GeoGlossary
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M
Macrofossil
Fossils that are large enough to be studied without the aid of a microscope.
Mafic
Silicate minerals that are dark and rich in iron and magnesium or the rocks that contain a lot of mafic minerals. The opposite of felsic.
Magma
Molten rock naturally formed within the earth and able to be intruded or extruded, which may or may not contain solid crystals or rock fragments, or gas phases. Igneous rocks are formed from the solidification of various compositions of magma at varying depths or at the surface.
Magma Chamber
A chamber of magma that forms the source or reservoir of a volcano, and occurs on a few km or tens of km below the surface of the earth. The source of the magma is much deeper in the crust, long lived volcanoes may have multiple injections of magma into the magma chamber, and the composition of the eruptions may vary to reflect the changing composition of the magma in the chamber.
Magnetic Anomaly
An area that has unexpected or unexplained magnetic properties. Detecting magnetic properties is a useful prospecting tool, as many rocks and geological structures (like faults) have properties that either change the signal of the earth’s magnetic field or generate their own magnetic field. Magnetic maps are usually obtained by aeromagnetic surveying where the total intensity of the earth's geomagnetic field is measured with a magnetometer carried in an aircraft or helicopter.
Magnetostratigraphy
The magnetic field of the earth has switched polarity in the past many times. Magnetostratigraphy maps the changes of the magnetic field from ‘normal’ (as it is now) to ‘reversed’ through time. Minerals that contain iron tend to crystallize within the earths magnetic field so that they are orientated in sympathy with that field. Provided no significant alteration or tectonic deformation has occurred since that crystallization the ‘remnant magnetism’ of a rock can be measured, and can be used to determine what orientation the magnetic field was in when the rock was formed. Measuring the magnetism of a succession of rocks can help date the succession (magnetochronology).
Mantle
The mantle is the part of the earth between the crust and the core (at around 2900 km depth). It is divided into the upper and the lower mantle with a transition zone, and other minor discontinuities may be present. The composition of the mantle is similar to olivine, but minor amounts of sulphides and other minerals may be present in the upper part, and iron-nickel alloys may be present in the lower part. It is hypothesized that slow convection cells in the mantle driven by heat from the core and density differences may be one of the driving forces behind the movement of the plates around the surface of the earth. Plumes of hot material rise up through the mantle, and may drive spreading (large plumes, such as exists under Iceland), or create hot spots (smaller plumes like under Hawaii). The plumes are thought to originate from discontinuities within the mantle or from the core-mantle boundary (the Gutenburg Discontinuity).
Marble
Metamorphic rock formed by either thermal (heating only) or regional (heating a pressure) metamorphism. It is the result of metamorphosing limestones, and is composed of fine to coarse grained recrystallized calcite. Often all trace of the original stone is lost, but certain marbles retain the shapes of shell fragments and other textures. Impurities present in the original limestone can result in brightly coloured layers or patches in the marble.
Marl
A sedimentary rock composed of almost equal amounts of clay or silt and calcium carbonate. It is frequently found interbedded with or grading into limestones.
Massive
A description of any rock that has a more or less homogeneous texture, or a sameness about outcrops or handspecimens. Massive rocks do not show flow structures, foliation, cleavage, joints, bedding etc. In mineral deposits, massive is used to describe ores that are concentrated into one place, rather than deposited in a thin vein-like structure, for example massive sulphide deposits.
Mélange
A fancy name for a mappable unit that is composed of blocks in a range of sizes, usually including some house sized blocks, embedded in a sheared and very fine-grained, crushed matrix. The blocks can be of local origin (the surrounding units) or of exotic origin (no correlative rocks nearby). A mélange can be sedimentary in origin, but they are often caused by intense deformation along a major tectonic boundary or fault.
Metamorphic Rock
A rock that has been derived from pre-existing rocks by changes in the mineralogy, structure and fabric and sometimes chemistry, while the rock remains in solid state (no melting). The process of change in solid state other than those changes that occur at the earth’s surface due to weathering or cementation (see diagenesis) is called metamorphism. The changes are in response to changes in temperature (thermal metamorphism), pressure and temperature (regional metamorphism), shearing stress (cataclastic metamorphism) and injections of hot fluids, usually from nearby magmatic bodies (metasomatism). Different grades of metamorphism are linked to particular pressure and temperature conditions, and marked by which minerals grow under these conditions, although the composition of the original rock largely governs what minerals will crystallize. A metamorphic rock may have undergone many such changes under different conditions, for example in regional metamorphism a rock is subjected to increasing pressure with causes different minerals to grow as the pressure and temperature conditions change (prograde metamorphism), and then if the rock is uplifted and exposed at the surface then it goes through a new series of conditions of decreasing temperature and pressure as it rises, causing new reactions to occur (retrograde metamorphism).
Metasomatism
Solid state reactions that result from hot fluids passing though a rock mass and altering the minerals and the rocks overall composition. Metasomatic minerals include tourmaline, topaz and serpentine. The fluids can be derived from igneous intrusions or be heated by passing though zones of high heat flow, and carry minerals dissolved from other areas which are then concentrated in some zones (hydrothermal systems). Some metasomatic mineral deposits include serpentinite (greenstone), clay, and gem stones such as topaz and beryl.
Meteorites
Solid objects from space which have fallen to the earths surface, without being vaporized in the atmosphere. Meteorites are usually stony or metallic and are though to be fragments of comets and similar to the material from which the earth was originally assembled. Meteors are objects that vaporize high in the earth’s atmosphere, without falling to earth. Late meteorite impacts create large craters and throw large volumes of dust and fragmented material into the atmosphere, if they land in oceans they cause large volumes of water to rise into the atmosphere. Thus large meteorite impacts have global effects on the climate.
Micas
A group of minerals called sheet silicates or phyllosilicates, where the silica tetrahedral form layers that are connected by other ions, such as potassium, sodium, iron, magnesium and aluminium. The structure means that the minerals have one perfect cleavage, and split easily into thin elastic sheets. The minerals range in colour from colourless (muscovite) to black (biotite) including various green (chlorite) and brown (phlogopite) shades. Micas are a common accessory mineral in igneous rocks, are relatively rare in sedimentary rocks, but extremely common in metamorphic rocks, especially regional metamorphic rocks, where the preferred orientation of mica (perpendicular to the maximum pressure) creates cleavage, schistosity and foliation. Large ‘books’ of mica occur in pegmatites, and are used as insulation, and in paint.
Micrite
Very fine grained (less than 4 microns) particles of calcium carbonate, usually as a matrix in a limestone, interpreted to be lithified lime mud, formed by nannofossil skeletons, direct precipitation of minute crystals, or grinding of larger calcium carbonate grains by animal boring activity.
Microfossil
Any fossils too small to be studied without the aid of a microscope, such as foraminifera, diatoms, dinoflagellates and ostracods.
Migmatite
An intimate association of igneous rock and coarsely crystalline metamorphic rock, so that it becomes difficult to distinguish between them. Migmatites may be derived from the intrusion of a magma into the layers of a metamorphic rock, involving some mingling and recrystallization of the metamorphic rock, or they may be derived from partial melting followed by resolidifcation of part of the metamorphic rock.
Milankovitch Cycles
In 1941, Milankovitch described and calculated the fluctuations in the orbit of the earth around the sun. These are the precession cycles (rotation of the earths axis; cycles have periods between 19 and 24 kyr (thousand years)), obliquity cycles (tilt of the earths axis relative to the orbital plane; periods between 40 and 54 kyr) and eccentricity cycles (how elliptical the earths orbit is, major periods around 100 and 400 kyr). These cycles affect the climate of the earth by varying the amount of sunlight to reach the atmosphere and the earth’s surface (insolation). This then causes relatively cool or relatively warm periods (there are various complicated feedback and canceling processes) which can trigger glaciations or interglacials. Studies of the recent glaciations have shown that since about 800 kyr ago, the glaciation-interglaciation fluctuations have been on 100 kyr periodicities, so are being controlled by eccentricity cycles. However, before this period climatic fluctuations were controlled by obliquity cycles. Milankovitch cycles have now been identified through much of the history of the earth, although the periodicities have changed through time as the earth-moon-sun system has evolved.
Mine
n. An underground or open-cast excavation for the extraction of mineral deposits, as distinguished from surface and small scale excavation for stones or earth which are called quarries. V. To excavate for and extract mineral deposits.
Mineral
An inorganic element or compound which has an ordered, repeated internal structure, and is characterized by specific chemical composition, crystal form, structure and physical properties.
Mineralization
Used to describe the process by which valuable minerals are deposited or crystallized within a rock, resulting in an ore deposit. There are various kinds of mineralization, including replacement of pre-existing minerals, filling of veins, cracks or fissures, metasomatism and so on. Alternatively mineralization can refer to the process of replacing organic remains by inorganic materials – a process of fossilization.
Mineralogy
The study of the formation, occurrence, properties, composition and classification of minerals.
Mohorovicic Discontinuity
The abrupt change in density at the base of the crust, the boundary between the crust and the mantle. The boundary was discovered because it marks a change in the velocity that earthquake waves travel at, and studying earthquakes led to the discovery of this layer. The discontinuity lies between 5 and 15 km beneath ocean floor, and around 35 km beneath continental crust, although up to 60 km beneath mountain ranges.
Mohs Scale
A set of ten minerals that is used to determine the hardness of unknown minerals. The scale, from softest to hardest, is talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum, diamond.
Monocline
A local increase in the dip of a surface or an anticline-syncline pair where the direction of dip on all limbs remains the same.
Monzonite
Plutonic igneous rocks which contain equal amounts of alkali (potassium rich) feldspar and plagioclase, with little or no quartz, and augite.
Moonstone
A variety of alkali feldspar (adularia) or fine intergrowths of alkali feldspar and quartz, that has a bluish to milky white colour, pearly to opaline luster, is transparent to translucent and is used as a gemstone.
Moraine
A mound or ridge of glacial debris or till, mainly unstratified and deposited as the direct result of glacial ice. Variations include terminal moraine (deposited at the front end of a glacier) and lateral moraine (deposited at the sides of a glacier). Moraines are frequently reworked by glaciers advancing over them, or by stream action redepositing the material and carrying the very fine sediment away.
Morphology
Literally, shape. Geomorphology is the study of the shape of the earth’s surface. Paleontologists study the morphology of fossils, animals or plants, and especially looking at the relationships and growth of different parts of animals or plants.
Mud
A wet mixture of silt and clay sized particles, generally containing more than 10% clay and less than 50%.
Mud Cracks
Cracks in the surface of mud, clay or silt, in an irregular or roughly polygonal shape, and caused by shrinkage associated with drying at the surface (dessication cracks), or shrinkage associated with changes in salinity (syneresis cracks).
Mudflow
See earthflows.
Mudstone
Mud that has been lithified.
Mylonite
A rock produced by cataclastic metamorphism, where the original rock
has been intensely sheared and ground up by large or long lasting shear
stresses. Mylonites are produced in deep, extensive or old fault systems.