THE NATURAL ENVIRONMENT

Geography 101

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Geomorphology

Geomorphology studies the landforms that pattern Earth's surface and the processes that shape them. Landform refers to any individual feature, or group of features, that are made from the planet's native rock. Mountains toward the interior of the Islands provide prominent examples of landforms, including the smooth shapes of Mauna Loa, Hualalai, and Haleakala on the Big Island and Maui and the jagged, serrated peaks of O'ahu and Kaua'i. Although all of these mountains have a common origin, their shapes are radically different. A geomorphologist trying to understand this difference would document the structure of the different mountains (shape, location, composition) and try to explain their similarities and differences in terms of the processes (forces, actions, time) that formed them.

While thousands of different landforms populate the Earth, a relatively small number of processes build and sculpt them. We will organize our study, then, around the geomorphic process that shape landforms. At the most fundamental level, geomorphic processes can be separated into two major groups based on their energy source.

Endogenic (internal origin) processes are driven by the internal heat of the Earth, which in turn results from the radioactive decay of elements deep beneath the surface. This heat bubbles upward providing a huge driving force that bends, cracks, lifts, and moves Earth's rigid outer layer. Occasionally we see this rising energy empty directly onto the surface in molten lava. Typically, endogenic forces are mountain building processes.

Exogenic (external origin) processes are driven by the energy in sunlight. Sunlight causes air to move, water to be lifted into mountains and ocean waves to rise. These moving fluids attack the solid surface, eroding it, carrying the broken pieces away and depositing them to fill low places in the landscape. In other words, exogenic forces are mountain destroying processes.

Overall, mountain building endogenic processes act much faster than mountain destroying exogenic processes. Otherwise, we would not have mountains at all. Can you see why? Consider the opposite scenario: that of Jupiter's moon, Europa. It has a relatively smooth surface because the processes that destroy mountains act much more quickly than the processes that build them.

The very slow pace of geomorphic change creates a problem with studying landform development. Most landforms are the product of millions of years of change, but the geomorphologist studying them can only observe change for a few years at most. Thus, they must make some rather grand assumptions. At the heart of the study of geomorphology lies the deeply held belief that the geomorphic processes we observe today have been active throughout the geologic history of our planet, and that physical laws, as we understand them, have been constant in time. This is called the principle of uniformitarianism. Based on this principle, geomorphologists make short term measurements of erosion rates, weathering rates, plate tectonic motion, and so on, and then extrapolate backwards in geologic time to understand the long-term development of landforms. Geomorphology research is based on the premise that "the present is the key to the past."

An older theory, called catastrophism, suggested that the major work of landform shaping was done in sudden, planetary-scale upheavals, rather than through gradual, uniform change. This belief seemed to fit both religious doctrine and earlier geological models, but was dismissed as scientists developed a deeper understanding of both earth processes and geological time scales. Recently, however, it has become clear that occasional cataclysms can play a role in planetary development. Abrupt mass extinctions during the past half billion years, for example, appear to have been caused by extreme events like meteor impacts and widespread volcanism. On Venus, the entire planet may have been resurfaced about 300 million years ago in a global volcanic convusion that erased most surface relief.

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