Geol 33 Environmental Geomorphology
J Bret Bennington
Landform Construction and Destruction
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
More recently, geomorphologists have come to view all landscapes
as the products of some combination of three factors:
- tectonics (orogeny, epeirogeny, extension, volcanism)
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
- 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.
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.
- 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.
- 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.
- 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
(ref: Pinter, N. and Brandon, M. T., 1997, How erosion builds
mountains., Scientific American, 276:74-81.)
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.
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.