Glossary - Environmental Science - page 333

Rule of 70 The mathematical method used to determine a population's doubling time. The formula is
, where r is the growth rate of the population in percent. Doubling time is commonly
expressed in years.
runoff Water that flows along the Earth's surface without entering the soil; also called surface runoff.
salinization Occurs where salts accumulate on the soil's surface. Evaporation pulls water and its dissolved salts
toward the surface from deeper within the soil horizon; when the water evaporates, the salts are left behind on
the soil surface.
seafloor spreading The process that keeps the tectonic plates in motion. As magma rises through the midocean
ridge, the magma hardens and creates new land. Convection currents in the asthenosphere are pushing apart the
plates that create the seafloor.
season A division of the year that is marked by changes in weather, ecology, and sunlight intensity, resulting
from the yearly revolution of the Earth around the sun and the tilt of the Earth on its axis. Temperate and polar
regions usually experience four seasons (fall, winter, spring, and summer), while tropical and subtropical re-
gions more commonly having only a wet and a dry season or sometimes hot, rainy, and cool seasons.
secondary growth New growth that emerges in a forest after the original, virgin forest has been cut down.
secondary treatment The stage in a sewage treatment system in which oxygen enters the water from continual
mixing and movement, encouraging aerobic decomposition.
sedimentary rock Rock formed when sediment from erosion and weathering, biogenic decomposition, or
chemical deposition is compressed and/or cemented together, or "lithified."
seed bank A place for housing and preserving many seed types as a way to protect seed diversity and safe-
guard the food supply in the event of disaster.
seed tree The forestry practice of leaving mature and seed-producing trees standing, providing the seeds neces-
sary for the regrowth of forest trees after harvesting.
selection system A partial harvest method in which only a few trees are harvested from an area at a time.
sex ratio The ratio of number of males to number of females in a population.
shelterbelt A protective border created when tall plants or trees are planted along the edges of fields or farms
in order to reduce erosion from wind.
shelterwood system The forestry practice of leaving a few full-grown trees in order to create shelter for emer-
gent seedlings. Cutting is done on a regular basis with select trees taken each time.
shield volcano A slowly erupting volcano with a broad side, gradual slopes, and usually a crater at the top; the
least explosive volcano type, with slow-moving liquid lava due to its low silica content.
sink A natural or artificial reservoir that stores an element, such as carbon, for an indefinite period. Natural
sinks tend to be much larger than man-made sinks. Natural sinks include water, biomass, fossil fuels, rocks and
soil. Artificial sinks include proposed carbon capturing and storage in a process called carbon sequestration.
soil permeability The ability of a liquid to flow through the soil.