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Geological Terms Beginning With "M"



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Mafic

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

Magma

Molten rock material that occurs below Earth's surface.

Magma Chamber

Magma Chamber

A full or emptied magma reservoir in the shallow portion of the lithosphere.

Magmatic Water

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

Magnetic Anomaly

An increase or decrease in the local magnetic field compared to the normally expected value.

Magnetic Declination

Magnetic Declination

The horizontal angular difference between True North and Magnetic North.

Magnetic Inclination

Magnetic Inclination

The vertical angular difference between a horizontal plane and the orientation of Earth's magnetic field.

Magnetic North

Magnetic North

The direction that a compass points. The location where Earth's magnetic field dips vertically into the Earth.

Magnetic Reversal

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

Magnetic Stratigraphy

The correlation of rock units and study of Earth's history using magnetic events and magnetic epochs as a time reference.

Magnetometer

Magnetometer

An instrument designed to measure the strength and character of Earth's magnetic field.

Magnitude

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

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

Mantle

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

Mantle Plume

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

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

Massive

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

Mass Wasting (also Mass Movement)

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

Mcf

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

MMcf

MMcf

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

Meandering Stream

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

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

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

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

Metamorphism

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

Meteoric Water

Meteoric Water

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

Meteor

Meteor

A streak of light momentarilly visible in the night sky when a meteoroid penetrates Earth's atmosphere. The meteoroid is travelling at a speed of at least 20 kilometers per second. That great speed causes it to impact air molecules with enough force to heat it to a glowing temperature and to vaporize particles from its surface. A trail of these hot particles is left behind the meteoroid. Because of their high temperature they glow momentarily, producing a streak of light. This streak of light is known as a "shooting star" or a "meteor".

Meteorite

Meteorite

A particle of iron or rock that has fallen to Earth's surface from inter-planetary space. They sometimes have concave pits in their surface caused by ablation as they fall through the atmosphere. Meteorites are also found on the Moon and other bodies in our solar system. In fact, the NASA Mars Rovers have found many Martian meteorites.

Meteoroid

Meteoroid

A particle of iron or rock found in inter-planetary space. Distinguished from planets or asteroids by its much smaller size.

Microseism

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

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

Milling Capacity

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

Mineral

Mineral

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

Mineral Interest

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

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

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

Mineralogy

The study of minerals - their composition, structure, formation, uses, properties, occurrence and geographic distribution.

Mohorovicic Discontinuity

Mohorovicic Discontinuity

The boundary between the crust and the mantle. Frequently referred to as the Moho.

Mohs Hardness Scale

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

Monocline

An area of increased dip in otherwise gently dipping strata.

Moraine

Moraine

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

Mountain

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

Mudflow

A downslope movement of wet soil and rock debris that is mostly composed of clay-size particles and water. At the base of the slope the flow spreads out over the run-out area in the form of a lobe. The internal particle motion is that of a flow rather than of a rotating or translating mass. Many mudflows move at a rate of a few feet per year or less but some reach speeds in excess of 60 miles per hour. Movement is usually triggered during times of heavy rainfall or rapid snow melt.

Mudstone

Mudstone

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

Multiple Completion Well

Multiple Completion Well

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

>M.Y.

>M.Y.

Million years - abbreviation.

M.Y.A.

M.Y.A.

Million years ago - abbreviation.

Mylonite

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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