Geoscience Reference
In-Depth Information
Earth is nearest the sun in January. In about 10,000 years
it will be nearest the sun in July.
Precipitation
All types of falling and direct deposition
of water or ice from the atmosphere at Earth's surface; also
the process whereby dissolved solids are deposited from
a fluid.
Pressure-melting point
The temperature at which ice
can melt at a given pressure; this is central to a modern
understanding of glacier behaviour, since the
overburden
pressure
at the base of a glacier is frequently high enough
to melt basal ice at sub-zero temperatures and form
supercooled meltwater.
Primary succession
The sequence of plant communites
which, successively, occupy a natural area previously
devoid of vegetation.
Primary (P) wave
The fastest-moving type of seismic
wave, propagated by alternating compression and
extension of material in the direction of movement (see
also
secondary wave
).
Principal stress
A stress which acts perpendicular to
each of the three pairs of faces of a cube in a rock mass.
Principle of competitive exclusion
Principle which states
that two species with identical niches cannot coexist, and
one will outcompete the other.
Principle of competitive replacement
The principle
which states that in successions plant species tend to make
conditions more favourable for a competing species which
will replace them.
Principle of uniformitarianism
The principle that
present-day analogues are used as a basis for the interpre-
tation of observed features in the past geological record.
Prisere
A
primary succession
.
Prismatic
A type of soil structure consisting of vertical
units with straight tops, usually in the subsoils of clay soils.
Process-response system
A combination of morpho-
logical and cascading systems so that the system
demonstrates the manner in which form is related to
process.
Producers
Autotrophic organisms capable of
photo-
synthesis
, i.e. organisms in the first
trophic level
of an
ecological pyramid.
Progradation
The seaward extension of river flood plains
by downstream sediment transfer into estuarine and delta
environments.
Protalus rampart
A steep-sided ridge formed at the foot
of a permanent snow bed by the accumulation of coarse,
angular, frost-weathered debris which has slid over, or
been washed under, the snow bed.
Psammosere
A series of
plant communities
which,
successively, occupy and stabilize an area of unconsol-
idated sand.
Plutonic
Said of igneous rock mass formed at great
depth in the lithosphere by slow cooling, and charac-
terized by granitic texture and mineralogy.
Polar glacier
See
cold glacier
.
Pollen analysis
The technique of reconstructing
vegetation covers of the past by studying the pollen grains
and spores of plants preserved in oxygen-deficient peats,
sediments and soils.
Pollen diagram
An important graph used in pollen
analysis, which plots the frequency of the pollen grains or
spores of specific types of plant against the depth (i.e. time
or sequence) of their occurrence in peat, soil or sediment.
Polyclimax theory
The theory of climax vegetation
which emphasizes that in any region a variety of climaxes
will develop in relation to soil and topographic conditions.
Polymerization
The formation of large framework min-
erals by the replication of smaller constituent minerals,
involving the sharing of atoms and consequential streng-
thening of mineral structure.
Polymorphic
The description of a single mineral capable
of assuming two crystalline forms.
Polyploidy
A genetic adaptation in plants, endowing
them with more than two sets of chromosomes, which
appears to make them particularly vigorous and successful
colonizers of hostile environments.
Pool
A depression formed by erosive scour in a stream
bed.
Porosity
The volume of voids in Earth materials,
measured as a percentage of their bulk volume and
comprising interparticle pores and fractures.
Porphyritic
An igneous rock texture characterized by
larger, slower-cooling crystals in a finer matrix.
Positive feedback
A feedback effect in which the initial
change in the system is amplified to bring about even
greater changes. It can result in instability.
Potential energy
The energy possessed by a body because
of its position. A common example is the potential energy
possessed by a boulder on a mountainside, which has
potential energy relative to the valley bottom and
eventually to sea level.
Potential evapotranspiration
The amount of moisture
which would be evaporated and transpired from a short
vegetation surface with no moisture deficit.
Pothole
A cylindrical hole developed in a rocky stream
bed by evorsion or a shaft taking surface water under-
ground.
Power threshold
The minimum power required to
overcome frictional resistance to movement.
Precession of the equinoxes
The movement of the
timing of the equinoxes around Earth's orbit. Currently
