Geoscience Reference
In-Depth Information
Grey dune zone
A stage in a
psammosere
on sand dunes
when mosses and lichens colonize the ground surface, and
darken the colour of the yellow sand.
Gross primary productivity (GPP)
The total amount of
solar energy fixed in
photosynthesis
by
autotrophs
per unit
area per unit time.
Ground heave
Small-scale ground expansion and uplift
in unconsolidated materials through hydration, ice
formation and influent water seepage, with consequent
disturbance of any incipient structure and its strength.
Ground ice
Any form of frozen water below the land
surface, irrespective of its origin and whether it is
interstitial
or
segregated ice
.
Ground moraine
A surface veneer of glacial
till
which is
not differentiated by individual glacial depositional
landforms.
Grounding line
The water depth at which an
ice shelf
or
tidewater glacier
begins to float by virtue of its lower
density than water.
Groundwater
The portion of all subsurface water stored
in saturated rock below the water table; sometimes exten-
ded to include water in the overlying unsaturated layer.
Gulf Stream
The warm, north-east-flowing current of
the clockwise
gyre
in the North Atlantic Ocean, flowing
from the Florida coast into the Arctic basin past Britain
and Norway; also known as the North Atlantic Drift.
Gully
A modest, steep-sided channel eroded by inter-
mittent stream flow with a frequency and vigour capable
of keeping the channel open.
Guyot
A flat-topped submarine mountain or
seamount
.
Gyre
A system of surface, wind-driven ocean currents
forming a closed or partially closed circulation which
transfers heat from warmer to colder surface waters.
Halocline
A zone of marked change in salinity with
ocean depth.
Halophyte
A plant adapted to growth in saline
environments.
Halosere
The sequence of
plant communities
which,
successively, occupy a salt marsh.
Hamada
An upland desert land surface of wind-scoured
bare rock with patches of
lag gravels
.
Hardware model
A model of a system composed of real
objects. A flume is a hardware model of a river; a wind
tunnel is a hardware model of air flow near the ground.
Headward retreat
The upslope migration of the point
of initiation of channel flow as it continues to attract ever
more surface or saturated
overland flow
.
Heinrich event
A surge of marine-based portions of the
former Laurentide ice sheet in Canada which sent a pulse
of icebergs, ice-rafted debris and cold fresh water into the
north-west Atlantic Ocean.
Helical flow
A spiral motion superimposed on the
general direction of stream flow or air flow which causes
lateral transport of energy and entrained materials.
Heliophyte
A sun-loving plant, adapted to high exposure
to sunlight in its mature form.
Hemicryptophytes
Tussock plants whose buds are
located at or just below the surface of the soil.
Herbivores
Organisms which eat plants and therefore
occupy the second trophic level in ecosystems.
Holocene
The second and 'wholly modern' epoch of the
Quaternary
period, which began 11,600 years ago and in
which we now live.
Horst
An elevated, large-scale fault block, flanked by
down-faulted
graben
.
Horton overland flow
Surface water discharge in sheet
rather than channel form, occurring when rainfall
intensity exceeds soil infiltration rates on non-vegetated
surfaces.
Human biometeorology
The scientific study of human
health and biological relationships with the atmosphere,
weather and climate, especially through respiratory, heat
and moisture exchanges and their regulation.
Hydration
The incorporation of water into the chemical
composition of a mineral, converting it from an
anhydrous to a hydrous form; the term is also applied to
a form of weathering in which hydration swelling creates
tensile stress within a rock mass.
Hydraulic conductivity
The rate at which water is able to
move through a soil or rock.
Hydraulic drop
An abrupt step over which a stream
surface falls as the stream enters a steeper segment.
Hydraulic efficiency
The conservation of potential
energy in stream flow, achieved by minimizing friction
losses against the channel, etc.
Hydraulic jump
An abrupt step or standing wave over
which a stream surface rises as the stream enters a less
steep segment or encounters a submerged obstacle.
Hydraulic radius
The relationship between the wetted
perimeter to the cross-sectional area of a stream; the
higher the value, the more efficient the channel.
Hydrogenic
The description of rocks formed at the sea
bed by the
hydrothermal circulation
of sea water through
mid-ocean ridges.
Hydrogeology
The study of the terrestrial part of the
hydrological cycle and the association between its
vegetation, Earth materials and stream flow.
Hydrograph
A plot of the variation of stream discharge
with time at a selected point in a catchment and capable
of separation into estimates of the various components
of flow.
