Engineering geology

How Is Isostasy Related to Changes in Elevation?

How Is Isostasy Related to Changes in Elevation?

Visualize what would happen if another small block of wood were placed a top one of the blocks in Figure 1.

Figure 1 – The principle of isostasy
This drawing shows how wooden blocks of different thicknesses
float in water. In a similar manner, thick sections of crustal material float higher than thinner crustal slabs.

The combined block would sink until it reached a new isostatic (gravitational) balance. At this point, the top of the combined block would be higher than before, and the bottom would be lower.

This process of establishing a new level of gravitational balance by loading or unloading 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 will rebound when weight is removed.

(Visualize what happens when a ship’s cargo is loaded or 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-kilometer- (2 mile-) thick masses of ice caused downwarping of Earth’s crust by hundreds of meters.

In the 8000 years since this last ice sheet melted, gradual uplift of more than 300 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 a mountain range, the crust rises in response to the reduced load (Figure 2).

Figure 2 – The effects of isostatic adjustment and erosion on mountainous topography
This sequence illustrates how the combined effects of erosion and isostatic adjustment result
in a thinning of the crust in mountainous regions.

The processes of uplift and erosion continue until the mountain block reaches “normal” crustal thickness. When this occurs, these once-elevated structures will be 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 (see Figure 2).

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