weathering

oxygen

Oxygen strongly reacts with elements at the Earths surface. You are probably most familiar with the rust that forms when iron reacts with oxygen (Figure 9.4). Many minerals are rich in iron. They break down as the iron changes into iron oxide. This makes the red color in soils. Plants and animals also cause chemical weathering. As plant roots take in nutrients, elements are exchanged.

weathering happens at different rates

Each type of rock weathers in its own way. Certain types of rock are very resistant to weathering. Igneous rocks tend to weather slowly because they are hard. Water cannot easily penetrate them. Granite is a very stable igneous rock. Other types of rock are easily weathered because they dissolve easily in weak acids. Limestone is a sedimentary rock that dissolves easily. When softer rocks wear away, the more resistant rocks form ridges or hills. Devils Tower in Wyoming shows how different types of rock weather at different rates (Figure 9.5). The softer materials of the surrounding rocks were worn away. The resistant center of the volcano remains behind. Minerals also weather differently. Some minerals completely dissolve in water. As less resistant minerals dissolve away, a rocks surface becomes pitted and rough. When a less resistant mineral dissolves, more resistant mineral grains are released from the rock.

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water

Water is an amazing molecule. It has a very simple chemical formula, H2 O. It is made of just two hydrogen atoms bonded to one oxygen atom. Water is remarkable in terms of all the things it can do. Lots of things dissolve easily in water. Some types of rock can even completely dissolve in water! Other minerals change by adding water into their structure.

carbon dioxide

Carbon dioxide (CO2 ) combines with water as raindrops fall through the air. This makes a weak acid, called carbonic acid. This happens so often that carbonic acid is a common, weak acid found in nature. This acid works to dissolve rock. It eats away at sculptures and monuments. While this is normal, more acids are made when we add pollutants to the air. Any time we burn any fossil fuel, it adds nitrous oxide to the air. When we burn coal rich in sulfur, it adds sulfur dioxide to the air. As nitrous oxide and sulfur dioxide react with water, they form nitric acid and sulfuric acid. These are the two main components of acid rain. Acid rain accelerates chemical weathering.

abrasion

Abrasion is another type of mechanical weathering. With abrasion, one rock bumps against another rock. Gravity causes abrasion as a rock tumbles down a slope. Moving water causes abrasion it moves rocks so that they bump against one another (Figure 9.3). Strong winds cause abrasion by blasting sand against rock surfaces. Finally, the ice in glaciers cause abrasion. Pieces of rock embedded in ice at the bottom of a glacier scrape against the rock below. If you have ever collected beach glass or pebbles from a stream, you have witnessed the work of abrasion.

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plants and animals in mechanical weathering

Sometimes biological elements cause mechanical weathering. This can happen slowly. A plants roots grow into a crack in rock. As the roots grow larger, they wedge open the crack. Burrowing animals can also cause weathering. By digging for food or creating a hole to live in the animal may break apart rock. Today, human beings do a lot of mechanical weathering whenever we dig or blast into rock. This is common when we build homes, roads, and subways, or quarry stone for construction or other uses.

mechanical weathering and chemical weathering

Mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases. With more surfaces exposed, there are more places for chemical weathering to occur. Lets say you wanted to make some hot chocolate on a cold day. It would be hard to get a big chunk of chocolate to dissolve in your milk or hot water. Maybe you could make hot chocolate from some smaller pieces like chocolate chips, but it is much easier to add a powder to your milk. This is because the smaller the pieces are, the more surface area they have. Smaller pieces dissolve more easily.

chemical weathering

Chemical weathering is different than mechanical weathering. The minerals in the rock change. The rock changes composition and becomes a different type of rock. Most minerals form at high pressure or high temperatures deep within Earth. But at Earths surface, temperatures and pressures are much lower. Minerals that were stable deeper in the crust are not stable at the surface. Thats why chemical weathering happens. Minerals that formed at higher temperature and pressure change into minerals that are stable at the surface. Chemical weathering is important. It starts the process of changing solid rock into soil. We need soil to grow food and create other materials we need. Chemical weathering works through chemical reactions that change the rock. There are many agents of chemical weathering. Remember that water was a main agent of mechanical weathering. Well, water is also an agent of chemical weathering. That makes it a double agent! Carbon dioxide and oxygen are also agents of chemical weathering. Each of these is discussed below.

what is weathering

Weathering changes solid rock into sediments. Sediments are different sizes of rock particles. Boulders are sedi- ments; so is gravel. At the other end, silt and clay are also sediments. Weathering causes rocks at the Earths surface to change form. The new minerals that form are stable at the Earths surface. It takes a long time for a rock or mountain to weather. But a road can do so much more quickly. If you live in a part of the world that has cold winters, you may only have to wait one year to see a new road start to weather (Figure

mechanical weathering

Mechanical weathering breaks rock into smaller pieces. These smaller pieces are just like the bigger rock; they are just smaller! The rock has broken without changing its composition. The smaller pieces have the same minerals in the same proportions. You could use the expression a chip off the old block to describe mechanical weathering! The main agents of mechanical weathering are water, ice, and wind.

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ice wedging

Rocks can break apart into smaller pieces in many ways. Ice wedging is common where water goes above and below its freezing point (Figure 9.2). This can happen in winter in the mid-latitudes or in colder climates in summer. Ice wedging is common in mountainous regions. This is how ice wedging works. When liquid water changes into solid ice, it increases in volume. You see this when you fill an ice cube tray with water and put it in the freezer. The ice cubes go to a higher level in the tray than the water. You also may have seen this if you put a can of soda into the freezer so that it cools down quickly. If you leave the can in the freezer too long, the liquid expands so much that it bends or pops the can. (For the record, water is very unusual. Most substances get smaller when they change from a liquid to a solid.)

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instructional diagrams

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questions

Sediments produced by weathering include

a. boulders.

b. gravel.

c. silt.

-->  d. all of the above

If pieces of a rock flake off due to extreme temperature differences, it would be

a. erosion

-->  b. mechanical weathering

c. chemical weathering

d. transportation

What is the first step in the process of ice wedging?

a. Ice melts in cracks in rocks.

b. Water freezes in cracks in rocks.

-->  c. Water seeps into cracks in rocks.

d. Ice wedges apart cracks in rocks.

Chemical weathering

a. is unrelated to mechanical weathering

-->  b. can go faster when there has been mechanical weathering

c. is slowed down after there has been mechanical weathering

d. none of these

Abrasion may be caused by all of the following except

a. gravity.

b. glaciers.

c. moving water.

-->  d. carbon dioxide.

If a mineral changes to a different type it has experienced

a. erosion

b. physical weathering

-->  c. chemical weathering

d. transportation

Minerals undergo chemical weathering because

-->  a. they formed at different pressure and temperature

b. they first undergo mechanical weathering

c. they break apart by mechanical weathering

d. water takes away some of their ions

Plants can cause

a. mechanical weathering.

b. chemical weathering.

c. ice wedging.

-->  d. two of the above

How does mechanical weathering increase the rate of chemical weathering?

a. It makes rocks softer.

b. It changes the minerals in rocks.

-->  c. It increases the surface area of rocks.

d. all of the above

Because carbon dioxide combines with water in the atmosphere

a. average global temperatures are rising

b. plants die off

c. the atmosphere is warmer

-->  d. rainwater is a weak acid

Agents of chemical weathering include all of the following except

-->  a. ice.

b. water.

c. nitric acid.

d. sulfuric aci

Which rock weathers quickly?

a. basalt

b. granite

-->  c. limestone

d. none of the above

agent of chemical weathering

a. chemical weathering

-->  b. carbonic acid

c. abrasion

d. mechanical weathering

e. sediment

f. erosion

g. ice wedging

weathering process that occurs when water freezes in cracks in rocks

a. chemical weathering

b. carbonic acid

c. abrasion

d. mechanical weathering

e. sediment

f. erosion

-->  g. ice wedging

weathering process that occurs when rocks and rock particles scrape other rocks

a. chemical weathering

b. carbonic acid

-->  c. abrasion

d. mechanical weathering

e. sediment

f. erosion

g. ice wedging

Important agents of chemical weathering include oxygen, carbon dioxide and sulfur.

-->  a. true

b. false

rock particle created by weathering

a. chemical weathering

b. carbonic acid

c. abrasion

d. mechanical weathering

-->  e. sediment

f. erosion

g. ice wedging

If temperature increases by 10C, the rate of chemical reactions will double.

-->  a. true

b. false

type of weathering that breaks rock into smaller pieces

a. chemical weathering

b. carbonic acid

c. abrasion

-->  d. mechanical weathering

e. sediment

f. erosion

g. ice wedging

movement of weathered rock particles

a. chemical weathering

b. carbonic acid

c. abrasion

d. mechanical weathering

e. sediment

-->  f. erosion

g. ice wedging

All rocks weather at the same rate.

a. true

-->  b. false

type of weathering that changes the minerals in rock

-->  a. chemical weathering

b. carbonic acid

c. abrasion

d. mechanical weathering

e. sediment

f. erosion

g. ice wedging

Abrasion is a type of chemical weathering.

a. true

-->  b. false

Water can dissolve salt.

-->  a. true

b. false

Natural weathering is usually a very slow process.

-->  a. true

b. false

Grains of sands are weathered particles of rock.

-->  a. true

b. false

Agents of mechanical weathering include wind and gravity.

-->  a. true

b. false

Ice wedging occurs only in extremely cold climates.

a. true

-->  b. false

Pebbles in a stream are worn smooth by abrasion.

-->  a. true

b. false

Burrowing animals cause mechanical weathering.

-->  a. true

b. false

Only acids can dissolve rocks.

a. true

-->  b. false

Carbon dioxide makes a weak acid when it combines with oxygen in the air.

a. true

-->  b. false

Water is an agent of both mechanical and chemical weathering.

-->  a. true

b. false

All rocks weather at the same rate.

a. true

-->  b. false

diagram questions

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