Home | Session 4 | Product of Weathering Pg 1, 2, 3, 4

The Products of Weathering and Erosion

Presented by Lisa White
San Francisco State University

2. Experiment with the different properties of the clay samples montmorillonite and kaolinite by placing samples in a beaker, and adding small amounts of water to each. (Click on photos to zoom in.)

Questions to answer: What are the characteristics of each material when wet? How differently do they hold water? Do you think one of the clays is more susceptible to landsliding and slope failure? Answer: Weathering is the ultimate source for nearly all clay. The type of clay that forms is a function of what ions are abundant in the source rock, and the extent to which those ions are retained in the soil (in an arid climate) or leached out of the soil (in a warm, humid climate). With a little water, clay flakes bond strongly and have considerable cohesive strength. With the addition of a lot of water, that bond diminishes in strength, and clays become heavy and weak. In an arid climate, with incomplete leaching, K and Mg remain in the soil. If basic rocks are being weathered, abundant Mg ions are made available and montmorillonite, a Mg-rich clay, forms. Montmorillonite (shown on the right in the third photo above) is a very expansive clay that can absorb up to 1400 times its own volume of water. However, it gets very heavy when it is weathered making it most prone to slope failure. In a humid climate, with complete leaching, both K and Mg are removed from the soil by percolating ground water, and kaolin, which lacks both K and Mg, forms. Kaolinite clay is shown on the left in the third photo above.
What are the characteristics of each when dry? Closely examine the mudcracks in the photos above and see if you can guess which ones are produced from the montmorillonite clay vs. the kaolinite clay. (Click to zoom in on photos, and for the answer.)

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updated March 6, 2002

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