Weathering describes the breaking down or dissolving of rocks and minerals on the surface of the Earth. Water, ice, acids, salts, plants, animals, and changes in temperature are all agents of weathering.
 
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 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. 
 
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.
 
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.
 
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.
 
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 sheeting
 
Another type of mechanical weathering occurs when clay or other materials near rock absorb water. Clay, more porous than rock, can swell with water, weathering the surrounding, harder rock. 
 
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. 
 
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.
 
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.
 
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. 
 
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. 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. 
 
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 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.
 
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.
 
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.
 
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.
 
Weathering and People
 
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.
weathering
Weathering by water's freeze-thaw cycle has split this rock in two.
Spheroidal Weathering
Spheroidal weathering is a form of chemical weathering that occurs when a rectangular block is weathered from three sides at the corners and from two sides along its edges. It is also called “onion skin” weathering.

Weathered Mountains
The Appalachian Mountains in eastern North America once towered more than 9,000 meters (30,000 feet) high—taller than Mount Everest! Over millions of years, weathering and erosion have worn them down. Today, the highest Appalachian peak reaches just 2,037 meters (6,684 feet) high.

acid
Noun

chemical compound that reacts with a base to form a salt. Acids can corrode some natural materials. Acids have pH levels lower than 7.

acid rain
Noun

precipitation with high levels of nitric and sulfuric acids. Acid rain can be manmade or occur naturally.

Noun

harmful chemicals in the atmosphere.

alluvial
Adjective

having to do with matter deposited by flowing water (alluvium).

anhydrite
Noun

(CaSO4) grey-white mineral found in sedimentary rocks. Also known as anhydrous calcium sulfate.

arid
Adjective

dry.

Noun

layers of gases surrounding a planet or other celestial body.

biological weathering
Noun

process in which living or once-living organisms contribute to the disintegration of rocks and minerals (weathering).

bornhardt
Noun

isolated rock outcropping shaped as a steep-sided dome at least 30 meters (100 feet) tall.

Buddha
Noun

(c. 563-483 BCE) Indian prince, spiritual leader, and founder of the Buddhist religion. Also called Prince Siddhartha and Gautama Buddha.

byproduct
Noun

substance that is created by the production of another material.

Noun

deep, narrow valley with steep sides.

carbonation
Noun

absorption of, or reaction with, carbon dioxide.

carbonic acid
Noun

chemical produced as carbon dioxide dissolves in water.

cave
Noun

underground chamber that opens to the surface. Cave entrances can be on land or in water.

chemical bond
Noun

attraction between atoms, ions or molecules that enables the formation of chemical compounds.

chemical weathering
Noun

process changes the composition of rocks, often transforming them when water interacts with minerals to create various chemical reactions.

clay
Noun

type of sedimentary rock that is able to be shaped when wet.

Noun

dark, solid fossil fuel mined from the earth.

concave
Adjective

curving inward.

contract
Verb

to shrink or get smaller.

crevice
Noun

crack in a rock.

cryofracturing
Noun

chemical weathering process in which the freeze-thaw cycle of ice cracks and disintegrates rock. Also called frost weathering.

crystal
Noun

type of mineral that is clear and, when viewed under a microscope, has a repeating pattern of atoms and molecules.

decay
Verb

to rot or decompose.

Noun

area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

disaggregation
Noun

process of rocks crumbling due to rain, wind, or other atmospheric conditions. Also called mechanical weathering and physical weathering.

dissolve
Verb

to break up or disintegrate.

drainage system
Noun

series of pipes, gutters, or other waterways used to carry off excess water.

Noun

act in which earth is worn away, often by water, wind, or ice.

evaporate
Verb

to change from a liquid to a gas or vapor.

exfoliation
Noun

process describing the peeling away of outer layers, such as tree bark or rock sheeting.

expand
Verb

to grow or get larger.

fertile
Adjective

able to produce crops or sustain agriculture.

flared slope
Noun

C-shaped landform consisting of a concave rock wall formed by weathering and erosion of subsurface rocks. Also called a “wave rock.”

freeze
Noun

weather pattern of temperatures below 0 degrees Celsius (32 degrees Fahrenheit).

frost weathering
Noun

chemical weathering process in which the freeze-thaw cycle of ice cracks and disintegrates rock. Also called cryofracturing.

geologic
Adjective

having to do with the physical formations of the Earth.

gravestone
Noun

stone marking a person's burial place, often engraved with the person's name and dates of birth and death.

Noun

water found in an aquifer.

gypsum
Noun

(hydrated calcium sulfate, CaSO4) soft, colorless or white mineral.

haloclasty
Noun

type of physical weathering caused by the growth of salt crystals in and around rocks.

hydration
Noun

process of a substance or solution chemically combining with water.

hydrolysis
Noun

process in which a compound is split into other compounds by reacting with water.

iconic
Adjective

event or symbol representing a belief, nation, or community.

innovative
Adjective

new, advanced, or original.

inscription
Noun

record that has been cut, impressed, painted, or written on a hard surface.

Noun

landscape made of limestone.

landmark
Noun

a prominent feature that guides in navigation or marks a site.

Noun

the geographic features of a region.

lava
Noun

molten rock, or magma, that erupts from volcanoes or fissures in the Earth's surface.

limestone
Noun

type of sedimentary rock mostly made of calcium carbonate from shells and skeletons of marine organisms.

Noun

windblown soil or silt.

marble
Noun

type of metamorphic rock.

mechanical weathering
Noun

process of rocks crumbling due to rain, wind, or other atmospheric conditions. Also called physical weathering.

mineral
Noun

nutrient needed to help cells, organs, and tissues to function.

mitigate
Verb

to lower the severity of a natural or human condition.

molecular
Adjective

having to do with the smallest physical unit of a substance.

monument
Noun

large structure representing an event, idea, or person.

Noun

type of fossil fuel made up mostly of the gas methane.

network
Noun

series of links along which movement or communication can take place.

Noun

substance an organism needs for energy, growth, and life.

oxidation
Noun

chemical process of a substance combining with oxygen to change the substance's physical and molecular structure.

Noun

fossil fuel formed from the remains of ancient organisms. Also called crude oil.

physical weathering
Noun

process of rocks crumbling due to rain, wind, or other atmospheric conditions. Also called mechanical weathering.

pockmarked
Adjective

scarred with many small indentations.

porous
Adjective

full of tiny holes, or able to be permeated by water.

pressure
Noun

force pressed on an object by another object or condition, such as gravity.

quartz
Noun

common type of mineral.

remains
Noun

materials left from a dead or absent organism.

rock
Noun

natural substance composed of solid mineral matter.

root
Noun

part of a plant that secures it in the soil, obtains water and nutrients, and often stores food made by leaves.

rust
Verb

to dissolve and form a brittle coating, as iron does when exposed to air and moisture.

salt
Noun

(sodium chloride, NaCl) crystalline mineral often used as a seasoning or preservative for food.

salt dome
Noun

structure formed as water evaporates from a salty lake or sea. The remaining salt is buried by sediments, but eventually pierces through the rock, forming a hill.

salt upwelling
Noun

process in which underground salt domes expand, impacting surrounding rock layers.

Noun

solid material transported and deposited by water, ice, and wind.

Noun

rock formed from fragments of other rocks or the remains of plants or animals.

seed
Noun

part of a plant from which a new plant grows.

sheeting
Noun

type of physical weathering in which a single layer of rock is broken off. Also called contour weathering.

Noun

hole formed in a rock or other solid material by the weight or movement of water.

soil
Noun

top layer of the Earth's surface where plants can grow.

solution
Noun

substance in which a gas, liquid, or solid is evenly distributed in another medium.

subsurface
Adjective

beneath the surface or upper layer.

Noun

degree of hotness or coldness measured by a thermometer with a numerical scale.

thermal stress
Noun

strain on material usually associated with expansion and contraction due to temperature changes.

till
Noun

rock, earth, and gravel left behind by a retreating or melting glacier.

tropical
Adjective

existing in the tropics, the latitudes between the Tropic of Cancer in the north and the Tropic of Capricorn in the south.

vast
Adjective

huge and spread out.

vulnerable
Adjective

capable of being hurt.

Noun

the breaking down or dissolving of the Earth's surface rocks and minerals.

wedge
Noun

triangle shape.