The angular blocks interlock and jam together. As a result, talus typically has a steep angle of repose, up to 45 degrees. In contrast, rounded sand grains do not interlock and therefore have a lower angle of repose—about 30 to 35 degrees

loam

The most fertile soil, a mixture especially rich in sand and silt with generous amounts of organic matter. Such soils are well drained and may contain abundant organic matter, making them especially fertile and productive.

Aspect

The orientation of a slope with respect to the Sun; the direction toward which the slope faces.

reworks

The process by which sediment is deposited, then re-eroded and transported further.

capillary action

The process by which water is pulled upward through the soil due to the natural attraction of water molecules to soil particles and the cohesion of water. In a soil, capillary action is caused by the attraction of the water molecules to the soil particles and by the cohesiveness of the water itself.

percentage base saturation

The proportion of a soil's cation exchange capacity that is saturated by basic cations K+, Na+, Ca++, or Mg++.

eluviation

The removal and downward movement of dissolved ions and clays from the O, A, and E horizons by infiltrating water.

Erosion

The removal of weathered rocks that occurs when water, wind, ice, or gravity transports the material to a new location.

B horizon

The soil layer just below the A horizon, containing less organic matter and where ions and clays leached from the A and E horizon accumulate; also called subsoil. The B horizon is a transitional zone between topsoil and the weathered parent rock below. Roots and other organic material can grow in the B horizon, but the total amount of organic matter usually is low.

Parent Rock

The texture and composition of soil depends partly on its parent rock. For example, when granite decomposes, the feldspar converts to clay and the rock releases quartz as sand grains. If the clay leaches from the E horizon into the B horizon, a sandy soil forms. In contrast, because basalt contains no quartz and is rich in finely crystalline feldspar, soil formed from basalt typically is rich in clay and contains little sand.

regolith

The thin layer of loose, unconsolidated, weathered material that overlies bedrock. Some earth scientists and engineers use the terms regolith and soil - interchangeably; soil scientists identify soil as only the upper layers of regolith.

Soils

The upper layers of regolith that support plant growth. Some earth scientists and engineers use the terms soil and regolith interchangeably.

O horizon

The uppermost layer of soil, named for its organic component; the combined O and A horizons are called topsoil. This layer consists mostly of organic litter and humus, with a small proportion of minerals.

translocation

The vertical, usually downward, movement of physical or chemical soil constituents from one horizon to another.

Water and Vegetation

To understand how water affects slope stability, think again of a sand castle. Even a novice sand castle builder knows that sand must be moistened to build steep walls and towers but too much water causes the walls to collapse. If only small amounts of water are present, the water collects only where one sand grain touches another. The surface tension and cohesion of the water binds the grains together. However, excess water fills the pores between the grains and exerts an outward pressure—called pore pressure—that pushes the grains apart. The excess water also lubricates the sand and adds weight to a slope. When some soils become water saturated, they flow downslope, just as the sand castle collapses. In addition, if groundwater collects on an impermeable layer of clay, shale, or even on permafrost, it may cause overlying rock or soil to become saturated and move easily.

cloud condensation nuclei

Very small particles suspended in the atmosphere on which water vapor condenses. Air pollution creates condensation nuclei that are very acidic. For example, the burning of coal produces tiny sulfur-rich particles that serve as cloud condensation nuclei and that produce sulfuric acid when combined with water. More sulfur-rich particles are incorporated into the raindrops as they grow. By the time the raindrops are big enough to fall to Earth's surface, they have become acid rain.

Weathering and Erosion

Weathering involves little or no movement of the decomposed rocks and minerals. The weathered material simply accumulates where it forms. However, loose soil and other weathered material offer little resistance to rain or wind and are easily eroded. Weathering, erosion, transport, and deposition typically occur in an orderly sequence. For example, water freezes in a crack in granite and loosens a grain of quartz. A hard rain erodes the grain and washes it into a stream. The stream then transports the quartz to a beach at the seashore, where it is tumbled in the surf and becomes rounded. During a large storm, the now rounded grain of quartz is swept offshore and deposited on the seafloor, where it may reside for hundreds of years or more.