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.

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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.

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