Geoscience Reference
In-Depth Information
particular sediment sample, expressed by its standard
deviation.
Passive margin
A tectonically passive continental
margin associated with divergent plate boundaries and
marking the zone of initial rifting.
Patterned ground
A collective term for a variety of plan-
form patterns on a
permafrost
land surface formed by
turbulent heaving, sifting and collapse in the
active layer
;
symmetrical patterns develop in more homogeneous
earth materials and flat surfaces, becoming irregular
elsewhere.
Peak discharge value
The highest water discharge in a
stream channel stimulated by a precipitation event and
appearing as a peak on its
hydrograph
.
Peat
Dark organic material composed of plant residues
accumulating under wet or waterlogged conditions.
Ped
A natural soil
aggregate
consisting of primary
particles and colloidal material.
Pediment
A concave erosion surface sloping gently
down to a lowland plain from rather more abrupt contact
with a mountain front.
Pediplain
The coalescence of one or more
pediments
to create a more extensive lowland, considered by
proponents of
denudation
cycles to develop through
parallel slope retreat in semi-arid climates.
Pedological process
Any process associated with the
formation and development of soil.
Pegmatitic
An igneous rock texture characterized by
very large crystals representing the final
magma
fraction.
Pelagic
Of the open ocean environment, as opposed to
the ocean margin and coastline.
Pelagic sediments
Sediments associated with the
pelagic
zone and excluding terrigenous material; they consist of
the remains of marine organisms and
red clays
.
Peneplain
A lowland plain on which erosion of whatever
nature has progressively obliterated structural and
morphological features; considered to be the final stage
of a humid fluvial
denudation cycle
.
Percolation
Water transfer through the voids of
unsaturated soil or rock.
Peridotite
The coarse-grained, olivine-rich
ultramafic
rock which forms the
asthenosphere
and is the raw material
of oceanic crust.
Periglacial
A term formerly used to describe the environ-
ment and processes around the margins of a glacier or ice
sheet and strongly influenced by its proximity; this use is
too restrictive and may be misleading, so a definition
which emphasizes the predominance of
cryospheric
processes - without the need for glaciers - is preferred.
Perihelion
Orthogonal
Said of a system with right-angle relation-
ships between its components.
Outlet glacier
A steep, fast-flowing
glacier
discharging
large ice volumes from inland portions of
ice sheets
through a confining rock-walled channel.
Output
The flow of energy and matter out of a system.
Outwash plain
An extensive land surface covered by
glaciofluvial sediments and braided meltwater streams
released from a glacier terminus, especially in unconfined
piedmont zones.
Overbank discharge
That portion of stream discharge
not confined to the channel during a flood.
Overburden pressure
A compressive stress exerted on
Earth material by the mass of overlying rock, soil, water
or ice.
Overland flow
Non-channelled surface water flow where
precipitation intensity exceeds infiltration capacity, taking
two forms:
Horton overland flow
develops as
sheet flow
on
unvegetated surfaces before infiltrating or concentrating
in channels, whereas
saturated overland flow
emerges
towards valley floors on vegetated slopes.
Oxidation
A chemical weathering process involving the
combination of oxygen with a mineral accompanied by a
positive shift in its valency.
Palaeocurrent
A historical current, aspects of whose
former direction, form and energy level may be inferred
from sedimentary structures and textures preserved in
rocks deposited at the time.
Palaeogene
See
Cenozoic
Palaeosol
An ancient soil formed under past environ-
mental conditions (climate and vegetation). It occurs
either at the present land surface as relict soil or buried
beneath later material (e.g.
sediment,
peat,
human
earthworks) as fossil or buried soil.
Palsa
A small mound of peat and ice formed by
segregated ice growth in a peat bog.
Palynology
The science of
pollen analysis
.
Pangaea
Earth's most recent supercontinent, formed by
the coalescence of most continental plates
c.
300 million
years ago and rifted apart
c.
200 million years ago.
Parabolic dune
A tight crescent-shaped dune with
elongated arms pointing upwind, often developing from
a
blow-out
.
Paraglacial
Descibes a range of geomorphic processes
including those relating to slopes, rivers, coasts,
sedimentation etc., conditioned by current or recent
glaciation and deglaciation.
Particle sorting
The segregation of debris particles
according to their size and the competence or power of a
moving medium; also, the range of particle sizes in a
The point on Earth's orbit when it is closest
to the sun.
