Erosion: What Goes Up Must Come Down

In this lesson, students will complete a hands-on activity in which they will discover that mountains erode at different rates and form different shapes as a result of their underlying composition.

Concepts and learning outcomes	Students will understand that: Geologic forces that form and shape mountains are both constructive and destructive. Erosion is a destructive force that sculpts and shapes all mountains. Mountains are usually made up of many types of rock. Some rock types erode more easily than others, producing differential rates of erosion in neighboring rocks. Differential erosion of rocky material produces jagged mountains as well as mesas, buttes, and exposing batholiths.
Time requirements	Two 50-minute class periods
Vocabulary	mesa , butte, plateau, batholith, erosion, alluvial fan, weathering
Background for teachers	Erosion is a general term that refers to the breaking down of material by physical, chemical, and biological factors and the transport of material away from its original site by water, gravity or wind. Weathering is part of the process that breaks rocks into smaller pieces. Weathering can occur by the action of streams, glaciers, ice, waves, wind and groundwater, by biological forces such as those exerted by lichens, tree roots or burrowing organisms, and by chemical reactions that 'rust' (oxidize) or dissolve minerals. Mechanical (physical) weathering tends to be the most important process in shaping mountains, but other processes can go a long way to facilitate the process. See Weathering and Erosion: What's the difference between these two terms? for more information. For this activity, students should focus on the process and resulting shapes of mountains. Students record their ideas in the Mountain Building Journal and we provide suggested answers in the Mountain Building Journal: Teacher's Guide.

Materials / Preparation

Supplies for 12 teams:
- 8 cups of sand
- 6 cups of pea or aquarium gravel
- 2 cups small rocks (driveway style crushed rock, approximately almond to cherry sized)
- 3 large rocks the size of a large plum
- Water
- Watering can (shower-head style pouring tip) or adjustable spray nozzle for a hose
Each team will need:
- Styrofoam plates (pre-cut, see below), 1 per team
- 250 ml (1 cup) of water for mixing
- 8 oz. plastic cups and spoons, 1 each per team
- Erosion Photos: These can be accessed online on the Mountain Building Student Web Page
- Mountain Photo Archive : These can be accessed online on the Mountain Building Student Web Page
Students will use pages 14& 15 of their Mountain Building Journals.

Preparation:

Pre-measure and prepare all materials for each group prior to the students starting this activity
Cut one or two wedges out of the edge of each plate so that the water can flow off the plate (otherwise the upturned edge of the plate will trap the water).
Teams are assigned one of the following mountain-building mixtures. You can prepare their cups ahead of time or you can have the students make their own (this will take more time).
Mountain 1:1 cup sand
Mountain 2: 1/3 cup sand + 2/3 cup gravel
Mountain 3: 1/3cup gravel + 1/3 cup sand + 1/3 cup rocks
Mountain 4: 1/2 cup sand + 1/2 cup gravel + 1 large rock underneath it all (representing a batholith)

Grouping

Groups of two or three

Teacher tips

Do this activity outdoors if possible. If not, elevate the mountain models on the plates in a large plastic tub to contain the run-off. Do not over pour the water or you might wash the entire model away! The water needs to be sprinkled onto the model.

You may want to point out the alluvial fans that form at the cut out wedges of the plate as erosion progresses.

You can also demonstrate the formation of mesas and buttes using this same method. Simply place a flat object (a coin, flat stone or piece of pottery will do), representing a layer that is more resistant to erosion, on top of some sand. Gently pour water from a watering can over this. The sand will erode away more quickly from the unprotected areas creating flat-topped mesas and buttes.

Students record their ideas in the Mountain Building Journal and we provide suggested answers in the Mountain Building Journal: Teacher's Guide.

Some perceptive students will observe that the COMPOSITION of the sand and gravel is actually quite similar -- the difference between the different trials in the experiment is the SIZE of individual pieces (grains). This is absolutely true. We use the different size rocks as analogs to rocks with different mineral composition, structure, and grain size -- all three are essential factors in determining resistance to weathering and erosion.

There are two big ideas that students should come away with, and they are easy to confuse. One focuses on time and rates, while the other addresses shapes of mountains:

Rates: Softer and weaker rocks erode more quickly, resulting in mountains that change shape rapidly over time. (Given enough time, even strong and resistant batholiths will turn into a smooth pile of sand. In fact, many piles of sand on California beaches were eroded from the Sierra Nevada batholith).
Shape: Materials that are more resistant to erosion can maintain steeper faces without being torn down, resulting in steep and dramatic mountains. Softer, weaker rocks result in smoother, rounder mountains with fewer jags (like the pile of sand).

Procedures

Class discussion: In the last Lesson, we focused on how mountains get built up by forces of volcanism and plate tectonics. However, as mountains get higher and higher, what happens to them? (trees and plants grow, they get pounded by wind and rain, snow freezes and then thaws) These forces actually start tearing the mountain down. We call this process "erosion."
Show the student the Erosion Photos. These photos can be viewed on line, or displayed at the front of the classroom using a projection device. Note: These images can be accessed from the Mountain Building Student Web Page. Ask students:
- How do you think these mountain features were formed? (erosion)
- Do you think that all mountains erode at the same rate? (no)
- Why not? (They are composed of different kinds of rock.)
- How might the ability of different kinds of rocks to withstand erosion contribute to the shape of these mountains?
Explain to students that erosion causes the mountain to change as it sculpts and shapes the mountain. Shapes of the mountain get complicated because most mountains are made up of many different kinds of rocks and each of them differ in how well they withstand the forces of erosion. The differential erosion of rocky material makes every mountain unique in shape.
Explain to students: 'Today we'll investigate how erosion affects mountains made of different types of rocks.'
a. Divide the class into teams of two. Assign each team to construct one of the 4 different types of mountains.
b. Tell the students to use their spoon to mix enough water into their cup to make the materials “sticky” enough to make a mountain (sand castle consistency). Excess water is easily poured off.
c. Instruct the students to empty the contents of their cup onto the plate forming a mountain. This can be done most gracefully as you would take a cake from a pan: Place the paper plate over the top of the cup. Carefully turn the plate and cup over so that the cup is inverted. Then, carefully lift the cup away leaving a perfectly shaped mountain to start with. For some mixtures, the mountain may not retain its shape too well and will collapse into a pile. If this happens, see if students can make the connection between this behavior and landslides on real mountains (both are caused by gravity!) Then they can form it back into a “mountain shape” before proceeding with the erosion.
d. Students should draw each mountain type (four of them) before continuing to the next step. Mountains should be drawn in side view (not birds eye view) on page 14 in their journals.
Either the teacher or the students then use the watering can to make their mountains erode. Make sure that you use a gentle flow of water to prevent the erosion from occurring too rapidly.

Students should observe and draw (side view not birds eye view) the same four mountains after erosion. They should record their observations on page 14 of their journals.
Note: Point out that easily eroded material (sand) produced a low rounded hill while Mountain # 4 with its single large rock will be pretty steep and dramatic -- more steep and dramatic than the models made of just gravel or sand. The steepness of the sides is one of the biggest differences in shape.

Review and Reflection: Have students answer the questions on page 15 in their journals:
It is important that students recognize that EROSION CHANGES THE SHAPES OF ALL MOUNTAINS. Even a volcano that starts out as a perfect cone like Mount Fuji will have its shape changed. When they are doing the culminating activity, they will need to remember this fact.