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MAFIC: Adjective describing dark-colored minerals rich in iron and magnesium and relatively poor in silica (for example, pyroxene, amphibole, or olivine); also, describing rocks rich in mafic minerals.
MAGMA: Molten rock material that forms igneous rocks upon cooling. Magma that reaches the surface is lava.
MAGMA CHAMBER: A magma-filled cavity within the lithosphere.
MAGNETIC STRATIGRAPHY: The study and correlation of polarity epochs and events in the history of the Earth's magnetic field as contained in magnetic rocks.
MAGNITUDE: A measure of earthquake size, determined by taking the common logarithm (base 10) of the largest ground motion observed during the arrival of a P wave or seismic surface wave and applying a standard correction for distance to the epicenter.
MANTLE: The main bulk of the solid Earth, between the crust and the core, ranging from depths of about 40 km to 2900 km. It is composed of dense, mafic silicate minerals and divided into concentric layers by phase changes that are caused by the increase in pressure with depth.
MANTLE PLUME: Rising jet of hot, partially molten material, thought to emanate from the deep mantle and responsible for intraplate volcanism.
MARBLE: The metamorphosed equivalent of limestone or other carbonate rock.
MARINE EVAPORITE SEDIMENT, ROCK: A sediment or sedimentary rock consisting of minerals precipitated by evaporating seawater. Includes salt and gypsum.
MASS MOVEMENT: A downhill movement of soil or fractured rock under the force of gravity.
MASS WASTING: The sum of all mass movements and related erosional phenomena.
MEANDER: A broad, semicircular curve in a stream that develops as the stream erodes the outer bank of a bend and deposits sediment (as point bars) against the inner bank.
MÉLANGE: A formation found at convergent plate boundaries consisting of a heterogeneous mixture of rock materials in which fragments of diverse composition, size, and texture were mixed and consolidated by tremendous deformational pressure.
MESA: A flat-topped, steep-sided upland topped by a resistant formation.
METALLIC BOND: A type of covalent bond in which freely mobile electrons are shared and dispersed among ions of metallic elements, which have the tendency to lose electrons and pack together as cations.
METAMORPHIC FACIES: Characteristic assemblages of minerals in metamorphic rocks that are indicative of the range of pressures and temperatures experienced during metamorphism.
METAMORPHIC ROCK: A rock whose original mineralogy, texture, or composition has been changed by the effects of pressure, temperature, or the gain or loss of chemical components.
METASOMATISM: A change in the bulk chemical composition of a rock by fluid transport of some chemical components into or out of the rock.
METEORIC WATER: Rainwater, snow, hail, and sleet.
MICROPLATE TERRANE: A block within an orogenic belt containing rock assemblages that contrast sharply with those in the surrounding areas, interpreted as small continents, seamounts, or island arcs that were accreted onto the larger continent at a convergent plate boundary.
MID-OCEAN RIDGE: A major elevated linear feature of the seafloor consisting of many small, slightly offset segments, with a total length of 200 to 20,000 km. A mid-ocean ridge occurs at a divergent plate boundary, a site where two plates are being pulled apart and new oceanic lithosphere is being created.
MIGMATITE: A rock with both igneous and metamorphic characteristics that shows large crystal and laminar flow structures. Probably formed metamorphically in the presence of water and without complete melting.
MINERAL: A naturally occurring, inorganic, crystalline solid with a specific chemical composition.
MINERALOGY: The study of mineral composition, structure, appearance, stability, occurrence, and associations. The mineralogy of a rock is the mineral assemblage contained within that rock.
MOHOROVICIC´ DISCONTINUITY, MOHO: The boundary between crust and mantle, at a depth of 5 to 45 km, marked by a rapid increase in seismic wave velocity to more than 8 km per second.
MOHS SCALE OF HARDNESS: An empirical, ascending scale of mineral hardness. (See Table 2.2.)
MOMENT MAGNITUDE: A measure of earthquake size, determined by the slip of the fault, the area of the break, and the rigidity of rock. (See also Richter magnitude.)
MORAINE: A glacial deposit of till left at the margins of an ice sheet. Subdivided into ground moraine, lateral moraine, medial moraine, and end moraine.
MUDFLOW: A mass movement of material mostly finer than sand, along with some rock debris, lubricated with large amounts of water. The water tends to make mudflows move faster than earthflows or debris flows.
MUDSTONE: The lithified equivalent of mud; a fine-grained sedimentary rock similar to shale but less finely laminated.
MYLONITE: A very fine grained metamorphic rock commonly found in major thrust faults and produced by shearing and rolling during fault movement.