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
Mechanical weathering, also called physical weathering and disaggregation, causes rocks to crumble.
Cryofracturing:
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- Water, in either liquid or solid form, is often a key agent of mechanical weathering.
- For instance, liquid water can seep into cracks and crevices in rock. If temperatures drop low enough, the water will freeze.
- When water freezes, it expands.
- 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.
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Exfoliation due to thermal stress:
- 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 contributes to the formation of bornhardts, 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.
Exfoliation due to pressure release or unloading:
- Changes in pressure can also contribute to exfoliation due to weathering. In a process called unloading, overlying materials are removed.
- The underlying rocks, released from overlying pressure, can then expand.
- As the rock surface expands, it becomes vulnerable to fracturing in a process called
Haloclasty:
- Salt also works to weather rock in a process called haloclasty.
- Saltwater sometimes gets into the cracks and pores of rock.
- If the saltwater evaporates, salt crystals are left behind.
- As the crystals grow, they put pressure on the rock, slowly breaking it apart.
- Haloclasty is not limited to coastal landscapes.
- Salt upwelling, the geologic process in which underground salt domes expand, can contribute to weathering of the overlying rock.
- Structures in the ancient city of Petra, Jordan, were made unstable and often collapsed due to salt upwelling from the ground below.
Honeycomb weathering :
- Honeycomb weathering is associated with haloclasty.
- As its name implies, honeycomb weathering describes rock formations with hundreds or even thousands of pits formed by the growth of salt crystals.
- Honeycomb weathering is common in coastal areas, where sea sprays constantly force rocks to interact with salts.
Biological weathering:
- Plants and animals can be agents of mechanical weathering.
- The seed of a tree may sprout in soil that has collected in a cracked rock.
- As the roots grow, they widen the cracks, eventually breaking the rock into pieces.
- Over time, trees can break apart even large rocks.
- Even small plants, such as mosses, can enlarge tiny cracks as they grow.
- Animals that tunnel underground, such as moles and prairie dogs, also work to break apart rock and soil.
- Other animals dig and trample rock aboveground, causing rock to slowly crumble.
Chemical Weathering
Chemical weathering changes the molecular structure of rocks and soil. Clay minerals, including quartz, are among the most common byproducts of chemical weathering. Clays make up about 40% of the chemicals in all sedimentary rocks on Earth.
Carbonation:
- Carbon dioxide from the air or soil sometimes combines with water in a process called carbonation. This produces a weak acid, called carbonic acid , that can dissolve rock.
- Carbonic acid is especially effective at dissolving limestone.
- When carbonic acid seeps through limestone underground, it can open up huge cracks or hollow out vast networks of caves.
- Carlsbad Caverns National Park, in the U.S. state of New Mexico, includes more than 119 limestone caves created by weathering and erosion. The largest is called the Big Room. With an area of about 33,210 square meters (357,469 square feet), the Big Room is the size of six football fields.
Solution:
- Sometimes, chemical weathering dissolves large portions of limestone or other rock on the surface of the Earth to form a landscape called karst.
- In these areas, the surface rock is pockmarked with holes, sinkholes, and caves.
- One of the world’s most spectacular examples of karst is Shilin, or the Stone Forest, near Kunming, China. Hundreds of slender, sharp towers of weathered limestone rise from the landscape.
Oxidation:
- Another type of chemical weathering works on rocks that contain iron.
- These rocks turn to rust in a process called oxidation.
- Rust is a compound created by the interaction of oxygen and iron in the presence of water.
- As rust expands, it weakens rock and helps break it apart.
Hydration :
- Hydration is a form of chemical weathering in which the chemical bonds of the mineral are changed as it interacts with water.
- One instance of hydration occurs as the mineral anhydrite reacts with groundwater.
- The water transforms anhydrite into gypsum, one of the most common minerals on Earth.
Hydrolysis:
- Another familiar form of chemical weathering is hydrolysis.
- In the process of hydrolysis, a new solution (a mixture of two or more substances) is formed as chemicals in rock interact with water.
- In many rocks, for example, sodium minerals interact with water to form a saltwater solution.
- Hydration and hydrolysis contribute to flared slopes, another dramatic example of a landscape formed by weathering and erosion.
- Flared slopes are concave rock formations sometimes nicknamed “wave rocks.” Their c-shape is largely a result of subsurface weathering, in which hydration and hydrolysis wear away rocks beneath the landscape’s surface.
Biological weathering:
- Living or once-living organisms can also be agents of chemical weathering.
- The decaying remains of plants and some fungi form carbonic acid, which can weaken and dissolve rock.
- Some bacteria can weather rock in order to access nutrients such as magnesium or potassium.
