M Index Glossary geology Science
M Index Glossary geology Science
Mafic
A term used to describe an igneous rock that has a large percentage of dark-colored minerals such as amphibole, pyroxene and olivine. Also used in reference to the magmas from which these rocks crystallize. Mafic rocks are generally rich in iron and magnesium. Basalt and gabbro are examples of mafic rocks. (See felsic to contrast.)
Magma Chamber
A full or emptied magma reservoir in the shallow portion of the lithosphere.
Magmatic Water
Water that is dissolved in a magma or water that is released from a magma. Some magmas can contain up to several percent dissolved water by weight.
Magnetic Anomaly
An increase or decrease in the local magnetic field compared to the normally expected value.
Magnetic Declination
The horizontal angular difference between True North and Magnetic North.
Magnetic Inclination
The vertical angular difference between a horizontal plane and the orientation of Earth's magnetic field.
Magnetic North
The direction that a compass points. The location where Earth's magnetic field dips vertically into the Earth.
Magnetic Reversal
A change in the polarity of Earth's magnetic field in which the north magnetic pole becomes the south magnetic pole and vice versa. Also known as geomagnetic reversal or polarity reversal. Earth's magnetic field has reversed many times in the past and the time intervals between these changes are known as polarity epochs.
Magnetic Stratigraphy
The correlation of rock units and study of Earth's history using magnetic events and magnetic epochs as a time reference.
Magnetometer
An instrument designed to measure the strength and character of Earth's magnetic field.
Magnitude
A measure of earthquake strength based upon the amount of ground motion experienced and corrected for the distance between the observation point and the epicenter. There are several magnitude scales in use.
Manganese nodule
A rounded concretion, rich in manganese minerals with minor concentrations of cobalt, copper and nickel. These occur in abundance on some parts of the deep ocean floor and have been considered as a potential source of manganese.
Mantle Plume
A rising mass of hot mantle material that can create an area of volcanic activity in the center of a lithospheric plate.
Massive
A term used in reference to a rock unit that is homogeneous in texture, fabric and appearance.
Mass Wasting
A general term used for any downslope movement of rock, soil, snow or ice under the influence of gravity. Includes landslides, creep, rock falls and avalanches. (also Mass Movement)
Mechanical Weathering
A general term applied to a variety of weathering processes that result in the particle size reduction of rock materials with no change in composition. Frost action, salt crystal growth and pressure relief fracturing are examples. Also known as physical weathering.
Medial Moraine
A streak of till in the center of a glacier. These are found downslope from the junction of two glaciers and are a merging of their lateral moraine deposits.
Medical Geology
The study of human health related to geology. Examples would include the correlation of disease or vitality with residences over specific types of bedrock or health problems associated with exposure to specific mineral materials.
Meteor
A meteoroid that penetrates Earth's atmosphere, producing a streak of bright light caused by incineration.
Meteorite
A particle of iron or rock that has fallen to Earth's surface from inter-planetary space.
Meteoroid
A particle of iron or rock found in inter-planetary space. Distinguished from planets or asteroids by its much smaller size.
Microseism
A vibration of the Earth that is unrelated to earthquake activity - instead it is caused by wind, moving trees, ocean waves or human activity.
Mineralogy
The study of minerals - their composition, structure, formation, uses, properties, occurrence and geographic distribution.
Mohorovicic Discontinuity
The boundary between the crust and the mantle. Frequently referred to as the Moho.
Mohs Hardness Scale
A collection of minerals ranging from very soft to very hard. Use as a comparison scale during mineral identification. From softest to hardest, the ten minerals are talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, and diamond 10. Developed by Friedrich Mohs, a German mineralogist in the early 1800's.
Monocline
An area of increased dip in otherwise gently dipping strata.
Mountain
A general term used in reference to an area that is at a conspicuously higher elevation than surrounding lands. Mountains are larger than hills and are significant enough in relief that they are given names by local residents.
Mudstone
A sedimentary rock composed of clay-size particles but lacking the stratified structure that is characteristic of a shale.
map
A picture of a place that is usually drawn to scale on a flat surface
Maritime Climate
A climate strongly influenced by an oceanic environment, found on islands and the windward shores of continents. It is characterized by small daily and yearly temperature ranges and high relative humidity.
Mediterranean Climate
A climate characterized by moist, mild winters and hot, dry summers.
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.
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.
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, non foliated 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.
mass movement
The process by which such Earth materials as bedrock, loose sediment, and soil are transported down slopes by gravity.
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 flood plain.
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.
melange
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.
meltwater
Water formed from the melted ice of a glacier.
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, fol lowing 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 facies
1. A group of minerals customarily found together in metamorphic rocks and indicating a particular set of temperature and pressure conditions at which metamorphism occurred. 2. A set of metamorphic rocks characterized by the presence of such a group of minerals.
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
Rock that has been physically altered by heat and/or pressure. 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.
mid-ocean ridge
An underwater mountain range that develops between the margins of two lithospheric plates, formed 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.
mineral zone
An area of rock throughout which a given metamorphic index mineral is found, presumed to have undergone metamorphism under uniform temperature and pressure conditions.
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 Mohorovicic -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.
molecule
The smallest particle that retains all the chemical and physical properties of a given compound, consisting of a stable group of bonded atoms.
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.
mudflow
The rapid flow of typically fine-grained regolith mixed with water. There may be as much as 60% water in a mudflow.
An isolated, relatively flat-topped natural elevation, usually more extensive than a butte and less extensive than a plateau.
A spiny deep-rooted leguminous tree or shrub that forms extensive thickets in the southwestern
Metes and Bounds
A system of land survey that defines land parcels according to visible natural landscape features and distance. The resultant field pattern is usually very irregular in shape.
Metropolitan Coalescence
The merging of the urbanized areas of separate metropolitan regions; Megalopolis is an example of this process.
maximum contaminant level
(MCL)-the designation given by the U.S. Environmental Protection Agency (EPA) to water-quality standards promulgated under the Safe Drinking Water Act. The MCL is the greatest amount of a contaminant that can be present in drinking water without causing a risk to human health.
Milligram
(mg)-One-thousandth of a gram.
milligrams per liter
(mg/l)-a unit of the concentration of a constituent in water or wastewater. It represents 0.001 gram of a constituent in 1 liter of water. It is approximately equal to one part per million (PPM).
million gallons per day
(Mgd)-a rate of flow of water equal to 133,680.56 cubic feet per day, or 1.5472 cubic feet per second, or 3.0689 acre-feet per day. A flow of one million gallons per day for one year equals 1,120 acre-feet (365 million gallons).
Mafic mineral
A dark-colored mineral rich in iron and magnesium, especially a pyroxene, amphibole, or olivine.
Magnetic coupling
The transfer of momentum between celestial bodies, especially dust and gas clouds, through magnetic forces.
Magnetic north pole
(1) The point where the Earth's surface intersects the axis of the dipole that best approximates the Earth's field. (2) The point where the Earth's magnetic field dips vertically downward.
Massive rock
A rock that is little or not at all broken by joints, cracks, foliation, or bedding, tending to present a homogeneous appearance.
Mass spectrometer
An instrument for separating ions of different mass but equal charge (mainly isotopes in geology) and measuring their relative quantities.
Maturity
A stage in the geomorphic cycle in which maximum relief and well-developed drainage are both present.
Meander
Broad, semicircular curves in a stream that develop as the stream erodes the outer bank of a curve and deposits sediment against the inner bank.
Mesophere
The lower mantle.
Micrometeorite
A meteorite less than 1 millimeter in diameter.
Miogeosyncline
A Geosyncline that is situated near a craton and receives chemical and well-sorted elastic sediments from the continent.
Mohorovic discontinuity
The boundary between crust and mantle, marked by a rapid increase in seismic wave velocity to more than 8 kilometers per second. Depth: 5 to 45 kilometers. Abbreviated "Moho" or "M-discontinuity."
Mohs scale of hardness
An empirical, ascending scale of mineral hardness with talc as 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, and diamond 10.
mining water use
water use during quarrying rocks and extracting minerals from the land.
municipal water system
a water system that has at least five service connections or which regularly serves 25 individuals for 60 days; also called a public water system
Monadnock
An isolated hill or mountain of resistant rock rising above an eroded lowland.
Multilingual
The ability to use more than one language when speaking or writing (see Bilingual). This term often refers to the presence of more than two populations of significant size within a single political unit, each group speaking a different language as their primary language.
Municipal Waste
Unwanted by-products of modern life generated by people living in an urban area.
M.Y
Million years - abbreviation.
M.Y.A
Million years ago - abbreviation.
Mylonite
A brecciated metamorphic rock frequently found in a fault zone. The fractured texture is thought to form by the crushing actions of fault movement.