Geol 33 Environmental Geomorphology

J Bret Bennington

Landform Construction and Destruction

Megageomorphology

In keeping with the regional scale of our examination of major geomorphic provinces we will begin this course by considering tectonic landscapes on the scale of whole mountain ranges and some of the general processes that shape them.

Geomorphologists have also traditionally divided landforms into constructional and destructional categories.

Constructional landforms are those that have been or are being built (increasing in mass, height, or area). Examples include tectonic forms such as fault scarps, fault block mountains and valleys (horsts and grabens), rift valleys, monoclines, salt domes, and a variety of volcano landforms. In general, constructional landforms are more common in regions of active tectonism, usually associated with plate margins.

Destructional landforms are those that are decreasing in mass, height, or area over time and are primarily the products of erosion and weathering. In truth, all landforms are in part destructional because erosion affects all landforms that lie above sea level on the Earth's surface. However, many landforms are primarily the result of such processes.

More recently, geomorphologists have come to view all landscapes as the products of some combination of three factors:

None of these factors operates independently - all are in some way functions of the others:

Isostatic mountain building

Climate and erosion can also work together to create uplifted mountain peaks. An arid plateau will support few rivers and tend to erode slowly and uniformly. As material is removed across the plateau isostatic readjustment will cause some uplift and the average land surface will slowly be lowered. If however, the plateau is in moist climate with abundant rivers or glaciers, then abundant, deep valleys can form, removing a large mass of crust, but leaving some crust standing as peaks. Isostatic readjustment will elevate the plateau so that its average mass will be lower than before erosion, but this can result in the peaks being elevated to a higher level than before.

Example: the Himalayan Uplift

An excellent modern example of the complicated interrelationship between tectonics, erosion and climate is provided by the Himalayan mountains and the Tibetan Plateau. The Himalayas are the highest mountains on Earth, rising to a maximum height of 8,848 km at Mt. Everest. North of the mountains the Tibetan Plateau is a desert plain that is currently sitting above 5000 meters. This region began rising 50 million years ago as the Indian landmass came into collision with south Asia. For the past 50 million years India has continued to plow into Asia at a

rate of about 5 cm per year. The collision has squeezed both India and Tibet, creating a series of thrust faults that have almost doubled the thickness of the crust in the collision zone.

The Himalayan Orogeny initiated the uplift of in the region. This resulted in an intensification of the Asian monsoon by 8 million years ago because the uplifted region created a high altitude heat sink and associate summertime low pressure system. Sediment transport through the Indus and Bengal river systems increased 13-fold as erosion intensified due to increased rainfall along the uplift front. This rapid erosion resulted in deep valleys and isostatic readjustment that greatly increased the elevation of the peaks in the Himalayan range. At the same time, the high mountains created a rainshadow over the Tibetan Plateau the greatly slowed its rate of erosion, allowing uplift to elevate it to its present extreme height. Uplift of the plateau probably has served to maintain and even amplify the intensified monsoon.

The Three Stages of Mountains

It is probably incorrect to say the mountains are uplifted and then eroded back to sea level. Rather, isostacy, climate and erosion interact to create three stages in the life of a mountain range.

  1. Formative Stage - a tectonic event thickens or heats the crust and creates uplift with rates that exceed erosion rates. As the topography rises erosion rates increase.
  2. Steady State - erosion rates rise to match uplift rates or uplift slows to match erosion. A steady state results with uplift and erosion rates balanced. Because uplift rates and erosion rates create a negative feedback loop (increasing uplift raises the mountains which intensifies erosion and lowers the mountains) the equilibrium elevation of the mountains may remain stable for millions of years.
  3. Decline - if uplift diminishes sufficiently erosion may come to dominate and slowly lower the elevation of the range. This is a slow process because isostatic rebound will replace much of the elevation lost to erosion. By the time a mountain range is worn back to the average level of continental crust, a thickness of crust several times greater than the height of the mountains at their highest can be removed by erosion. For example, it is estimated from mineral studies that the metamorphic rocks of New York City and Connecticut were once buried under about 7 miles of overlying rock. This is not to say that the mountains above them were 7 miles high, only that this thickness of crust has been pulled upward by isostatic readjustment over the last 450 million years.

(ref: Pinter, N. and Brandon, M. T., 1997, How erosion builds mountains., Scientific American, 276:74-81.)

Major Geomorpho-tectonic Regions

Although fault block mountains are primarily tectonic features, they can be maintained by isostatic readjustment as the eroding upturned portion of the block rises and the sediment cover over the down-rotated portion thickens.