<b>Amateur<br>Geologist</b> Enter our Shop  
 

Geological Glossary - M

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Mauna Loa "An intermittently active volcano, elevation 4170 m (13,680 ft) on the island of Hawaii (19.475 N, 155.608 W); the last eruption was in 1984. Also see Mauna Loa record."
Mesozoic An era in geologic time that spans from 245 to 65 million years before the present and was the age of the dinosaurs.
Milankovitch theory An astronomical theory formulated by the Yugoslav mathematician Milutin Milankovitch that associates climate change with fluctuations in the seasonal and geographic distribution of insolation determined by periodic variation of the Earth's eccentricity and obliquity and the longitude of perihelion.
Miocene An Epoch that includes the time interval of about 23.7 to 5.3 million years ago.
Mississippian A period in the geologic time scale that spans from 360 to 320 million years ago.
Modified Mercalli intensity scale "Proposed in 1902 by Giuseppi Mercalli of Italy; updated in 1931 by H.O. Wood and F. Neumann as Modified Mercalli; further revised in 1956 by C. F. Richter and presented here in abridged form. Earthquake intensity on the MM scale is given as Roman numerals to prevent confusion with Richter magnitude and refers to the earthquake maximum intensity."
Mohorovicic discontinuity "The boundary surface or sharp seismic-velocity discontinuity that separates the Earth's crust from the subjacent mantle. It marks the level in the Earth at which P-wave velocities change abruptly from 6.7 to 7.2 km/s (in the lower crust) to 7.6 to 8.6 km/s or average 8.1 km/s (at the top of the upper mantle); its depth ranges from about 5 km beneath the ocean floor to about 35 km below the continent, although it may reach 60 km or more under some mountain ranges. The discontinuity probably represents a chemical change from basaltic or simatic materials above to peridotitic or dunitic materials below, rather than a phase change (basalt to eclogite); however, the discontinuity should be defined by seismic velocities alone. It is variously estimated to be between 0.2 and 3 km thick. It is named in honor of its discoverer, Andrija Mohorovicic (1857-1936), Croatian seismologist. Abbrev. Moho"
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.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
m.y. Abbreviation for million years.
maar volcano A volcanic crater without a cone, believed to have been formed by an explosive eruption of trapped gases.
macroturbulence Flood flows of great depth and high velocity are characterized by special turbulent flow phenomena that are exceedingly powerful. Macroturbulence phenomena include powerful upward vortices called kolksintences shocks resulting from sudden pressure change known as cavitation and powerful roller vortices oriented parallel to flow. These features provide enormous shear stresses and stream power to flood flows.
mafic A term used to describe mineral or igneous rocks that are rich in iron and/or magnesium. Mafic igneous rocks have a high percentage of dark-colored (mafic) minerals.
mafic mineral A dark-colored mineral rich in iron and magnesium, especially a pyroxene, amphibole, or olivine.
magma Naturally occurring liquid rock often containing volatile, such as water or carbon dioxide. Igneous rocks form by solidification of magma, usually by freezing. Molten silicate rock with dissolved gases. If erupted and degassed it is termed lava.
magma chamber The subterranean cavity containing the gas-rich liquid magma which feeds a volcano. When this chamber cools and solidifies, it is called a pluton.
magmatic water Water that is dissolved in a magma or that is derived from such water.
magnetic anomaly A local increase or decrease in the strength of the earth's magnetic field caused by the magnetism of nearby sediment or rocks.
magnetic anomaly The value of the local magnetic field remaining after the subtraction of the dipole portion of the Earth's field.
magnetic field lines Mathematical construction: a family of curves in space giving at each point the direction (by the tangent of the line) and the strength (by the local density of lines) of a magnetic field.
magnetic jerk Change in the slope of the geomagnetic secular variation. The change occurs within a few years and is global in extent.
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.
magnetic reversal Earth’s magnetic field occassionally flips or reverses polarity. This means that, if a polarity reversal happened today, your compass would point south instead of north! An important clue to plate tectonics.
magnetic stratigraphy The study and correlation of polarity epoch and events in the history of the Earth's magnetic field as contained in magnetic rocks.
magnetite Iron oxide mineral (Fe3O4). Usually tiny black, metallic crystal. Magnetite will attract a magnet and sometimes, in a rock, a hiker’s compass needle.
magnetometer An instrument for measuring either one orthogonal component or the entire intensity of the Earth's magnetic field at various points.
magnitude A measure of the strength of an earthquake or strain energy released by it as determined by seismographic observations. This is a logarithmic value originally defined by Charles Richter (1935). An increase of one unit of magnitude (for example, from 4.6
major earthquake An earthquake having a magnitude of 7 to 7.99 on the Richter scale.
manganese nodule A small, rounded concretion found on the deep ocean floor that may contain as much as 20 percent manganese and smaller amounts of iron, copper, and nickel oxides and hydroxides.
mantle The main bulk of the Earth, between the crust and core, ranging from depths of about 40 to 3480 kilometers. It is composed of dense mafic silicate and divided into concentric layer by phase changes that are caused by the increase in pressure with depth.The mantle is made up of dense, iron and magnesium rich (ultramafic) rock uch as dunite and peridotite. The uppermost part of the mantle is rigid and, along with the crust, form the 'plates' of plate tectonics.
marble A metamorphic rock of made of calcium carbonate. Marble form from limestone by metamorphic recrystallization.
marine terrace A level surface formed by wave erosion of coastal bedrock to the bottom of the turbulent breaker zone. May appear above sea level if uplifted.
mass balance "The application of the principle of the conservation of matter. For example, the mass of a glacier is not destroyed or created; the mass of a glacier and all its constitutive components remains the same despite alteration in their physical states. The mass balance of a glacier is calculated with the input/output relationships of ice, firn and snow, usually measured in water equivalent. Output includes all ablative processes of surface melting basal melting evaporation wind deflation calving and internal melting. Input includes direct precipitation avalanching and the growth of superimposed ice."
mass movement A downhill movement of soil or fractured rock under the force of gravity.
mass spectrometer An instrument for separating ion of different mass but equal charge (mainly isotope in geology) and measuring their relative quantities.
mass wasting Movement of rock and soil downslope under the influence of gravity.
massif An isolated block within a group of mountain. It indicates that a mountain is a segment of crystalline basement, or a pluton which survived the erosional processes that wore down the rock that originally surrounded it.
massive rock A rock that is little or not at all broken by joint, cracks, foliation, or bedding, tending to present a homogeneous appearance.
massive sulfide ore deposit Occurrence of a concentrated mass of sulfide mineral such as pyrite, sphalerite or chalcopyrite in one place, as opposed to their being disseminated or occurring in vein.
matrix Fine-grained material surrounding larger grains in a sedimentary rock.
mature A stage in the geomorphic cycle in which maximum relief and well-developed drainage are othen present.
mean sea level The average height of the sea surface based upon hourly observation of the tide height on the open coast or in adjacent water that have free access to the sea. In the United States, it is defined as the average height of the sea surface for all stage of the tide over a nineteen year period. Mean sea level, commonly abbreviated MSL and referred to simply as sea level, serves as the reference surface for all altitudes in upper atmospheric studies.
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.
mechanical weathering The set of all physical processes by which an outcrop is broken up into small particles.
medial moraine A long stripe of rock debris carried on or within a glacier resulting from the convergence of lateral moraine where two glaciers join.
medical geology The application of geologic science to problems of health, especially those relating to mineral sources of toxic or nutritious elements and natural dispersal of toxic pollutants.
mesa An area of high flat land, a tableland, with steep sides. It has horizontal bed of hard bock, which resist erosion, on top.
mesophere The lower mantle.
metaconglomerate Metamorphosed conglomerate.
metamorphic From the Greek meta (change) and morph (form). A rock that has undergone chemical or structural changes produced by increase in heat or pressure, or by replacement of elements by hot, chemically active fluids.Also applies to the conversion of snow into glacial ice.
metamorphic facies A set of rocks having a particular mineral composistion and having been metamorphosed under similar conditions of heat and pressure.
metamorphic zones An area of metamorphic rock that can be mapped by some special characteristic such as the presence of an index mineral.
metamorphism The changes of mineralogy and texture imposed on a rock by pressure and temperature in the Earth's interior. Meteoric water
meteorite A stoney or metallic object from inter-planetary space that penetrates the atmosphere to impact on the surface.
mica "Group of silicate mineral composed of varying amounts of aluminum, potassium, magnesium, iron and water. All mica form flat, plate-like crystal. Crystals cleave into smooth flakes. Biotite is dark, black or brown mica; muscovite is light-colored or clear mica"
micaceous A general term for mica-rich rocks.
microearthquake An earthquake having a magnitude of 2 or less on the Richter scale.
micrometeorite A meteorite less than 1 millimeter in diameter.
micromorphology Texture of a soil as viewed in petrographic thin section.
microseism A weak vibration of the ground that can be detected by seismograph and which is caused by wave, wind, or human activity, but not by an earthquake.
midden In geology, a mound of organic debris or organic-rich soil created by an animal. In archeology, a mound of human refuse.
migmatite Mixed rock. A metamorphic rock that form in one of two ways. The metamorphic rock may be heated enough to partially melt, but not completely. The molten mineral resolidify within the metamorphic rock, producing a rock that incorporates both metamorphic and igneous features. Migmatites can also form when metamorphic rock experiences multiple injections of igneous rock that solidify to form a network of cross-cutting dike.
mineral A naturally occurring element or compound with a precise chemical formula and a regular internal lattice structure. Organic products are usually not included.
mineral precipitate A mineral deposited from a water solution in pores or other openings in rocks. Chemical reaction with the surrounding rock, changes in pressure or temperature, or just drying up (evaporation) can cause a mineral to precipitate out of solution. Quartz vein are common products of mineral precipitation.
mineral precipitate A mineral deposited from a water solution in pores or other openings in rocks. Chemical reaction with the surrounding rock, changes in pressure or temperature, or just drying up (evaporation) can cause a mineral to precipitate out of solution. Quartz vein are common products of mineral precipitation.
mineral resource analysis Probabilistic modeling of occurrences and sizes of undiscovered deposits for mineral and energy resources evaluation and exploration.
mineralization The formation of mineral. New minerals may be added to fracture and empty spaces in a rock or by replacing preexisting minerals with different ones.
mineralogy The study of mineral composition, structure, appearance, stability, occurrence, and associations.
miogeosyncline A geosyncline that is situated near a craton and receives chemical and well-sorted elastic sediment from the continent.
modeling An investigative technique that uses a mathematical or physical representation of a system or theory that accounts for all or some of its known properties. Models are often used to test the effects of changes of system components on the overall performance of the system.
moho The boundary separating the base of the Earth’s crust and the top of the mantle. The Moho occurs at a depth of 5-10 kilometers beneath oceanic crust and about 35-65 kilometers below continental crust. The term moho is an abbreviation for Mohorovicic discontinuity, named for Andrija Mohorovicic, a Croatian seismologist.
monadnock An isolated hill or mountain rising above a peneplain.
monocline The S-shaped fold connecting two horizontal parts of the same stratum at different elevation. Its central limb is usually not overturned.
moraine A hill-like pile of rock rubble located on or deposited by a glacier. An end moraine form at the terminus of a glacier. A terminal moraine is an end moraine at the farthest advance of the glacier. A lateral moraine forms along the sides of a glacier.
morphology The study of shape or form.
most abundant elements

Element Percentage
O 46.6 Mg 2.09
Si 27.62 Ti .044
Al 8.13 H 0.14
Fe 5.00 Mn 0.1
Ca 3.63 Mn 0.1
Na 2.83 S 0.05
K 2.59 C 0.03
mountain " A steep-sided topographic elevation larger than a hill; also a single prominence forming part of a ridge or mountain range. "
mud Wet clay and silt-rich sediment.
mudflow A flow of water-saturated earth material possessing a high degree of fluidity during movement. A less-saturated flowing mass is often called a debris flow. A mudflow originating on the flank of a volcano is properly called a lahar.
mudstone A very fine-grained sedimentary rock formed from mud.
muscovite One of the mica family of mineral. Muscovite is light-colored or clear mica, sometimes called isingglass.
mylonite A very fine lithified fault breccia commonly found in major thrust fault and produced by shearing and rolling during fault movement.
mélange Mixture of rocks formed by tectonic disruption, such as multiple faulting, which brings disparate rock types together. Usually consists of a matrix of weak material, like shale, with hard pieces of exotic rocks, such as gneiss or igneous rocks.
© 2002,2003 Amateur Geologist™, All rights reserved.