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ANSWERS TO STUDY QUESTIONS CHAPTER 6 - Weathering and Soils

1. The mechanically weathered rock would be broken into smaller pieces, but the chemically weathered rock would change into new chemical compounds like clay and iron oxide.

2. frost wedging, frost heaving, abrasion, pressure release (sheet jointing & exfoliation), root wedging, thermal expansion/contraction

3. To expose plutonic rocks at the surface, miles of overlying rock must first be removed by erosion. This results in pressure release, expansion, formation of sheet joints parallel to the surface, and finally the spalling-off of the onion-like layers bounded by sheet joints.

4. Mechanical weathering adds to the effectiveness of chemical weathering by increasing surface area.

5. Oxygen and carbon dioxide from the atmosphere dissolve in rain as it falls. Dissolved carbon dioxide chemically bonds to water to form carbonic acid.

When K-feldspar (e.g. orthoclase) reacts with carbonic acid, the only solid product formed is clay. The soluble products are silica (soluble) and potassium ions.

When olivine reacts with carbonic acid, the only solid product is iron oxide (hematite and/or limonite). The soluble products are silica and magnesium ions.

6. Chemical weathering proceeds slowly in a hot desert because water is not abundant.

7. a. Chemical weathering will predominate in a hot, wet region.
    b. Basalt will weather more rapidly because it contains less silica.

8. Soil is capable of sustaining plant life whereas regolith is just any layer of weathered rock.

9. San Diego soils are generally poorly-developed because the arid environment slows chemical weathering. Furthermore San Diego is not flat. Soils tend to get washed-off slopes as quickly as they form.

10. Horizon O: Thin surface layer, usually dark brown to black, composed almost entirely of decomposing organic material (leaves, twigs, grass, etc.).

Horizon A: Burrowing organisms (commonly earthworms) mix mineral matter (quartz, feldspar, ferromagnesians) with the organic material in the overlying A horizon. As rain percolates down through this layer it becomes progressively more acidic as it picks up carbon dioxide from the decay of the organic material. Think of the "A" as standing for "Acid" because that's what forms in this layer.

Horizon E: The acids that are formed in the A horizon eventually become strong enough to chemically weather feldspar and ferromagnesian minerals. Soluble ions are leached and clays and iron oxide particles are formed as residual solids. The fine clay and iron oxide particles are elluviated (washed out), so ultimately only quartz will remain in the E horizon - usually making this layer rather light in color.

Horizon B: Accumulation of clay and iron oxide particles elluviated down from the E Horizon occurs here, so soils are clay rich and reddish colored.

Horizon C: At this depth, soil forming processes are less vigorous, alteration of bedrock is incomplete and some non-weathered bedrock remains.

 

Last Updated: 01/13/2015
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