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USGS Geology in the Parks

Geologic Glossary

This glossary contains simplified definitions for technical terms used within this site.
For more complete, technical definitions, click here.

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

mafic

A term used to describe minerals or igneous rocks that are rich in iron and/or magnesium. Mafic igneous rocks have a high percentage of dark-colored (mafic) minerals.

magma

Molten rock. Magma may be completely liquid or a mixture of liquid rock, dissolved gases and crystals. Molten rock that flows out onto the Earth's surface is called lava.

magma chamber

A body of molten rock and solid crystal mush beneath the Earth's surface. When this chamber cools and solidifies, it is called a pluton.

magnetic anomalies

See magnetic reversals.

magnetic reversals

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!

magnetite

Iron oxide mineral (Fe3O4). Usually tiny black, metallic crystals. Magnetite will attract a magnet and sometimes, in a rock, a hiker's compass needle.

magnitude

A measure of the total amount of energy released by an earthquake.
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magnetometer

Device for measuring magnetism.

mantle

The layer of the Earth below the crust and above the core. The uppermost part of the mantle is rigid and, along with the crust, forms the 'plates' of plate tectonics. The mantle is made up of dense, iron and magnesium rich (ultramafic) rock such as dunite and peridotite.
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marble

A metamorphic rock of made of calcium carbonate. Marble forms from limestone by metamorphic recrystallization.

mass wasting

Movement of rock and soil downslope under the influence of gravity.

matrix

Fine-grained material surrounding larger grains in a sedimentary rock.
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mechanical weathering

The mechanical break-up or disintegration of rock into smaller fragments.
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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.

metaconglomerate

Metamorphosed conglomerate.

metamorphic rock

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.

mica

Group of silicate minerals composed of varying amounts of aluminum, potassium, magnesium, iron and water. All micas form flat, plate-like crystals. Crystals cleave into smooth flakes. Biotite is dark, black or brown mica; muscovite is light-colored or clear mica.
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micaceous

A general term for mica-rich rocks.

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 forms in one of two ways. The metamorphic rock may be heated enough to partially melt, but not completely. The molten minerals 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 dikes.

Migmatites are usually conspicuously mixed light- and dark-colored rocks, generally formed of dark-colored gneiss and/or schist and lighter igneous intrusive dikes and sills intruded during deformation; they look to have been mushed around while still hot and plastic, and display prominent folding.

mineral

A naturally occurring chemical compound or limited mixture of chemical compounds. Minerals generally form crystals and have specific physical and chemical properties which can be used to identify them.
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mineralization

The formation of minerals. New minerals may be added to fractures and empty spaces in a rock or by replacing preexisting minerals with different ones.

mineralogy

The study of minerals.

Miocene

An Epoch that includes the time interval of about 23.7 to 5.3 million years ago.
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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.

moraine

A hill-like pile of rock rubble located on or deposited by a glacier. An end moraine forms 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. See till.

morphology

The study of shape or form. See geomorphology.

mud

Wet clay and silt-rich sediment.

mudflow

Same as debris flow.

mudstone

A very fine-grained sedimentary rock formed from mud.

muscovite

One of the mica family of minerals. Muscovite is light-colored or clear mica, sometimes called isingglass.
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N

nonconformity

An type of unconformity in which young sedimentary rocks lie on top of older metamorphic or intrusive igneous rocks.

nonsilicate minerals

A mineral without silicon (Si). See silicate minerals.

normal fault

A fault that drops rock on one side of the fault down relative to the other side.

nuee ardente

This is a type of volcanic activity whose name is french, meaning firey cloud. Associated with pyroclastic flows, they are very hot, sometimes glowing clouds of gas and volcanic debris that is forcefully ejected down the sides of a volcano.

normal polarity

A magnetic field produced by the Earth that is the same as exists today.

 

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