Geology Reference
In-Depth Information
are dominant, the rock is siltstone. Mudstone, claystone,
and siltstone are all members of the argillaceous group of
clastic sedimentary rocks.
pores
Small voids within rocks, unconsolidated sedi-
ments, and soils.
pore water pressure
The force that builds up owing to
the action of gravity on water in the pore spaces in soils
and sediments.
muskeg
In Canada, a swamp or bog composed of
accumulated bog moss (
Sphagnum
).
porosity
The amount of pore space or voids in a rock,
unconsolidated sediment, or soil body. Usually expressed
as the percentage of the total volume of the mass occupied
by voids.
nappe
A large body or sheet of rock that has been
moved 2 km or more from its original position by fold-
ing or faulting. It may be the hanging wall of a low-angle
thrust fault or a large recumbent fold.
porphyry
Any igneous rock that contains conspicuous
phenocrysts in a fine-grained groundmass.
Old Red Sandstone
A thick sequence of Devonian
rocks formed on land in north-west Europe some 408
to 360 million years ago.
pressure melting point
The temperature at which ice
can melt at a given pressure. The greater the pressure, the
lower the pressure melting point.
orogeny
The creation of mountains, especially by
folding and uplift.
profile curvature
The curvature (rate of change of
slope) at a point along a slope profile.
orthoclase
Potassium aluminium silicate, an essential
constituent of more acid igneous rocks, such as granite
and rhyolite.
pyrite (pyrites, iron pyrites)
Iron sulphide, FeS
2
;a
mineral.
Pangaea
The Triassic supercontinent comprising
Laurasia to the north and Gondwana to the south.
pyroxene
A group of minerals, most of which are gen-
erally dark-coloured anhydrous ferromagnesian silicates.
Characteristically occur in ultrabasic and basic rocks as
the mineral augite.
passive margin
The margin of a continent that is not
associated with the active boundary of a tectonic plate
and, therefore, lies within a plate.
quartz
A widely distributed mineral with a range of
forms, all made of silica.
pedosphere
The shell or layer of the Earth in which
soil-forming processes occur. The totality of soils on the
Earth.
quartzite
Sandstone that has been converted into
solid quartz rock, either by the precipitation of silica
from interstitial waters (orthoquartzite) or by heat and
pressure (metaquartzite). Quartzite lacks the pores of
sandstone.
perennial stream
A stream that flows above the surface
year-round.
peridotite
A coarse-grained, ultrabasic, plutonic rock,
mainly
made
of
olivine
with
or
without
mafic
quartzose
Containing quartz as a chief constituent.
minerals.
radiolarian
A unicellular organism (Kingdom Pro-
tista), usually with a silica skeleton that possess a beautiful
and intricate geometric form.
plan curvature
Contour curvature, taken as nega-
tive
(convex)
over
spurs
and
positive
(concave)
in
hollows.
rectilinear
Characterized by a straight line or lines.
plinthite
A hardpan or soil crust, normally rich in iron.
rhyolite
A fine-grained, extrusive, acid igneous rock
composed mainly of quartz and feldspar and commonly
mica, mineralogical equivalent of granite.
podzolization
A suite of processes involving the chem-
ical migration of aluminium and iron (and sometimes
organic matter) from an eluvial (leached) horizon in
preference to silica.
rock salt
Halite (sodium chloride, NaCl).
