Through hydrolysis, a mineral such as potassium feldspar is leached of potassium and changed into a clay mineral. Carbon dioxide CO 2 combines with water as raindrops fall through the atmosphere. This makes a weak acid, called carbonic acid. Carbonic acid is a very common in nature where it works to dissolve rock. Pollutants, such as sulfur and nitrogen, from fossil fuel burning, create sulfuric and nitric acid.
Sulfuric and nitric acids are the two main components of acid rain, which accelerate chemical weathering. Oxidation is a chemical reaction that takes place when oxygen reacts with another element.
Oxygen is very strongly chemically reactive. The most familiar type of oxidation is when iron reacts with oxygen to create rust. Minerals that are rich in iron break down as the iron oxidizes and forms new compounds. Iron oxide produces the red color in soils. Now that you know what chemical weathering is, can you think of some other ways chemical weathering might occur? Chemical weathering can also be contributed to by plants and animals.
As plant roots take in soluble ions as nutrients, certain elements are exchanged. Plant roots and bacterial decay use carbon dioxide in the process of respiration. Skip to main content. Weathering, Erosion, and Deposition. Search for:. Weathering Processes Weathering is the process that changes solid rock into sediments.
Mechanical Weathering Mechanical weathering , also called physical weathering, breaks rock into smaller pieces.
Gravity causes abrasion as a rock tumbles down a mountainside or cliff. When ice melts, the liquid water moves further into the widened spaces. Repeated cycles of freezing and melting eventually pry the rocks apart. The cycles can occur daily when fluctuations of temperature between day and night go from freezing to melting.
Like frost wedging , root wedging happens when plant roots work themselves into cracks, prying the bedrock apart as they grow. Occasionally these roots may become fossilized. Rhizolith is the term for these roots preserved in the rock record.
Tunneling organisms such as earthworms, termites, and ants are biological agents that induce weathering similar to root wedging. Salt expansion, which works similarly to frost wedging , occurs in areas of high evaporation or near- marine environments.
Evaporation causes salts to precipitate out of solution and grow and expand into cracks in rock. Salt expansion is one of the causes of tafoni , a series of holes in a rock. Tafonis, cracks, and holes are weak points that become susceptible to increased weathering. Another phenomena that occurs when salt water evaporates can leave behind a square imprint preserved in a soft sediment , called a h opper crystal.
Chemical weathering is the dominate weathering process in warm, humid environments. It happens when water, oxygen, and other reactants chemically degrade the mineral components of bedrock and turn them into water-soluble ions which can then be transported by water.
Higher temperatures accelerate chemical weathering rates. Chemical and mechanical weathering work hand-in-hand via a fundamental concept called surface-area-to-volume ratio. Chemical weathering only occurs on rock surfaces because water and reactants cannot penetrate solid rock. Mechanical weathering penetrates bedrock , breaking large rocks into smaller pieces and creating new rock surfaces.
This exposes more surface area to chemical weathering , enhancing its effects. In other words, higher surface-area-to-volume ratios produce higher rates of overall weathering. Carbonic acid H 2 CO 3 forms when carbon dioxide, the fifth-most abundant gas in the atmosphere , dissolves in water.
This happens naturally in clouds, which is why precipitation is normally slightly acidic. Carbonic acid is an important agent in two chemical weathering reactions, hydrolysis and dissolution. Hydrolysis occurs via two types of reactions. In another type of hydrolysis , carbonic acid molecules react directly with minerals , especially those containing silicon and aluminum i.
Feldspars , to form molecules of clay minerals. Hydrolysis is the main process that breaks down silicate rock and creates clay minerals. The following is a hydrolysis reaction that occurs when silica-rich feldspar encounters carbonic acid to produce water-soluble clay and other ions:. Clay minerals are platy silicates or phyllosilicates see Chapter 3, Minerals similar to micas, and are the main components of very fine-grained sediment.
The dissolved substances may later precipitate into chemical sedimentary rocks like evaporite and limestone , as well as amorphous silica or chert nodules. Dissolution is a hydrolysis reaction that dissolves minerals in bedrock and leaves the ions in solution , usually in water. Some evaporites and carbonates , like salt and calcite , are more prone to this reaction; however, all minerals can be dissolved. Non-acidic water, having a neutral pH of 7, will dissolve any mineral , although it may happen very slowly.
Water with higher levels of acid, naturally or man-made, dissolves rocks at a higher rate. Natural rainwater can be highly acidic, with pH levels as low as 2. Dissolution can be enhanced by a biological agent, such as when organisms like lichen and bacteria release organic acids onto the rocks they are attached to. Regions with high humidity airborne moisture and precipitation experience more dissolution due to greater contact time between rocks and water. Minerals at the top of the Bowen series crystallize under high temperatures and pressures, and chemically weather at a faster rate than minerals ranked at the bottom.
Olivine and pyroxene are rarely found as end products of weathering because they tend to break down into elemental ions. Dissolution is also noteworthy for the special geological features it creates. In places with abundant carbonate bedrock , dissolution weathering can produce a karst topography characterized by sinkholes or caves see Chapter 10, Mass Wasting.
The figure shows a cave formation created from dissolution followed by precipitation — groundwater saturated with calcite seeped into the cavern, where evaporation caused the dissolved minerals to precipitate out.
Oxidation , the chemical reaction that causes rust in metallic iron, occurs geologically when iron atoms in a mineral bond Two or more atoms or ions that are connected chemically. Any minerals containing iron can be oxidized. The resultant iron oxides may permeate a rock if it is rich in iron minerals. Oxides may also form a coating that covers rocks and grains of sediment , or lines rock cavities and fractures. If the oxides are more susceptible to weathering than the original bedrock , they may create void spaces inside the rock mass or hollows on exposed surfaces.
These iron oxides coat and bind mineral grains together into sedimentary rocks in a process called cementation , and often give these rocks a dominant color. These oxides can permeate a rock that is rich in iron-bearing minerals or can be a coating that forms in cavities or fractures. When the minerals replacing existing minerals in bedrock are resistant to weathering , iron concretions may occur in the rock. When bedrock is replaced by weaker oxides , this process commonly results in void spaces and weakness throughout the rock mass and often leaves hollows on exposed rock surfaces.
Erosion is a mechanical process, usually driven by water, gravity, see Chapter 10 , wind, or ice see Chapter 14 that removes sediment from the place of weathering. This is well demonstrated in the cliffs of the Grand Canyon. Rocks with different levels erosion resistant also create the unique-looking features called hoodoos in Bryce Canyon National Park and Goblin Valley State Park in Utah.
If erosion does not remove the sediment significantly, organisms can access the mineral content of the sediments. These organisms turn minerals , water, and atmospheric gases into organic substances that contribute to the soil A type of non-eroded sediment mixed with organic matter, used by plants. The organic component of soil A type of non-eroded sediment mixed with organic matter, used by plants. Nitrogen is the most common element in the atmosphere , but it exists in a form most life forms are unable to use.
Special bacteria found only in soil A type of non-eroded sediment mixed with organic matter, used by plants. These nitrogen-fixing bacteria absorb nitrogen from the atmosphere and convert it into nitrogen compounds. These compounds are absorbed by plants and used to make DNA, amino acids, and enzymes. Animals obtain bioavailable nitrogen by eating plants, and this is the source of most of the nitrogen used by life.
That nitrogen is an essential component of proteins and DNA. Freshly created volcanic soil A type of non-eroded sediment mixed with organic matter, used by plants. The nature of the soil A type of non-eroded sediment mixed with organic matter, used by plants.
For example, soil A type of non-eroded sediment mixed with organic matter, used by plants. The quantity and chemistry of organic matter of soil A type of non-eroded sediment mixed with organic matter, used by plants. Temperature and precipitation , two major weathering agents, are dependent on climate. Fungi and bacteria contribute organic matter and the ability of soil A type of non-eroded sediment mixed with organic matter, used by plants.
In well-formed soil A type of non-eroded sediment mixed with organic matter, used by plants. These soil A type of non-eroded sediment mixed with organic matter, used by plants. Each soil horizon reflects climate , topography, and other soil A type of non-eroded sediment mixed with organic matter, used by plants. The horizons are assigned names and letters. Differences in naming schemes depend on the area, soil A type of non-eroded sediment mixed with organic matter, used by plants.
The figure shows a simplified soil profile that uses commonly designated names and letters. O Horizon : The top horizon is a thin layer of predominantly organic material, such as leaves, twigs, and other plant parts that are actively decaying into humus.
A Horizon : The next layer, called topsoil , consists of humus mixed with mineral sediment. As precipitation soaks down through this layer, it leaches out soluble chemicals. In wet climates with heavy precipitation this leaching out produces a separate layer called horizon E, the leaching or eluviation zone.
B Horizon : Also called subsoil , this layer consists of sediment mixed with humus removed from the upper layers. The subsoil is where mineral sediment is chemically weathered. The amount of organic material and degree of weathering decrease with depth. The upper subsoil zone, called regolith , is a porous mixture of humus and highly weathered sediment.
In the lower zone, saprolite , scant organic material is mixed with largely unaltered parent rock. C Horizon : This is substratum and is a zone of mechanical weathering. Here, bedrock fragments are physically broken but not chemically altered. This layer contains no organic material. R Horizon : The final layer consists of unweathered, parent bedrock and fragments. The United States governing body for agriculture, the USDA, uses a taxonomic classification to identify soil A type of non-eroded sediment mixed with organic matter, used by plants.
Xoxisols or laterite soil A type of non-eroded sediment mixed with organic matter, used by plants. Ardisol forms in dry climates and can develop layers of hardened calcite , called caliche.
Andisols originate from volcanic ash Volcanic tephra that is less than 2 mm in diameter. Alfisols contain silicate clay minerals. These two soil A type of non-eroded sediment mixed with organic matter, used by plants.
In general, color can be an important factor in understanding soil A type of non-eroded sediment mixed with organic matter, used by plants. Black soil A type of non-eroded sediment mixed with organic matter, used by plants. This is true for many sedimentary rocks as well. Not only is soil A type of non-eroded sediment mixed with organic matter, used by plants. Careless or uninformed human activity can seriously damage soil A type of non-eroded sediment mixed with organic matter, used by plants.
A prime example is the famous Dust Bowl disaster of the s, which affected the midwestern United States. The damage occurred because of large-scale attempts develop prairieland in southern Kansas, Colorado, western Texas, and Oklahoma into farmland. The prairie soil A type of non-eroded sediment mixed with organic matter, used by plants.
With government encouragement, settlers moved in to homestead the region. They plowed vast areas of prairie into long, straight rows and planted grain. The plowing broke up the stable soil profile and destroyed the natural grasses and plants, which had long roots that anchored the soil A type of non-eroded sediment mixed with organic matter, used by plants. The grains they planted had shallower root systems and were plowed up every year, which made the soil A type of non-eroded sediment mixed with organic matter, used by plants.
The plowed furrows were aligned in straight rows running downhill, which favored erosion and loss of topsoil. The local climate does not produce sufficient precipitation to support non- native grain crops, so the farmers drilled wells and over-pumped water from the underground aquifers. The grain crops failed due to lack of water, leaving bare soil A type of non-eroded sediment mixed with organic matter, used by plants. Particles of midwestern prairie soil A type of non-eroded sediment mixed with organic matter, used by plants.
Huge dust storms called black blizzards made life unbearable, and the once-hopeful homesteaders left in droves. The lingering question is whether we have learned the lessons of the dust bowl, to avoid creating it again. What is the difference between weathering and erosion?
Remember that weathering is breakdown of rocks, erosion is movement of resulting materials. How do chemical and mechanical weathering work together to create sediment? By increasing the surface area, mechanical weathering allows chemical weathering to take place more readily. Which of these is NOT a component of soil A type of non-eroded sediment mixed with organic matter, used by plants. Which of the following is NOT an example of chemical weathering? Oxidation rusting , dissolution , hydrolysis , and formation of soil A type of non-eroded sediment mixed with organic matter, used by plants.
Exfoliation is an example of mechanical weathering. Sedimentary rock is classified into two main categories: clastic and chemical. Clastic or detrital sedimentary rocks are made from pieces of bedrock , sediment , derived primarily by mechanical weathering. Clastic rocks may also include chemically weathered sediment. Clastic rocks are classified by grain shape , grain size , and sorting. Chemical sedimentary rocks are precipitated from water saturated with dissolved minerals.
Chemical rocks are classified mainly by composition of minerals in the rock. Lithification turns loose sediment grains, created by weathering and transported by erosion , into clastic sedimentary rock via three interconnected steps.
Deposition happens when friction and gravity overcome the forces driving sediment transport, allowing sediment to accumulate. Compaction occurs when material continues to accumulate on top of the sediment layer, squeezing the grains together and driving out water. The mechanical compaction is aided by weak attractive forces between the smaller grains of sediment.
Groundwater typically carries cementing agents into the sediment. These minerals , such as calcite , amorphous silica, or oxides , may have a different composition than the sediment grains. Cementation is the process of cementing minerals coating the sediment grains and gluing them together into a fused rock.
Diagenesis is an accompanying process to lithification and is a low- temperature form of rock metamorphism see Chapter 6, Metamorphic Rock. During diagenesis , sediments are chemically altered by heat and pressure. A classic example is aragonite CaCO 3 , a form of calcium carbonate that makes up most organic shells. When lithified aragonite undergoes diagenesis , the aragonite reverts to calcite CaCO 3 , which has the same chemical formula but a different crystalline structure.
In sedimentary rock containing calcite and magnesium Mg , diagenesis may transform the two minerals into dolomite CaMg CO 3 2. Diagenesis may also reduce the pore Empty space in a geologic material, either within sediments, or within rocks.
Can be filled by air, water, or hydrocarbons. The processes of cementation , compaction , and ultimately lithification occur within the realm of diagenesis , which includes the processes that turn organic material into fossils. Detrital or clastic sedimentary rocks consist of preexisting sediment pieces that comes from weathered bedrock. Most of this is mechanically weathered sediment , although some clasts may be pieces of chemical rocks.
This creates some overlap between the two categories, since clastic sedimentary rocks may include chemical sediments. Detrital or clastic rocks are classified and named based on their grain size. Detrital rock is classified according to sediment grain size , which is graded from large to small on the Wentworth scale see figure.
Grain size is the average diameter of sediment fragments in sediment or rock. Grain sizes are delineated using a log base 2 scale. For example, the grain sizes in the pebble class are 2. These include, boulders, cobbles, granules, and gravel. Sand has a grain size between 2 mm and 0. Sediment grains smaller than sand are called silt.
Silt is unique; the grains can be felt with a finger or as grit between your teeth, but are too small to see with the naked eye. Sorting describes the range of grain sizes within sediment or sedimentary rock.
It is important to note that soil A type of non-eroded sediment mixed with organic matter, used by plants. When reading the story told by rocks, geologists use sorting to interpret erosion or transport processes, as well as deposition energy. For example, wind-blown sands are typically extremely well sorted, while glacial deposits are typically poorly sorted.
These characteristics help identify the type of erosion process that occurred. Coarse-grained sediment and poorly sorted rocks are usually found nearer to the source of sediment , while fine sediments are carried farther away.
In a rapidly flowing mountain stream you would expect to see boulders and pebbles. In a lake fed by the stream , there should be sand and silt deposits.
If you also find large boulders in the lake, this may indicate the involvement of another sediment transport process, such as rockfall caused by ice- or root-wedging. Rounding is created when angular corners of rock fragments are removed from a piece of sediment due to abrasion during transport. Well-rounded sediment grains are defined as being free of all sharp edges. Very angular sediment retains the sharp corners.
More rounded grains imply a longer erosion time or transport distance, or more energetic erosional process. Mineral hardness is also a factor in rounding. Composition describes the mineral components found in sediment or sedimentary rock and may be influenced by local geology, like source rock and hydrology. Other than clay, most sediment components are easily determined by visual inspection see Chapter 3, Minerals. The most commonly found sediment mineral is quartz because of its low chemical reactivity and high hardness , making it resistant to weathering , and its ubiquitous occurrence in continental bedrock.
Other commonly found sediment grains include feldspar and lithic fragments. Lithic fragments are pieces of fine-grained bedrock , and include mud chips , volcanic clasts, or pieces of slate. This is because the local rock is composed almost entirely of basalt and provides an abundant source of dark colored clasts loaded with mafic minerals.
According to the Goldich Dissolution Series , clasts high in mafic minerals are more easily destroyed compared to clasts composed of felsic minerals like quartz. Geologists use provenance to discern the original source of sediment or sedimentary rock. Provenance is determined by analyzing mineral composition and types of fossils present, as well as textural features like sorting and rounding. In quartz sandstone , sometimes called quartz arenite SiO 2 , provenance may be determined using a rare, durable clast mineral called zircon ZrSiO 4.
Zircon , or zirconium silicate , contains traces of uranium, which can be used for age-dating the source bedrock that contributed sediment to the lithified sandstone rock see Chapter 7, Geologic Time. Clastic rocks are classified according to the grain size of their sediment. Coarse-grained rocks contain clasts with a predominant grain size larger than sand. Typically, smaller sediment grains, collectively called groundmass or matrix, fill in much of the volume between the larger clasts, and hold the clasts together.
Conglomerates are rocks containing coarse rounded clasts, and breccias contain angular clasts see figure. Both conglomerates and breccias are usually poorly sorted. Medium-grained rocks composed mainly of sand are called sandstone , or sometimes arenite if well sorted. Sediment grains in sandstone can having a wide variety of mineral compositions, roundness, and sorting.
Quartz sandstone contains predominantly quartz sediment grains. Sandstone that contains feldspar , which weathers more quickly than quartz , is useful for analyzing the local geologic history. Weathering breaks down and loosens the surface minerals of rock so they can be transported away by agents of erosion such as water, wind and ice.
There are two types of weathering: mechanical and chemical. Mechanical Weathering Mechanical weathering is the disintegration of rock into smaller and smaller fragments. Frost action is an effective form of mechanical weathering. When water trickles down into fractures and pores of rock, then freezes, its volume increases by almost 10 percent. This causes outward pressure of about 30, pounds per square inch at Frost action causes rocks to be broken apart into angular fragments.
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Encyclopedic Entry Vocabulary. Weathering describes the breaking down or dissolving of rock s and mineral s on the surface of the Earth. Water, ice, acids, salts, plants, animals, and changes in temperature are all agents of weathering.
Once a rock has been broken down, a process called erosion transports the bits of rock and mineral away. No rock on Earth is hard enough to resist the forces of weathering and erosion. Together, these processes carved landmark s such as the Grand Canyon, in the U. This massive canyon is kilometers miles long, as much as 29 kilometers 18 miles wide, and 1, meters 1 mile deep. Weathering and erosion constantly change the rocky landscape of Earth.
Weathering wears away exposed surfaces over time. The length of exposure often contributes to how vulnerable a rock is to weathering. Rocks, such as lava s, that are quickly buried beneath other rocks are less vulnerable to weathering and erosion than rocks that are exposed to agents such as wind and water. As it smoothes rough, sharp rock surfaces, weathering is often the first step in the production of soil s.
Tiny bits of weathered minerals mix with plants, animal remains, fungi, bacteria, and other organisms. A single type of weathered rock often produces infertile soil, while weathered materials from a collection of rocks is richer in mineral diversity and contributes to more fertile soil. Soils types associated with a mixture of weathered rock include glacial till , loess , and alluvial sediment s. Weathering is often divided into the processes of mechanical weathering and chemical weathering.
Biological weathering , in which living or once-living organisms contribute to weathering, can be a part of both processes. Mechanical weathering , also called physical weathering and disaggregation, causes rocks to crumble.
Water, in either liquid or solid form, is often a key agent of mechanical weathering. For instance, liquid water can seep into cracks and crevice s in rock. If temperatures drop low enough, the water will freeze. When water freezes, it expand s.
The ice then works as a wedge. It slowly widens the cracks and splits the rock. When ice melts, liquid water performs the act of erosion by carrying away the tiny rock fragments lost in the split. This specific process the freeze-thaw cycle is called frost weathering or cryofracturing. Temperature changes can also contribute to mechanical weathering in a process called thermal stress.
Changes in temperature cause rock to expand with heat and contract with cold. As this happens over and over again, the structure of the rock weakens. Over time, it crumbles.
Rocky desert landscapes are particularly vulnerable to thermal stress. The outer layer of desert rocks undergo repeated stress as the temperature changes from day to night. Eventually, outer layers flake off in thin sheets, a process called exfoliation. Exfoliation contributes to the formation of bornhardt s, one of the most dramatic features in landscapes formed by weathering and erosion.
Bornhardts are tall, domed, isolated rocks often found in tropical areas. Sugarloaf Mountain, an iconic landmark in Rio de Janeiro, Brazil, is a bornhardt.
Changes in pressure can also contribute to exfoliation due to weathering. In a process called unloading, overlying materials are removed.
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