Chemical erosion involves a chemical reaction (one or mores substances changed into others). Most chemical erosion is the result of acid rain. Acid rain occurs when rain mixes with elements and gasses in the air, specifically pollutants like sulfur oxides and nitrogen oxides. These pollutants come from vehicles, factories and power-generation plants (which burn fossil fuels). See Figure 2 for an illustration of the acid rain cycle. This causes the water to have an acidic pH of approximately 5. (Note: On the pH scale, 7 is neutral (i.e., "pure" water), with any number towards 1 being more of an acid, and any number higher than 7 being more of a base. Visit http://www.epa.gov/acidrain/ph.html for more information.) Wind can then carry this contaminated water hundreds of miles away from the original pollution site, which then harms streams, lakes, forests and any water life which resides there (i.e., frogs and fish, among others). In addition to hurting nature, acid rain also destroys man-made structures, such as historical buildings and monuments. Some other examples of chemical erosion are Mammoth Cave in Kentucky or Carlsbad Cavern in New Mexico (both carved out by this process).

Figure 2. Acid rain cycle. click for copyright

Figure 3. Stalactites and stalagmites found in Carlsbad Caverns in New Mexico. click for copyright

Environmental engineers try to neutralize acid rain by planting trees, as they can have a buffering effect on acid rain. Forests reduce the acidity through the leaching process on leaves and the ion exchange in the soil. Engineers are also working on decreasing pollution by creating cleaner burning fuels and machines (e.g., cars) that do not release as much pollution into the air. Engineers try and protect structures from chemical erosion by using building material that are resistant to acid rain

Water erosion is perhaps the most familiar type of erosion. Water erosion can take many different forms. Sheet erosion is when a fairly uniform layer of topsoil is removed by raindrop splash and run-off during storms. This can cause soil to lose nutrients, which affects the land's ability to produce crops. Rill erosion is caused by run-off water gathering in flow lines (usually on recently cultivated land). It leaves small channels along the soil and often deposits this soil in streams or reservoirs, which can damage the surrounding ecosystem. If rill erosion is not addressed, water will continue to deepen these channels into gullies (gully erosion). Gully erosion results in the loss of significant amounts of land. Tunnel erosion occurs when water removes soil below the Earth's surface. This causes cavities to form underground, which weakens the surface. If these tunnels collapse, they become deep gullies. Tunnel erosion is dangerous because these cavities could collapse at anytime and cannot be detected from above. Stream-bank erosion is caused by moving streams or river water wearing away the surrounding banks. This can drastically change the course of a river and ultimately the surrounding landscape. Shore erosion deposits sediment into the body of water and changes coastline (storms, such as hurricanes, can accelerate this process). Water erosion has also shaped canyons (e.g., the Grand Canyon in Arizona) and rock formations (e.g., Rainbow Bridge in Utah).

Engineers try and minimize erosion by directing the flow of water using dams and levees. Planting grass and trees (to form a strong root system) or laying out netting can also decrease water erosion. Scouring is the erosion of streambed material that supports bridges and has been the cause of disasters and near-disasters throughout the nation. Engineers have developed several systems to measure scouring, including remote sensors and active sonar that monitors stream scouring as it happens. Figure 4 illustrates different types of water erosion.

Figure 4. Examples of erosion (from left to right) sheet erosion, rill erosion, tunnel erosion and stream-bank erosion click for copyright

Wind erosion creates and moves entire sand dunes, shapes rocks and other structures, and causes dust storms. Sand dunes are always being reshaped and shifting due to the wind. Dunes start out as mere ripples in the sand and gradually increase in size. Sand is moved up to the top of the dune until the pile becomes so steep that it collapses and slumps down the other side (the slip face; see Figure 5). Sand dunes are particularly vulnerable to wind erosion because they are completely composed of loose sand without anything anchoring them in place.

Figure 5. Sand dune formation. click for copyright

Wind erosion also shapes rocks and deteriorates structures, such as the ancient pyramids in Egypt. The wind picks up grainy particles — such as sand — and literally becomes airborne sandpaper that wears away (sands down) surrounding surfaces. The famed Devil's Tower in Wyoming (our nation's first national monument; see Figure 6) was actually created by wind erosion. It used to be a volcano, but now all that remains is the solidified, lava core. (The wind was powerful enough to cut away an entire volcano!)

Figure 6. Devil's Tower National Monument in Wyoming. click for copyright

One of the biggest problems associated with wind erosion is dust storms. The wind picks up dust, leaving crops exposed and blocking out the sun with a big black cloud. Farmers protect their crops (particularly potatoes) by planting larger crops (like rye) surrounding them or covering them with straw and foliage. Figure 7 shows unprotected and protected crops.

Figure 7. Unprotected (left image) and protected (right image) potato fields. click for copyright

A glacier is created by fallen snow that has accumulated and been compressed into massive ice formations on land. They make up about ten percent of the world's total land area. These masses are slowly pulled downhill by gravity, eroding the surface of the Earth as they move. Glacier erosion occurs by abrasion, crushing and fracturing material in the path of a glacier. This moving glacier picks up rock and sand along the way causing their surfaces to become extremely rough, which wears away even more material. Glacier erosion shaped Yosemite Falls in California, created the U-shaped valleys in Rocky Mountain National Park in Colorado (see Figure 8), and formed the Grand Tetons in Wyoming. The Matterhorn, a mountain in the European Alps, was also shaped by glacier erosion. It is unique in that it has four faces (slopes), which face the four compass directions.

Figure 8. Sheep Lakes, Rocky Mountain National Park (Colorado) click for copyright

Temperature erosion causes materials to crack and break apart. When material such as concrete or steel heats up (from the sun, for example), it expands; when it gets cold it shrinks in size. This fluctuation in size due to temperature can cause the material to crack. (Colorado is an excellent example of a climate where temperature is a problem for materials, since one day it can be 70 degrees and the next day it can snow, and be only 40 degrees outside.)

Engineers have accounted for these changes by putting groves in sidewalks (see Figure 9) and expansion joints in bridges. This minimizes the damage caused by thermal erosion, which is important since broken concrete on the road can be dangerous.

Figure 9. Sidewalk grooves click for copyright

Discussion Questions: Ask a couple of discussion questions to get students thinking about the upcoming lesson. After soliciting answers, explain that these questions will be answered during the lesson.

• What causes the Earth to look the way it does?
• Has the Earth always looked the same since the beginning of time?

Voting: Ask a true/false question and have students vote by holding thumbs up for true and thumbs down for false. Count the votes and write the totals on the board. Give the right answer.

• True or False: Erosion can wear away the surface of the Earth by chemicals, wind, water, glaciers and temperature (Answer: True)
• True or False: Chemical erosion is a reaction that usually involves water that has been polluted by chemicals, such as acid rain. (Answer: True)
• True or False: Wind erosion is rivers, streams or even rain, breaking away at bits of land while moving over it. (Answer: False, water erosion is described here.)
• True or False: Wind erosion can create and move entire sand dunes, shape rocks and other structures and cause dust storms. (Answer: True)
• True or False: Engineers do not need to know about erosion. (Answer: False, engineers use knowledge of erosion for many different applications, including the safe design and building of structures, highways, sidewalks and also for the protection of natural monuments.)
• True or False: Engineers also learn about erosion to help protect existing land formations and monuments that people want to keep around. (Answer: True)
• True or False: Water erosion causes sand, acting as sandpaper, to wear away surfaces as it moves over something. (Answer: False, wind erosion is described here.)

Can You Picture It? Show the students pictures of different things that have been affected by erosion and see if they can guess which type of erosion caused it. For example, the Grand Canyon, Carlsbad Caverns, Ayers Rock, the Grand Tetons, Devils' Towers, an eroded statue, cracks in pavement, etc.

Idea Web: Ask students to develop a list of the different types of erosion. What effects does erosion have on us and our environment? What are possible solutions for reducing these effects? What can engineers do to help?

Engineering Poster: Using the knowledge they learned in the lesson on the types of erosion, have students create a poster of a design for a protective structure for a statue or natural landform that would withstand one of the types of erosion (water, wind, temperature, glacier or chemical). Have them title their posters with an engineering firm name that they make up (for example, Erosion Engineering Firm). Have the students work in teams if possible.