Glossary - Fundamentals of the Physical Environment - page 897

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 components 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 biomodal 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 and 1 m, formed by high energy flow in shallow waters such as tidal estuaries.
Mélange A chaotic mixture of rock material from a variety of sources, commonly
associated with subduction zones , where it consists of subducted oceanic crust, ocean
floor sediment and, maybe, adjacent continental crust.
Melt - out The subglacial release of ice-transported debris during a temporary or
permanent melting phase.
Metamorphic aureole The zone of rock surrounding an igneous intrusion which is
altered by contact metamorphism .
Metamorphism The mineralogical and structural alteration of rock in response to
thermal and pressure conditions substantially different from those in which it formed,
whilst remaining in solid state; it lies between mild dia-genesis and the presence of a
liquid phase required by metasomatism and migmatization .
Metasomatism The alteration of existing minerals and formation of new species by
fluids and gases circulating through rock mass in metamorphic belts and mid-ocean
ridges.
Metastable equilibrium A condition whereby small changes in system variables can
have a major effect once they reach a certain value.