Geoscience Reference
In-Depth Information
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
diagenesis
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.
Microclimate
The climate of the land surface, extending
no more than a few metres above ground and strongly
influenced by its material, morphological and organic
components.
Mid-ocean ridge
A broad, linear ridge emerging from the
ocean floor along rising
mantle plumes
and the focus of
rifting and sea-floor spreading; basalt effusion forms new
oceanic crust at this
constructive margin
.
Migmatization
The mineralogical and structural altera-
tion of rock at extreme ranges of temperature and
pressure, causing significant remelt.
Mineralization
The decomposition of organic com-
pounds, which results in the production of mineral
nutrients in ionic form.
Minerogenic sediment
Sediment (or soil) derived solely
from inorganic, mineral sources.
Minor nutrient
A plant nutrient needed in relatively
small amounts (e.g. calcium, sulphur, magnesium).
Misfit stream
A stream which appears to underfit its
valley, as indicated by its diminutive size and its meander
wavelengths being much shorter than those of the valley
itself; attributed to a climatically related reduction in
stream discharge.
Mistral
A dry, cold air outflow moving south and
channelled along the lower Rhône valley in France.
Moder
Surface organic matter, intermediate in form
between mull and mor, and consisting mostly of partly
humified plant remains.
Mohr-Coulomb criteria
The failure criteria for Earth
materials which defines a state of
limiting equilibrium
when the
shear stress
acting on the material exactly equals
the material's internal
shear strength
comprising cohesion,
internal friction and normal stress.
Molasse
The sedimentary product of syntectonic or
early post-tectonic erosion of a new
orogen
, consisting of
coarse clastic, and mostly terrestrial sediments.
Mollisols
A soil order (USDA classification) character-
ized by base-rich soils with a dark, organic-rich surface
horizon.
Monadnock
An isolated mountain in a
peneplain
,
representing a residual feature of extensive denudation.
Monoclimax theory
The theory of climax vegetation
which emphasizes that only one type of climax will
ultimately develop in a specified climatic region; it will be
in stable equilibrium with climate and soil.
Montane
Said of the mountain forest belt or used more
generally to denote a characteristic of mountainous
terrain.
Montane forest
A cool, mountain forest community.
Montmorillonite
An expanding 2 : 1 type of clay mineral;
isomorphous replacement of aluminium by iron and
magnesium is common in the alumina sheet.
Mor
An acid organic matter horizon consisting of
litter (leaves, twigs, wood) overlying partly decomposed,
fermenting plant remains.
Moraine
Ridge-like accumulations of glacier debris,
carried as glacier surface medial and lateral moraines or
deposited at the ice margin by a variety of passive release
or active push processes; it is mostly composed of glacial
till
with admixed glaciofluvial sediment and is also
known as
ground moraine
when deposited in extensive,
amorphous sheets.
Morphogenesis
The conversion of weathered regolith
into a soil profile by processes of soil formation.
Morphogenetic
Said of a geographical region or process
where climate is believed to have created a distinct suite
of landforms.
Morphological system
A type of system in which the
morphological expression is examined rather than the
dynamics of interactions and flows.
Morphotectonic landform
A landform created by
tectonic processes, such as an
island arc, orogen, rift valley
or
passive margin
coast or any of
their principal
components.
Morphotectonics
The construction of large-scale surface
landforms by tectonic processes.
Mosaic theory
The theory of climax vegetation which
emphasizes the mosaic patterning of the climax cover.
