observations been made concerning such features as flood intensities and frequencies, debris-flow distribution, subsidence, and landslides. Thus, predictions are based on recent to current conditions that are known to have been altered by human actions. Predictions of longer-term events, such as 1,000-year floods, are very unreliable because they must be based on models with uncertain numerical characteristics.

Landforms are sensitive to climatic change because the operating rates of the processes that mold landforms vary dramatically with climate. Fluvial processes dominate in sculpting landforms in semiarid regions, while wind processes are more significant in arid regions. Under very wet climatic conditions, landslides and downhill movements of surface rocks are dominant. Each of these geomorphic processes leaves distinctive evidence in the geological record. When climate changes, the dominant land-forming processes change in response. Threshold values of rainfall and temperature can be defined at which a change from the dominance of one land-forming process to another is likely to occur. It is therefore possible to forecast how agricultural regions might shift size and location in response to global warming and related changes in rainfall.

On an even longer time scale, low-lying coastal areas are subject to episodic flooding as sea level oscillates. Most remarkable are the paleo-landscapes locally exposed by erosion beneath extensive blankets of sedimentary rocks that have been deposited on the continents during flooding episodes. Glacial valleys 450-million-years old are evident at central Saharan sites now occupied by desert wadis, a 170-million-year-old sea stack lies fallen on a modern beach in Scotland, and a tropical beach 450-million-years old can be visited on the outskirts of Quebec City. The long-term durability of low-lying continental surfaces—less than 1 km above sea level and less than 0.2 km below sea level—that is demonstrated by landscape exhumation can be seen on much shorter time scales by the slow rates of erosion that characterize such flat areas.

The occurrence of these extensive areas of low relief, coupled with a suitable climate, makes regions such as the American Midwest prime land resources. The lush agricultural production of such regions can be maintained only if the landscape is treated with the same conservation ethic that inspires reverence for parks and wilderness areas. Prevention of soil erosion and respect for natural ecological balances in low-lying, low-relief regions can ensure their productivity far into the future. Geological characterization forms an essential basis for planned preservation and maintenance.

Human society exists in the biosphere, which thrives at the boundary layer between the solid-earth and its fluid envelopes, perched between the two engines of mantle convection and solar energy that drive geological processes. The biosphere, composed of chemical elements that are cycled among the reservoirs of the atmosphere, hydrosphere, crust, and mantle, also contributes to cycles of rapid chemical turnover and thus functions as a part of the geological processes. The more vigorous manifestations of those processes, however, regularly destroy the parts of the biosphere—and its human constructions—located in their paths. As society has increased its utilization of earth resources and expanded its population and area of colonization, the frequency of its encounters with vigorous geological processes has increased. Society has adopted a term, geological hazards, for these perfectly normal processes that began occurring long before humans arrived on the scene.

Many of the most tragic episodes in the natural history of humans have been related to geological hazards such as disastrous floods, earthquakes, sea waves, landslides, and volcanic eruptions. The fact is that most geological hazards can be avoided or mitigated through proper land-use planning, engineered design and construction practices, building of containment facilities such as dams, use



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