Chemical weathering involves the interaction of rock with mineral solutions (chemicals) to change the composition of rocks. In this process, water interacts with minerals to create various chemical reactions and transform the rocks.
Chemical weathering is a gradual and ongoing process as the mineralogy of the rock adjusts to the near-surface environment. Secondary minerals develop from the original primary minerals of the rock.
In this the processes oxidation and hydrolysis are the most frequent chemical processes that take place.
Chemical weathering is enhanced by such geological agents as the presence of water and oxygen, as well as biological agents as the acids produced by microbial and plant root metabolism.
What is Chemical Weathering?
Chemical Weathering Chemical weathering changes the molecular structure of rocks and soil. For instance, 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. Sometimes, chemical weathering dissolves large portions of limestone or other rock on the surface of the Earth to form a landscape called karst.
How Chemical Weathering Happens
Chemical weathering does not break rocks into smaller fragments through wind, water, and ice (that’s physical weathering). Nor does it break rocks apart through the action of plants or animals (that’s biological weathering). Instead, it changes the chemical composition of the rock, usually through carbonation, hydration, hydrolysis or oxidation.
Chemical weathering alters the composition of the rock material toward surface minerals, such as clays. It attacks minerals that are relatively unstable in surface conditions, such as the primary minerals of igneous rocks like basalt, granite or peridotite. It can also occur in sedimentary and metamorphic rocks and is an element of corrosion or chemical erosion.
Water is especially effective at introducing chemically active agents by way of fractures and causing rocks to crumble piecemeal. Water may also loosen thin shells of material (in spheroidal weathering). Chemical weathering may include shallow, low-temperature alteration.
Let’s take a look at the four main types of chemical weathering that were mentioned earlier. It should be noted that these are not the only forms, just the most common.
Types of Chemical Weathering
There are different types of chemical weathering processes, such as solution, hydration, hydrolysis, carbonation, oxidation, reduction, and chelation. Some of these reactions occur more easily when the water is slightly acidic.
There are different types of chemical weathering processes, such as solution, hydration, hydrolysis, carbonation, oxidation, reduction, and chelation. Some of these reactions occur more easily when the water is slightly acidic.
Carbonation
Carbonation occurs when rain, which is naturally slightly acidic due to atmospheric carbon dioxide (CO2), combines with calcium carbonates (CaCO3), such as limestone or chalk. The interaction forms calcium bicarbonate, or Ca(HCO3)2.
Rain has a normal pH level of 5.0-5.5, which alone is acidic enough to cause a chemical reaction. Acid rain, which is unnaturally acidic from atmospheric pollution, has a pH level of 4 (a lower number indicates greater acidity while a higher number indicates greater basicity).
Carbonation, sometimes referred to as dissolution, is the driving force behind the sinkholes, caverns, and underground rivers of karst topography.
Hydration
Hydration occurs when water reacts with an anhydrous mineral, creating a new mineral. The water is added to the crystalline structure of a mineral, which forms a hydrate.
Anhydrite, which means “waterless stone,” is a calcium sulfate (CaSO4) that is usually found in underground settings. When exposed to water near the surface, it quickly becomes gypsum, the softest mineral on the Mohs hardness scale.
Hydrolysis
Hydrolysis is the opposite of hydration; in this case, water breaks down the chemical bonds of a mineral instead of creating a new mineral. It is a decomposition reaction.
The name makes this one particularly easy to remember: The prefix “hydro-” means water, while the suffix “-lysis” means decomposition, breakdown, or separation.
Oxidation
Oxidation refers to the reaction of oxygen with metal elements in a rock, forming oxides. An easily recognizable example of this is rust. Iron (steel) reacts easily with oxygen, turning into reddish-brown iron oxides. This reaction is responsible for the red surface of Mars and the red color of hematite and magnetite, two other common oxides.
Acidification
You’ve probably heard of acid rain. However, most people don’t know what it is or how it contributes to chemical weathering. Acid rain is water with sulfuric and nitric acids from the burning of coal and fossil fuels, along with volcano eruptions.
The acids create a reaction when they hit a stone, causing the surface to wear and the composition to soften. Acidification can also be caused by organisms like lichens, which are created from algae and fungi.
One well-known case of rapid weathering and blackening of stone is the weathering on the 1,000-year-old Leshan Giant Buddha in China. The 232-foot-tall Buddha required six months of repair for weathering after only 12 years due, in part, to acidification.
What type of chemical weathering is enhanced by acid rain?
Carbonation is a type of chemical weathering that is aided by acid rain. When rocks and minerals are altered by hydrolysis, acids may be produced. Acids may also be produced when water reacts with the atmosphere, so acidic water can react with rocks.
The effect of acids on minerals is an example of solution weathering. Solution weathering also covers other types of chemical solutions, such as basic rather than acidic ones.
One common acid is carbonic acid, a weak acid that is produced when carbon dioxide reacts with water. Carbonation is an important process in the formation of many caves and sinkholes. Calcite in limestone dissolves under acidic conditions, leaving open spaces.
Carbonation is a form of chemical weathering.
- When carbon dioxide is released into the environment by automobiles, it can be converted to carbonic acid.
- These acids then descend to earth as rain, removing important compounds from the crystalline structures that make up rocks.
- Simultaneously, sulfur dioxide is transformed into sulphuric acid, and nitrogen dioxide is turned to nitric acid as a result of fossil fuel burning.
- At high temperatures, these acids continue as vapor, but when the temperature drops, they begin to condense.
- These acids mingled with rain on their way down to the ground, causing it to become acidic.
- These acids are particularly harmful to marble, chalk, and limestone, and they cause damage to tombstones and statutes that are made with these kinds of materials.
- The case study of the yellowing of the Taj Mahal gives the perfect example of this.