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:

• tectonics (orogeny, epeirogeny, extension, volcanism)
• erosion
• climate

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

• Tectonics are clearly affected by erosion - removal of crustal mass by erosion leads to isostatic rebound and uplift. In general, about 80% of the height of a landmass removed by erosion will be replaced from below by isostatic readjustment. Some geologists refer to the "pull of erosion" to describe this phenomenon.
• Uplift accelerates erosion, which, as a general rule, increases in intensity with increasing elevation. High elevations also host less vegetation, which armors the land against erosion at low elevations.
• Erosion is a function of climate in many ways. Humid climates as a rule erode faster than arid climates and the effects of erosion on the landscape differ. Humid climates tend to result in the formation of deeply incised valleys. Arid regions erode more evenly. Cold climates result in mountain glaciers that can carve extremely steep topographic features. In fact, mountain glaciers may be Earth's most effective agents of erosion. Extremely frigid climates host large ice sheets that do very little erosion because they are frozen to the bedrock on which they rest.
• Climate itself is greatly affected by tectonism. High landmasses change the balance of heating on the surface and block or redirect the flow of weather systems. As a landmass is elevated its climate cools and may become more humid. This in turn can accelerate erosion. Windward of a mountain range the climate is usually humid as ascending air masses cool and drop their moisture. Leeward of the range is a rain shadow and very arid conditions.

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.

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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.

Major Geomorpho-tectonic Regions

• Coastal Plains - these are extensive regions of flat-lying to gently seaward dipping sediments that build up as a passive continental margin subsides over millions of years. They are areas of very low relief. Generally, coastal plains are absent in tectonically active areas because active uplift of the coastline prohibits their development.
• Orogenic Belts - these are elongate regions that form from collisional tectonics along active continental margins. In presently active orogenic belts the metamorphic rocks may not yet be exposed on the surface - only after extensive erosion of the overlying, intensely deformed sedimentary cover are the metamorphic roots of the mountains exposed. Where they are generated by subduction of oceanic crust beneath the continent they are highly intruded by magmatic plutons.
• Fold and Thrust Belts - marginal to orogenic belts are regions where the sedimentary cover has been folded and thrust faulted by compressional stresses without metamorphism. Low angle thrust faults create blocks of sedimentary crust stacked one on another, suggesting large amounts of shortening of the upper crust (often refered to as "thin-skined tectonics"). Erosion of ancient fold and thrust belts near sea level produces a distinctive ridge and valley topography with surface expressions of underlying syncline and anticline folds.
• Plateaus - these are tectonically elevated regions of undeformed sedimentary rock (the Tibetan Plateau and the Chilean Altiplano are a somewhat different type of plateau - elevated regions within an orogenic belt created by slow erosion due to rain shadow climates). Plateaus developed in in humid regions can be very mountainous due to stream dissection. An extreme form of stream dissection is seen in the canyonlands of the Colorado Plateau, where rivers such as the Colorado have carved mile-deep canyons such as the Grand Canyon.
• Stable Interior / Shield - the interior regions of continents are generally low relief areas with very mild stream dissection. Shield areas in the center of a continent are underlain by ancient, deeply eroded metamorphic and metaigneous rocks. Surrounding the shield regions are areas of the stable interior covered with a thin veneer of nearly horizontal sedimentary strata. These strata are very subtly deformed into broad structurally upwarped regions called domes and downwarped regions called basins which generally do not have any topographic expression.
• Extensional Regions - regions of high mantle heat flow beneath continental crust can cause broad crustal doming and the formation of fault block mountains bounded by grabens and half grabens. Large grabens are also known as rift valleys when they are associated with continental rifting. These features form as the crust is thinned and stretched, creating tensional forces and normal faults. The extensive Basin and Range region of Utah and Nevada in the western US has been generated in the last 17 million years by extension of the western US by as much as 100 miles in the east-west direction. Extensional tectonics are often accompanied by igneous intrusives and surface eruptions as magma works its way upward along faults in the thinned crust.