Geoscience Reference
In-Depth Information
Permafrost
The enduring and continuous presence of
freezing temperatures at and below the land surface in
which all soil and ground water is frozen except for a thin
surface
active layer
of summer melting.
Permafrost soil processes
Pipe
A narrow water conduit in soil formed through the
connection of
macropores
or the removal of
swelling clays
.
Placer
A terrestrial or shallow marine deposit of heavy
minerals in a body of
clastic
sediments, sourced by erosion,
transport and gravity deposition from a parent mineral
body.
Plagioclimax
A vegetational state where burning or
grazing modifies the natural state of the vegetation.
Planar discontinuity
A plane surface (such as a fold,
fracture, fault, thrust, joint, lamination, etc.) in rock mass
or other Earth materials at which the continuous or
homogeneous properties of material on either side of the
plane - providing its
intact strength
- are momentarily
interrupted or lost.
Planar slide
A sliding failure along a single
planar
discontinuity
, inclined at an angle less than that of the
slope on which it occurs but greater than the internal
friction angle of the material.
Plant community
A group of plants which form a
distinct combination of species in the landscape and
which interact with each other.
Plant uptake
The amount of a nutrient absorbed by a
plant, or the process of the absorption.
Plastic
A condition in which material is capable of con-
tinuous and permanent deformation without fracturing.
Plastic limit
The threshold water content of a solid
sediment at the point at which its behaviour changes to a
plastic state.
Plate
A large and rigid 'raft' of Earth's lithosphere which
is mobilized by mantle convection currents and whose
boundaries are marked by the formation or destruc-
tion of oceanic crust and the creation of new continental
crust.
Plate tectonics
The global-scale movement and
deformation of Earth's lithospheric plates, representing
the surface expression of Earth's long-term geological
evolution and responsible for global-scale landforms.
Platy
Soil processes occurring in
frozen ground (
permafrost
).
Permeability
The capacity of Earth materials to circulate
and transmit fluids (water, solutions, air, etc.) through
their pores and fractures and measured as the fluid volume
passing through a unit cross-section area.
P-form
'Plastically sculptured' sinuous bedrock grooves
of uncertain origin but believed to be formed by high-
pressure subglacial meltwater rather than glacier abrasion.
pH
The measure of acidity or alkalinity of a substance,
measured by the number of hydrogen ions per litre, on a
logarithmic scale where neutrality = 7·0; acid and alkaline
substances have a pH of less than 7·0 and more than 7·0
respectively.
Phanerozoic
The Eon of geological time, commencing
c.
540 Ma ago, characterized by 'evident life' preserved in
the fossil record, in part because of the appearance of hard,
skeletal body parts.
Photic zone
The thin surface layer of a water body
penetrated by sunlight.
Photosynthesis
The synthesis of organic compounds
from water and carbon dioxide, using energy absorbed by
chlorophyll from the radiant energy of the sun.
Phreatic water
Water in the saturated zone of an aquifer.
Phreatophytes
Plants which survive aridity by develop-
ing deep root systems to exploit soil water reserves.
Phytoplankton
The plant (primary producer) form of
marine micro-organisms which form the base of the
marine food web.
Piedmont fan
A fan-shaped lobe of alluvium or other
debris accumulated at the break of slope along a mountain
front.
Piedmont glacier
A glacier which fans out across the
unconfined surface of a piedmont zone as it leaves the
confined channel of an outlet or valley glacier.
Piedmont lobe
A type of soil structure consisting of horizontal
units.
Playa
An enclosed, ephemeral lake basin and its residual
mud or evaporite floor in an arid or semi-arid environ-
ment.
Pleistocene
The first epoch of the
Quaternary
period,
which lasted from 1·8 million to 10,000 years before the
present time, and when Earth's glaciers frequently covered
double their present area.
Plinthite
A reddish clay in tropical and subtropical soils
which hardens irreversibly on drying; it consists of
sesquioxides of iron and aluminium, and
kaolinite
.
Pluton
An igneous
intrusion
of
plutonic
character which
has cooled and solidified below ground.
The lobate terminal zone of a
piedmont
glacier
.
Piezometric surface
An imaginary surface defined by the
level to which water rises in a well and representing the
static 'head' of water.
Pingo
A large ice-cored mound elevated by hydrostatic
pressure and
segregated ice
growth on a flat
permafrost
land
surface which is waterlogged in summer.
Pioneer community
The first set of plant species to
colonize a newly available site which was previously
unvegetated.
