Geoscience Reference
In-Depth Information
rafted sources and rain-out of marine organic debris;
their red colour comes from ferric oxide coating.
Reduction
A chemical reaction involving the dissoci-
ation of oxygen from a mineral and accompanied by a
negative shift in valency.
Redundancy model
Model proposed to describe the
relationship between ecosystem functioning and species
richness in species-rich communities, whereby there is a
high probability that 'spare' species will be able to assume
the functional role of species lost from that ecosystem.
Reef
A shallow-water marine bench or mound con-
structed mostly of the carbonate-rich skeletal secretions
and remains of organisms; also known as a
bioherm
.
Reflected wave
A wave which has rebounded from
coastal features in its path into an incoming wave.
Refracted wave
A wave front which has been diverted
from its original path as it encounters shallow water or a
coastal current.
Refugia
Isolated geographical locations whose
distinctive environments permit the survival of formerly
widespread plant and animal species during periods of
adverse environmental change; they may act as centres of
dispersal in any subsequent amelioration.
Reg
A stony desert surface or
desert pavement
.
Regelation
The refreezing of supercooled water by the
reduction of glacier
overburden pressure
, or of surface
meltwater which has percolated into colder ice.
Regional metamorphism
The chemical and textural
alteration of rock by widespread compression and heating
or burial in, for example, a
subduction zone
.
Regolith
A general term for the superficial layer of
disaggregated Earth material at the land surface, irrespec-
tive of its specific origins.
Regression
A seaward retreat of the coastline caused by
a relative fall in sea level and its stratigraphic expression
in the advance of terrestrial sedimentation.
Rejuvenation
The stimulation of denudation processes
to renewed activity, normally by the increase in potential
energy caused by tectonic or isostatic uplift; also regarded
as the impetus for the first stage of a new
denudation cycle
.
Release surface
A
planar discontinuity
in Earth materials
along which slope failure has occurred; it may act singly,
in the case of planar slides, or with other release surfaces
in more complex failure.
Relic soil
A type of fossil soil currently at the surface, and
therefore showing properties formed by past and present
soil-forming processes
.
Remnant arc
An extinct volcanic arc abandoned by the
migration of a
subduction zone
.
Residual soil
Pumice
A low-density, highly porous volcanic rock
material formed by explosive degassing of its parent
magma
.
Pycnocline
A zone of marked density change with ocean
depth.
Pyroclast(ic)
Molten or solid explosive volcanic products
in the form of
ash, lapilli
or
tephra
; volcanic rocks are
formed by their deposition and, frequently, hot welding.
Pyroclastic rocks
Rocks formed by the cooling and
lithification of
the
pyroclastic
products of
volcanic
eruptions.
Quarrying
The large-scale excavation of large bedrock
blocks and fragments by moving glacier ice and high-
pressure basal meltwater; replaces the now defunct term
'plucking'.
Quaternary
The youngest period of the
Cenozoic
era,
which began 2.6 Ma ago, and in which we now live;
consists of the
Pleistocene
and
Holocene
epochs.
Quick clay
A clay whose
shear strength
collapses on
disturbance.
Quickflow
The component of stream discharge
contributed by water reaching the channel rapidly by
direct channel precipitation,
overland flow
and subsurface
throughflow
and forming a transient
peak discharge
.
Radiant energy
Energy transmitted in the form of
electromagnetic waves. The waves do not need molecules
to transmit them and in a vacuum they travel at the speed
of light.
Radiation
Another term for
radiant energy
.
Radiometric dating
A means of determining the age of
materials from the rate of decay of unstable, radioactive
isotopes they contain.
Raised beach
A former beach abandoned by
isostatic
uplift or a
eustatic
fall in sea level and retaining mineral
and organic sedimentary evidence of its origin.
Raised platform
A former marine, wave-cut rock
platform abandoned by a change in sea level (see
raised
beach
).
Range of tolerance
The intervening range between the
upper and lower thresholds of survival for a species.
Reagent
A substance which is capable of causing a
chemical reaction.
Realized niche
That proportion of the fundamental
niche which is actually occupied by a plant species.
Recurrence interval
The predicted frequency or return
period of a particular value of stream discharge, measured
in years.
Red clay
Open ocean (
pelagic
) sea-bed sediments
derived from long-distance aeolian or ocean current
transport of clay minerals from volcanic, meteoric and ice-
A soil whose parent material is solid
bedrock.
