Geoscience Reference
In-Depth Information
Rift Valley.
A zone of near-vertical fracturing of the
Earth's surface that causes one segment to drop
substantially below another.
Rossby Waves.
Any wave motion that develops with a
long wavelength of hundreds or thousands of kilo-
meters in a fluid moving parallel to the Earth's surface
and controlled by Coriolis force. Generally restricted
to the upper westerlies attached to the polar jet stream
at mid-latitudes.
Run-Up.
When any ocean wave reaches the shoreline, its
momentum tends to carry a mass of water landward.
The distance landward of the shoreline is the run-up
distance, while the elevation above sea level is the
run-up elevation.
Rhyolite.
A fine-grained, extruded, volcanic rock with a
wavy, banded texture. Mineralogically it is similar to
granite, consisting of quartz, feldspar, and mica.
Set-Up.
The enhanced elevation of sea level at a coastline
due mainly to the process of wave-breaking across
a surf zone, but also aided by wind, shelf waves,
Coriolis force, upwelling, and current impinging along
a coast.
Shallow Focus.
Earthquakes occurring within 60 km of
the surface.
Shear Sorting.
Any process that causes coarse particles to
be transported more efficiently in a liquid or air flow
that is thin, full of sediment, and traveling at high
velocity.
Shear Strength.
The internal resistance of a body of
material to any stress applied with a horizontal
component.
Shear Stress.
The force applied to a body of material,
which tends to move it parallel to the contact with
another solid or fluid.
Shearing.
A stress caused by two adjacent moving objects
tending to slide past each other parallel to the plane of
contact.
Slump.
The tendency for some landslides to undergo
rotation, with the greatest failure of material at the
downslope end.
Solar Flares.
Large ejections of ionized hydrogen
from the sun's surface accompanied by a pulse of
electromagnetic radiation.
Solifluction.
The slow movement of water-saturated
material downslope, usually in environments where
freezing and thawing are the dominant processes
affecting the surface.
Southern Oscillation.
The usual movement of air in the
tropical Pacific is strong easterly trades reinforced by
low pressure over Indonesia-Australia and high
pressure over the south-east Pacific. Every two to
seven years, this low and high pattern weakens or even
reverses, causing the trades to fail or become wester-
lies. It is this switching from one pattern to another
that is the Southern Oscillation.
See also
North Atlantic
Oscillation
and
North Pacific Oscillation.
Spot Fires.
Small fires ignited by flying embers carried on
updrafts generated by heated air from the main core of
a forest or bushfire.
Steric.
Refers to the change in volume of seawater due to
heating or cooling.
Storm Surge.
The elevation of sea or lake levels above
predicted or normal levels resulting mainly from a
drop in air pressure, or from the physical piling up
of water by wind associated with a low-pressure
disturbance or storm.
S
Sahel.
The seven African countries - Gambia, Senegal,
Mauritania, Mali, Burkina Faso, Niger, and Chad - at
the southern edge of the Sahara Desert.
Saltation.
The tendency for sediment particles to move
from the bed into air- or water-flow and back to the bed
in zigzag paths, where the distance beneath upward
movement is much shorter than the distance beneath
downward movement.
Scoriae.
Fused volcanic slag or ash that results from a
pyroclastic flow and contains a high degree of gas.
Seamounts.
Volcanoes that have formed in the ocean,
planed off at the top because of close proximity to the
ocean's surface, and have then been lowered below the
sea as the Earth's crust moves away from the center of
volcanic activity.
Seiching.
Excitation of a regular oscillation of water
waves within a basin or embayment - caused by an
earthquake, the passage of an atmospheric pressure
wave, or the occurrence of strong winds.
Seismic.
Related to sudden and usually large movement
of the Earth's crust.
Seismic Gap.
That part of an active fault zone that has
not experienced moderate or major earthquake activity
for at least three years.
Seismic Risk.
The probability of earthquakes of given
magnitude occurring in a region.
Set-Back Line.
A line established shoreward of a coastline,
incorporating 50-100 years of postulated shoreline
retreat, and in front of which no development should
proceed.
