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M


magma
Molten (melted) rock that forms naturally within the Earth. Magma may be either a liquid or a fluid mixture of liquid, crystals, and dissolved gases.
magnetic field
The region within which the magnetism of a given substance or particle affects other substances.
magnetic reversal
The process by which the Earth's magnetic north pole and its magnetic south pole reverse their positions over time.
magnetism
The property, possessed by certain materials, to attract or repel similar materials. Magnetism is associated with moving electricity.
mantle
The middle layer of the Earth, lying just below the crust and consisting of relatively dense rocks. The mantle is divided into two sections, the upper mantle and the lower mantle; the lower mantle has greater density than the upper mantle. See also core and crust.
marble
A coarse-grained, nonfoliated metamorphic rock derived from limestone or dolostone.
marine magnetic anomaly
An irregularity in magnetic strength along the ocean floor that reflects sea-floor spreading during periods of magnetic reversal.
massive sulfide deposit
An unusually large deposit of sulfide minerals.
meandering stream
A stream that traverses relatively flat land in fairly evenly spaced loops and separated from each other by narrow strips of floodplain.
mechanical exfoliation
A form of mechanical weathering in which successive layers of a large plutonic rock break loose and fall when the erosion of overlying material permits the rock to expand upward. The thin slabs of rock that break off fall parallel to the exposed surface of the rock, creating the long, broad steps that can be found on many mountains.
mechanical weathering
The process by which a rock or mineral is broken down into smaller fragments without altering its chemical makeup; weathering that affects only physical characteristics. See also chemical weathering.
mélange
A body of rock that forms along the inner wall of an ocean trench and is made up of fragments of lithosphere and oceanic sediment that have undergone metamorphism.
Mercalli intensity scale
A scale designed to measure the degree of intensity of earthquakes, ranging from I for the lowest intensity to XII for the highest. The classifications are based on human perceptions.
Mesozoic Era
The intermediate era of the Phanerozoic Eon, following the Paleozoic Era and preceding the Cenozoic Era, and marked by the dominance of marine and terrestrial reptiles, and the appearance of birds, mammals, and flowering plants.
metallic bonding
The act or process by which two or more atoms of electron-donating elements pack so closely together that some of their electrons begin to wander among the nuclei rather than orbiting the nucleus of a single atom. Metallic bonding is responsible for the distinctive properties of metals.
metamorphic differentiation
The process by which minerals from a chemically uniform rock separate from each other during metamorphism and form individual layers within a new metamorphic rock.
metamorphic grade
A measure used to identify the degree to which a metamorphic rock has changed from its parent rock. A metamorphic grade provides some indication of the circumstances under which the metamorphism took place.
metamorphic index mineral
One of a set of minerals found in metamorphic rocks and used as indicators of the temperature and pressure conditions at which the metamorphism occurred. A metamorphic index mineral is stable only within a narrow range of temperatures and pressures, and the metamorphism that produces it must take place within that range.
metamorphic rock
A rock that has undergone chemical or structural changes. Heat, pressure, or a chemical reaction may cause such changes.
metamorphism
The process by which conditions within the Earth, below the zone of diagenesis, alter the mineral content, chemical composition, and structure of solid rock without melting it. Igneous, sedimentary, and metamorphic rocks may all undergo metamorphism.
meteoric water
The precipitation of condensed water from clouds as rain, snow, sleet, or hail.
microcontinent
A section of continental lithosphere that has broken off from a larger, distant continent, as by rifting.
migmatite
A rock that incorporates both metamorphic and igneous materials.
mineral
A naturally occurring, usually inorganic, solid consisting of either a single element or a compound, and having a definite chemical composition and a systematic internal arrangement of atoms.
mineraloid
A naturally occurring, usually inorganic, solid consisting of either a single element or a compound, and having a definite chemical composition but lacking a systemic internal arrangement of atoms. See also mineral.
Moho (abbreviation for Mohorovi?cic´)
The seismic discontinuity between the base of the Earth's crust and the top of the mantle. P waves passing through the Moho change their velocity by approximately one kilometer per second, with the higher velocity occurring in the mantle and the lower in the crust.
Moment-magnitude scale
A recently developed alternative to the Richter scale used to measure more accurately the amount of energy released by large earthquakes. This scale involves measurement of an earthquake's seismic moment.
moraine
A single, large mass of glacial till that accumulates, typically at the edge of a glacier.
mudcrack
A fracture that develops at the top of a layer of fine grained, muddy sediment when it is exposed to the air, dries out, and then shrinks.
mudstone
A detrital sedimentary rock composed of clay-sized particles.

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