Geoscience Reference
In-Depth Information
Moulin
A cylindrical, supraglacial 'pothole' marking the
englacial transfer of a surface meltwater stream.
Mountain circulation winds
A general regional pattern
of wind circulation determined or modified by the
insolational and mechanical character of mountainous
terrain.
Mud flow
The moderate to fast downslope movement
of a fluidized mass of very fine debris, or its resultant
landform.
Mull
Well decomposed and well humified organic
matter, thoroughly mixed with mineral soil by earth-
worms.
Mutualism
Symbiotic relationship between two species
which benefits both.
Mutualistic symbiotic relationship
See
mutualism
.
Nappe
A
fold
which has experienced such intense
deformation as to have become recumbent (horizontal)
and sheared along its axis.
Natural fire
Nitrate-vulnerable Zone (NVZ)
An area where there are
restrictions on the use of nitrogen fertilizers.
Nitrification
The conversion of organic nitrogen
compounds in soil organic matter into nitrates by soil
micro-organisms.
Nitrogen fixation
The conversion of atmospheric
nitrogen into organic nitrogen compounds by soil micro-
organisms, either free-living or in nodules of plant roots.
Nivation
The erosion of surface depressions by the
combined processes of rock weathering and mass wasting
associated with the growth and decay of snowpack.
Normal stress
The portion of rock or soil strength
dependent on the anchoring effect of the mass of a particle
or intact block, normal (at right-angles) to a surface on
which it rests; this is at a maximum if the surface is
horizontal but diminishes as slope angle increases.
Nuée ardente
An incandescent (fiery) cloud of
ash
and
volcanic gas developing as near-surface
pyroclastic
gravity
flow after volcanic eruption,
A fire ignited by natural means (e.g.
capable of
destroying
lightning).
Natural selection
Process in which genetic charac-
teristics become more common in a population over time
because individuals are more successful in adapting to
their environment if they possess those characteristics.
Neap tide
The twice-monthly tidal period when the
gravitational pull of sun and moon are opposed (at right-
angles to each other) and minimize tidal range.
Nearshore
The zone of shoreline-wave interaction,
subdivided landwards into
breaking wave
, surf and
swash
zones.
Negative feedback
A feedback effect in which the initial
change in the system is damped down.
Neogene
See
Cenozoic
Neotectonic
anything in its path.
Nunatak
An isolated mountain or hill protruding
through, and completely surrounded by, glacier ice.
Obligate symbiotic relationships
Relationships which
are restricted to specified conditions only.
Obliquity of the ecliptic
The tilt of Earth's axis of rotation
relative to the plane of its orbit. It varies between 21·8°
and 24·4° over a period of about 40,000 years.
Occupational fire
A fire started by humans in order to
manage an
ecosystem
for economic gain (e.g. grazing,
land clearance or herding wild animals).
Offshore
A zone of deeper water lying on the inner
margins of the continental shelf, beyond the
nearshore
zone.
Ooze
Fine-grained, marine sediment comprised of more
than 30 per cent skeletal remains of
pelagic
organisms and
clay minerals.
Open systems
Systems which are characterized by the
exchange of both matter and energy with their surround-
ings. The majority of natural systems are open.
Ophiolite
A sliver of oceanic crust caught up in an
accretionary prism
and found out of place in a subsequent
orogen
.
Organismic community
The concept which regards the
plant community
as a 'super-organism', with properties not
present in its individual constituent organisms.
Orogen
A linear continental mountain range elevated
mechanically or thermally by plate collision, crustal
shortening and uplift.
Orogenesis
The formative processes of an orogen.
Oroshi
A cold-air drainage current blowing from the
mountains of central Japan.
Plate tectonic activity during the late
Cenozoic era.
Net Ecosystem Exchange (NEE)
Net flux of carbon
dioxide (CO
2
) within an ecosystem.
Net ecosystem production (NEP)
The change in the
biomass
of an
ecosystem
per unit time; equivalent to
net
primary productivity
minus losses due to grazing by
herbivores.
Net primary productivity (NPP)
The amount of energy
fixed by plant
photosynthesis
, taking losses by respira-
tion into account; it represents growth by the plant or
ecosystem, and is measured per unit area per unit time.
Net radiation
The difference between the total incoming
and outgoing radiation terms. A positive value would
indicate greater incoming than outgoing energy and so a
warming; a negative value would indicate the reverse.
Net radiation deficit
The situation in which Earth is
losing more
radiant energy
than it is gaining.
