Types of Weathering: Physical, Chemical & Biological

There are three types of weathering: physical (or mechanical) weathering, which breaks rock into smaller pieces without changing its minerals; chemical weathering, which alters the minerals themselves through reactions with water, air and acids; and biological weathering, in which plants, lichens, microbes and animals do either or both.

Weathering is the breakdown of rock where it sits, at or near the Earth's surface. In practice the three types rarely act alone. A crack opened by ice lets in water that dissolves the minerals lining it; a root that pries a joint apart also leaks acids into it. Geographers separate them because each has a distinct mechanism, favours a distinct climate, and leaves a distinct signature on the landscape, from limestone pavements in Ireland to the red skin of Uluru.

Physical Weathering: Breaking Rock Without Changing It

Mechanical weathering produces smaller fragments of the same material. A shattered block of granite is still quartz, feldspar and mica; it just has far more surface exposed. Five mechanisms account for most of it.

Frost wedging

Water expands by about 9 percent when it freezes. If it fills a crack and freezes, it presses on the walls; when it thaws, it seeps deeper, and the next freeze pushes further. The process is most effective where temperatures swing across 0 °C many times a year, which is why high mountains and subarctic uplands are littered with angular scree. The talus slopes below cliffs in the Alps, the Rockies and the Scottish Highlands are largely frost-wedged debris. Recent research suggests the growth of ice lenses, which draw in water from surrounding pores, may matter as much as simple expansion.

Exfoliation and pressure release

Granite forms kilometres underground under enormous confining pressure. When erosion strips away the overlying rock, the granite expands slightly and cracks into curved sheets parallel to the surface, a process called unloading. The sheets peel away like onion skins, leaving rounded domes. Half Dome and the other granite domes of Yosemite in California are the textbook example, as is Stone Mountain in Georgia.

Thermal stress

Desert rock can heat to well over 50 °C in afternoon sun and cool sharply at night. Because different minerals expand at different rates, repeated cycles were long thought to shatter rock on their own. Laboratory experiments from the 1930s onward cast doubt on that, finding that dry heating did little damage; most researchers now think thermal stress works mainly in combination with moisture, and its importance remains debated.

Salt crystallisation

In deserts and along coasts, water carrying dissolved salts soaks into porous rock and evaporates. Salt crystals grow in the pores and exert pressure strong enough to loosen individual grains. Over time this hollows out honeycomb pits and larger cavities called tafoni, common on coastal sandstones and in arid regions. Salt weathering is also a major threat to stone buildings, including the rock-cut facades of Petra.

Abrasion

Wind-blown sand, rolling river pebbles and ice dragging rock debris all grind surfaces down. Many textbooks list abrasion under mechanical weathering, but strictly it requires moving material, so others classify it as erosion.

Chemical Weathering: Reactions That Remake the Minerals

Chemical weathering changes rock into new substances, many of them more stable at surface temperatures than the minerals that crystallised deep underground. Water is the essential ingredient, and warmth speeds every reaction, so it dominates in humid tropical climates.

Hydrolysis

Feldspar, the most common mineral group in the crust, reacts with slightly acidic water to form clay minerals such as kaolinite, releasing potassium, sodium or calcium in solution. This is why weathered granite crumbles into a gritty sand called grus: the feldspar has turned soft while the resistant quartz grains are left loose. The china clay deposits around St Austell in Cornwall are rich in kaolinite derived from decomposed granite, although hot fluids within the granite also played a part there.

Carbonation

Rainwater absorbs carbon dioxide from the air and soil to form weak carbonic acid. It reacts with calcite, the main mineral in limestone and marble, converting it to calcium bicarbonate, which is soluble and simply washes away. Carbonation builds karst landscapes. In the Burren in County Clare, Ireland, it has etched the bare limestone into a pavement of blocks (clints) separated by deep fissures (grikes). In Guangxi, southern China, the same chemistry in a hot, wet climate has left the steep tower karst around Guilin and the Li River. Caves, sinkholes and disappearing streams are all products of carbonation.

Oxidation

Iron-bearing minerals react with oxygen dissolved in water to form iron oxides such as haematite and goethite, the same chemistry as rust. The result is a red, orange or yellow staining. Uluru in central Australia is made of arkose, a feldspar-rich sandstone that is grey where freshly exposed; its famous red colour is a thin weathered skin of iron oxide. The deep red laterite soils of tropical Africa, India and Brazil are another product of long-running oxidation.

Solution

Some minerals dissolve directly in water without any acid at all. Rock salt (halite) and gypsum are the obvious cases, which is why they survive at the surface mainly in deserts.

Biological Weathering: Roots, Lichens and Burrowers

Living things weather rock both mechanically and chemically, which is why some geographers treat biological weathering as a category that overlaps the other two rather than standing wholly apart.

Root wedging

Tree roots grow into joints in search of water and widen them as they thicken. Pavements lifted by street trees are the urban version; on a larger scale, the temples of Angkor in Cambodia show how fig and silk-cotton tree roots can split masonry apart.

Lichens, mosses and microbes

Lichens that coat bare rock release organic acids, including oxalic acid, and pull metal ions out of mineral surfaces. Their fungal threads also penetrate tiny cracks and swell when wet. Bacteria and fungi in soil speed the chemical breakdown of the rock beneath, and decaying vegetation adds carbon dioxide that strengthens carbonic acid.

Burrowing animals

Earthworms, ants, termites and rodents bring rock fragments to the surface and expose fresh material to air and water. Charles Darwin's last book, published in 1881, was devoted to how earthworms turn over soil and slowly bury stones and ruins.

What Sets the Pace of Weathering

The same rock can survive for millennia in one setting and crumble within a century in another. Four controls do most of the work.

• Climate. In 1950 the geomorphologist Louis Peltier plotted weathering against mean annual temperature and rainfall. His diagram shows chemical weathering strongest where it is hot and wet, frost action strongest where it is cold but moist enough to supply water, and very slow weathering of any kind in cold, dry deserts such as the Antarctic Dry Valleys.
• Rock type. Minerals that crystallise at high temperature, such as olivine, are least stable at the surface, while quartz is extremely resistant. Samuel Goldich set out this order in 1938, and it mirrors Bowen's reaction series. Limestone is vulnerable to carbonation; quartzite hardly reacts at all.
• Surface area. Cut a cube into eight smaller cubes and you double its exposed surface. Jointed, fractured rock weathers far faster than massive blocks, so physical weathering accelerates chemical weathering.
• Time. Weathering is slow; deep weathering profiles in stable tropical regions can extend tens of metres down and represent a very long exposure.

Cleopatra's Needle on the Thames Embankment in London is often used to illustrate climate's effect. The granite obelisk was quarried at Aswan around 1450 BC, stood in Egypt for roughly 3,300 years, and was shipped to London in 1878. It is frequently said to have weathered more in its first century of damp, coal-smoke London air than in all its time in Egypt. The story is a useful teaching example, but it should be treated with caution: some of the damage probably occurred while it lay near the sea in Alexandria, where salt could attack it, and its twin in New York's Central Park has a similarly contested history.

Weathering Versus Erosion

The two words are often used together, and they are easy to confuse. The distinction rests on movement.

Weathering prepares material and erosion removes it, and each helps the other: stripping away the weathered layer exposes fresh rock to further attack. The blanket of broken, rotted rock that stays behind is called regolith, and its upper, biologically active part becomes soil. Our guide to the types of erosion covers the transport side in detail, and both halves fit into the wider rock cycle, where eroded sediment eventually becomes new sedimentary rock.

Reading Weathering in Real Landscapes

Once you know the mechanisms, their fingerprints are everywhere. The arch in the photograph above is carved from the sandstone of Wadi Rum in southern Jordan. Sandstone is sand grains held together by a natural cement, and when that cement is weakened by water chemistry and salt crystallisation, grains simply fall away. Weaker or more porous layers decay first, undercutting stronger beds above until only a bridge of rock remains. Wind and occasional flash floods then carry the loose grains off, which is where weathering hands over to erosion. The desert's sandstone cliffs, domes and rock bridges are explored further in our Wadi Rum desert guide.

Graveyards are an unexpectedly precise laboratory. Headstones carry a date, come in a few standard stones, and stand in known climates, so researchers have used them to estimate weathering rates. Marble stones, being calcite, lose their lettering to carbonation and acid rain, and in industrial cities the loss has been measured in millimetres per century; slate and granite stones of the same age often remain crisp. Comparing marble headstones in a smoky city with those in a clean rural churchyard shows the effect of air pollution directly.

Other clues are easy to spot. Rounded boulders sitting in a bed of crumbly grus point to spheroidal weathering, where corners decay faster than faces. Red-stained rock faces signal oxidation, grey lichen crusts signal biological attack, and fluted, pitted limestone points to carbonation, the process behind the sinkholes and caves described in our look at karst landscapes around the world.