Weathering

How do you think weathering on Earth compares to weathering on the moon?

Weathering does not take place on the moon because an atmosphere is necessary to produce weathering. The moon doesn't have an atmosphere. The surface of the moon has remained relatively unchanged for millions of years.

This process is called oxidation,

and in rocks, oxidation often causes them to crumble into soft pieces.

But nothing on Earth escapes change

and one of the types of change you will learn about in this section is called weathering.

Like physical weathering, chemical weathering can be accomplished by

biological activity.

This moss-covered rock is undergoing

biological weathering as chemicals and acids from the moss combine with water from the atmosphere to react with the rock, weaken it, and break it down. pg 13

Carbonation results in a change in the chemical form of the rock and weakens the rock so that it

breaks down more easily.

Physical weathering does not cause chemical changes in the original substance like chemical weathering does

but both physical and chemical weathering wear down rocks into smaller pieces.

carbonation

chemical weathering

In weathering, the atmosphere usually interacts

directly with rocks (such as wind directly shaping a rock by constantly beating against it)

This acid reacts easily with the minerals in some rocks,

especially limestone

Under the larger categories of the causes of physical weathering (water, wind, ice) are smaller categories that include:

exfoliation, frost wedging, pressure release, haloclasty, biological activity, and hydraulic action, which also cause physical weathering

You also may recall that many rocks contain the mineral

iron

Biological weathering

is when a living thing produces some form of weathering on an object. It really is a sub-type of both physical and chemical weathering because both physical and chemical weathering can be produced by the activity of living things.

When atmospheric carbon dioxide combines with water in the atmosphere,

it forms carbonic acid

Weathering is said to be an in situ process

meaning it takes place without transporting materials from one place to another

Like the artist uses different processes to form a sculpture

nature uses different types of weathering as tools

Just as the artist uses tools, like chisels and hammers, to whittle away a block of marble into a statue of Lincoln,

nature uses water, wind, ice, or other climatic factors to shape its sculptures. Like the marble in the statue, Earth materials are broken down into smaller pieces.

When water molecules are added to a mineral's crystal structure,

new minerals are usually formed, so it is considered a chemical process.

Chemical weathering:

occurs when chemicals and water in the environment act on rock, minerals, or soil to produce new chemical substances within the original material. For example, oxygen and water in the atmosphere may combine with iron in soils to produce rust. Such soils or rocks turn deep orange or red from the rust within them.

. The atmosphere is the "artist" of all weathering

on Earth

Exfoliation

Exfoliation is a type of physical weathering that is characterized by rocks breaking apart in thin layers. In this picture you can see how an outer layer is being weathered off of the larger rock. Exfoliation happens because of thermal stress on a rock. It is most common in desert areas where daytime temperatures are very hot but nighttime temperatures are remarkably colder. The rocks expand by day but cool and contract at night, putting pressure on the outside layers, which then begin to peel off.

When carbon dioxide leaves the mixture

, the soda loses its fizz and water is left behind.

Climate

Because climate determines temperature and the availability of water, weathering rates are greatly dependent on local climate conditions. The amount of rain that an area receives affects the degree to which these processes can take place. Warmer temperatures usually result in more of the chemical reactions involved in chemical weathering, as well as more biological activity. Thus, chemical weathering happens to the greatest degree in warm, humid climates, such as those near the equator. It is limited, however, in cold dry climates. Study the chart to see where different types of weathering peak in terms of annual rainfall and average temperature. Remember that 0°C is equal to 32°F.

Chemical Weathering

Biological Activity

What kind of physical weathering is shown when tree roots grow within the cracks of a rock?

Biological Activity

Occurs when acids from plants react with a rock and break the rock down

Biological activity

Breaks rocks into smaller pieces

Both

Can be accomplished by biological activity

Both

Whats the connection between hydration and hydrolysis?

Both pertain to how water interacts with something.

Reaction of carbonic acid with rocks, especially limestone

Carbonation

Accomplished by chemicals acting on a rock

Chemical Weathering

Alters the chemical form of a rock

Chemical Weathering

While climate is the biggest determining factor in how fast and to what degree weathering takes place, several other factors also contribute to weathering rates

Click through the tabs to learn how each of these affect weathering rates.

Abraham Lincoln

Daniel Chester French used rock as his medium for the sculpture of Abraham Lincoln. Artists shape rock or clay into intricate works of art using tools like chisels, grinders, and polishers.

Can you think of how climate might affect these weathering processes?

Different parts of the world have different climates. They also have different types and rates of weathering. For example, places with more rain have more chemical weathering because water is necessary to accomplish chemical weathering. Colder places have more physical weathering from frost-wedging.

What type of physical weathering is common in deserts and is characterized by rocks breaking in thin outer layers?

Exfoliation

What type of physical weathering is common in mountains and is caused by ice forming between the cracks of a rock and exerting pressure on the rock?

Frost Wedging

Frost Wedging

Frost wedging is a type of physical weathering that involves rocks or soil minerals being broken apart by repeated cycles of freezing and thawing. It happens when water accumulates in tiny cracks within rocks or in soil pores, freezes, contracts, and then exerts pressure on the rocks and soils. Frost wedging is common in mountainous areas where temperatures often fall below freezing at night.

What kind of physical weathering is caused by the growth of salt crystals?

Haloclasty

When water is added to the mineral structure of rocks,

they are said to be hydrated.

Hydrolysis is a kind of chemical weathering that takes place when

water reacts with silicate minerals in soils and rocks

There are three main types of weathering:

: physical, chemical, and biological. Each depends on the forces involved.

Haloclasty

Haloclasty is a type of physical weathering caused by salt crystal growth. It commonly occurs along ocean shorelines where salt water can creep into the cracks in rocks. The water later evaporates, but it leaves salt crystals behind. When the salt crystals are heated they expand and exert pressure on the rock. Haloclasty leaves rocks with a distinctive honeycomb appearance.

Addition of water to the minerals in a rock

Hydration

Chemical Weathering

Hydration

What kind of physical weathering takes place when waves beat on a rock and force air into small cracks in the rock, which then becomes trapped and exerts a force on the rock?

Hydraulic Action

Chemical Weather

Hydrolysis

Reaction of silicate minerals with water

Hydrolysis

How does weathering compare to erosion?

In the natural sculpture, for example, the rock you see was weathered in the same location in which it is now located. In this way, weathering is different from erosion, which involves the transport of materials from one place to another.

Suppose you live in an area underlain by lots of limestone rocks. Use your knowledge of weathering to explain why the ground might be very uneven in your area.

Limestone is easily weathered away by carbonation, or the reaction of carbonic acid in the atmosphere with the minerals in the limestone. The uneven ground is indicative of the weathering away of limestone.

Biological Activity

Living things can also be agents of physical weathering. Plant roots can exert pressure on rocks and soil when they grow within cracks or soil pores. As the roots grow, they press outward on the rock, eventually leading to the disintegration of the rock. Burrowing animals can also contribute to physical weathering by digging through the soil and opening up channels that accelerate other types of weathering.

natural rock sculpture

Nature is the artist of this sculpture. This rock formation from Bolivia has been sculpted by years and years of beating from the wind (and the particles within). What began as an ordinary mass of rock has been shaped into an intriguing landscape feature by the forces of nature. This process of shaping the landscape by beating down Earth materials is an example of weathering.

Chemical Weathering

Oxidation

Reaction of metals with oxygen; softens a rock and causes it to crumble

Oxidation

Accomplished by abrasion, heat, or pressure

Physical Weathering

Does not involve a change in chemical form

Physical Weathering

What kind of physical weathering is caused when rocks are relieved of overlying pressure, expand outward, and break apart in sheets?

Pressure Release

Pressure Release

Pressure release is a type of physical weathering that occurs when a rock is relieved of overlying pressure and expands in response. As the rock expands, outer layers begin to break off in sheets, as you can see in this image. For example, if erosion processes remove a large mass of overlying material from a rock, the rock is suddenly under much less pressure than it was before. Being under less pressure allows it to expand, and this expansion leads to the fracturing you see in this image.

Human Activity

Several human activities can accelerate weathering rates. One of these is the production of air pollution that results in acid rain. Acid rain contains chemicals that eat away at rocks and soils, as well as weaken and break them down. Limestone rocks are especially prone to attack from acid rain.

Avatar: The Importance of Soil Structure

Soil structure affects how well water and nutrients move downward through a soil. The more structured it is, the more it encourages good drainage, or the downward flow of water. More peds in the soil encourage the development of space for air and water within it. Good soil structure also allows seedlings to emerge. In this picture, the surface of the soil lacks the good structural development of the subsoil. The emerging seed ling is unable to reach the surface and grow. Excessive tilling of the land or compacting of the soil by heavy traffic can destroy soil structure and hinder the movement of water and nutrients through it. But adding organic matter to soil can encourage good structure, because organic matter acts like an adhesive to bind soil particles together.

What was the essential ingredient for making the iron nails rust?

That is right; the key ingredient is oxygen

Rock Composition and Structure

The composition and structure of rocks also affect weathering rates. Limestone is particularly prone to weathering by carbonation, and silicate minerals are easily weathered by hydrolysis. Some rocks are just naturally more or less resistant to weathering than others. Also, rocks that are highly jointed are weathered more easily and more quickly because the joints are areas of weakness that break easily. Essentially, the more surface area that is exposed the more readily the rock weathers. Smaller particle sizes generally means more surface area exposed to the elements and thus lead to higher rates of weathering.

The soda that you may drink contains carbonation

The fizz comes when carbon dioxide is dissolved in water to form carbonic acid.

Weathering Rates

Think of how many weathering processes like hydration, hydrolysis, and frost wedging are dependent on water in the environment

Devil's Marbles near Wauchope, Australia

This area is known for large boulders like the ones you see here, delicately poised atop the rocks beneath them

In what types of climate do you find thermal stress as a common type of weathering?

This kind of weathering is only found in environments that have a wide range in day and nighttime temperatures, such as a desert.

Vegetative Cover

Vegetation affects weathering rates in various ways. Chemical weathering can be accelerated under dense vegetation because of increased acids from plants that react with rock and soil minerals. Plants can also accelerate physical weathering by root action. In other cases, vegetation can slow down physical weathering. For example, dense vegetative cover can protect rocks and soils from the effects of freeze-thaw cycles, or prevent exposure to heat and rain.

Hydraulic Action

Waves that beat along a shoreline are a type of physical weathering. In this type of weathering, water is forced into cracks within a rock. As the water is forced inward, so is a small amount of air, which becomes trapped within the cracks. When the wave retreats, the trapped air is suddenly released with explosive force, wearing down the rocks.

Like all other features of the natural landscape

these rocks have a story to tell, and the scientists that study them have pieced together enough clues to learn that each "marble" began millions of years ago as granitic magma deep within Earth

Physical weathering:

or mechanical weathering, is the breaking down of rocks, minerals, or soils into smaller pieces without changing them chemically. Rubbing, heat and pressure all cause physical weathering and disintegrate earth materials into small pieces. An example of physical weathering is a rock developing cracks and breaking under the stress of heat or pressure.

This picture shows the mineral, kaolinite, a soft clay mineral that is formed when the harder mineral, orthoclase, undergoes hydrolysis.

pg 11

All weathering takes place

place near Earth's surface

Carbonation in the atmosphere also acts on

rocks

). Sometimes, however, the atmosphere plays a more secondary role

such as when plants produce chemicals to speed up the weathering of rocks. These plants could only survive with the atmosphere around them.

When oxygen interacts with the iron in soils and rocks

the result is rusting metals, which give the characteristic orange or reddish look

In a hydrolysis reaction

the silicate minerals are broken down or weakened

In this example

the water from rain changed the mineral, which in turn changed the rock's color. The green area represents an area of hydration. (pg 10)