Geoscience Reference
In-Depth Information
world suffered a decline of temperature but not so
marked.
Lodgement till A subglacial till deposit formed by direct
plastering-on of debris to the substrate by a moving
glacier, where the deforming boundary lies along the
till-ice contact.
Loess An accumulation of wind-blown dust which may
have undergone mild diagenesis and lithification .
Longshore current A coast-parallel or oblique current in
the surf zone comprising water pushed by edge waves and
the swash from refracted waves which periodically drains
seawards as rip currents .
Longshore drift
Manning equation An equation which calculates the
velocity of uniform stream flow in relation to channel
slope, hydraulic radius and bed roughness.
Mantle Earth's internal sphere, sandwiched between the
core and the crust, whose outer lithosphere and
asthenosphere are instrumental in plate tectonic processes.
Mantle plume A rising limb of slow flow/creep of hot
rock driven by convection in the mantle; marked initially
by surface hot spots, persistent motion leads eventually to
surface rifting and sea-floor spreading.
Marginal sea A marine basin impounded on oceanic
crust, associated usually with the development of a back
arc and partially separated from an adjacent ocean by an
island volcanic arc .
Mass balance The mass input (accumulation), storage
and output (ablation) of ice, snow and water which
constitute the mass budget of a glacier per unit time,
normally a 'budget' year commencing at the end of the
summer ablation season.
Mass movement An alternative term for mass wasting
in common use but passed over here because it
inaccurately implies the coherent movement of material
en masse .
Mass wasting The downslope movement of Earth
materials solely under the influence of gravity and without
the active aid of other moving materials such as water, ice
and air.
Mathematical model A model in which all the compo-
nents of the system are represented by mathematical
symbols and the relations between them by equations.
Matric force A soil suction force due to adsorption and
capillarity in the soil matrix which resists gravity drainage.
Matrix The finer-grained component of earth material
which surrounds and infills pores between larger clasts
and crystals; together they create a bimodal texture and
reduce porosity .
Mechanical weathering The disaggregation of rock
mass caused by the development of internal tensile
stress through thermal expansion, hydration and ice
growth in the conditions prevailing at or near the land
surface.
Medieval Warm Epoch A period of northern
hemispherical, and possibly global, climatic warming for
some three centuries between AD 800 and AD 1300 but
reaching a peak at different times in different places;
summer temperatures were approximately 1°C warmer
than they are today, enough to trigger substantial climatic,
environmental and socioeconomic change.
Megaripple A large sand wave, with wavelengths 1-100
m and wave heights between 0·1 m and 1 m, formed by
high energy flow in shallow waters such as tidal estuaries.
Sediment transfer along the coast by
longshore currents .
Long-wave radiation Electromagnetic radiation between
about 3 μm and 100 μm. Earth radiates only in this
waveband, so it is sometimes called terrestrial radiation.
Lower limit of tolerance The lower threshold of the
tolerance range of a plant species, i.e. the point at which
the species will not survive.
Lysimeter Instrument used to obtain evapotranspira-
tion. It incorporates growing vegetation and compares
changes in the water content of the soil column beneath
the vegetation.
Macropore An intergranular pore in earth material
which is too large (greater than 50 mm in diameter) to
hold capillary water and acts instead as a conduit for
gravity drainage.
Mafic A mnemonic from ma gnesium and f err ic which
identifies the ferromagnesian-rich igneous rocks, charac-
terized by their dark-coloured, basic minerals.
Magma Partially molten rock material, usually with
solid minerals and/or gas pockets suspended in a liquid
silicate mass, from which igneous rocks solidify.
Magnetic polarity Earth's magnetic field, characterized
by two poles of opposite tendency, with the dipole
normally arranged so that its south pole lies in the
northern hemisphere (corresponding to the north
magnetic pole) and vice versa; this is periodically reversed
on time scales of 10 3-6 years and provides a valuable dating
tool.
Major nutrient A plant nutrient needed in relatively
large quantities (e.g. nitrogen, potassium, phosphorus).
Managed retreat A pragmatic approach to the coastal
management of rising sea levels which rejects the use of
hard defences in favour of allowing the coastline to retreat
at favourable sites with the landward migration of salt
marsh or dune barriers as soft defences.
Mangrove swamp A tropical intertidal ecosystem on
low-energy coasts, capable of high productivity, wide
diversity and coastline protection against erosion.
 
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