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Geological Terms Beginning With "M"



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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:

Molten rock material that occurs below Earth's surface.

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:

A major subdivision of Earth's internal structure. Located between the base of the crust and overlying the core.

Mantle Plume:

A rising mass of hot mantle material that can create an area of volcanic activity in the center of a lithospheric plate.

Marble:

A non-foliated metamorphic rock that is produced from the metamorphism of limestone. It is composed primarily of calcium carbonate. Picture of Marble.

Massive:

A term used in reference to a rock unit that is homogeneous in texture, fabric and appearance.

Mass Wasting (also Mass Movement):

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.

Mcf:

One thousand cubic feet - the standard sales volume for natural gas.

MMcf:

One million cubic feet - the standard reporting volume for daily natural gas well production.

Meandering Stream:

A stream that has many bends (meanders). This type of drainage pattern usually develops on a nearly level landscape and where the banks of the stream are easily eroded.

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.

Metamorphism:

Alteration of the minerals, textures and composition of a rock caused by exposure to heat, pressure and chemical actions.

Meteoric Water:

Water from the atmosphere, such as rain, snow, hail, or sleet.

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.

Milling:

The activities of preparing an ore for market. This can include: crushing, grinding, concentration, separation of impurities, and conversion into a transportable state.

Milling Capacity:

The maximum amount of material that a mill can produce in a unit of time.

Mineral:

A naturally occurring, inorganic solid with a definite chemical composition and an ordered internal structure.

Mineral Interest:

An ownership, lease, concession, or other contractual interest that gives a party the right to explore and extract mineral resources on a property.

Mineral Lease:

A contract in which a mineral interest owner conveys to another party a right to explore for, develop, and produce mineral resources. The lessee acquires a working interest and the lessor retains a royalty interest of a specified percentage.

Mineral Rights:

The ownership of rocks, minerals and fluids beneath an area of land. The owner has the freedom to sell, lease, gift or bequest these rights individually or entirely to others. 

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.

Moraine:

A mound, ridge or ground covering of unstratified and unsorted till, deposited by ice action or by melting away of a glacier.

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.

Mudflow:

A type of mass movement composed mainly of clay-size materials with a high enough water content that it flows readily..

Mudstone:

A sedimentary rock composed of clay-size particles but lacking the stratified structure that is characteristic of a shale.

Multiple Completion Well:

A well equipped to produce oil and/or gas from more than one reservoir.

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.

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