Glossary - Fundamentals of the Physical Environment - page 904

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 underground.
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 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 stage The first stage in a plant succession .
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 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 interpretation 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 morphological and cascading systems so
that the system demonstrates the manner in which form is related to process.
Producers Autotrophic organisms capable of photosynthesis , i.e. organisms in the first
trophic level of an ecological pyramid.