The processes that produced Earth’s varied topography are complex. Beyond the tectonic forces that move rocks laterally and thicken them vertically to produce mountainous topography, additional processes help to shape Earth’s surface. As weathering and erosion work to lower mountains, a compensating process called isostasy causes them to rise, so they remain mountainous long after the tectonic processes that initially created them have ceased. Also, if tectonic processes raise a mountain belt “too high,” the rock at its core will become too weak to support the load, and the mountain will spread.

The Principle of Isostasy

During the 1840s, researchers discovered that Earth’s low-density crust “floats” on top of the high-density rocks of the mantle, much as wood floats in water. We will use the floating wooden blocks in Figure 1 to explore this idea. You can think of these blocks as a model mountain belt, floating in the mantle. Notice that only about one-quarter of each block projects above the water and three-quarters is submerged. This is because wood is about three-quarters as dense as water. Similarly, most of the vertical thickness of a mountain belt forms a buoyant root that is “submerged” in the mantle; the remainder projects above the surrounding crust. This concept—that the crust floats in gravitational balance in the mantle—is called isostasy.

Notice that the tallest of the floating blocks in Figure 1 stands the highest above the water surface and also sits the deepest. Similarly, the greater the crustal thickness of a mountain belt, the higher it will stand above sea level, and also the deeper its roots will be. Thus, the Himalayas, as the tallest range on Earth, also have the deepest roots.

Now, visualize what would happen if you placed a second small block on top of one of the blocks in Figure 1. The combined block would sink until it reached a new isostatic (gravitational) balance, at which point its top would be higher than before, and its bottom would be lower. This process of establishing a new gravitational balance in response to loading or unloading is called isostatic adjustment. Notice also that as a block rises or sinks, the surrounding water flows to accommodate it. Similarly, the highly viscous mantle will flow, albeit at an excruciatingly slow rate, when weight is added to or subtracted from the overlying crustal blocks.

Applying the concept of isostatic adjustment, we should expect that when weight is added to the crust, the crust will respond by subsiding, causing the underlying mantle rocks to flow away. When the weight is removed, the crust will rebound, and the mantle rock will flow back underneath. (Visualize what happens to a ship when its cargo is loaded or unloaded.) Scientists have found evidence for crustal subsidence followed by isostatic rebound in areas formerly overlain by Ice Age glaciers. When continental ice sheets covered portions of North America during the Pleistocene epoch, ice masses that averaged about 3 kilometers (about 2 miles) thick added weight to the crust and caused downwarping by hundreds of meters. In the 8000 years since this ice sheet melted, gradual uplift of as much as 330 meters (about 1100 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 cuts into a mountain range, removing mass, the range rises in response to the reduced load (Figure 2). In fact, because erosion removes material mainly by carving canyons and valleys rather than by uniformly wearing down mountain peaks, isostasy may actually “push” the peaks higher than their original height.

The processes of uplift and erosion continue until the mountain block reaches average crustal thickness. When this occurs, these once-elevated structures are near sea level, and the once-deeply buried interior of the mountain is exposed at the surface. In addition, as mountains are worn down, the eroded sediment is deposited on adjacent landscapes, causing these areas to subside (refer to Figure 2).

How High Is Too High?

Where compressional forces are great, such as those driving India into Asia, lofty mountains such as the Himalayas result. Is there a limit on how high a mountain can rise? As mountaintops are elevated, gravity-driven processes such as erosion and mass movement accelerate, carving the deformed strata into rugged landscapes. Equally important, however, is the fact that gravity also acts on the rocks within the mountain belt. The higher the mountain, the greater the downward force on rocks near the base. Eventually, the rocks deep within the developing mountain, which are relatively warm and weak, begin to flow laterally, as shown in Figure 3. This process of gravitational collapse is analogous to what happens when a ladle of very thick pancake batter is poured onto a hot griddle. In addition to causing ductile spreading at depth, this process leads to normal faulting and subsidence in the upper, brittle portion of Earth’s crust.

Considering these factors, 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, when India’s northward trek ends, the downward pull of gravity, weathering, and erosion will become the dominant forces acting on this mountainous region.

• Earth’s crust floats in the denser material of the mantle the way wood floats in water. This principle is termed isostasy. If additional weight is placed on the crust (an ice sheet, for example), the crust sinks, and if weight is removed (glacial melting), the crust rebounds. This process of maintaining gravitational equilibrium is called isostatic adjustment. For a mountain belt, isostasy partially offsets the effect of erosion, pushing the mountains up as erosion wears them down.

• When compressional forces raise a mountain belt too high, the rock at the belt’s core becomes warm and weak, and the belt spreads, becoming broader and lower.

• gravitational collapse: The gradual subsidence of mountains caused by lateral spreading of weak material located deep within these structures.

• isostasy: The concept that Earth’s crust is floating in gravitational balance on the material of the mantle.

• isostatic adjustment: Compensation of the lithosphere when weight is added or removed. When weight is added, the lithosphere responds by subsiding, and when weight is removed, there is uplift.