Chemical weathering occurs when minerals within rocks change as a result of exposure to air and other environmental chemicals.
Silicates found in igneous rocks may change to clays or become oxidized over time, weakening them and making erosion easier to break them apart.
Physical
Physical weathering entails several processes which weaken and disintegrate rocks and other solid materials, including temperature fluctuations, abrasion, freeze-thaw action (freezing then thawing of rock), salt crystal growth and cracking.
Water seeping through cracks in rock, and then freezing, will expand and fracture it apart, which is known as frost wedging. Another form of physical weathering, called root wedging, occurs when plant roots grow into existing fractures and expand them as they expand.
Physical weathering refers to any process by which material from harder materials, such as rocks, is gradually removed through erosion by air currents or wind action, eroding away at soft materials like silt and sand from them – this form of physical weathering is known as mechanical weathering.
Chemical
Chemical weathering occurs when minerals in rocks react with water, air and organic acids to form complex chemical reactions that alter its molecular structure and physical characteristics. Some minerals are very resistant to weathering while others deteriorate quickly – for instance feldspar easily forms clays with oxygen and water and easily oxidizes (combusts).
Minerals that are covered by lava flows may be more protected from weathering than those exposed on the surface, yet over time these lava flows will still break down, with its speed depending on exposure to both warm temperatures or cold ones, as well as how long it is exposed for.
Carbonation is one of the primary forms of chemical weathering. Rain that is naturally acidic due to atmospheric carbon dioxide reacts with limestone and chalk to produce calcium bicarbonate (Ca(HCO3)2) which, when seeped into cracks of rocks, dissolves them – thus creating caves like those seen at Carlsbad Caverns National Park in New Mexico.
Biological
Biological weathering refers to the breakdown of rocks by plants, animals and microbes such as bacteria. Bacteria, lichens (symbiotic colonies of fungi and algae) and mosses all thrive on rocks, making them more fragile while producing weak acids which dissolve rock minerals; their acids also etch holes and cracks into rock surfaces more rapidly in humid tropical regions where limestone rocks dissolve faster under exposure to heat than others. Biological weathering processes tend to accelerate more quickly where temperatures exceed 90 F (32 C).
Tree roots have the power to wreak havoc on rock surfaces. Animals that burrow underground such as badgers and moles may also impact them; even prolonged walking on them may wear away portions. We frequently see this phenomenon at cemeteries where the root systems have caused headstones to be worn away over time by tree roots.
Human
Human activity influences both the rate and type of weathering. Burning fossil fuels releases carbon dioxide back into the atmosphere and causes changes to Earth’s temperature that increase chemical weathering rates. Air pollutants produce acid rain containing sulfuric and nitric acids which combine with raindrops to form acid rain which quickly deteriorates limestone, marble, and other types of stone structures – damaging gravestones while making reading their inscriptions harder than ever.
Tree roots can mechanically weather rocks by growing into cracks in them and widening them over time, while biological weathering results in honeycomb-shaped rock structures known as haloclasty, or cave-like depressions known as tafoni. Chemical weathering takes place when salt water seeps into an existing rock crack and reacts with it, creating a chemical that expands when heated putting pressure on nearby rock surfaces and breaking them apart; such is what forms Karst landscapes such as those found in Shilin China.