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Lecture14 weathering
1. Announcements Problem Set 1 Answer Key is posted. Check your graded set. If there are any discrepancies talk to me. Exam 1 review session: Monday at 5pm, here in McG-S room 20. Bring questions! Exam 1 itself: Wednesday the 23 rd
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3. Endogenic vs. exogenic Internal vs. external systems plate tectonics folding faulting volcanism weathering water erosion wind erosion Building up Breaking down tug of war glacial erosion
Preferential weathering along joints can create very important influences on landforms. Arch at left is NOT the product of eolian abrasion, as many people think (wind-blown sand seldom rises more than 2-3 m above the ground surface; transport at greater heights is primarily silt and clay, which isnât very abrasive), but rather is solutional weathering along joints. In upper right, massive joints help keep upper layer relatively resistant to weathering and erosion, allowing it to serve as a caprock for weaker underlying siltstone/shale units, and limiting rate of cliff retreat
Examples of spheroidal weathering, which is typically best expressed in granite, sandstone, or basalt (all of which tend to have orthogonal joints). Drawing is from Ritter textbook
Examples of orthogonal joints, which are very common in granites, although obviously these are not evenly spaced or truly orthogonal. Can emphasize that differences in joint density at scales of tens to thousands of meters in granite terrains creates hills/tors/high points and depressions/valleys (more densely spaced joints = greater surface area/volume ratio = faster weathering). Photo scale at upper left. Wyoming photos are at Veda Vu
These are all photos of the combined effects of different physical weathering processes (thermal expansion, unloading, hydration) that are expressed as joints parallel to the ground surface. This is a typical expression of differential pressures because surface expands more rapidly and completely under unloading, hydration, etc than does subsurface. Where relatively closely spaced orthogonal joints are present, this can create 3d checkerboard effect, with corners and edges weathering most rapidly because of greater surface area/volume ratio, leading to spheroidal weathering
These are carbonate-cemented sandstones in arid environments, so thereâs enough water to dissolve some of the matrix, but not enough for large-scale dissolution (as there likely would be in a wetter climate). Also note preferential dissolution along bedding planes and joints. This type of solutional weathering is known as taphoni. In the Australian example, there is a weathered surface layer in front of the solutional weathering, that has been locally removed (particularly at top of rock face).
![Weathering ,[object Object]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)