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Geomorphology
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Geomorphology
I. INTRODUCTION
Geomorphology, scientific study of landforms and landscapes. The term usually applies to the origins and dynamic
morphology (changing structure and form) of the earth's land surfaces, but it can also include the morphology of the
seafloor and the analysis of extraterrestrial terrains. Sometimes included in the field of physical geography,
geomorphology is really the geological aspect of the visible landscape. The science has developed in two distinctive ways
that must be integrated in order for the whole picture of landscapes to emerge.
II. HISTORICAL GEOMORPHOLOGY
One approach to the science of landforms is by means of historical, cyclic geomorphology. The concepts involved were
worked out at the turn of the 20th century by the American geologist William Morris Davis, who stated that every
landform could be analyzed in terms of structure, process, and stage. The first two are also treated by process
geomorphology, discussed below; but the third, by introducing the element of time, is subject to a far greater degree of
interpretation. Davis argued that every landform underwent development through a predictable, cyclic sequence: youth,
maturity, and old age.
Historical geomorphology relies on various chronological analyses, notably those provided by stratigraphic studies of the
last 2 million years, known as the Quaternary period. The relative chronology usually may be worked out by observation
of stratigraphic relationships, and the time intervals involved may then be established more precisely by dating methods
such as historical records, radiocarbon analysis, tree-ring counting (dendrochronology), and paleomagnetic studies. By
applying such methods to stratigraphic data, a quantitative chronology of events is constructed that furnishes a basis for
calculating long-term rates of change.
III. PROCESS GEOMORPHOLOGY
This second branch of geomorphology analyzes contemporary dynamic processes at work in landscapes. The
mechanisms involved—weathering and erosion—combine processes that are in some respects destructive and in others
constructive. The bedrock and soil provide the passive material, whereas the climatic regime and crustal dynamics
together provide the principal active variables.
IV. UNDERLYING DYNAMICS
In geomorphological processes, gravity is an all-pervading, essentially invariable energy factor; a second variable, energy
flow is provided by solar radiation. The latter is expressed either as a direct thermal variable or, indirectly, through the
hydrologic cycle, which involves evaporation of water from the ocean, atmospheric transport of water, precipitation as
rain or snow, and a return to the ocean by various processes. A third energy factor is heat flow from the earth's interior.
Although of a magnitude considerably less than solar energy, this heat flow ultimately is responsible for creating major
geological structures such as faults, but rates of change tend to be quite low (usually less than 1 mm per year).
Nonetheless, in particular zones—for example, along crustal-plate boundaries (see Plate Tectonics) such as the San
Andreas fault—stress may build up until released catastrophically in violent displacements of up to 12 m (40 ft). Locally,
heat flow from the interior is concentrated in eruptions of magma (molten rock), which produce a variety of volcanic
landforms.
V. WEATHERING AND EROSION
Weathering is often a combination of three processes: the mechanical process, as in the growth of ice or salt crystals or in
thermal heating and cooling; the chemical process, as in acid-water solutions that tend to dissolve minerals such as
calcite and feldspar; and the biological process, as in the effect of plant roots, which generate both mechanical and
chemical energy. Erosion is the dislodging, removal, and transport of material, either in solution or in particle form. The
energy to accomplish this may be provided in the form of raindrops, running water, wind, waves, or simply gravity (as in
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a landslide).
An eroding landmass tends to rise to compensate for the removal of the load, but it eventually stabilizes as land relief
decreases and stream gradients decline. The resulting surface, almost flat, is called a peneplain. It may be interrupted,
here and there, by isolated hills called monadnocks consisting of rocks especially resistant to erosion. The theoretical
base level of such a surface—the ultimate grade of streams—is mean sea level. For a peneplain to form and not be
destroyed by renewed erosion, sea level must remain stable for millions of years. However, since the end of the
Quaternary Ice Age, 10,000 years ago, sea level has risen hundreds of feet.
Human-induced soil erosion is a feature of the present day and of the last few millennia, because clearing land of native
vegetation or excessive grazing by domesticated animals exposes the soil to massive erosion. In this way some 3 billion
metric tons of particulate material are washed from the surface of the U.S. alone each year. In undisturbed natural
settings, on the other hand—notably in low-relief continental interiors—erosion rates are very slow (except in semiarid
areas where thunderstorms produce flash floods). In structurally active belts such as in youthful mountains, which as a
rule coincide with plate boundaries that recently collided or rifted, erosion rates may be enormous.
Of all the different processes acting on the earth's surface, rain and rivers are the most vigorous erosive agents. By
contrast, although wave action on a rocky coast is often impressive, the rate of retreat of the shoreline is generally very
slow. Sand dunes in the Sahara are also impressive, but the sand is only a relatively thin veneer; and the moraines left by
giant continental glaciers are likewise only superficial scrapings of ancient soils. In general, without human interference,
the landscape is stable.
Contributed By:
Rhodes W. Fairbridge, M.A., Ph.D.
Professor Emeritus of Geology, Columbia University. Contributor to Nature, Science, and Scientific American.
"Geomorphology," Microsoft® Encarta® Online Encyclopedia 2005
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