Geoscience Reference
In-Depth Information
Strain.
The deformation or movement caused by applying
a force to material on a slope.
Stratosphere.
That part of the atmosphere lying
12-60 km above the Earth, and characterized by
stability and an increase in temperature with altitude.
Stratospheric Fountains.
Little air is exchanged between
the troposphere and stratosphere. However, intense
convection - generated by thunderstorms in the
tropics at the west side of the Pacific - breaches this
barrier and, like a fountain, injects air originating from
the ground into the stratosphere.
Strip Farming.
An agricultural practice used to conserve
soil moisture whereby crops are alternated in strips
with fallow land.
Subcritical Flow.
Tranquil river flow in which velocities
relative to the channel depth are low enough that
sediment transport is not excessive.
Subduction Zones.
The zone down which an oceanic
plate passes beneath a continental plate to be
consumed into the mantle.
Subsidence.
The downward failure of the Earth's surface
brought about mainly by removal of material from below.
Sunspot.
An area 20 000 km in diameter on the sun's
surface featuring strong magnetic disturbance so strong
that convective movement of heat to the surface is
curtailed. Hence, the spot is darker because it is cooler.
Supercell Thunderstorm.
A particularly large, convective
thunderstorm cell with a diameter of 50-100 km.
Such a storm has a greater chance of producing hail,
tornadoes, strong wind, or copious rainfall.
Supercritical Flow.
River flow in which velocities relative
to the channel depth are so high that the flow begins to
shoot or jet. Such flow is very erosive.
Surcharging.
The escape of stormwater through
access-hole covers that are designed to lift off under
excess pressures caused by flooding. This prevents
stormwater pipes from bursting underground, which
would necessitate costly repairs.
Suspension Load.
That portion of sediment being carried
in air or water flow by turbulence.
Teleconnection.
The linkage of a measured climatic time
series to another one at some distance across the globe.
For instance, rainfall in Florida can be related, through
the Southern Oscillation, to sea-surface temperature in
northern Australia.
Teleseismic Tsunami.
A tsunami generated by an earth-
quake from a distant source. The Chilean earthquake of
1960 generated a teleseismic tsunami that impacted on
most coasts surrounding the Pacific Ocean.
Telluric Currents.
Electric currents moving through the
Earth.
Tephra.
Volcanic ash that is disaggregated and blown by
the force of an eruption, usually vertically, into the
atmosphere.
Tertiary.
The geological period from 65 to 2-3 million
years BP.
Thermal Expansion.
Water is unusual in that it has its
greatest density at 4°C. Thus, it expands readily when
heated. Ice, however, has its greatest volume around
the freezing point. It shrinks and cracks around -10°C.
If water can enter the cracks and refreeze, such ice will
expand and buckle as it warms.
Thermocline.
The abrupt change in water temperature
with depth within a large body of water.
Thixotropy.
The property of becoming fluid when stirred
or agitated, especially by compressive shock waves.
See
also
Liquefaction.
Threshold.
The point at which the application of a force
triggers a dramatic response that did not exist before-
hand. For example, increasing stress applied to rock
may produce no observable effect until a threshold is
reached and the rock fractures.
Tidal Inlet.
The opening that forms across a barrier island
in which there is a free and substantial exchange of
saline water between the ocean and a landward lagoon
during each tide cycle.
Tidal Waves.
A misnomer applied to tsunami and storm
surges because these phenomena sometimes produce a
slow drop in sea level followed by a rapid rise, analogous
to tides.
Tilt Steps.
Tilting of the Earth's crust that occurs in a
series of discrete steps rather than smoothly.
Topographic Control.
Any airflow whose path is deflected
or altered by terrain that is higher than the surround-
ing landscape. For example, a moving mass of air may
be blocked by high mountains and go around the
obstruction.
Tornado.
An extremely violent vortex or whirlwind of air
measuring only hundreds of meters in diameter and
with wind speeds up to 1300 km hr
-1
.
T
Taiga.
Swampy, coniferous, evergreen forest of sub-Arctic
Siberia.
Tectonic.
The process characterizing the deformation of
the Earth's crust because of earthquakes.
Tectonic Creep.
Deformation of the Earth's crust that
occurs in such small increments that it is virtually
undetectable, except over time.
