Weathering

Lesson Objectives

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

Vocabulary

  • abrasion
  • chemical weathering
  • erosion
  • ice wedging
  • mechanical weathering

Introduction

Weathering breaks rocks apart. Some types of weathering alter some minerals. Erosion moves the broken pieces.

What is Weathering?

Weathering changes solid rock into sediments. Sediments are different sizes of rock particles. Boulders are sediments; so is gravel. At the other end, silt and clay are also sediments. Weathering causes rocks at the Earth’s surface to change form. The new minerals that form are stable at the Earth’s 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 below).

A hard winter has damaged this road.

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.

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 below). This can happen in winter in the mid-latitudes or in colder climates in summer. Ice wedging is common in mountainous regions.

(A) Diagram showing ice wedging. (B) Ice wedging along the joints in this rock helped to break it apart.

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.)

Ice wedging happens because water expands as it goes from liquid to solid. When the temperature is warm, water works its way into cracks in rock. When the temperature cools below freezing, the water turns to ice and expands. The ice takes up more space. Over time, this wedges the rock apart. Ice wedging is very effective at weathering. You can find large piles of broken rock at the base of a slope. These rocks were broken up by ice wedging. Once loose, they tumbled down the slope.

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 below). 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.

Rocks on a beach are worn down by abrasion as passing waves cause them to strike each other.

Plants and Animals in Mechanical Weathering

Sometimes biological elements cause mechanical weathering. This can happen slowly. A plant’s 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. Let’s 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 Earth's surface, temperatures and pressures are much lower. Minerals that were stable deeper in the crust are not stable at the surface. That’s 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.

Water

Water is an amazing molecule. It has a very simple chemical formula, H2O. 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.

Oxygen

Oxygen strongly reacts with elements at the Earth’s surface. You are probably most familiar with the rust that forms when iron reacts with oxygen (Figure below). Many minerals are rich in iron. They break down as the iron changes into iron oxide. This makes the red color in soils.

Iron ore oxidizes readily.

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.

Devil’s Tower in Wyoming shows how different types of rock weather at different rates (Figure below). The softer materials of the surrounding rocks were worn away. The resistant center of the volcano remains behind.

Devil

Minerals also weather differently. 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.

Lesson Summary

  • Mechanical weathering breaks rocks into smaller pieces. Their composition does not change.
  • Ice wedging and abrasion are two important processes of mechanical weathering.
  • Chemical weathering breaks down rocks by forming new minerals. These minerals are 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.

Lesson Review Questions

Recall

1. Name two types of mechanical weathering. Explain how each works to break apart rock.

2. What are three agents of chemical weathering? Give an example of each.

Apply Concepts

3. How do acids form in the atmosphere? What increases the acidity of rainfall?

4. What are the effects of acid rain?

Think Critically

5. Describe what you think weathering would be like in an arid region. What would weathering be like in a tropical region?

6. What type of surface weathers faster: a smooth surface or a jagged surface?

Points to Consider

  • What types of surfaces other than rock are affected by weathering?
  • What might the surface of the Earth look like if weathering did not occur?
  • 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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