Continent - GEOL 114 The Earth's Dynamic Interior Lecture...

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GEOL 114 The Earth's Dynamic Interior Lecture Notes (Copyright © 2006 by Jeffrey S. Barker) 14. The Continental Crust Morphology In our discussion of the Oceanic Crust, we found that most of the Earth's ocean basins were at a depth of 3-4 km (the Abyssal Plains), with Mid-Ocean Ridges forming high elevation and deep- sea trenches providing deeper depths than the highest mountains are high. Now let's turn our attention to the part of the Crust above sea level, the Continental Crust. Again, most of the surface of the Earth has fairly low elevation. In particular, look at the histogram of elevation in the lower part of the figure to the right. There are some rather remarkable areas of high elevation (mountains), and we'll spend quite a bit of time talking about them, but they do not represent most of the surface of the continents. The elevation here in Vestal is just under 300 meters (0.3 km), which is fairly average for the continents. Continuing our comparison with the Oceanic Crust, we see from figure C (below) that the Oceanic Crust is thin relative to the Continental Crust. We have already referred to this in terms of isostasy: since Oceanic Crust has higher density than Continental Crust, it is less buoyant, so it "floats" lower on the "fluid substratum" beneath (figure B). [Note: here “fluid” refers to the material's ability to flow; it is not a liquid!] Oceanic Crust is topographically low, so it is covered with oceans. Conversely, Continental Crust is less dense, more buoyant, "floats" higher, so it is topographically high, and so is dry (most of the time anyway). You may have GEOL 114 1 14. The Continental Crust From Brown and Mussett.
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Continental Crust is drawn thickest under mountain areas and thinner under plains and coastal areas (although still thicker than the Oceanic Crust). Due to isostasy, the mass of the mountains is exactly balanced by the buoyancy of the thick "root" beneath the mountains (which displaces the higher density material of the Mantle). We know continental thickness from seismic surveys carried out in a number of continental areas, some of which are summarized to the right (and we'll go into more detail for a couple of examples later). Shown are layers with P-wave velocities for a) a stable shield area: Wisconsin; b) the Basin and Range area of the western U.S.; c) northern Scotland, an old mountain belt; d) southern California; e) the central Andes; f) the central Alps. As you can see, the Crust consists of 2-3 layers overlying the Mantle (in black) and has an average thickness of 30-40 km. However, under mountain belts such as the Andes or Alps, the crustal thickness nearly doubles to 60-70 km. GEOL 114
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Continent - GEOL 114 The Earth's Dynamic Interior Lecture...

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