Activity 4.1 - Review of Sedimentary Processes

Upon completion of this segment, students should be able to:

• Describe the processes that act to make sedimentary rocks, specifically mechanical weathering, chemical weathering, and erosion.

Students often have a hard time grasping the concept of weathering, even with the simple examples that they see every day (such as dissolution and oxidation, a.k.a. rust). This activity will allow students to think about weathering and erosion, and also consider how humans (due to mining activities) increase weathering and erosion rates.

Part 1

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The colored shapes represent minerals, and white space is pore space (space in rock that is filled with air or water). The original sediment (or rock, on left) at the surface is exposed to rain (the arrows from above) and hence, chemical weathering. The gray minerals are not soluble, so remain unchanged. The purple minerals are made of red and blue elements. The purple minerals react with the water, which preferentially removes one element (the red) from the mineral and carries that element deeper into the ground, where it crystallizes as a new mineral. Weathering therefore separated the blue from the red elements, and concentrated those elements. A mining company interested in the blue element will mine rocks/sediments right at the surface, whereas a mining company interested in the red element will extract deeper rocks/sediments.

Provenance: Image created by Joy Branlund, Southwestern Illinois College.
Reuse: This item is offered under a Creative Commons Attribution-NonCommercial-ShareAlike license http://creativecommons.org/licenses/by-nc-sa/3.0/ You may reuse this item for non-commercial purposes as long as you provide attribution and offer any derivative works under a similar license.

Chemical weathering affects minerals differently. This activity addresses this by showing a drawing of a hypothetical rock on Earth's surface and the underlying sediment. That hypothetical rock contains two minerals, one stable (it resists weathering) and one that dissolves. The soluble (unstable) mineral in the example doesn't completely dissolve but rather one element is extracted. This is similar to the alteration of ilmenite in the Mining Sand activity; iron is removed from the mineral by chemical weathering, leaving a more titanium-rich mineral behind.

It helps to review this drawing (found in the Unit 4 background reading and on this page) in class. Then, ask students to imagine a Peanut M&M.

1. What will happen if you put some water on the Peanut M&M? Answer: The color will wash off.
   
2. What will happen to the remaining M&M? Answer: The sugar coating will eventually dissolve but not the peanut.
   

The Peanut M&M is similar to the rock in the example. The color part of the sugar coating is like the red element in the purple mineral. The peanut is like the grey mineral.

Additionally, you can point out that mechanically weathering often accelerates chemical weathering. For example, if you chew the M&M, then the candy-coating will dissolve faster.

Part 2

When humans extract mineral resources, they increase weathering and erosion rates. Given that students learned about mining in the previous unit (Unit 3), they can be asked to link mining and weathering/erosion. This can be done verbally with the entire class. Instructors should stress that this is a review/application of concepts from the reading assignment.

1. What do we call it when mining companies blast apart the rock to get to a mineral resource? Answer: Mechanical weathering.
   
2. What will happen to the rate of chemical weathering in a place that has been mined, with waste rock either in piles or filling the mine site? Answer: Chemical weathering will occur at a faster rate.
   
3. What is a consequence of this higher chemical weathering rate?
   
4. Large trucks are filled with gravel at a quarry and then carry this gravel to build a road on the other side of town. This is an example of ____________ Answer: Erosion.
   

Note: The questions in Part 2 can be presented as multiple choice questions and used with clickers.

• Instructor may need to spend additional time with this review and activity if students have not completed the Unit 4 background reading.