2.11-A Uplift, Isostasy, and Isostatic Adjustment : Vertical Movements of the Crust

In addition to the large crustal displacement driven mainly by plate tectonics, gradual up-and-down motions of the continental crust are observed at many locations around the globe. Although much of this vertical movement occurs along plate margins and is associated with active mountain building, some of it is not.

Evidence for crustal uplift occurs along the west coast of the United States. When the elevation of a coastal area remains unchanged for an extended period, a wave-cut platform develops. In parts of California, ancient wave-cut platforms can now be found as terraces hundreds of meters above sea level. Such evidence of crustal uplift is easy to find. Unfortunately, the reason for uplift is not always as easy to determine.

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Isostasy: Early workers discovered that Earth’s less dense crust floats on top of the denser and deformable rocks of the mantle. The concept of a floating crust in gravitational balance is called isostasy. Etymology: ISO is equal. STASIS is standing. Perhaps the easiest way to grasp the concept of isostasy is to envision a series of wooden blocks of different heights floating in water. Note that the thicker wooden blocks float higher than the thinner blocks.

Similarly, many mountain belts stand high above the surrounding terrain because of crustal thickening. These compressional mountains have buoyant crustal roots that extend deep into the supporting material below, just like the thicker wooden blocks.

Isostatic Adjustment. Visualize what would happen if another small block of wood were placed atop one of the blocks. The combined block would sink until a new isostatic, gravitational, balance was reached. However, the top of the combined block would actually be higher than before, and the bottom would be lower. This process of establishing a new level of gravitational equilibrium is called isostatic adjustment.

Applying the concept of isostatic adjustment, we should expect that when weight is added to the crust, it will respond by subsiding, and when weight is removed, the crust will rebound. Visualize what happens to a ship as cargo is being loaded and unloaded. Evidence for crustal subsidence followed by crustal rebound is provided by Ice Age glaciers. When continental ice sheets occupied portions of North America during the Pleistocene epoch, the added weight of 3 km thick masses of ice caused downwarping of Earth’s crust by hundreds of meters. In the 8,000 years since the last ice sheet melted, uplifting of as much as 330 meters,1000 feet, has occurred in Canada’s Hudson Bay Region, where the thickest ice had accumulated.

One of the consequences of isostatic adjustment is that as erosion lowers the summits of mountains, the crust will rise in response to the reduced load. However, each episode of isostatic uplift is somewhat less than the elevation loss due to erosion. The processes of uplifting and erosion will continue until the mountain block reaches normal crustal thickness. When this occurs, the mountains will be eroded to near sea level, and the once deeply buried interior of the mountain will be exposed at the surface. In addition, as mountains are worn down, the eroded sediment is deposited on adjacent landscapes, causing these areas to subside.

Where compressional forces are great, such as those driving India into Asia, mountains such as the Himalayas result. But is there a limit on how high a mountain can rise? As mountain tops are elevated, gravity driven processes such as erosion and mass wasting accelerate, carving the deformed strata into rugged landscapes. Just as important, however, is the fact that gravity also acts on the rocks within these mountainous masses. The higher the mountain, the greater the downward force on the rocks near the base. Visualize a group of cheerleaders at a sporting event building a human pyramid. At some point the rocks deep within the developing mountain, which are comparatively warm and weak, will begin to flow laterally. This is analogous to what happens when a ladle of very thick pancake batter is poured on a hot griddle. As a result the mountain will experience a gravitational collapse, which involves normal faulting and subsidence in the upper, middle portion of the crust and ductile spreading at depth.

You then might ask, what keeps the Himalayas standing? Simply, the horizontal compressional forces that are driving India into Asia are greater than the vertical force of gravity. However, once India’s northward trek ends, the downward pull of gravity will become the dominant force acting on this mountainous region.

Playlist of Geology and Earth Science Videos from Snow Mountain
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