Weathering

Weathering is breaking down rocks, soil, and minerals as well as wood and artificial materials by contacting the atmosphere, water, and biological organisms of the Earth. Weathering takes place in situ, i.e. in the same place, with little or no movement. It should therefore not be confused with erosion involving the movement of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity, and then transported and deposited elsewhere.

  • 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 landmarks such as the Grand Canyon, in the U.S. state of Arizona. This massive canyon is 446 kilometers (277 miles) long, as much as 29 kilometers (18 miles) wide, and 1,600 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 lavas, 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 soils.
  • 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 sediments.

Weathering rates depend on several factors. These include the composition of the rock and the minerals it contains as well as the climate of a region.

1.     Rock and Mineral Type

      • Different rock types weather at different rates.
      • Certain types of rock are very resistant to weathering.
      • Igneous rocks, especially intrusive igneous rocks such as granite, weather slowly because it is hard for water to penetrate them.
      • Other types of rock, such as limestone, are easily weathered because they dissolve in weak acids.
      • Rocks that resist weathering remain at the surface and form ridges or hills. Devil’s Tower in Wyoming is an igneous rock from beneath a volcano. As the surrounding less resistant rocks were worn away, the resistant center of the volcano remained behind.
      • Different minerals also weather at different rates.
      • Some minerals in a rock might completely dissolve in water but the more resistant minerals remain. In this case, the rock’s surface becomes pitted and rough.
      • When a less resistant mineral dissolves, more resistant mineral grains are released from the rock.

2.     Climate

A region’s climate strongly influences weathering. Climate is determined by the temperature of a region plus the amount of precipitation it receives. Climate is weather averaged over a long period of time. Chemical weathering increases as:

        • Temperature increases: Chemical reactions proceed more rapidly at higher temperatures. For each 10oC increase in average temperature, the rate of chemical reactions doubles.
        • Precipitation increases: More water allows more chemical reactions. Since water participates in both mechanical and chemical weathering, more water strongly increases weathering.

So how do different climates influence weathering?

      • A cold, dry climate will produce the lowest rate of weathering.
      • A warm, wet climate will produce the highest rate of weathering.
      • The warmer a climate is, the more types of vegetation it will have and the greater the rate of biological weathering . This happens because plants and bacteria grow and multiply faster in warmer temperatures.
      • Some resources are concentrated by weathering processes.
      • In tropical climates, intense chemical weathering carries away all soluble minerals, leaving behind just the least soluble components. The aluminum oxide, bauxite, forms this way and is our main source of aluminum ore.

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:

      • 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.

cyrofracturing

 

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 thermal stress

 

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.
  • Weathering is the first step in formation of soils.
  • Weathering of rocks and deposits helps in the enrichment and concentrations of certain valuable ores of iron, manganese, aluminium, copper etc.
  • Weathering helps in soil enrichment.
  • Without weathering, the concentration of the same valuable material may not be sufficient and economically viable to exploit, process and refine. This is what is called enrichment.
  • Weathering is a natural process, but human activities can speed it up.
  • For example, certain kinds of air pollution increase the rate of weathering.
  • Burning coal, natural gas, and petroleum releases chemicals such as nitrogen oxide and sulfur dioxide into the atmosphere.
  • When these chemicals combine with sunlight and moisture, they change into acids. They then fall back to Earth as acid rain.
  • Acid rain rapidly weathers limestone, marble, and other kinds of stone. The effects of acid rain can often be seen on gravestones, making names and other inscriptions impossible to read.
  • Acid rain has also damaged many historic buildings and monuments.
  • For example, at 71 meters (233 feet) tall, the Leshan Giant Buddha at Mount Emei, China is the world’s largest statue of the Buddha. It was carved 1,300 years ago and sat unharmed for centuries. An innovative drainage system mitigates the natural process of erosion. But in recent years, acid rain has turned the statue’s nose black and made some of its hair crumble and fall.

The main difference between weathering and erosion is that there is weathering whereas erosion involves moving to a new location. Both are caused by wind, water, ice, temperature, and even biological action similar factors. They can also take place together.