Weathering describes the breaking down or dissolving of rocks and minerals on the surface of 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. Together, these processes have carved famous landmarks, such as the Grand Canyon in the U.S. state of Arizona. This massive canyon is 446 kilometers (277 miles) long, up to 29 kilometers (18 miles) wide and around 1.6 kilometers (one mile) deep.

Weathering and erosion constantly change the rocky landscape of Earth. Weathering wears away exposed surfaces over time. The duration of exposure often contributes to how vulnerable a rock is to weathering. Rocks such as lavas are quickly buried beneath other rocks and are less vulnerable to weathering and erosion than rocks that are exposed to agents such as wind and water.

As it smooths rough, sharp rock surfaces, weathering contributes to the production of soil. Soil forms as tiny bits of weathered minerals mix with plants, animal remains, fungi, bacteria and other organisms. Weathered materials from a single type of rock typically produce infertile soil, while materials from a collection of rocks is richer in mineral diversity and creates more fertile soil. Soil types associated with a mixture of weathered rock include glacial till, loess and alluvial sediments.

Weathering is often divided into two processes: 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. When temperatures drop low enough, liquid water turns into ice. As the water freezes, it expands, causing the ice to work as a wedge. It slowly widens the cracks and splits the rock. When the 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 wedging 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 often subject to thermal stress because of the large temperature fluctuations from day to night.

Salt also works to weather rock in a process called haloclasty. Saltwater sometimes gets into the cracks and pores of rock. When the water evaporates, it leaves behind salt crystals. 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 that look like honeycombs, 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 weathering occurs in deserts as well, creating formations known as salt upwellings or salt anticlines. In this geologic process, underground salt domes expand and contribute to weathering of the overlying rock. Salt can even impact the built environment. Structures in the ancient city of Petra, Jordan, were made unstable and often collapsed due to haloclasty.

Weathering can cause the entire outer layers of rock formations to flake off in sheets. This is known as exfoliation, and it can create dramatic landscape features, such as isolated dome rock formations. Yosemite National Parks’ Half Dome—an iconic landmark in the U.S. state of California—was created through exfoliation.

A process called unloading removes overlying materials, which then gives the underlying rocks room to expand. As the rock surface expands, it becomes vulnerable to fracturing in a process called sheeting.

Plants and animals can be agents of mechanical weathering and erosion. 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 smaller plants, such as mosses, can enlarge tiny cracks as the mosses 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 and erode.

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.

A similar type of dissolution can happen with sulfuric acid. Carlsbad Caverns National Park, in the U.S. state of New Mexico, includes more than 119 limestone caves created by weathering and erosion from sulfuric acid. The largest is called the Big Room. With an area of about 33,200 square meters (357,361 square feet), the Big Room is the largest cave chamber in North America.

Sometimes chemical weathering dissolves large portions of limestone or other rock to form a landscape called karst. In these areas, 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 can 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 forms, it weakens rock and makes it more vulnerable to weathering.

Hydration is a form of chemical weathering in which water forms chemical bonds with minerals as they interact. One instance of hydration occurs as the mineral anhydrite reacts with groundwater. The water binds to anhydrite to form gypsum, one of the most common minerals on Earth.

Another familiar form of chemical weathering is hydrolysis. In the process of hydrolysis, a new mineral forms as chemicals in minerals 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 water erosion. Flared slopes are concave rock formations. An example of a flared slope is Wave Rock in Australia, where water dissolved the landform’s granite for millions of years and created a wave-like shape in the rock.

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 weather rock in order to access nutrients. Microorganisms, such as bacteria, appear in soil in even the harshest environments. Beyond using these nutrients for their own benefit, bacteria are also key to unlocking nutrients for other organisms, such as plants. In fact, bacteria can even be added to soil to improve fertility and agricultural productivity.

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 over 1,200 years ago. 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 statue has undergone several restorations, with a major repair project completed in 2019.

The impacts of weathering can be felt on culturally significant landmarks and structures around the world, putting prehistoric artifacts, like ancient rock art, at risk. This is especially true for the cave art in Sulawesi, Indonesia, which is about 45,000 years old. The cave art is some of the oldest rock art in the world, and it provides insights into ancient people’s migrations patterns and hunting practices in the region. Salt crystals are the main culprit of the erosion, and warmer temperatures combined with saltwater intrusion from extreme weather are accelerating the process.

This is part of a larger trend in which Aboriginal art in Australia and Oceania are succumbing to damage from weathering, erosion and climate change. These artifacts often have spiritual and cultural significance to Aboriginal populations today, making preservation a priority. While archaeologists are working to preserve this art, ultimately, slowing climate change is necessary to create a lasting impact. It’s crucial to incorporate Indigenous perspectives in this effort or, at the very least, follow the lead of Indigenous peoples. With ancient rock art in Australia, Aboriginal people have established history and knowledge of preserving these works, and relying on their traditional knowledge would empower the communities most impacted by the weathering of this art.

Weathering and erosion also destroy infrastructure, including people’s homes. Soil erosion contributes to food and economic instability by weakening the ability of soils to grow crops. Weathering even connects to human health. In the 1930s, about 7,000 people in the Southern Plains of the United States died of “dust pneumonia.” During this event, known as the “Dust Bowl,” harmful agricultural practices damaged soils so much that they broke down into fine dust particles that harmed the lungs, leading to dust pneumonia.

Weathering and climate change are interrelated processes. Climate change from burning fossil fuels is exacerbating the impacts of both chemical and mechanical weathering. Warmer and wetter climates cause more mechanical weathering. Emissions from coal power plants and vehicles release chemical pollutants that can form acid rain.

While weathering damages rocks and minerals, there are also some benefits to weathering. Farmers use broken down rocks to add needed minerals to their soil, improving the soils’ pH level and overall health. This process—known as enhanced weathering—helps the soil absorb important minerals like calcium and iron, which increases nutrient availability for crops and can improve crop yield. Typically, farmers use silicate rocks, but the needs of the land must be considered when selecting the appropriate rock.

Weathered rocks can also help remove carbon from the atmosphere to combat climate change. When exposed to the air, crushed rock absorbs carbon dioxide, reacting with it to form bicarbonate ions, removing the carbon dioxide from the environment. The amount of carbon dioxide removed is dependent on the amount of land using enhanced weathering. The cost and ease of the project depends on where the rocks are sourced. Using existing local supply chains can be low cost and relatively easy to implement. But the process of mining and grinding the rocks has the potential to create more carbon emissions than the rock can save. That’s why it is important to carefully consider all steps in the process before implementing this practice.