Rate Factors in Geologic Processes (World of Earth Science)
The rates at which geologic processes occur range from imperceptibly slow to exceptionally fast.
At the slow end of the spectrum, mountain ranges rise, basins subside, and tectonic plates move over time periods that span many millions of years. Recent research using repeated GPS (global positioning satellite) measurements of crustal deformation has shown, for example, that the Los Angeles basin is being shortened at a rate that is on the order of a few millimeters per year as the result of movement along faults throughout the basin. Likewise, the rate at which the earth's crust rebounds after the retreat of continental glaciers is generally on the order of a few millimeters per year and decreases with time. Measurements of offset strata cut by faults, combined with radiometric age dating, also suggest that crustal deformation rates are on the order of millimeters per year.
At the opposite end of the spectrum, acoustic waves traveling through rock (for example, from earthquakes, blasting, or seismic exploration surveys) typically travel at velocities of one to ten thousand meters per second. Thus, an observer standing 62 mi (100 km) from the epicenter of a large earthquake may have to wait 10 or 20 seconds before ground shaking begins. The catastrophic debris avalanche that occurred in conjunction with the 1980 eruption of Mount St. Helens traveled at a velocity greater than 149 mph (240 kph), which is fast in terms of human perception but is on the order of one-millionth of the velocity of typical seismic waves.
The rate at which any given geologic process occurs can vary significantly. Different kinds of landslides, for example, can move at rates as slow as a few centimeters per year to rates as rapid as tens of meters per second, a range that extends over eight orders of magnitude. Similarly, the flow of groundwater through porous aquifers is controlled by a combination of the hydraulic gradient and the permeability of the aquifer, which can vary over many orders of magnitude. In general, however, groundwater and other subsurface fluids generally flow at velocities so low that their kinetic energy (proportional to the square of the velocity) can safely be ignored in calculations. The viscosity, or resistance to flow, of fluids such as lava and crude oil depend strongly on the temperature and chemical composition of the fluid. For example, the viscosity of basalt lava falls by a factor of 100 to 1,000 as its temperature increases from 2,100°F to 2,550°F (1,150°C to 1,400°C). Therefore basalt lava, which is typical of Hawaiian volcanoes, flows as easily as honey (honey at room temperature has the same viscosity as basalt at 2,550°F/1,400°C) when it first erupts but slows and eventually solidifies in place as it cools. The rates of chemical reactions, for example those associated with metamorphism and ore deposit formation, are also strongly dependent upon pressure and temperature.
The rates at which rocks are subjected to stress can also control their response. A rock struck sharply with a hammer will behave as a brittle substance, deforming elastically and, if the blow is strong enough, breaking into pieces. Earthquakes are an example of naturally occurring elastic deformation of rocks. Over the lengths of time required to build mountain ranges, however, rocks appear to deform as very viscous fluids via a process known as slow creeping flow. The viscosity of deforming rocks is also influenced by pressure and temperature conditions during mountain building episodes.
Geologic process rates must also be viewed in relationship to the rates at which society functions. In many parts of the world, for example, groundwater pumping rates exceed those at which aquifers are naturally replenished, resulting in land subsidence that can occur at the rate of tens of centimeters per year. Humans have also become significant geologic agents and are responsible for the movement of 37 billion tons of soil and rock per year throughout the world. Likewise, the rates at which mineral and energy resources are consumed by society need to be balanced by the rates at which humans discover new deposits and develop new technologies for extraction, as well as the extremely slow rates at which mineral and energy resources accumulate over geologic time.
See also Catastrophism; Half-life; Historical geology; Uniformitarianism