Geoscience Reference
In-Depth Information
P
Parent Cloud.
A large cloud characterized by turbulent
airflow, and subdividing into separate areas of
convection that can give rise to tornadoes, intense
rainfall, and hail.
Partial Pressure.
The contribution made by a gas to the
total pressure of the atmosphere.
Permafrost.
Permanently frozen ground.
Permeability.
The degree to which water can flow through
the voids in sediment deposits or micro-fractures in
rock.
Persistence.
The tendency for an abnormal climatic
situation to continue over time.
Photosynthesis.
The process whereby plants convert
carbon dioxide and water in the presence of sunlight,
and aided by chlorophyll, into carbohydrates or biomass.
Piezoelectric.
The ability of a mineral to generate
an electric current when pressure or stress is applied
to it.
Piezomagnetism.
The generation of magnetism in iron-
rich magma by increases in pressure or stress.
Plastic.
A property of any solid that permanently
deforms without breaking up or rupturing when
subject to a load.
Plate.
The Earth's crust is subdivided into six or seven
large segments or plates that move independently of
each other at rates of 4-10 cm per year over the under-
lying mantle. These plates can collide with (converge),
or separate from (diverge) each other.
Pleistocene.
The geological age in the Quaternary
beginning approximately 2-3 million years BP. It was
characterized by periods of worldwide glaciation.
Pliocene.
The last period in the Tertiary, 12-2 million
years BP. The Pliocene terminated when large-scale
glaciation began to dominate the Earth.
Pluviometers.
Instruments used to measure the rate at
which rain falls over very short time spans.
Polar Front.
Dense air, cooled by long wave emission
of heat in the polar regions and flowing towards
the equator, differs substantially in temperature
and pressure from the air it displaces. This differ-
ence occurs over a short distance termed the 'polar
front'.
Pore Water Pressure.
The pressure that builds up
because of gravity in water filling the spaces or voids
between sediment particles.
Power.
The rate at which energy is expended.
Precursors.
Variables that give an indication of the
occurrence of some future event.
Prescribed Burning.
The deliberate and controlled
burning of vegetation growing close to or on the ground
to minimize the fuel supply for future bush or forest
fires. Also called (in Australia especially) 'burning-off'.
Probability of Exceedence.
The probability or likelihood
of an event - with a certain magnitude or size -
occurring, expressed in the range of 0-100 per cent.
Pumice.
Volcanic ash filled with gas particles and hence
able to float on water.
Pyroclastic.
Referring to the fine, hot ash or sediment
produced from the fragmentation of magma in the
atmosphere by the force of a volcanic eruption, and
the gases contained within.
Pyroclastic Ejecta.
Pyroclastic material blown out into the
atmosphere.
See also
Tephra.
Pyroclastic Flow.
See
Nuée Ardente.
Q
Quaternary.
The most recent geological period, beginning
approximately 2-3 million years BP and consisting of the
Pleistocene and the Holocene.
Quick Clays.
Clays that have been deposited with a high
degree of sodium cations that tend to glue particles
together, giving the deposit a high degree of stability.
However, sodium cations are easily leached by ground-
water, with the result that the clay deposit becomes
progressively unstable over time.
R
Recurrence Interval.
The average time between repeat
occurrences of an event of given magnitude.
Reg Desert.
The residual, stony desert formed after clay
and sand-sized particles are blown away.
Regolith.
The layer of weathered or unweathered, loose,
incoherent rock material of whatever origin forming
the surface of the Earth.
Regression Coefficient.
A term used in statistics to define
the relationship between two variables. The coefficient
can vary from -1.0, a perfect inverse relationship, to
1.0, a perfect direct relationship.
Reservoir.
(1) A structure deliberately constructed to
impound water. (2) A pocket of resistance that exists in
the natural environment in which an organism or
pathogen can exist with impunity and can spread
outwards under favorable conditions.
Resonance.
A process involving the development of air
or water waves in which the wavelength is equal to, or
some harmonic of, the physical dimensions of a basin
or embayment.
