Glossary

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M

mafic

Igneous rocks that contain a group of dark-colored minerals, with relatively high concentrations of magnesium and iron compared to felsic igneous rocks.

magma

Molten rock located below the surface of the Earth. Magma can cool beneath the surface to form intrusive igneous rocks. However, if magma rises to the surface without cooling enough to crystallize, it might break through the crust at the surface to form lava.

magnetic

Affected by or capable of producing a magnetic field.

magnetite

A mineral form of iron oxide (Fe3O4). It is the most magnetic naturally occurring mineral. The molecules in magnetite align with the North and South poles when rocks containing magnetite ore are formed. By examining the alignment today, scientists can reconstruct how the rocks have moved since their formation, giving them clues about the previous arrangement of the continents.
Magnetite lodestones were used as an early form of compass. Huge deposits of magnetite have been found in Precambrian banded iron formations.

magnitude (earthquake)

A logarithmic scale used to measure the seismic energy released by an earthquake. Magnitudes follow a numerical scale, with M1 earthquakes classified as micro, M2 earthquakes classed as minor, and earthquakes of M8 or greater being classified as great.

mammoth

An extinct terrestrial vertebrate animal belonging to the Order Proboscidea of the Class Mammalia. Mammoths are from the same line of proboscideans that gave rise to African and Asian elephants. They had tall bodies with a rather high “domed” skull, and teeth with numerous parallel rows of ridges. Mammoths are among the most common Pleistocene vertebrate fossils in North America, Europe, and Asia.

manganese

A metallic chemical element (Mn). Manganese is used in the production of steel.

mantle

The layer of the Earth between the crust and core. It consists of solid silicate rocks that, over long intervals of time, flow like a highly viscous liquid. Convection currents within the mantle drive the motion of plate tectonics.

marble

A metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite. Not everything commercially called a marble is “true marble,” which lacks fossils and is recrystallized from limestone or dolostone.

marblenotes

The limestone in northwestern Vermont was only very weakly metamorphosed, if at all. Actual marble is found between Manchester and Middlebury Vermont, and parts of western Massachusetts and Connecticut. Green serpentine rock from the Ultramafic Belt is also quarried and sold as a marble under the name Verde Antique. Serpentine is hardly a marble. It was formed from the metamorphism of slices of oceanic crust and upper mantle that were scraped off of the oceanic plate being subducted beneath the North American continent during the Ordovician. The Ultramafic Belt, stretching the length of the Appalachian Mountains along the Taconic volcanic island suture zone, also has concentrations of the soft rock soapstone, made primarily of the mineral talc.

marl

A fine-grained sedimentary rock consisting of clay minerals, calcite and/or aragonite, and silt.

mass extinction

The extinction of a large percentage of the Earth’s species over a relatively short span of geologic time.
Unfortunately, this is not just a phenomenon of the past: it is estimated that the extinction rate on Earth right now may be as much as 1000 times higher than normal, and that we are currently experiencing a mass extinction event.

mass wasting

A process in which soil and rock move down a slope in a large mass. This can occur both on land (such as a landslide) or underwater (such as a turbidity current).

mastodon

An extinct terrestrial vertebrate animal belonging to the Order Proboscidea of the Class Mammalia, and characterized by an elephant-like shape and size, and massive molar teeth with conical projections. Mastodons are among the most common Pleistocene vertebrate fossils in North America.

matrix

A fine-grained mass of material around and embedding larger grains or crystals. The term matrix can also describe sediment or rock in which a fossil is embedded.

megashear

A large shear, typically tens to hundreds of kilometers (miles) in length, formed when rocks have been continuously fractured due to compressive stress.

megathrust

Powerful earthquakes occurring at subduction zones, where one plate is forced beneath another. Since 1990, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes.
See also: earthquake, subduction

mlange

A mixture of fragmented rocks produced in a subduction zone.
See also: subduction

mesosphere

The solid part of the Earth’s mantle lying between the asthenosphere and the core.See also asthenosphere.

Mesozoic

A geologic time period that spans from 252 to 66 million years ago. This period is also called the “age of reptiles” since dinosaurs and other reptiles dominated both marine and terrestrial ecosystems. During this time, the last of the Earth’s major supercontinents, Pangaea, formed and later broke up, producing the Earth’s current geography.
The Mesozoic contains the Triassic, Jurassic, and Cretaceous periods.

metamorphism, metamorphic rocks

Rocks formed by the recrystallization and realignment of minerals in pre-existing sedimentary, igneous, and metamorphic rocks when exposed to high enough temperature and/or pressure. This can be a result of plate movements, very deep burial, or contact with molten rock or superheated water. This process destroys many features in the rock that would have revealed its previous history, transforming it into an entirely new form.
Tectonic forces can cause minerals to realign perpendicularly to the direction of pressure, layering them in a pattern called foliation, as exemplified in gneiss and schist. Recrystallization, as seen in marble and quartzite, results as rock is heated to high temperatures, and individual grains reform as interlocking crystals, making the resulting metamorphic rock harder than its parent rock.

metasedimentary

A sediment or sedimentary rock which shows evidence of metamorphism.See also metamorphism, sedimentary rocks.

meteorite

A stony or metallic mass of matter that has fallen to the Earth’s surface from outer space.

methane

The simplest alkane (CH4) and the main constituent of natural gas, a colorless, odorless, flammable gas that is used as fuel.

mica

A large group of sheet-like silicate minerals.

Michigan Basin

An inland basin centered on Michigan’s Lower Peninsula, which formed when Baltica approached North America in the Ordovician.
The rocks of the Michigan Basin are a commercial source of petroleum.

Michigan Formation

A ring-like stratum in the rock of the Michigan Basin, where most of the state’s gypsum is mined. It can be found in a nearly continuous band around the center of the Lower Peninsula.
See also: gypsum, Michigan Basin

microcontinent

A piece of continental crust, usually rifted away from a larger continent. Microcontinents and other smaller fragments of continental crust ( terranes ) each had their own, often complex, geologic history before they were tacked onto the margin of another continent.

Milankovitch Cycles

Cyclical changes in the amount of heat received from the sun, associated with how the Earth’s orbit, tilt, and wobble alter its position with respect to the sun. These changes affect the global climate, most notably alterations of glacial and interglacial intervals.

mineral

A naturally occurring inorganic solid with a specific chemical composition and a well-developed crystalline structure. Minerals are identified based on their physical properties, including hardness, luster, color, crystal form, cleavage, density, and streak.
There are over 4900 identified minerals. However, the number of common rock-forming minerals is much smaller. The most common minerals that form igneous, metamorphic, and sedimentary rocks include quartz, feldspars, micas, pyroxenes, and amphiboles.

mineral deposit

Although quartz occurs in several colors, it is most commonly white, gray or clear. Feldspar may be a variety of colors, including pink, white, and black or gray. Mica, a thinly sheeted, flaky mineral, is most commonly either light in color (muscovite) or black (biotite). Pyroxene and amphibole are dark green to black, generally needle-like crystals.

mineral deposit

An accumulation of a mineral on the surface or in the Earth’s interior that results from geological processes and whose quantity, quality, and conditions of bedding make it suitable for industrial use.

mineral deposit

See also feldspar, igneous rocks, metamorphism, mineral, quartz, sedimentary rocks, weathering deposit.

mineral deposit

The most common minerals that form igneous, metamorphic and sedimentary rocks (and the ones that you will most commonly see) include quartz, feldspar, micas, pyroxenes, and amphiboles.

mineralogy

The branch of geology that includes study of the chemical and physical properties and formation of minerals.

Miocene

A geologic time unit extending from 23 to 5 million years ago. During the Miocene, the Earth experienced a series of ice ages, and hominid species diversified. The Miocene is the first epoch of the Neogene period.

mirabilite

A saline evaporite mineral, sodium sulfate (NaSO4), also known as “Glauber salts” in its processed form. This mineral is used in the manufacture of detergents, paper, and chemical processing, especially in the production of hydrochloric and sulfuric acids.

Mississippi Embayment

A topographically low-lying basin in the south-central United States, stretching from Illinois to Louisiana. The Mississippi Embayment originated as far back as the Precambrian, during the breakup of Rodinia. During this time, many smaller rifts in the crust formed adjacent to the major rift that split away North America—one of these smaller rifts is located beneath the modern day Mississippi Embayment.
During the breakup of Pangaea, the area subsided, forming a trough that was flooded during the Cretaceous. When sea level fell, the Mississippi River was born. Thousands of meters of Cretaceous to Recent sediment were deposited in the river valley. Recurrent activity along faults associated with the deeply buried ancient rifts beneath the embayment caused the 1811 - 1812 New Madrid Earthquakes, one of the largest earthquakes ever recorded in North America.

Mississippian

A subperiod of the Carboniferous, spanning from 359 to 323 million years ago.

Mohs Scale of Hardness

The scale of relative hardness of minerals, developed by the Austrian mineralogist Frederich Mohs in 1824. The scale is very useful as a means for identifying minerals or quickly determining hardness. A piece of glass has a hardness of approximately 5 on the scale; our fingernails are just over 2; a knife blade is just over 5. Diamond ranks at 10 as the hardest mineral.

Mollisols

A soil order ; these are agricultural soils made highly productive due to a very fertile, organic-rich surface layer.

mollusk (var. mollusc)

A marine, freshwater, or terrestrial invertebrate animal belonging to the phylum Mollusca, and characterized by a hard, calcareous shell secreted by a specialized tissue (mantle) that encloses a cavity for respiration and excretion, and a rasp-like feeding ribbon (radula).

mollusk (var. mollusc)

See also bivalve, cephalopod, gastropod.

mollusk (var. mollusc)

This diverse group includes clams, snails, slugs, chitons, tusk shells, octopuses, squid, and the Chambered Nautilus).

molybdenum

A metallic chemical element (Mo) which has the sixth-highest melting point of any element at 2623°C (4753°F). Molybdenum is mainly used in the creation of alloys, such as stainless steel and cast iron, and its strong ability to withstand heat makes it useful in applications that utilize extreme heat, such as the manufacture of motors and aircraft parts.

monadnock

An isolated hill or small mountain on a plain, formed from rock more resistant to erosion than that of the rest of the surrounding landscape. These features are named after Mount Monadnock (New Hampshire), which is made of schist and quartzite.

monoplacophoran

Any mollusk belonging to the class Monoplacophora, characterized by serially repeating organs and roughly bilateral symmetry. Once known only from fossils of the Paleozoic era, living monoplacophorans were discovered in 1952. They are typically found in the deep ocean.

Monterey Formation

A distinctive light-colored sedimentary rock unit that formed in the Miocene seas. Its buff color comes from its high silica content, derived from microfossils such as diatoms. Outcrops from the Monterey Formation are visible along California’s coast and peninsula, and on some of the offshore islands. It is composed primarily of shale, and it is the source rock for most of California’s oil.
See also: Miocene, oil, sedimentary rock, shale, silica

moraine

An accumulation of unconsolidated glacial debris ( soil and rock) that can occur in currently glaciated and formerly glaciated regions, such as those areas acted upon by a past ice age. The debris is scraped from the ground and pushed forward by the glacier, to be left behind when the ice melts. Thus, many moraines mark the terminus or edge of a glacier. Lateral moraines can also occur in between and at the sides of glaciers or ice lobes.

Morton Gneiss

The oldest rock in the United States. It was formed 3.6 billion years ago, during the Archean.
See also: Archean, gneiss

mosasaur

An extinct, carnivorous, marine vertebrate reptile. Mosasaurs were characterized by a streamlined body for swimming, a powerful fluked tail, and reduced, paddle-like limbs. They were common in Cretaceous seas and were were powerful swimmers, reaching 12 - 18 meters (40 - 59 feet) in length.

mysticete

A member of the group of whales (cetaceans), such as finback and humpback whales, which have baleen for feeding upon some combination of plankton, krill, and small fish. The earliest mysticetes occur in the late Eocene.