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Ma	Millions of years ago (abbreviation).
macrofossil	A fossil large enough to be seen with the naked eye.
mafic	A term used in reference to magmas or igneous rocks that are relatively poor in silica and rich in iron and magnesium.
magma	Molten rock beneath the Earth’s surface.
magma chamber	A space below ground filled with magma.
magma contamination	The process in which flowing magma incorporates components of the country rock through which it passes.
magmatic deposit	An ore deposit formed when sulfide ore minerals accumulate at the bottom of a magma chamber.
magnetic anomaly	The difference between the expected strength of the Earth’s magnetic field at a certain location and the actual measured strength of the field at that location.
magnetic declination	The angle between the direction a compass needle points at a given location and the direction of true north.
magnetic field	The region affected by the force emanating from a magnet.
magnetic field lines	The trajectories along which magnetic particles would align, or charged particles would flow, if placed in a magnetic field.
magnetic force	The push or pull exerted by a magnet.
magnetic inclination	The angle between a magnetic needle free to pivot on a horizontal axis and a horizontal plane parallel to the Earth’s surface.
magnetic reversal	The change of the Earth’s magnetic polarity; when a reversal occurs, the field flips from normal to reversed polarity, or vice versa.
magnetic-reversal chronology	The history of magnetic reversals through geologic time.
magnetization	The degree to which a material can exert a magnetic force.
magnetometer	An instrument that measures the strength of the Earth’s magnetic field.
magnetosphere	The region protected from the electrically charged particles of the solar winds by Earth’s magnetic field.
magnetostratigraphy	The comparison of the pattern of magnetic reversals in a sequence of strata, with a reference column showing the succession of reversals through time.
manganese nodules	Lumpy accumulations of manganese-oxide minerals precipitated onto the sea floor.
mantle	The thick layer of rock below the Earth’s crust and above the core.
mantle plume	A column of very hot rock rising up through the mantle.
marble	A metamorphic rock composed of calcite and transformed from a protolith of limestone.
mare	The broad darker areas on the Moon’s surface, which consist of flood basalts that erupted over 3 billion years ago and spread out across the Moon’s lowlands.
marginal sea	A small ocean basin created when sea-floor spreading occurs behind an island arc.
maritime tropical air mass	A mass of air that originates over tropical or subtropical oceanic regions.
marsh	A wetland dominated by grasses.
mass movement (or mass wasting)	The gravitationally caused downslope transport of rock, regolith, snow, or ice.
mass-extinction event	A time when vast numbers of species abruptly vanish.
matrix	Finer-grained material surrounding larger grains in a rock.
mean sea level	The average level between the high and low tide over a year at a given point.
meander	A snake-like curve along a stream’s course.
meander neck	A narrow isthmus of land separating two adjacent meanders.
meandering stream	A reach of stream containing many meanders (snake-like curves).
medial moraine	A strip of sediment in the interior of a glacier, parallel to the flow direction of the glacier, formed by the lateral moraines of two merging glaciers.
Medieval warm period	A period of high temperatures in the Middle Ages.
melt	Molten (liquid) rock.
meltdown	The melting of the fuel rods in a nuclear reactor that occurs if the rate of fission becomes too fast and the fuel rods become too hot.
melting curve	The line defining the range of temperatures and pressures at which a rock melts.
melting temperature	The temperature at which the thermal vibration of the atoms or ions in the lattice of a mineral is sufficient to break the chemical bonds holding them to the lattice, so a material transforms into a liquid.
meltwater lake	A lake fed by glacial meltwater.
Mercalli intensity scale	An earthquake characterization scale based on the amount of damage that the earthquake causes.
mesa	A large, flat-topped hill (with a surface area of several square km) in an arid region.
mesopause	The boundary that marks the top of the mesosphere of Earth’s atmosphere.
mesosphere	The cooler layer of atmosphere overlying the stratosphere.
Mesozoic	The middle of the three Phanerozoic eras; it lasted from 245 Ma to 65 Ma.
metal	A solid composed almost entirely of atoms of metallic elements; it is generally opaque, shiny, smooth, and malleable, and can conduct electricity.
metallic bond	A chemical bond in which the outer atoms are attached to each other in such a way that electrons flow easily from atom to atom.
metamorphic aureole	The region around a pluton, stretching tens to hundreds of meters out, in which heat transferred into the country rock and metamorphosed the country rock.
metamorphic facies	A set of metamorphic mineral assemblages indicative of metamorphism under a specific range of pressures and temperatures.
metamorphic foliation	A fabric defined by parallel surfaces or layers that develop in a rock as a result of metamorphism; schistocity and gneissic layering are examples.
metamorphic mineral assemblage	A group of minerals that form in a rock as a result of metamorphism.
metamorphic rock	Rock that forms when preexisting rock changes into new rock as a result of an increase in pressure and temperature and/or shearing under elevated temperatures; metamorphism occurs without the rock first becoming a melt or a sediment.
metamorphic zone	The region between two metamorphic isograds, typically named after an index mineral found within the region.
metamorphism	The process by which one kind of rock transforms into a different kind of rock.
metasomatism	The process by which a rock’s overall chemical composition changes during metamorphism because of reactions with hot water that bring in or remove elements.
meteoric water	Water that falls to Earth from the atmosphere as either rain or snow.
meteorite	A piece of rock or metal alloy that fell from space and landed on Earth.
micrite	Limestone consisting of lime mud (i.e., very fine-grained limestone).
microfossil	A fossil that can be seen only with a microscope or an electron microscope.
mid-latitude (wave) cyclone	The circulation of air around large, low-pressure masses.
mid-ocean ridge	A 2-km-high submarine mountain belt that forms along a divergent oceanic plate boundary.
migmatite	A rock formed when gneiss is heated high enough so that it begins to partially melt, creating layers, or lenses, of new igneous rock that mix with layers of the relict gneiss.
Milankovitch cycles	Climate cycles that occur over tens to hundreds of thousands of years, because of changes in Earth’s orbit and tilt.
mine	A site at which ore is extracted from the ground.
mineral	A homogenous, naturally occurring, solid inorganic substance with a definable chemical composition and an internal structure characterized by an orderly arrangement of atoms, ions, or molecules in a lattice.
mineral classes	Groups of minerals distinguished from each other on the basis of chemical composition.
mineral resources	The minerals extracted from the Earth’s upper crust for practical purposes.
Mississippi Valley–type (MVT) ore	An ore deposit, typically in dolostone, containing lead- and zinc-bearing minerals that precipitated from groundwater that had moved up from several km depth in the upper crust; such deposits occur in the upper Mississippi Valley.
Moho	The seismic-velocity discontinuity that defines the boundary between the Earth’s crust and mantle.
Mohs hardness scale	A list of ten minerals in a sequence of relative hardness, with which other minerals can be compared.
mold	A cavity in sedimentary rock left behind when a shell that once filled the space weathers out.
monocline	A fold in the land surface whose shape resembles that of a carpet draped over a stair step.
monsoon	A seasonal reversal in wind direction that causes a shift from a very dry season to a very rainy season in some regions of the world.
moraine	A sediment pile composed of till deposited by a glacier.
mountain (alpine) glacier	A glacier that exists in or adjacent to a mountainous region.
mountain front	The boundary between a mountain range and adjacent plains.
mountain ice cap	A mound of ice that submerges peaks and ridges at the crest of a mountain range.
mouth	The outlet of a stream where it discharges into another stream, a lake, or a sea.
mud pot	A viscuous slurry that forms in a geothermal region when hot water or steam rises into soils rich in volcanic ash and clay.
mudflow	A downslope movement of mud at slow to moderate speed.
mudstone	Very fine-grained sedimentary rock that will not easily split into sheets.
mylonite	Fine-grained rock formed by the nonbrittle subdivision of larger grains; formed during dynamic metamorphism and characterized by foliation that lies roughly parallel to the fault (shear zone) involved in the shearing process.