Weathering facts for kids

Kids Encyclopedia Facts

This page is about weathering of rocks and minerals. For weathering of polymers, see Polymer degradation and Weather testing of polymers. For the public health concept, see Weathering hypothesis.

A natural arch formed by rocks breaking down in Jebel Kharaz, Jordan.

Weathering is how rocks, soil, and minerals break down over time. This happens when they come into contact with water, air, sunlight, and living things. It's like how an old toy might get rusty or faded if left outside.

Weathering happens right where the rock is, without moving it much. This is different from erosion, which is when wind, water, or ice carry away pieces of rock.

There are two main types of weathering:

Physical weathering breaks rocks into smaller pieces.
Chemical weathering changes what the rocks are made of.

Water is a big player in both types. Air (with oxygen and carbon dioxide) and living organisms also play important roles.

When rocks break down, their pieces mix with dead plants and animals to create soil. Many of Earth's amazing landforms, like mountains and valleys, are shaped by weathering and erosion. Weathering is a key part of the rock cycle, helping to form sedimentary rock which covers most of our planet's land.

How Rocks Break Down Physically

Physical weathering (also called mechanical weathering) is when rocks break into smaller bits without changing their chemical makeup. Think of it like breaking a cookie into crumbs. This often happens because of temperature changes, water, or pressure.

Physical weathering can make cracks bigger. These bigger cracks then allow more water and air to get in, which can speed up chemical weathering too.

Freezing and Thawing

A rock in Abisko, Sweden, cracked by freezing water or temperature changes.

One of the most important ways rocks break physically is through frost weathering. This happens when water gets into cracks in rocks and then freezes. When water turns into ice, it expands, taking up about 9% more space. This expansion pushes on the sides of the crack, like a wedge. If this happens over and over, the pressure can be strong enough to split the rock apart.

This process works best in places where temperatures go above and below freezing often, like in mountains or places with cold winters.

Heat and Cold Changes

Rocks can also break down from thermal stress weathering. This is caused by rocks expanding when they get hot and shrinking when they get cold. Imagine a hot day followed by a cold night in the desert. The outside of a rock heats up and expands faster than the inside. This creates stress. Over many cycles, these stresses can cause the rock to crack and break into pieces.

This type of weathering is common in deserts, where temperatures change a lot between day and night. Wildfires can also cause very fast thermal stress weathering because of the sudden, intense heat.

Pressure Release

Layers of granite peeling off in Texas, likely due to pressure release.

Pressure release happens when rocks that were once buried deep underground are brought to the surface. Rocks deep down are under huge pressure from all the rock above them. When erosion removes this overlying material, the pressure is released. The rock then expands, especially its outer layers. This expansion causes cracks to form parallel to the surface, making the rock peel off in sheets, like layers of an onion. This process is called exfoliation.

You can see this in places where large amounts of rock have been uncovered, like after a glacier melts away.

Salt Crystals at Work

Tafoni rock formations at Salt Point State Park, California, shaped by salt weathering.

Salt crystallization (or salt weathering) happens when salty water seeps into cracks in rocks. As the water evaporates, it leaves behind salt crystals. These crystals grow over time, pushing against the rock and making the cracks wider. Sodium and magnesium salts are especially good at this.

This type of weathering is very common in arid (dry) climates where water evaporates quickly, and along coastlines where sea spray brings salt to the rocks. It can create interesting hollowed-out rock shapes called tafoni.

Plants and Animals Helping Break Rocks

Living things can also help break rocks physically.

Plant roots can grow into tiny cracks in rocks. As the roots get bigger, they act like wedges, pushing the rock apart.
Animals like worms or burrowing creatures can dig into soil and rock, helping to break it up.
Lichens (a mix of fungi and algae) that grow on rocks can also gently pull off tiny mineral grains from the surface.

How Rocks Change Chemically

A comparison of fresh (left) and chemically changed (right) limestone.

Chemical weathering is when water, oxygen, carbon dioxide, and other chemicals react with rocks to change what they are made of. It's like how iron rusts when it's exposed to air and water. This process turns the original minerals in the rock into new minerals that are more stable at the Earth's surface.

Water is the main agent for chemical weathering. Oxygen and carbon dioxide from the air, and chemicals produced by living things, are also very important.

Dissolving Rocks

Limestone core samples showing different levels of chemical change.

Dissolution is when a mineral completely dissolves in water, just like sugar dissolves in your tea. Rainwater can dissolve easily soluble minerals like halite (rock salt). Even very tough minerals like quartz can dissolve over a very long time.

A common type of dissolution is carbonate dissolution, which affects rocks like limestone and chalk. This happens when carbon dioxide from the air mixes with rainwater to form a weak acid called carbonic acid. This carbonic acid then dissolves the calcium carbonate in the limestone, turning it into a soluble substance called calcium bicarbonate. This process is slower in cold water because cold water holds more dissolved carbon dioxide.

This is why limestone areas often have unique landscapes with deep cracks and caves.

Acid rain, caused by pollution, makes this process much faster. Gases like sulfur dioxide and nitrogen oxides in the air mix with rainwater to form stronger acids, which can quickly dissolve rocks and damage buildings.

Water and Acid Reactions

Olivine mineral changing into iddingsite due to water and acid.

Hydrolysis is a type of chemical weathering where water and acids react with minerals to change them into new solid materials, often clay minerals. For example, some minerals in rocks can react with water to form new minerals like brucite and silicic acid.

The most common type is acid hydrolysis, where hydrogen ions (from acids in water) attack the bonds in mineral crystals. Carbonic acid from dissolved carbon dioxide is a major source of these hydrogen ions. This process is sometimes called carbonation when it leads to new carbonate minerals.

This process is important because it helps control the amount of carbon dioxide in the atmosphere, which can affect Earth's climate.

Rusting Rocks (Oxidation)

Pyrite cubes that have rusted (oxidized).

Oxidation is like rusting for rocks. It happens when oxygen in the air and water reacts with metals in rocks, especially iron. When iron in rocks reacts with oxygen, it forms iron oxides and hydroxides, which are reddish-brown. This makes the rock look rusty, crumble easily, and become weaker.

You can often see this reddish-brown color on the surface of rocks that have been exposed to the elements for a long time.

Water Joining Minerals (Hydration)

Hydration is when water molecules attach themselves to the atoms and molecules of a mineral. This doesn't dissolve the mineral, but it can make it swell or change its structure. For example, some iron oxides can become iron hydroxides when water molecules attach to them.

This process is often the first step in other chemical weathering reactions, as water molecules on the surface of a mineral make it easier for other chemical changes to happen.

Living Things and Chemical Changes

Lichens causing chemical changes on basalt rock in La Palma.

Living organisms can also speed up chemical weathering.

Soil microorganisms like bacteria can help break down minerals faster.
Lichens on rocks are very effective. They create a moist, acidic environment that helps dissolve the rock surface.
Plants release carbon dioxide and organic acids from their roots. These acids can dissolve minerals, especially those containing aluminum and iron.
Decaying plants in the soil also create organic acids that contribute to chemical weathering.
Some fungi and bacteria that live with plant roots can release nutrients from minerals, helping the plants grow while also weathering the rock.

Weathering in Different Places

Under the Ocean

Weathering of rocks on the ocean floor, mostly basalt, happens much slower than on land. The basalt becomes hydrated (absorbs water) and changes its chemical makeup over millions of years.

Weathering on Buildings

Concrete damaged by acid rain.

Buildings, statues, and monuments made of stone, brick, or concrete are also affected by weathering. The same processes that break down natural rocks can damage human-made structures. This is especially true in areas with acid rain, which can quickly dissolve and weaken building materials.

Fires can also cause significant damage to stone in buildings. The intense heat makes the stone expand and crack, and then water used to put out the fire can cause further damage from sudden cooling.

How Soil is Made

Weathering is the first step in making soil. When rocks break down, their tiny pieces mix with organic material (dead plants and animals) to form soil. Different types of rocks weather into different kinds of soil. For example, granite breaks down into a mix of clay minerals and iron oxides.

Soil formation is a slow process, taking hundreds to thousands of years. Scientists can even find "fossil soils" (called paleosols) in very old rock layers, showing how soil has formed throughout Earth's history.

Weathering Wood, Paint, and Plastic

It's not just rocks that weather! Wood, paint, and plastics can also break down. Sunlight, especially ultraviolet (UV) radiation, causes chemical reactions that degrade their surfaces. Water and other environmental factors also contribute to their breakdown.

Amazing Weathering Pictures

Salt weathering on a building stone in Gozo, Malta.
Salt weathering on sandstone near Qobustan, Azerbaijan.
A Permian sandstone wall near Sedona, Arizona, USA, shaped by weathering.
Weathering on a sandstone pillar in Bayreuth.
The effect of acid rain on statues.
Weathering on a sandstone statue in Dresden, Germany.
Physical weathering on pavements at Azad University Science and Research Branch in Tehran, Iran.