High School Earth Science/Weathering

Lesson Objectives

Define mechanical and chemical weathering.
Discuss agents of weathering.
Give examples of each type of weathering.

What is Weathering?

Weathering is the process that changes solid rock into sediments. Geologists use the word sediment to describe all different sizes of rock particles. Sediment includes really large pieces of rock, like boulders or gravel, but it also includes sand and much smaller particles, called silt and clay. In the process of weathering, rock is disintegrated and decomposed. The chemical composition breaks down, and the disintegration of rock happens as rock is broken into pieces. Once the pieces are separated from the rocks, erosion is the process that moves those pieces. Gravity is one way that pieces of rock move, as broken pieces of rock fall or tumble from high places to lower ones. Gravity causes large and small pieces to fall from cliffs, as well as moving water in rivers and streams from mountaintops to the ocean. Wind and glaciers also move pieces of rock from one place to another. Wind moves sand sized and smaller pieces of rock through the air. Glaciers can move all sizes of particles, from extremely large boulders to the tiniest fragments.

Weathering happens at the Earth's surface. When most rocks form, they are forming at very high temperatures and pressures. This is a very different environment than the low temperatures and pressures at Earth's surface. When rocks reach Earth's surface, weathering causes them to change form. The new form will include minerals that are stable at the low temperatures and pressures of Earth's surface. So while powerful forces on Earth, such as those resulting from plate tectonics, work to build huge mountains like the Himalayas or majestic volcanoes like Mt Fuji, the forces of weathering gradually wear away rocks, changing once tall mountains into hills and even plains. The Appalachian Mountains along the east coast of North America were once as tall as the Himalayas! So what happened?

No human being can watch for millions of years as mountains are slowly built, nor can we watch as those same mountains gradually wear away. However you probably have been able to ride your bike or walk along a brand new sidewalk or road. What do you experience? The new road or sidewalk is smooth and even. If it was made well, there won't be any cracks or bumps. Does that smooth surface stay that way? Certainly over millions of years, it will completely disappear, but we don't have to wait that long. If you live in a part of the world that has cold winters, you may only have to wait one year to start seeing changes. We will talk next about what types of weathering change that brand new, smooth and even sidewalk into areas that are rough or cracked, chipped or buckled (Figure 9.1).

Figure 9.1: You can see the once smooth road surface has cracks and fractures, plus a large pothole.

Mechanical Weathering

Mechanical weathering (also called physical weathering) is the breaking of rock into smaller pieces. These smaller pieces will be just like the bigger rock, the pieces will just be smaller. That means the rock has been changed mechanically (or physically) without changing its composition. The smaller pieces will have the same minerals, in just the same proportions as the original rock. You could actually use the expression, 'A chip off the old block' to describe mechanical weathering! The main agents of mechanical weathering are water, wind, ice, and gravity. You will see how each of these works to break rock into smaller pieces.

There are two main ways that rocks can break apart into smaller pieces. The way that is most common in cold climates is called ice wedging. Ice wedging is the main form of mechanical weathering in any climate that regularly cycles above and below the freezing point (Figure 9.2). Some places where this happens include Earth's polar regions and mid latitudes. It also happens in the colder climates of higher elevations, like mountainous regions.

Figure 9.2: Water seeps into cracks and fractures in rock. As it freezes, it expands which wedges the rock apart.

This is how it works. When water changes from a liquid into a solid (ice), it increases in volume. This is a very unusual property. Most substances contract (get smaller) as they change from a liquid to a solid, but water does just the opposite. You may have already experienced this if you ever filled an ice cube tray all the way to the top with water and then put it into the freezer. The ice cubes will be much larger than the amount of water you first put in. You may have also made the mistake of putting your favorite soda into the freezer to cool it down quickly. If you leave your drink in the freezer too long, it will expand so much that it bends or pops the can. Ice wedging happens for the same reason. Water works its way into cracks and fractures in rock, and then expands as that water freezes. The ice takes up more space than the water did, which wedges the rock apart, physically breaking the rock into pieces. Ice wedging breaks apart so much rock that you will find large piles of broken rock at the base of a cliff or mountain, as broken pieces separate and tumble down its sides. Ice wedging will work quickly, breaking apart lots of rock in areas that go above and below the freezing point every night and day, and also in areas that cycle with the seasons.

Abrasion is another form of mechanical weathering. Abrasion can happen anywhere. All that is needed is one rock bumping against another rock. Gravity can cause abrasion as a rock tumbles down a mountainside or cliff. Moving water causes abrasion as particles carried in the water collide and bump against one another. Strong winds can pick up pieces of sand and blast surfaces with those sand grains. Finally, the ice in glaciers carries many bits and pieces of rock. As the glacier moves, pieces of rock embedded in the ice scrape against the rocks below. Broken pieces of rock tumbling down a mountain stream or tossed about by waves crashing onto the shore, will become smooth and rounded as abrasions smooth and round the sharp or jagged edges. If you have ever collected beach glass or cobbles from a stream, you have benefited from the work of abrasion.

Scientists talk about a few other types of mechanical weathering but ice wedging and abrasion are the two most important types. Without these two types of mechanical weathering, very little rock would break apart and that would slow down the rate of chemical weathering as well. Sometimes biological elements can do the work of mechanical weathering. This could happen slowly as a plant's roots grow into a crack or fracture in rock and gradually grow larger, wedging open the crack. Burrowing animals can also break apart rock as they dig for food or to make living spaces for themselves. Today, of course, human beings do quite a bit of mechanical weathering, whenever we dig or blast into rock to build homes, roads, subways, or to quarry stone for construction or other uses.

Actually whenever there is mechanical weathering, it increases the rate of chemical weathering. This happens because as rock breaks into smaller pieces, the surface area of the pieces increases (Figure 9.3). With more surfaces exposed, there are more places for chemical weathering to occur. Let’s say you wanted to make some hot chocolate on a cold day. You can imagine how hard it would be 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 and the easier it is to dissolve in the milk.

Figure 9.3: As rock breaks into smaller pieces, overall surface area increases.

Salt weathering of building stone on the island of Gozo, Malta.

Chemical Weathering

Another important type of weathering that happens on the Earth's surface is chemical weathering. Chemical weathering is different than mechanical weathering because with this type of weathering, rock is changed, not just in size of pieces, but changed in composition. This means that one type of mineral changes into a different mineral. The reason chemical weathering happens is that most minerals form at high pressure or high temperatures, deep within the Earth. When rocks reach the Earth’s surface, they are now at very low temperatures and pressures. This is a very different environment from the one in which they formed. The environment at Earth's surface is so different that these minerals are no longer stable. That's where chemical weathering begins. Minerals formed deep within the Earth must change to minerals that are stable at Earth's surface. Chemical weathering is important because it starts the process of changing solid rock into the soil we need to grow food and for the plants we need for our clothing and medicine. The way that chemical weathering works is through chemical reactions that cause changes in the rock.

There are many types of chemical weathering because there are many agents of chemical weathering. You probably remember that mechanical weathering is caused by several agents, such as water, wind, ice, and gravity. Well, water is also an agent of chemical weathering, so that makes it a double agent! Two other important agents of chemical weathering are carbon dioxide and oxygen. We will talk about each of these one at a time.

The minerals that make up most of the Earth's crust are called silicate minerals. These minerals are mostly made of just eight elements; oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), potassium (K) and sodium (Na). When chemical weathering occurs, the elements that make up the minerals react to form new minerals. The minerals that form at the lowest temperatures and pressures (closest to the situation at the Earth's surface) are the most stable while minerals that form from very hot magmas or at very high pressures are the least stable. The elements sodium, calcium, potassium and magnesium actually dissolve easily in water. Iron reacts with oxygen, which leaves atoms of silicon, oxygen and aluminum to combine to form new minerals, like clay minerals.

Water is an amazing molecule. It has a very simple chemical formula, H₂O, which means it is made of just two hydrogen atoms bonded to one oxygen atom. Even though it is simple to remember, water is pretty remarkable in terms of all the things it can do. Water is an excellent solvent. The way that a water molecule joins together allows water to attract lots of other elements, separate them from their compounds and dissolve them. Water is such a good solvent that some types of rock can actually completely dissolve in water. Other minerals change by adding water into their structure.

Hydrolysis is a chemical reaction between a mineral and water. When this reaction takes place, water itself separates into ions. These ions grab onto other ions, dissolving them in water. As the dissolved elements are carried away, we say that these elements have been leached. Through hydrolysis, a mineral like potassium feldspar is changed into a clay mineral. Once clay minerals have formed, they are stable at the Earth's surface.

Carbon dioxide (CO₂) combines with water as raindrops fall through the air in our atmosphere. This makes a weak acid, called carbonic acid. This happens so often that carbonic acid is a very common, weak acid found in nature. This acid works to dissolve rock. It also slowly changes the paint on a new car or eats away at sculptures and monuments. The normal situation can be made worse 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, it forms nitric acid and sulfuric acid. These are the two main components of acid rain. Acid rain accelerates chemical weathering.

A monument damage by acid rain.

Oxidation is the type of chemical reaction that happens when oxygen reacts with elements at the Earth's surface. Oxygen is very strongly chemically reactive. The type of oxidation that you are probably most familiar with produces rust when iron reacts with oxygen (Figure 9.4). Many minerals are rich in iron. They break down as the iron oxidizes, forming new compounds. Iron oxide produces the red color in soils. Chemical weathering can also be contributed to by plants and animals. As plant roots take in soluble ions as nutrients, certain elements are exchanged. Plant roots and bacterial decay use carbon dioxide in the process of respiration.

Figure 9.4: When iron rich minerals oxidize, they produce the familiar red color found in rust.

Differential Weathering

Rates of weathering depend on several factors. Different types of rocks weather at different rates. Certain types of rock, like granite, are very resistant to weathering. Igneous rocks tend to weather slowly because it is hard for water to penetrate them. Other types of rock, like limestone and marble are easily weathered because they dissolve easily in weak acids. More resistant rocks remain at the surface and form ridges or hills. Devil's Tower in Wyoming is an interesting example of how different types of rock weather at different rates (Figure 9.5). As the softer materials of the surrounding rocks were worn away, the resistant center of the volcano remained behind. Different minerals also weather at different rates. Some minerals completely dissolve in water. As less resistant minerals dissolve away, a rock's surface becomes pitted and rough. When a less resistant mineral dissolves, more resistant mineral grains are released from the rock.

Figure 9.5: Devil's Tower is an amazing example of differential weathering. All that remains of the volcano today is this central plug of resistant lava that forms the tower.

Most importantly, the climate of a region influences weathering. Climate is determined by the temperature of a region plus the amount of rainfall it receives. As the amount of precipitation increases, so does the rate of solution and the number of chemical reactions. In general, as the amount of rainfall increases, so does the degree of weathering. Remember that water is an agent of both mechanical and chemical weathering, so when water is not available, the rate of weathering slows tremendously. Two amazing examples of preservation include mummification and freezing. Both of these situations occur in the absence of liquid water. Therefore a dry climate will produce the lowest rate of weathering, followed by a very cold climate, regardless of the amount of rainfall it receives. The rates of highest weathering would occur in a wet climate that is also warm or hot. As the temperature of a region increases, so does the rate of chemical reactions. For each 10°C increase in average temperature, the rate of chemical reactions doubles. The warmer a climate is, the more types of vegetation it will have and the greater the rate of biological weathering. This happens because plants and bacteria grow and multiply faster in warmer temperatures. If you want an easy way to remember these examples, think about where you would put your sandwich if you want it to stay fresh for a while. How quickly does it go bad in your lunch box? Where would you put food from the grocery store if you wanted to save it for a week or more?

Some resources are actually concentrated for us by the actions of weathering. In tropical climates, intense chemical weathering carries away all soluble minerals, leaving behind just the least soluble components. The aluminum oxide, bauxite forms this way and is our main source of ore for producing aluminum. The actions of moving water can also concentrate heavier minerals, like gold. This process fueled the gold rush out west in North America in the 1800s.

Lesson Summary

Mechanical weathering breaks existing rock into smaller pieces without changing the composition of the rock.
Ice wedging and abrasion are two important processes of mechanical weathering.
The main agents of mechanical weathering are moving water, wind, glacial ice and gravity.
Chemical weathering decomposes or breaks down existing rock, forming new minerals that are stable at the Earth's surface.
Water, carbon dioxide, and oxygen are important agents of chemical weathering.
Different types of rocks weather at different rates. More resistant types of rocks will remain longer.

Review Questions

Name two types of mechanical weathering. Explain how each works to break apart rock.
What are three agents of chemical weathering? Give an example of each.
What type of climate would likely produce the greatest degree of weathering? Explain.
Would a smooth even surface weather faster than an uneven, broken surface?
What type of rock would best be suited for making monuments?

Vocabulary

abrasion: A form of mechanical weathering that occurs whenever one rock hits another.
chemical weathering: The form of weathering which decomposes rock; minerals that form at high temperatures and pressures change to minerals that are stable at the Earth's surface.
erosion: The transport of weathered materials by water, wind, ice or gravity.
hydrolysis: Chemical reactions between minerals and water in which hydrogen or hydroxide ions replace the cations in the mineral.
ice wedging: The form of mechanical weathering that occurs as water expands as it freezes, wedging apart rock.
leaching: The process of removing dissolved minerals as they are carried to lower layers in soil.
mechanical weathering: The form of weathering which disintegrates rock; bigger pieces of rock are broken into smaller pieces of the same composition as the original rock.
oxidation: A form of chemical weathering in which oxygen reacts with elements; happens when an atom or ion loses an electron.
sediments: Bits and pieces of weathered rock; the largest pieces would be gravel or pebbles, then sand, silt, and clay sized particles.

Points to Consider

What other types of surfaces are affected by weathering other than rock?
What might the surface of the Earth look like if weathering did not occur on Earth?
Do you think that you would be alive today if water did not dissolve elements?
Would the same composition of rock weather the same way in three very different climates?

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