Chapter 9 - weathering
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.
Because quartz is so resistant to both mechanical and chemical weathering
it is a primary component in much of Earth's sand. After quartz is freed from its source rock, it is transported by water, wind, glacial ice, and gravity as sand-sized particles. Many of the quartz particles are transported to the shoreline where they are concentrated on beaches and deltas or are carried offshore by storm currents. Eventually, the sand lithifies to form sandstone.
The clays and ions lost from the E horizon are translocated downward and added to
the B horizon, also called the zone of accumulation or 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. The lowest layer, called the C horizon, consists of partially weathered bedrock that grades into unweathered parent rock. This zone contains little organic matter.
Organic acids and carbon dioxide produced by decaying organic matter in the topsoil are transported downward into
the E horizon. E stands for eluviation, referring to the process of leaching in which the acids produced in the topsoil dissolve ions such as calcium, silica, and iron from the E horizon and translocate them downward. Clays are also translocated from the E horizon into deeper layers of soil. 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.
Imagine that you are a geological consultant on a construction project. The developers want to build a road at the base of a hill, and they wonder whether the road will be affected by slides, flows, or falls. What factors should you consider?
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. 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, as shown in Figure 10.26A. If the base of the hill is undercut (Figure 10.26B), 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 (Figures 10.26C and 10.26D). 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. The Nature of Unconsolidated Materials The angle of repose is the maximum slope or steepness at which loose sediment remains stable. If the slope becomes steeper than the angle of repose, the sediment 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 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 (Figure 10.28); 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. Roots hold soil together and plants absorb water; therefore, a vegetated slope is more stable than a similar bare one. Many forested slopes that were stable for centuries slid when the trees were removed during logging, agriculture, or construction. Landslides are common in deserts and regions with intermittent rainfall. For example, Southern California has dry summers and occasional heavy winter rain. Vegetation is sparse because of summer drought and wildfires. When winter rains fall, bare hillsides often become saturated and slide. Landslides occur for similar reasons during infrequent but intense storms in deserts. 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 (see Figure 10.23). Many volcanic regions contain thick sedimentary sequences consisting almost exclusively of debris flow deposits.
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.
Mass wasting
The downslope movement of earth material, primarily caused by gravity. (See also landslide.)
topsoil
The fertile, dark-colored surface soil; the combined O and A soil horizons.
The main characteristics of each soil order, landscapes in which they occur, and example soil profile
(A) Andisol; (B) Vertisol; (C) Aridisol; (D) Alfisol; (E) Spodosol; (F) Mollisol; (G) Ultisol; (H) Oxisol; (I) Gelisol; (J) Histosol; (K) Entisol; and (L) Inceptisol
In mountains or deserts at midlatitudes, temperature may fluctuate from
-5*C to + 25*C 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.
Water expands by
7 to 8 percent when it freezes. If water accumulates in a crack and then freezes, the newly forming ice wedges the rock apart in a process called frost wedging. 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.
oxidation
A chemical weathering process in which a mineral decomposes when it reacts with oxygen.
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.
dissolution
A chemical weathering process in which mineral or rock dissolves, forming a solution.
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.
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.
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.
landslide
A general term for mass wasting (the downslope movement of rock and regolith under the influence of gravity) and the landforms it creates.
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.
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.
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.
frost wedging
A mechanical weathering process in which water freezes in a crack in rock, and the resulting expansion wedges the rock apart.
abrasion
A mechanical weathering process that consists of the grinding and rounding of rock and mineral surfaces by friction and impact.
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.
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.
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.
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.
inceptisol
A young soil exhibiting weak horizons and developed in subhumid to humid environments. Typically retains abundant unweathered material.
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.
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. Typically, large open pore spaces exist between the rocks.
loess
An accumulation of windblown silt derived from glacial erosion.
Vertisol
Clay-rich, dark-colored soil characterized by periodic desiccation and deep cracking. Nearly impermeable and sticky when wet.
Clay minerals perform several important functions in soils
Clays can retain water by incorporating it into their crystal structure or by attracting water molecules to the charged flat surface of the platy clay crystals. The charged surfaces of clay minerals also can attract and hold other ions, such as calcium (Ca++) or potassium (K+), commonly formed from the weathering of feldspars. By releasing H+ from organic acids, plant roots are able to exchange it for the Ca++ or K+ that the plant needs for growth.
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.
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.
mollisol
Grassland soil characterized by rich A horizon, high cation exchange capacity, and B horizon rich in base cation salts; very fertile soil.
Many elements react with atmospheric oxygen (o2)
Iron rusts when it reacts with water and oxygen. Rusting is an example of a more general weathering process called oxidation. 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.
litter
Leaves, twigs, and other plant or animal materials that have fallen to the surface of the soil but have not decomposed.
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.
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.
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.
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.
Mechanical weathering
The disintegration of rock into smaller pieces by physical processes without altering the chemical composition of the rock.
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.
soil order
The highest hierarchical classification of soils by the National Resource Conservation Service. Twelve soil orders are recognized.
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."
C horizon
The lowest soil layer, consisting of weathered bedrock.
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.
loam
The most fertile soil, a mixture especially rich in sand and silt with generous amounts of organic matter.
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.
percentage base saturation
The proportion of a soil's cation exchange capacity that is saturated by basic cations K+, Na+, Ca++, or Mg++.
translocation
The vertical, usually downward, movement of physical or chemical soil constituents from one horizon to another.
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. The removal of rock mass from an area by rain, running water, wind, glaciers, or gravity. Agents of erosion may then carry the weathered material great distances and later deposit it as layers of sediment.
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.
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.
Rocks at Earth's surface are exposed to daily and yearly cycles of heating and cooling
They expand when they are heated and contract when they cool. When temperature changes rapidly, the surface of a rock heats or cools faster than its interior, and as a result, the surface expands or contracts faster than the interior. The forces generated by this thermal expansion and contraction may fracture the rock.
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.
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. A slump occurs when blocks of material slide downhill over a gently curved fracture in rock or regolith. Trees remain rooted in the moving blocks. However, because the blocks rotate on the concave fracture, trees on the slumping blocks are tilted upslope (Figure 10.24). Thus, you can distinguish slump from creep because a slump tilts trees uphill, whereas creep tilts them downhill. At the lower end of a large slump, the blocks often break apart and pile up to form a jumbled, hummocky topography.
If soil collects in a crack in bedrock
a seed may fall there and sprout. The roots work their way into the crack, expand, and may eventually widen the crack as they grow. City dwellers often see the results of this type of organic activity when tree roots raise and crack concrete sidewalks.
Air pollution can make rain even more
acidic. When water vapor condenses to liquid, it requires a non-gaseous (liquid or solid) surface to do so. In the atmosphere, this surface is presented by tiny solid or liquid particles called cloud condensation nuclei. Cloud condensation nuclei are tiny particles of liquid or solid that typically are less than one micrometer (one millionth of a meter) in diameter. 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. Thus, water—especially if it is acidic or basic—dissolves ions from soil and bedrock and carries the dissolved material away. Groundwater also dissolves rock and can produce spectacular caverns in limestone
The mapped soil elements
are based on soil orders. A soil order is the highest hierarchical level of soil classification as defined by the NRCS and is 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 soil series is the lowest category of soil classification and is generally given a name based on the locality where that soil was first mapped and described. Today, over 20,000 different soil series have been identified and mapped by the NRCS within the United States. 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. Figure 10.17 shows typical examples of each of the twelve soil orders currently recognized by the NRCS, along with their associated landscapes and diagnostic characteristics. As explained below, each order is characteristic of a unique combination of the main soil-forming factors: parent rock, climate, biological activity, and topography.
It is useful to identify a slump
because it often recurs in the same place or appears on nearby slopes. Therefore, a slope that shows evidence of past slumping is not a good place to build a house. During a rockslide (or rock avalanche), bedrock slides downslope over a fracture plane. Characteristically, the rock breaks up as it moves, and a turbulent mass of rubble tumbles down the hillside. In a large rockslide, the falling debris traps and compresses air beneath and among the tumbling blocks. The compressed air reduces friction and allows some rockslides to attain speeds of 500 kilometers per hour. The same mechanism allows a snow or ice avalanche to cover a great distance at high speed.
Landslides cause
billions of dollars in damage every year. (A) An earthquake of magnitude 6.7 struck Japan's northern island of Hokkaido on September 6, 2018, triggering these landslides which destroyed several houses. (B) This landslide on the island Gran Canaria, Spain, occurred after heavy rain in February, 2017. (C) On July 7, 2018, heavy rainfall triggered a very large landslide in western Iceland. The slide was initiated on a steep slope and ran for about 1.5 kilometers horizontally, blocking the Salmon River and causing the lake in the foreground to form.
The angle of repose depends on
both the type of material and its water content. Dry sand forms low mounds, but moistened sand can form the familiar steep-sided hills and towers of sand castles because the surface tension among the grains causes them to stick together.
Five different soil horizons are commonly distinguished by
color, texture, and chemistry. These include the O and A horizon, characterized by abundant organic matter; the E horizon from which organic acids percolating downward have leached cations and clays; the B horizon in which those cations and clays accumulate; and the C horizon, which consists mainly of weathered bedrock. (B) Many soils contain only a subset of these five horizons such as this mollisol (grassland soil) which is lacking the E horizon.
Rocks, grains of sand, and silt collide with one another when
currents or waves carry them along a stream or beach. During these collisions, the sharp edges and corners of the particles are worn away and they become rounded. The mechanical wearing of rocks by friction and impact is called abrasion. Note that water itself is not abrasive—it is the collisions among rock, sand, and silt in the water that does the rounding.
Many igneous and metamorphic rocks form
deep below Earth's surface. Imagine, for example, that a granitic pluton solidifies from magma at a depth of 15 kilometers. At that depth, the pressure from the weight of overlying rock is about 5,000 times that at Earth's surface. Over millions of years, tectonic forces may uplift the granite while erosional forces strip off the overlying layers of rock, ultimately exposing the granite at the Earth's surface. As the pressure on the rock diminishes, the rock expands, but because the rock is now cool and brittle, it fractures as it expands. This process is called pressure-release fracturing. Many igneous and metamorphic rocks that formed at depth but now lie at Earth's surface have fractured in this manner.
Sedimentary rock layers
dip parallel to this slope. (B) If a roadcut undermines the slope, the dipping rock provides a good sliding surface and the slope may fail. (C) Sedimentary rock layers dip at an angle to this slope. (D) The slope may remain stable even if it is undermined.
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.
The atmosphere and hydrosphere weather
erode the surface rocks of the geosphere
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.
all other forms of weathering involve
interactions among rocks of the geosphere and the hydrosphere, atmosphere, and biosphere.
Granite typically fractures by
exfoliation, a process in which plates or shells of weathered material split away like the layers of an onion. 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.
Rapid forms of mass wasting fall broadly into three categories
flows, slides, and falls (Figure 10.21). To understand these categories, think of a sand castle. Sand that is saturated with water flows down the face of the structure. During flow, loose, unconsolidated soil or sediment moves as a fluid. Some slopes flow slowly—at a speed of 1 centimeter or less per year. In contrast, mud with high water content can flow nearly as rapidly as pure water. The three categories of rapid mass wasting are flow, slide, and fall. During a flow, loose soil or sediment moves as a fluid. During a slide, the entire slab of rock or soil moves downslope as a unit. In a fall, loose material bounces freely downslope.
pressure-release fracturing occurs by processes involving only the
geosphere
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. During hydrolysis, feldspars and other silicate minerals react with water to form clay. As a result of the addition of water, clays have a greater volume than the original minerals have. So a chemical reaction (hydrolysis) forms clay, and the mechanical expansion that occurs as the clay forms may cause exfoliation. This explanation is compatible with the observation that exfoliation concentrates near Earth's surface because water and chemical weathering are most abundant close to the surface.
The ability of a soil to absorb and exchange cations is called
its cation exchange capacity (CEC) and is an important component of soil classification. In general, the more clay and organic matter in the soil, the higher its CEC. That proportion of a soil's CEC that is satisfied by cations Ca++, Mg++, K+, and Na+ (called the basic cations) is referred to as percentage base saturation and is also important in soil classification. Organic matter in soils also contains nutrients necessary for plant growth. If you walk through a forest or prairie, you can find bits of organic litter—leaves, stems, and flowers that have not decomposed—on the soil surface. When this litter decomposes sufficiently that you can no longer determine the origin of individual pieces, it becomes humus. 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. In intensive agriculture, farmers typically plow the soil and leave it exposed for weeks or months. Humus oxidizes in air and decomposes, while rain dissolves soil nutrients and carries them away. Farmers replace the lost nutrients with chemical fertilizers but rarely replenish the humus. As a result, much of the soil's ability to absorb and regulate water and nutrients is lost. Rainwater flowing over the plowed surface transports soil particles, excess fertilizer, and pesticide residues, polluting streams and groundwater.
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.
Because of the close connection that soils have to humans living on Earth's surface
many different soil classification systems have been developed. Soils have been classified on the basis of their engineering properties as related to support for building foundations, their morphology, and their genesis. Many countries have established their own soil classification systems, although an international system also has been developed. In the United States, the National Resources Conservation Service (NRCS) is responsible for defining and mapping soils across the country. Figure 10.15 is the current soils map for the United States as developed by the NRCS, and Figure 10.16 is the soils map for the world.
Weathering occurs by both
mechanical and chemical processes. Mechanical weathering (also called physical weathering) reduces solid rock to small fragments but does not alter the chemical composition of rocks and minerals. Think of crushing a rock with a hammer: the fragments are no different from the parent rock except that they are smaller. In contrast, chemical weathering occurs when air and water chemically react with rock to alter its 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.
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. A loam is a soil with approximately equal parts of sand, silt, and clay. Such soils are well drained and may contain abundant organic matter, making them especially fertile and productive. Clay minerals perform several important functions in soils. Clays can retain water by incorporating it into their crystal structure or by attracting water molecules to the charged flat surface of the platy clay crystals. The charged surfaces of clay minerals also can attract and hold other ions, such as calcium (Ca++) or potassium (K+), commonly formed from the weathering of feldspars. By releasing H+ from organic acids, plant roots are able to exchange it for the Ca++ or K+ that the plant needs for growth.
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. Later, another powerful storm again erodes—or reworks—the grain off the seafloor and suspends it in the turbulent water along with other sedimentary particles. Collectively, the sediment particles are carried into a submarine canyon and funneled into deep water as part of a sediment-gravity flow, a bottom-hugging fluid mixture of sediment and water that flows downslope. After several days of transport down the submarine canyon, the sediment-gravity flow comes to rest on the ocean floor in deep water, and the grain is finally deposited.
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.
Six factors control soil characteristics
parent rock, climate, rates of plant growth and decay, slope aspect and steepness, time, and transport of soil materials.
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. Both of the examples above involve the formation of soil directly on bedrock and are called residual soils. In contrast, transported soils do not develop from weathering of local bedrock; they develop from parent material (regolith) brought in from somewhere else. Despite the name, it is not the soil itself that is transported, but rather the regolith from which the soil is formed. 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. Generally, soils formed on hillsides are residual soils and thin relative to valley bottoms, where thicker, transported soils typically form. 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. Andisol, one of the twelve soil orders, is linked specifically to its parent rock (Figure 10.17A). Andisols contain high proportions of volcanic rock material, including volcanic glass, and primarily form as residual soils from the direct weathering of volcanic bedrock. In the continental United States, andisols are restricted to the large volcanic provinces of the Pacific Northwest. They are also common in Hawaii. Because volcanic rocks are commonly rich in nutrients, andisols are capable of supporting intense agricultural production. Vertisol (Figure 10.17B) is a soil order also classified mainly on the basis of its parent material. Characterized by an abundance of vertical cracks, vertisols form in clay-rich soil in which the clay expands and contracts with the addition and removal of water. 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.
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. Such a soil, called a gelisol (from the Latin gelare, meaning "to freeze"), is characterized by permafrost within two meters of the surface and an A horizon that usually extends to the permafrost boundary (Figure 10.17I). Gelisols may contain preserved organic matter, but they are unfertile because nutrients are typically highly leached above the permafrost zone. Among the most fertile soils are the mollisols and alfisols of temperate prairies and forests. There, large amounts of plant litter drop to the ground in the autumn, but decay is slow during the winter. Plant growth during the growing season is not fast enough to extract all the nutrients from the soil. As a result, thick layers of nutrient-rich humus accumulate. Even more organic-rich are histosols (Figure 10.17J), which are dominated by organic soil materials. Histosols are usually formed in bogs or swamps. Slope Aspect and Steepness Aspect refers to the orientation, or facing direction, of a slope with respect to the Sun. 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 (Figure 10.19). 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.
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.
In many semiarid to humid climates
rainstorms are of longer duration and more rain falls. As a result, water seeps downward through the soil, leaching soluble ions from both the A and E horizons and translocating these along with clays downward to the B horizon. The less-soluble elements—such as aluminum, iron, and some silicon—remain behind to form a soil type called alfisol (Figure 10.17D). The subsoil in an alfisol is commonly rich in clay, which is mostly aluminum and silicon and has the reddish color of iron oxide; the prefix "alf" is derived from the chemical symbols for aluminum (Al) and iron (Fe). Alfisols typically form under a hardwood forest cover and contain relatively high percentages of base cations. For this reason, alfisols are relatively fertile and are widely used in agriculture. They are common in the Ohio River valley of the eastern United States. Closely related to alfisols are spodosols (Figure 10.17E), which are also typically formed in coniferous or mixed deciduous and coniferous forests. Spodosols form by intense translocation of base cations and clay downward, are relatively depleted of base cations, and typically have a well-developed E horizon. Spodosols are less fertile than alfisols and form mainly in the dense forests of northern New England and the Great Lakes region, as well as in parts of the southeastern United States. Much of central North and South America and central Asia is characterized by a semiarid to semihumid climate, grassland ecosystem, and a soil type called mollisol (Figure 10.17F). Mollisols are characterized by an A horizon that is nutrient enriched, has high organic content, and is typically 60 to 80 centimeters thick. The high organic content and thick A horizon results from the long-term addition of organic matter from plant roots, which also produce a soft, granular texture. Fire and disruption by ants and earthworms also exert strong influences on mollisols. The fertility, thickness, and texture of the A horizon make mollisols among the most agriculturally productive soil type. At the opposite extreme are soils formed in environments characterized by high temperatures and high rainfall. In such environments, so much water seeps through the forest litter that the organic acids formed leach away nearly all the soluble cations. Two soil orders, ultisols (Figure 10.17G) and oxisols (Figure 10.17H), are formed in such environments. Ultisols are commonly called "red clay soils" and are common in the southeastern United States, where heavy leaching has removed roughly two-thirds of the base cations from the soil. Ultisols can be used for agriculture, but typically must be fertilized. The name oxisol comes from "oxide" minerals, particularly the very insoluble iron oxides and aluminum oxides, which become concentrated. Oxisols are very infertile soils, are nearly devoid of soluble ions, and have little ability to retain nutrients. Thus, plants growing on oxisols must derive their nutrients almost entirely from decaying litter in the O horizon. Oxisols are typically colored yellow or red by iron oxide. Bauxite, the world's main type of aluminum ore (Chapter 5), is a highly aluminous oxisol. Oxisols are not common in the continental United States, but are found on Puerto Rico and Hawaii.
Cropland soils are
rapidly eroding, posing a significant threat to food sustainability for future human generations Weathering decomposes bedrock, and plants add organic material to the regolith to create soil at Earth's surface. However, soil does not accumulate and thicken throughout geologic time. If it did, Earth would be covered by a mantle of soil hundreds or thousands of meters thick and rocks would not exist at Earth's surface. Instead, all natural forms of erosion combine to remove soil about as fast as it forms. Interactions with flowing water, wind, and glaciers all erode soil, and some weathered material simply slides downhill under the influence of gravity, as we will explore below. Once soil erodes, the particles of clay, sand, and gravel are carried downhill by the same agents that eroded them: streams, glaciers, wind, and gravity. On their journey, they may come to rest temporarily on a floodplain or lake bottom, but eventually most particles are eroded again, or reworked, and carried further downslope to the sea, where they accumulate in deltas or are swept offshore by marine currents. There, the sediment is deposited, buried, and ultimately lithified to form sedimentary rocks. It can take thousands of years for an eroded soil to reform. Thus, for all practical purposes, soil is a nonrenewable resource: once it is eroded, it is gone for generations. Unfortunately, human activities have greatly accelerated soil erosion. One recent study concluded that humans move 10 times more sediment than the sum of all natural processes operating on the surface of the planet! The study also concluded that this massive displacement of sediment has resulted in a rate of cropland loss roughly 10 times faster than the rate at which cropland soils are formed. Improper farming, livestock grazing, and logging all accelerate erosion. Plowing removes plant cover that protects soil. Logging removes forest cover, and the machinery breaks up the protective litter layer. Intensive grazing strips away protective plants, while hoof prints disrupt the soils. Rain, wind, and gravity then can erode the exposed soil easily and at a much faster rate than new soil is formed, leading to net soil loss. Soil erosion also contaminates waterways with silt as well as with herbicides and pesticides. Soil can be difficult or expensive to renew once it has become degraded. Intense pressures on the world's soil resources have resulted in loss of productivity and biodiversity and increasing desertification of once-fertile lands. When farmers use proper conservation measures, soil can be preserved indefinitely or even improved. Some regions of Europe and China have supported continuous agriculture for centuries. However, in recent years, marginal lands on hillsides, in tropical rain forests, and along the edges of deserts have been brought under cultivation, greatly accelerating the global rate of soil loss. If left to continue, this global loss of soil resources will threaten the food sustainability for future human generations.
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. In the past few years, the knocking over of balanced rocks has become a sport to a few people, depriving many of viewing these unique structures. Glaciers also are powerful agents of abrasion, as they drag rock clasts, sand, and silt across bedrock.
craters
steep, circular depressions with a sharp berm and surrounding blanket of debris
As the name implies, creep is the
slow, downhill flow of rock or soil under the influence of gravity. A creeping slope typically moves at a rate of about 1 centimeter per year, although wet soil can creep more rapidly. 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 (see Figure 10.20). Trees have a natural tendency to grow straight upward. As a result, when downhill creep of soil and shallow bedrock tilts a growing tree, the tree develops a J-shaped curve in its trunk, called pistol butt (Figure 10.22). If you ever contemplate buying hillside land for a homesite, examine the trees. If they have pistol-butt bases, the slope is probably creeping, and creeping soil can slowly tear a building foundation apart.
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. Entisols are soils that are so young that little or no evidence for soil horizons is present (Figure 10.17K). In addition to being sandy or gravelly, entisols are typically very thin. As weathering of an entisol continues, it commonly will evolve to an inceptisol (Figure 10.17L), which is more mature but still lacks a B horizon in which downward translocated clays have accumulated. As weathering continues and clay and cations begin to translocate downward and accumulate, inceptisols will evolve to other soil orders containing a diagnostic B horizon.
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. Figure 10.25 shows three examples of mass wasting that have affected humans.
Bedrock breaks into
smaller fragments as it weathers, and much of it decomposes to form sand, silt, and clay. Therefore, on most land surfaces, a thin layer of loose rock fragments, clay, silt, and sand overlies bedrock. This material is called regolith. Soils are the upper layers of regolith that contain organic matter and can support rooted plants.
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. In addition, a process called capillary action draws water upward in the same way that water is drawn up into the holes of a sponge placed on a countertop spill 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. In arid and semiarid regions, rainstorms typically are of short duration and little rain falls. Consequently, when the rain stops, capillary action, plant roots, and subsurface evaporation draw most of the water upward, where it escapes or is taken up by plants. As the water escapes, many of its dissolved ions precipitate in the B horizon, encrusting the soil with salts and in some cases forming an impervious layer called a hardpan. 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. Most arid and semiarid climates produce soils classified as aridisols (Figure 10.17C), characterized by low concentration of organic matter, water deficiency, and accumulation of soluble 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. Because nutrients concentrate when water evaporates, many aridisols are fertile if irrigation water is available. However, 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 (Figure 10.18). The evaporative concentration of salts in soil is called salinization.
We are all familiar with the fact that
some minerals dissolve readily in water and others do not. If you put a crystal of halite (rock salt, or table salt) in water, the crystal will rapidly dissolve to form a solution. The process is called dissolution. 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. Water (H20) is a polar molecule, meaning that it has both a positively charged and a negatively charged end. This polarity arises because the oxygen atom in water has a great affinity for electrons, which move away from the hydrogen atoms to concentrate around the oxygen and give that end of the molecule a negative charge. The two hydrogen atoms are then left with a slight positive charge because their electrons have moved closer to the oxygen atom. When a crystal of halite (NaCl) is dropped in a glass of water, the negatively charged ends of water molecules pull the positively charged sodium ions away from the halite, while the positively charged ends of water molecules remove the negatively charged chlorine ions
The uppermost layer is called
the O horizon, named for its organic component. This layer consists mostly of organic litter and humus, with a small proportion of minerals. The next layer down, called the A horizon, is a mixture of humus, sand, silt, and clay. Together, the O and A horizons are called topsoil. 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.
Very commonly
the combined effects of mechanical and chemical weathering cause coarsely crystalline igneous rocks to form spheroidal shapes (Figure 10.13). Igneous rocks exposed at the surface typically undergo pressure-release fracturing as a result of unroofing and jointing as a result of tectonic activity. The intersection of these different fracture planes initially produces rocks with flat faces and sharp corners and edges. As the rocks weather, however, their corners and edges are more exposed and susceptible to attack by hydrolysis and the mechanical expansion of the resulting clays. Through time, the corners and edges are slowly rounded, producing a spheroidally shaped boulder. It is important to understand that spheroidal weathering is not due to mechanical abrasion, but rather takes place as a rock is weathered in place, without moving.
Mass wasting is
the downslope movement of earth material by gravity. The word landslide is a form of mass wasting that involves the creation of certain landforms. All rock and sediment on the side of a hill or mountain is constantly being pulled downward by gravity, and any process that causes this soil to expand and shrink promotes the incremental downhill creep of soil particles. 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
During creep
the land surface moves more rapidly than deeper layers, so objects embedded in rock or soil tilt downhill.
The angle of repose is
the maximum slope at which a specific material can remain stable. Because the chunks of rock forming talus interlock and jam together, talus has a steeper angle of repose than sand, whose rounded grains do not interlock.
The earliest impact craters on Earth's surface quickly disappeared because
the planet was so hot that plastic and molten rock oozed inward to fill the craters. As the crust thickened, cooled, and hardened, additional bombardment formed new craters.
If you undermine the base of a sand castle
the wall may fracture and a segment of the wall may slide downward. Movement of a coherent block of material along a fracture is called a slide. Natural slides usually move over timeframes of a few seconds or minutes, although some slides take days or weeks to finish moving. If you take a huge handful of sand out of the bottom of the castle, the whole tower topples. This rapid, free-falling motion of loose material is called fall. Fall is the most rapid type of mass wasting. In extreme cases, such as the face of a steep cliff, rock can fall at a speed dictated solely by the force of gravity and air resistance. Table 10.1 outlines the characteristics of flow, slide, and fall. Also, refer back to Figure 10.21 as you read details of these three types of mass wasting, described next.
If heavy rain falls on unvegetated soil
the water can saturate the soil to form a slurry of mud and rocks called a debris flow, earthflow, or mudflow depending on the size and sorting of the particles. 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. The advancing front of a debris flow, earthflow, or mudflow often forms tongue-shaped lobes (Figure 10.23). A slow-moving earthflow or mudflow travels at a rate of about 1 meter per year or slower, but others can move as fast as a car speeding along an interstate highway. A debris flow can pick up boulders and automobiles; it can also destroy houses, filling them with muddy sediment and even dislodging them from their foundations.
The combination of tectonic activity and erosion has eliminated
traces of early impact craters from Earth's surface. In contrast, the smaller Moon has lost most of its heat, so tectonic activity is nonexistent. In addition, the Moon's gravitational force is too weak to have retained an atmosphere or significant amounts of surface water. Thus, because neither wind nor flowing water is available to modify the Moon's surface, ancient impact craters have persisted there for billions of years.
Soils are subdivided into
visibly, chemically, or physically distinct layers called soil horizons that form and change as the soil evolves. These horizons mainly are due to the transformation of soil constituents by the chemical, biological, or physical processes taking place in a soil over time and the downward translocation of sediment and ions within the soil profile. Five soil horizons can occur: O, A, E, B, and C (Figure 10.14). Although some soils contain all five horizons as recognizable layers, many soils contain only a subset of the five horizons.
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. For example, limestone is made of the mineral calcite (CaCO3). Calcite barely dissolves in pure water but is quite soluble in acid. If you place a drop of strong acid on limestone, bubbles of carbon dioxide gas instantly form as the calcite dissolves. Water found in nature is never pure. Atmospheric carbon dioxide dissolves in raindrops and reacts to form a weak acid called carbonic acid. This acidity can be intensified when rainwater percolates down through leaf litter on the ground. As a result, even the purest rainwater falling in the Arctic or on remote mountains is slightly acidic.
In hydrolysis
water reacts with one mineral 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. In contrast to feldspar, quartz resists chemical weathering because it dissolves extremely slowly and only to a small extent. It is also hard and has no cleavage, and so resists abrasion during erosion. When granite weathers, the feldspar and other minerals decompose to form clay, but the unaltered quartz grains fall free from the rock. Hydrolysis has so deeply weathered some granites that quartz grains can be pried out with a fingernail at depths of several meters. The rock looks like granite but has the consistency of sand.
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. The growing crystals exert tremendous forces that loosen mineral grains and widen cracks in a process called salt cracking. 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
