Geoscience Reference
In-Depth Information
Draw-down
The process and extent of gravitational
or artificial withdrawal of water from
drainage basin
stores
.
Drought
Variable period of time during which an area
experiences well below expected levels of precipitation.
Drought-avoidance
Plants and animals which are able to
survive in arid and semi-arid environments by entering a
resting stage (e.g. bulbs in plants, aestivation in animals),
so drastically reducing their water demand.
Drumlin
A large, subglacial
bed form
composed mostly
of
till
and streamlined in the iceflow direction; it is
indicative of active ice flow and probable deformation in
the sediment body.
Dry valley
A surface valley showing evidence of erosion
by fluvial processes but rarely or never occupied by a
modern stream.
Dune
A mobile sand-wave
bed form
shaped by fluid
motion, found in a wide range of wavelengths (from
centimetres to kilometres) and environments, ranging
from stream channel beds to coasts and deserts.
Duricrust
A hard, crystalline crust found on tropical
land surfaces formed by
evaporite
minerals brought there
in solution by
capillary
action from underlying soil and
rock.
Dyke
A columnar igneous
intrusion
which cuts
discordantly through existing rock structures.
Dynamic equilibrium
A form of self-regulation in a
system which maintains a similar type of system.
Dynamic viscosity
Ecosystem stability
The behaviour of the entire sys-
tem in response to an external perturbation; can be defined
by several indicators, including resistance and resilience.
Ecotone
A zone of transition, and hence competition,
between two contiguous
plant communities
.
Edaphic climax
Climax vegetation maintained by soil
conditions (e.g. wetness, chemistry).
Eddy diffusion
The mixing of atmospheric matter and
properties which is brought about by eddies.
Eddy viscosity
The resistance to flow of a fluid caused
by friction between individual strands of the flow.
Edge wave
A wave moving approximately at right-
angles to the shore and breaking waves, as a result of the
need to drain water being pushed onshore which cannot
easily escape through the
swash
.
Eemian stage
The penultimate global
temperate stage
,
known as the Ipswichian in Britain and as the Sangamon
in North America, extending from
c.
135,000 to 115,000
years
BP
.
Effusive
Extrusive igneous activity characterized by a
steady outflow of basaltic material from a fissure (cf.
explosive).
El Niño
The appearance of unusually warm water off the
South American Pacific coast when the westward-driven
equatorial ocean current periodically falters, owing to a
reduction in trade-wind strength in the equatorial Pacific
Ocean. This, in turn, suppresses
upwelling
cold, deep water
induced by these atmospheric and ocean currents;
ocean-atmosphere coupling further disturbs pressure
and precipitation systems throughout the region (the
'Southern Oscillation'). The effect occurs every few years,
commencing around Christmas time - hence its Hispanic
allusion to the Christ child.
Elastic
A material condition in which strain (defor-
mation, or change of shape) is wholly and immediately
recoverable upon the removal of stress.
Elastic strain release
The restoration of the original
shape of a material which has experienced elastic defor-
mation, when the stress which caused it is released; thus
rock which deformed elastically when compressed will
expand on 'pressure release' and may fracture if the
recovery is imperfect.
Electrical conductivity (EC)
The ability of a soil to
conduct an electric current; used as an index of soil
salinity.
Eluviation
The removal of suspended solids or mineral
colloids from a higher to a lower soil horizon by water
percolation.
Emerson model
A model of soil aggregate formation
involving clay domains, organic linkages and quartz
particles.
The resistance to flow of a fluid.
Mounds at the soil surface formed by
Earth hummocks
a combination of
cryoturbation and frost heave in
periglacial regions.
Eccentricity of the orbit
The changing shape of Earth's
orbit around the sun from a more circular to a more
elliptical path. It varies over a cycle of almost 100,000
years.
Ecological amplitude
The overall range in which an
organism can function; the sum total of all individual
tolerance ranges for a particular species.
Ecological niche
The position of an individual species
within an ecosystem, in terms of function, space and time.
Ecological optimum
That part of the range of a plant
species where the plant's vigour is greatest.
Ecological status
The position of a
plant community
within the hierarchy of seral and climax communities.
Ecosphere
The biologically inhabited part of the Earth,
oceans and atmosphere.
Ecosystem
Open system comprising plants, animals and
their environments which is involved in the flow of energy
and the circulation of matter.
