Role Of Physical Weathering
The role of physical weathering, or erosion, on weathering profile development was discussed in Section 184.108.40.206.3. More fundamentally, chemical weathering, in combination with physical erosion, is the process that cycles elements globally. Berner and Berner estimated that the combined denudation rate of the continents is 252 tons km 1 year 1 while the average chemical denudation rate is about 20% of that value. As expected, the absolute rates and relative ratios of physical to chemical weathering are strongly scaled to differences in topography. For example, the percentage of chemical to physical denudation rates for watersheds of high relief such as the Ganges River, which drains the Himalayas, is less than 10% but approaches 45% for lowland rivers such as the Congo River. Global compilations of denudation rates including data from the steep, but tectonically quiescent, Sri Lanka have shown that tectonic activity, rather than high relief alone, produces the high physical denudation rates seen in many mountainous watersheds . Several researchers have identified a correlation between chemical weathering rates and total denudation or physical erosion rates from which they infer that processes that control or enhance physical erosion, such as uplift, also control chemical weathering rates.
J. Viers, … J. Gaillardet, in, 2014
Mechanical Weathering: Definition Process Types And Examples
Mechanical weathering is the process through which large rocks are broken into increasingly smaller pieces. Sometimes referred to as physical weathering, the process normally happens near the Earths surface. Can you believe that the tiny sand grains you see at the beach were once part of massive rocks?
It involves mechanical processes that disintegrate a rock, like tree roots growing in cracks in a rock and eventually breaking it up. Mechanical weathering doesnt change the chemical nature of the rocks.
In this article, we look at how mechanical weathering works, its types, and some examples. Lets dive in.
Answer In Brief What Is Mechanical Weathering
Weathering is the disintegration, breaking down and decomposition of components in the earths crust through the action of natural or other agents. Weathering can occur due to exposure to the earths atmosphere. Weathering is controlled and determined by various factors like temperature, rainfall, vegetation and movements of other components in the earths crust. Weathering can be mechanical, chemical or biological.
Mechanical or physical weathering is the disintegration and breaking down of components in the earths crust through the action of physical forces. It does not change the chemical composition of rocks. It occurs due to the changes in temperature during the day and the night in cold and dry places. Water, temperature, wind, pressure and other physical factors are the agents of disintegration. It can be again classified into different types- exfoliation, block disintegration and granular weathering.
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How Are Animals Involved In Mechanical Weathering
Animals are also responsible for mechanical weathering albeit to a relatively small extent. The rare mechanical weathering linked to animals is predominantly caused by burrowing animals who dig burrows beneath the earthâs surface, and these burrows cause the surface above them to weaken and susceptible to collapse. However, there are instances where marine invertebrates are known to cause mechanical weathering. Piddocks, also known as angel wings are mollusks which use their sharp teeth-like structures to burrow into the rock where they then reside in for their lifespan. Each piddock leaves behind a hole in the rock after it dies which caused such a rock to be susceptible to breaking down.
How Does Ice Cause Mechanical Weathering
Mechanical weathering can also occur in the form of frost weathering. Frost weathering is the process which involves the action of ice on a rock and is also known as frost wedging or cryofracture. Frost weathering is common in mountain peaks where extremely low temperatures allow for the freezing of water covering the rocks and in any cracks found on the rocks. When frozen the icy water expand causing a significant amount of stress on the rock which in the long-run degenerates. When frozen to ice, water expands by up to 10% in size and the entire size is confined inside the crevices of rock which lead to the exertion of high amount of pressure. Estimates put the pressure placed on the rock by the freezing water with a temperature of -7.6 degrees Fahrenheit at about 30,000 pounds per square inch. Ice is also a medium of mechanical weathering while on transit through moving ice in avalanches. These powerful forces of nature carry with them millions of tons of debris, snow, and ice down a mountain slope and any loose rock on their path quickly erodes away. The debris in avalanches also features rocks which are further disintegrated by the powerful pressure and force created by the rapidly moving ice and snow.
Historical Geology/mechanical Weathering And Erosion
In this article, we shall present a brief overview of erosion and mechanical weathering. It is possible to be brief because the erosional processes involved will be discussed at length in subsequent articles, so there is no need to do more than sketch out the topic in brief.
The Role Of Mechanical Erosion
In a recent paper, West et al. assembled a new global compilation of data on weathering rates. The overall variation in silicate weathering rates with physical erosion rates, rainfall, and temperature could be quantitatively described by a parameterization based on their limiting relationships. The main result is that mineral supply limits weathering in situations where total erosion rate is low whereas at higher erosion rates, temperature and runoff control the CWR, suggesting a kinetic control.
In some areas, such as in the High Himalayan Crystalline zone, the high rate of uplift causing steep relief and high elevation surfaces was considered by some studies to be a limiting factor for chemical erosion . Along with lithology and climate, rapid physical erosion instead of slow processes of soil development would be responsible for low CWRs in the High Himalayan Crystalline zone .
Figure 12. Loglog relation between chemical and physical weathering rates established by Millot et al. using data from granitoid watersheds and basaltic basins.
J. Viers, … J. Gaillardet, in, 2007
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What Is Abrasion In Mechanical Weathering
Abrasion is the process where rocks are eroded by continuous friction acting on the rock surfaces. Such friction is brought about by rapidly moving particles colliding with the rock while such particles are being carried by water, wind or glaciers. The velocity and size of the moving particles are responsible for the intensity of the mechanical weathering process. Abrasion is also present in the formation of coastal geography through ocean waves. These waves which consistently hit the shore carry with them fragments which ultimately lead to the erosion of rocks. Winds are also leading causes of mechanical weathering through abrasion as strong winds moving particles of sand and rock fragments collide with a rock and with time cause such rock to display denudation. Such forms of mechanical weathering is usually present in desert landscapes where massive sand storms which carry with them millions upon millions of dust particles which have eroded away the desert rocks.
Weathering Produces Other Natural Resources Such As Clay Which Is Used For Making Bricks
Clay is also used for pottery. Another natural resource found from weathering is bauxite which is aluminum ore used in the extraction of aluminum
Also weathering is significant since it prepares the land on which the agents of erosion and transportation act.
These agents then modify the original landforms which in turn influence the kind of human activities which can be undertaken in such regions. The process of weathering is therefore important in supporting life.
Last but not least, some weathered rocks like the granitic tors are very fascinating. They, therefore, act as a tourist attraction.
An example includes the Bismarck rock in Mwanza Tanzania. Some of these rocks look so unique that local people do not understand how came to be.
They have turned them into local shrines where they make offerings.
To sum up, weathering is very important to human life since it helps us to describe the formation of various landforms, leading to the formation of building materials like clay and the remnant of weathered rock like granitic tors can act as a tourist attraction.
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date: 13 January 2023
Physical Weathering Expansion Resulting From Unloading
The release of pressure in the rock also causes physical weathering. The pressure is released due to the removal of overlying material by erosion. This process is known as unloading. It is a typical phenomenon resembling the elastic property of rubber. As the pressure is released above the rock, the underlying rock elastically compensates for the erosional loss by expanding perpendicular to the direction of pressure release. This unloading of pressure by the rock results in a horizontal fracture. These fractures are arranged horizontally and are called sheet fractures or exfoliation sheets.
Read More: Biological Weathering
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How Do We Know
In this particular case, asking “how do we know?” seems almost superfluous, for the processes involved are neither hidden nor subtle. We can observe a sandstorm we can see how the head of a waterfall shifts year on year, or how a river shifting its course scours out a new bed we can see how cliffs crumble and the effects of landslides. The fact that glaciers carry boulders is evident, and the distance they travel each year can be measured as can the quantity of sediment discharged at the mouth of a river.
A more interesting question is, how do we know that these processes have happened when the agency that caused them is no longer present? How, for example, do we identify the courses of glaciers long since melted, or of rivers that have dried up or shifted their beds? These are questions that we shall review in subsequent articles.
Exfoliation Weathering Or Unloading
This type of weathering takes place when the cracks develop parallel to the land surface. As a result, a consequence of the reduction in pressure takes place during uplift and erosion. In exfoliation, rock breaks apart in layers that are parallel to the Earths surface. Exfoliation is common in plutonic igneous rocks since they are exposed to great pressure.
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What Is The Other Name For Physical Weathering
The other name of Physical weathering is Mechanical weathering.
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Chemical Weathering By Oxygen
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.
- Dynamic Earth: Introduction to Physical Geography. : R. Adam Dastrup. Located at: . Project: Open Geography Education. License: CC BY-SA: Attribution-ShareAlike
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What Is An Example Of Weathering
An example of weathering is salt water crystal growth. This is when salt water collects in cracks in rocks and cliff faces. When the water evaporates, salt crystals are left behind. When the temperature rises, the salt crystals expand, putting pressure on the rock, eventually causing it to break apart.
Physical Weathering Thermal Expansion & Contraction Or Insolation
Minerals have their limits of expansion and contraction. With rising temperatures, the mineral expands and pushes against the neighbouring rocks. On the other hand, the fall in temperature results in the contraction of the rocks. The diurnal temperature changes are accompanied by contraction and expansion regularly. The process is effective in dry climates and high-elevation areas. The exfoliated surface is more rounded because of the differential heating and cooling . For example granites.
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Process Of Mechanical Weathering
The main process in mechanical weathering is abrasion, a physical process by which rocks and clasts are reduced in size. Abrasion by ice, water, and wind processes loaded with sediments can have immense cutting power. The worlds greatest gorges, valleys, and ravines are largely a result of abrasion.
In glacial regions, massive masses of moving ice embedded with soil and rock particles grind down rocks in their path, carrying away large volumes of material. Tree roots sometimes penetrate cracks in rocks and wedge them apart, leading to disintegration.
Temperature fluctuations from daytime to nighttime may cause a rock to expand and contract. This weakens the rock, causing it to fracture and eventually disintegrate. To have a deeper understanding of these processes, lets explore the different types of mechanical weathering.
What Is Mechanical Weathering
Rocks break due to various reasons like wind, ice, weather, water, acids and chemical reactions. Even if an external force like the growing of plants takes place on rocks, the roots lead to weathering. Lets see how mechanical weathering takes place.
Mechanical weathering is also known as physical weathering. In this type of weathering, a large rock is disintegrated into smaller pieces of rocks. When rocks disintegrate or break up without experiencing any change in their chemical composition, it is known as mechanical weathering. Thermal expansion and contraction that happens due to the increase or decrease in temperature. This process causes the rock to break into fragments.
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Examples Of Mechanical Weathering
Real world examples of mechanical weathering are easy to spot. They include the following:
Chemical Weathering By Water
A water molecule has a very simple chemical formula, H2O, two hydrogen atoms bonded to one oxygen atom. But water is pretty remarkable in terms of all the things it can do. Water is a polar molecule the positive side of the molecule attracts negative ions and the negative side attracts positive ions. So water molecules separate the ions from their compounds and surround them. Water can completely dissolve some minerals, such as salt. Hydrolysis is the name of the chemical reaction between a chemical compound and water. When this reaction takes place, water dissolves ions from the mineral and carries them away. These elements have undergone leaching. Through hydrolysis, a mineral such as potassium feldspar is leached of potassium and changed into a clay mineral. Clay minerals are more stable at the Earths surface.
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How Does Salt Cause Mechanical Weathering
Salt can be more closely linked with chemical weathering than mechanical weathering considering the composition of the compound, and its dissolving ability in the water. However, salt is another primary cause of mechanical weathering in rocks. After being inside rock crevices primarily located along the shorelines of salt lakes or seas, increased heat causes the water to evaporate, leaving the salt inside the rockâs crevices. These salt crystals are some of the leading mechanical weathering causes whereby these crystals expand upon the increase in temperature and hence exerting tremendous pressure on the rock.
Mechanical Weathering Effects On Coasts
The table below summarises the information mentioned above and the effects on coasts in particular.
|Weathering process||Effect on coasts|
|Saltwater crystal growth||Saltwater collects in cracks in rocks and cliff faces warm temperatures evaporate the water and leave the salt crystals behind. The heat expands the crystals, exerting enormous pressure on the rock, causing it to disintegrate. This is the primary weathering that occurs in coastal areas.|
|Freeze-thaw||Water collects in cracks in rocks. This water freezes, making the water expand and creating a bigger crack. When it thaws, the water is released from the crack, leaving behind a bigger crack than before. Repeated freezing and thawing will eventually cause the rock to split/disintegrate. This type of weathering is most effective on high latitude coasts that have significant rainfall.|
|Wetting and drying||The clay-rich soil you often find on the coast expands when wet and contracts when dry. The constant expansion and contraction will lead the clay-rich rock to crack. This expansion and contraction of minerals are most effective on clay and in micro-tidal environments.|
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