The Earth is made up of a series of concentric rock zones; the crust, mantle outer core and inner core. The Crust is the main concern here. It varies greatly in thickness and composition; beneath the oceans it is as little as 5 Km. thick in places, but under some mountain ranges it extends to depths of 70 km. Rocks in the crust fall into two main groups. The ocean basins are predominantly underlain by basaltic rocks, containing much iron and magnesium, and having densities of between 2.8 and 3.0 (water =1.0). In contrast, the rocks that make up the continents are lighter in color and weight (densities of 2.7) and are rich in silicon and aluminum. Below the crust lie the denser rocks The upper part of the mantle, to depths of about 100 km. is solid and together with the crust forms a relatively rigid unit known as the lithosphere. At depths between 100 and 250 km. The 13 mantle is partially molten and capable of slow flowage; this is the asthenosphere.
Differences in density between the continental and oceanic areas of the lithosphere are thought to explain why continents stand high above ocean basis. Each continent is underlain by a root zone of similar light material projecting down into the asthenosphere by an amount proportional to the height of the continental area. The term-isostasy describes this state of balance. There are important implications to landforms of any disturbance to this structural equilibrium. If material is moved by erosion from an area and deposited on the sea floor, it will involve isostatic adjustment to both areas: a rise in the level of the area subject to erosion and subsidence of the sea floor. Similarly, the addition of weight to a continental area, in the form of ice or body of water for example, will cause the, crust to sink slightly. There may be a considerable timelag, perhaps thousands of years, between the external change and isostatic response of the crust. Much broad crustal warping today is related to isostasy. Measurable amounts of isostatic uplift are taking place in regions such as Scandinavia and Arctic Canada, which were depressed beneath ice caps during Pleistocene glaciations.
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