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Before You Read

Think of a time when you had soil on your hands. That soil

might once have been at the top of a mountain. Describe

how soil might make its way down a mountain.

Weathering and Its Effects

Tiny moss plants, earthworms, and even oxygen weaken

and break apart rocks at Earth’s surface. The surface

processes that break down rock are called weathering.

Weathering breaks rock into smaller and smaller pieces

called sediment. Sand, silt, and clay are three different sizes

of sediment. Sand grains are larger than silt. Silt is larger

than clay.

Over millions of years, weathering has changed Earth’s

surface. The process continues today. There are two

different types of weathering—mechanical weathering and

chemical weathering. Both types work together to change

Earth’s surface.

Mechanical Weathering

Mechanical weathering occurs when rocks are broken

apart by physical processes. Mechanical weathering breaks

rocks into small pieces, changing only the size and shape of

the rock. The chemical makeup of the small pieces is the

same as the chemical makeup of the original rock.

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37

How do plants and animals cause weathering?

Water and nutrients collect in the cracks of rocks. Seeds

that land in the cracks are able to grow. As a plant grows,

its roots grow larger and move deeper into the crack in the

rock. As the roots get bigger, they make the crack larger.

You may have seen how the roots of a tree can lift and

crack a piece of sidewalk. This is one way plants cause

mechanical weathering.

Animals also cause mechanical weathering. Look at the

figure above. Small burrowing animals, such as voles, dig

tunnels in the ground. Burrowing loosens small rocks and

sediment in soil. The animal pushes these small pieces of

rock to the surface. Once these small rocks and sediment are

out of the ground, other weathering processes act on them.

What is ice wedging?

Ice wedging is the mechanical

weathering process that occurs

when water freezes in the cracks

of rocks. Water may seep into a

crack in a rock. As the water turns

to ice, it expands and pushes

against the sides of the crack. The

crack gets wider and deeper, as

shown in the figure. The pressure

of the ice in the crack is so great it can break the rock apart.

When temperatures rise, the ice melts. Because the crack

is larger now, more water can enter the crack. When the

water freezes again, the ice will again put pressure on the

crack. After many years of this freezing and melting cycle,

the rock will break up completely.

88 Weathering and Soil

Where does ice wedging occur?

Ice wedging is often seen in mountains, where warm days

and cold night are common. Ice wedging is a process that

wears down mountain peaks. The cycle of freezing and

thawing also breaks up roads. When water seeps into cracks

in the pavement and freezes, it forces the pavement apart.

Ice wedging in roads is one cause of potholes.

How does mechanical weathering affect

surface area?

Mechanical weathering by plants, animals, and ice

wedging breaks rocks into smaller pieces. These small pieces

have more surface area than the original rock had. As the

surface area increases, more rock is exposed to water and

oxygen. This speeds up a different kind of weathering, called

chemical weathering.

Chemical Weathering

Chemical weathering occurs when chemical reactions

dissolve or alter the minerals in rocks or change them into

different minerals. Like mechanical weathering, chemical

weathering changes the size and shape of rocks. But it also

changes the chemical makeup of rock. These chemical

changes weaken the rock.

How do natural acids weather rock?

Naturally formed acids can weather rock. Carbonic acid is

a natural acid formed when water reacts with carbon

dioxide gas in the air or soil. Even though carbonic acid is a

weak acid, it causes chemical weathering in rocks.

Over thousands of years, carbonic acid can form caves in

limestone. Calcite is the main mineral in limestone. When

carbonic acid reacts with calcite, it causes the calcite to dissolve.

Over time, enough calcite in the limestone may dissolve to

form a cave.

Other naturally occurring acids weather other types of

rock. Granite, some types of sandstone, and other rocks

all contain the mineral feldspar. Over many years, feldspar

is broken down into a clay mineral called kaolinite

(KAY oh luh nite). Kaolinite clay is found in some soils.

Clay is an end product of weathering.

How do plant acids cause chemical

weathering?

Some plant roots give off acids. Rotting or decaying

plants also give off acids. These natural acids can dissolve

minerals in rock. When the minerals dissolve, the rock is

weakened. Over time, the rock cracks and breaks into

smaller pieces. As the rock weathers, nutrients become

available to plants.

How does oxygen cause chemical weathering?

Oxygen also causes chemical weathering. Oxidation (ahk

sih DAY shun) occurs when some materials are exposed to

oxygen and water. When minerals containing iron are

exposed to water and oxygen, the iron in the mineral reacts

to form a new material. This new material looks like rust.

Oxidation occurs in the mineral magnetite. When the iron

in magnetite is exposed to water and oxygen, it forms

limonite, a rustlike material. Oxidation of minerals gives

some rock layers a red or rust color.

Effects of Climate

Climate affects how quickly weathering occurs. Climate is

the pattern of weather in a region over many years. In cold

climates, with frequent freezing and thawing, mechanical

weathering happens rapidly. Freezing and thawing cause ice

wedging that breaks down rock.

Chemical weathering is more rapid in warm, wet climates.

So, chemical weathering occurs quickly in tropical areas, such

as rain forests. Chemical weathering is slower in deserts

where there is little water. The constant low temperatures in

polar regions also slow down chemical weathering.

Do all rocks weather at the same rate?

Different types of rock weather at different rates. In

wet climates, marble weathers rapidly and discolors. Granite

weathers more slowly in humid climates.

The weathering of rocks and the process of soil formation

alter rock minerals so that soil minerals are mostly of the

parent rock type. Weathering begins the process of forming

soil from rock and sediment and also affects particle size and

soil texture. Recall that sand, silt, and clay simply describe

the different particle sizes of the soil’s mineral content.