Macroclimate - The climatic characteristics over a region with an extensive spatial area. Compare with microclimate.
Macroflora - Plants occupying an area that are large enough to be seen without the aid of additional optical magnification. Compare with microflora.
Macrofauna - Animals occupying an area that are large enough to be seen without the aid of additional optical magnification. Compare with microfauna.
Macrofossil - A fossil that is large enough to be seen without the aid of optical magnification. Compare with microfossil.
Macrometeorology - The scientific study of the atmosphere and its associated phenomena on a large scale. A sub-discipline of meteorology.
Macronutrient - A nutritional element required by an organism in relatively large quantities. Compare with a micronutrient.
Maelstrom - A strong whirlpool or current in a stream or ocean waters.
Mafic - A rock that is rich in calcium, magnesium, and iron content.
Mafic Magma - A type of magma that is relatively poor in silica (45% to 52%) but rich in calcium, magnesium, and iron content. This type of magma solidifies to form dark-colored igneous rocks rich in calcium, magnesium, and iron but relatively poor in silica.
Magma - The molten rock found beneath the Earth's surface. Besides molten rock, this substance also contains suspended mineral crystals, dissolved gas, and gas bubbles. Magmas can range in temperature from 700°C to 1300°C (1300°F to 2400°F). Geologists recognize four types of magma (in brackets, the typical igneous rocks formed from these magmas): ultramafic (picritic), mafic (basalt), intermediate (andesite), and felsic (rhyolite). Compare with lava.
Magma Plume - A subsurface mass of vertically rising magma originating from the mantle.
Magmatic Water - See juvenile water.
Magnesium Limestone - An old British term used to describe dolomite.
Magnesite - A mineral with the chemical formula MgCO3 (magnesium carbonate). Magnesite is often found in ultramafic rocks.
Magnetic Anomaly - A local variation in the Earth's magnetic field often caused by interference generated by the magnetism or chemical composition of rocks found at this location.
Magnetic Declination - The horizontal angle between True North and Magnetic North or True South and Magnetic South.
Magnetic Field - The space influenced by magnetic force. The magnetic field is made visible with iron filings on a piece of paper. Magnetic fields are measured by the force they exert on charged subatomic particles, such as electrons. Magnetic fields have both a direction and a magnitude. Many celestial bodies in the Universe, including planets and stars, have magnetic fields. The Earth's magnetic field is believed to be generated by a process at the planet's core. The process that generates Earth's magnetic field is called a dynamo. A dynamo is a mechanical device that creates an electrical current through motion. The Earth's dynamo is created by a zone of liquid iron, nickel, and small amounts of other metals, about 2000 kilometers (1240 miles) thick, that moves dynamically above a hot (5,700°C) solid core made mainly of iron. The movement of the liquid metal is caused by two things: convection currents and the Earth's rotation on its axis. This movement of liquid iron generates circulating electric currents that create the Earth's magnetic field.
Magnetic Inclination - Compass angle made relative to the horizontal because of the Earth's magnetic field. Also called magnetic dip.
Magnetic Meridian - An imaginary line approximating a great circle connecting the South Magnetic Pole and North Magnetic Pole.
Magnetic North - See North Magnetic Pole.
Magnetic Reversal - A change in the polarity of the Earth's magnetic field. There have been nine magnetic reversals in the past 4 million years.
Magnetic South - See South Magnetic Pole.
Magnetic Storm - A disturbance in the Earth's atmosphere caused when solar wind and the charged particles it carries interact with the magnetosphere and magnetic field (Image Source: Wikimedia Commons, NASA). These disturbances can last for hours to several days. Magnetic storms can damage human electrical systems, including satellites, radio communications, power transmission systems, and navigation systems. In the last 200 years, two very severe magnetic storms have occurred: a storm from August 28 to September 2, 1859, and the second on March 13, 1989. The more recent event caused the collapse of the Hydro-Québec power grid, damaged some computers, and impaired several satellites in orbit.
Magnetic Stripes - Alternating parallel bands of differing polarity found in rocks moving away from mid-oceanic ridges. Magnetic stripes are a byproduct of sea-floor spreading.
Magnetic Survey - A tool used by geophysicists to create subsurface images of the spatial anomalies in the Earth's magnetic field. Magnetic surveys are commonly used in mineral, oil, and gas exploration.
Magnetite - A black or brownish-black, metallic mineral with the chemical formula Fe3O4. This magnetic mineral is one of the main types of iron ore and can be found in most igneous and metamorphic rocks. Also called loadstone.
Magnetometer - An instrument used to measure the direction, strength, or relative variation in a magnetic field. One common use of this instrument is to measure the Earth's magnetic field at a specific location.
Magnetosphere - The zone that surrounds the surface of the Earth that is influenced by the Earth's magnetic field.
Magnitude - (1) The quantifiable size of a natural event. (2) A quantitative measure of the size of an earthquake using the Richter scale.
Mammal - A group of warm-blooded vertebrate animals. Common characteristics of these organisms include hair, milk secretion, a diaphragm for respiration, a lower jaw composed of a single pair of bones, a middle ear containing three bones, and a single left systemic arch. See the Encyclopedia of Life for more information on this group of organisms. https://eol.org/pages/1642
Mammilated Surface - A smoothed and rounded rock surface due to the effects of erosion.
Manganese - A chemical element that makes up about 0.1% of the Earth's crust. It has an atomic number of 25 and is represented chemically with the symbol Mn. Manganese is often found in chemical combination with iron, forming various minerals. It has many industrial uses.
Mangrove - A wetland where specially adapted trees are the dominant plants. Mangroves are commonly located on the coastlines of warm tropical climates. There are about 110 species of mangrove shrubs and trees. One genus that has several representative species is Rhizophora. The World Mangrove Atlas, published by the United Nations Environment Programme (UNEP) in 2010, suggests that about 20 percent of the world's mangrove ecosystems have been lost since 1980.
Manning Equation - An empirical equation used in engineering to estimate the velocity of a liquid flowing uniformly in an open channel.
Mantle - A layer in the Earth's interior composed mostly of solid rock rich in silica. The mantle can, at times, be like a very viscous liquid due to tectonic activity and in localized rising mantle plumes. The mantle extends from the base of the crust to a depth of about 2,900 kilometers (1,800 miles) and accounts for about 84% of our planet's volume. Geologists have divided the mantle into several zones that are based on seismic measurements. These zones include the upper mantle beginning at the base of the Earth's crust at 7 to 35km (4.3 to 22miles) down to a depth of about 410km (250mi), a transition zone from 410 to 660km (250 to 410miles), the lower mantle from 660 to 2,900km (410 to 1,800miles), and finally a core/mantle transition zone with a thickness of about 200km (120mi). There are also a few places on Earth where mantle rock has reached the surface because of tectonic activity, such as the Tablelands region of Gros Morne National Park in Canada. The term mantle can also refer to the interior of another planet.
Mantle Plume - A localized column of hot rock that rises from the base of the mantle to the crust because of convection. The Hawaiian Island chain formed because of a mantle plume melting upward through oceanic crust. On continental crust, mantle plumes are believed to be responsible for basalt plateaus. Compare with the term hot spot.
Map - An abstraction of the real world that is used to depict, analyze, store, and communicate spatially organized information about physical and cultural phenomena important to humans.
Map Projection - A cartographic technique used to represent the three-dimensional surface of the Earth or some other celestial body (like a planet) onto a two-dimensional map. This process creates a distortion artifact on the map due to the projection of the three-dimensional surface onto a two-dimensional plane.
Map Reference - A grid coordinate system for locating a point on the Earth's surface. The most commonly used method is the Universal Transverse Mercator grid system. Also called a grid reference.
Map Scale - Ratio between the distance between two points found on a map compared to the actual distance between these points in the real world.
Marble - A metamorphic rock created by the recrystallization of calcite and/or dolomite.
March Equinox - One of two days during a year when the Sun's declination is at the equator. The March Equinox denotes the first day of spring in the Northern Hemisphere. In the Southern Hemisphere, this date marks the first day of fall. During the March Equinox, all locations on the Earth (except the poles) experience equal (12-hour) day and night. The March Equinox occurs on either March 20 or 21.
Margalitic - A soil A horizon that is rich in calcium and magnesium, giving it a high base cation status.
Marginal Channel - A meltwater-created stream that is found at the base of a glacier. Also called a meltwater stream.
Marginal Deep - An ocean trench located on the seaward side of an island.
Marginal Depression - An area of depressed land located around the edge of an inselberg.
Marginal Sea - An area of ocean that is partially enclosed by the coastline of islands or continents, yet part of it is still open to a major ocean. Some marginal seas include the Arabian Sea, Baltic Sea, Bay of Bengal, Bering Sea, Beaufort Sea, Black Sea, Gulf of California, Gulf of Mexico, Mediterranean Sea, Red Sea, Ross Sea, and the Weddell Sea.
Marine - With reference to ocean environments, processes, and things.
Marine Abrasion - The abrasion action caused by waves striking the shoreline. This process is aided by the sand and other solid materials suspended in seawater.
Marine Pollution - A type of pollution that occurs in a marine environment.
Maritime Climate - The typical climate of locations found on or near an ocean. Seasonal variations in temperature are small due to the thermal characteristics of large bodies of water. In middle and high latitudes, summers and winters tend to be significantly milder than those in the continental interior. Maritime climates generally experience more precipitation and fog due to their proximity to significant moisture sources. See maritime effect. Compare with a continental climate.
Maritime Effect - The effect that large ocean bodies have on the weather and climate of locations or regions. This effect results in a lower range in surface air temperature at both daily and annual scales. Compare with a continental effect.
Maritime Equatorial Air Mass (mE) - An air mass that forms over extensive ocean areas near the equator. Maritime Equatorial air masses are hot and humid year-round. These air masses are generally unstable and produce thunderstorms in the tropics. On weather maps, the symbol mE is used to identify a Maritime Equatorial air mass. Also see Continental Arctic air mass, Continental Antarctic air mass, Continental Polar air mass, Maritime Polar air mass, Maritime Tropical air mass, and Continental Tropical air mass.
Maritime Polar Air Mass (mP) - An air mass that forms over extensive ocean areas of the middle to high latitudes. Around North America, these air mass systems form over the Atlantic and Pacific Oceans at middle latitudes. Maritime Polar air masses are mild and humid in summer and cool and humid in winter. In the Northern Hemisphere, maritime polar air masses are normally unstable during the winter. In the summer, atmospheric stability depends on the position of the air mass relative to a continent. Around North America, Maritime Polar air masses found over the Atlantic are stable in summer, while Pacific systems tend to be unstable. On weather maps, the symbol mP is used to identify a Maritime Polar air mass. Also see Continental Arctic air mass, Continental Antarctic air mass, Continental Polar air mass, Maritime Tropical air mass, Continental Tropical air mass, and Maritime Equatorial air mass.
Maritime Tropical Air Mass (mT) - An air mass that forms over extensive ocean areas of the low latitudes. Around North America, these systems form over the Gulf of Mexico and the eastern tropical Pacific. Maritime Tropical air masses are warm and humid in both winter and summer. In the Northern Hemisphere, maritime tropical air masses are normally stable throughout the year if they form just west of a continent. If they form just east of a continent, these air masses will be unstable in both winter and summer. On weather maps, the symbol mT indicates a Maritime Tropical air mass. Also see Continental Arctic air mass, Continental Antarctic air mass, Continental Polar air mass, Maritime Polar air mass, Continental Tropical air mass, and Maritime Equatorial air mass.
Marker Horizon - A stratigraphic unit (bed) that contains a distinctive composition and appearance and is found in more than one location over an extensive geographical area. Marker horizons are often used to date the relative timing of past geologic events.
Markov Process - A cause and effect process where the probabilistic occurrence of an event in the future depends on the present state of the system. It was named after the Russian mathematician Andreyevich Markov.
Marl - A mud or mudstone that is rich in calcium carbonate or other calcium-based compounds. The individual particles found in marl are usually clay and silt.
Marsh - A type of wetland ecosystem that can be found along the edge of a watercourse (stream) or water body (lake, sea, or ocean), and is dominated by herbaceous rather than woody plants.
Marsh Gas - Methane emissions produced by organic matter decomposition in a marsh, wetland, swamp, or bog. Marsh gas may also contain carbon dioxide and hydrogen sulfide emissions. Also called swamp gas.
Mass - Refers to the amount of material found in an object (usually of unit volume).
Mass Balance - The relative balance between the input and output of material within a system. This term is often used to describe the flow dynamics of glaciers.
Mass Extinction - A catastrophic, widespread perturbation where major groups of species become extinct in a relatively short time compared to normal background extinctions.
Mass Movement - A general term that describes the downslope movement of sediment, soil, and rock material.
Mass Number - Total number of neutrons and protons in the nucleus of an atom. An approximate measure of the mass of an atom. Also see atomic number.
Mass Strength - A quantitative measure of a mass of rock's resistance to deformation and fracture. Mass strength measurements are used in engineering to determine the safety of slopes, building foundations, and underground excavations.
Mass Wasting - A general term that describes the downslope movement of sediment, soil, and rock material.
Mathematical Model - A mathematical representation of a system from which predictions or inferences can be made.
Matric Force - The force that holds soil water from 0.0002 to 0.06 millimeters (0.0000079 to 0.0024 inches) from the surface of soil particles. This force is due to two processes: molecular attraction (adhesion and absorption) between soil particles and water, and the cohesion between water molecules. This force decreases with increasing distance from the soil particle. The force becomes nonexistent past 0.06 millimeters (0.0024 inches).
Matter - Is the material (atoms and molecules) that constructs things on the Earth and in the Universe.
Mature Soil - A soil that is in equilibrium with the soil-forming factors in its environment. Mature soils have well-developed soil horizons.
Maunder Minimum - The period from 1645 to 1715 during which the Sun had very little sunspot activity.
Maximum Minimum Temperature System (MMTS) - A device that records the maximum and minimum temperatures over a set time interval, usually 24 hours (midnight to midnight). MMTS are usually electronic devices that employ a thermistor, a type of electronic resistor sensitive to temperature changes. In a thermistor, the resistance to the flow of electricity changes predictably with temperature, and this change can be calibrated, measured, and recorded.
Maximum Thermometer - A meteorological thermometer designed to record the maximum temperature over a set time interval, usually 24 hours (midnight to midnight). Liquid-in-glass type maximum thermometers have a bore that narrows between the reserve bulb and the graduated portion of the glass stem. With rising temperature, the mercury in the reserve bulb pushes past the constriction and up into the graduated section as long as the temperature continues to rise. The mercury in the graduated section does not fall back into the reserve bulb due to the constriction, and as a result, the highest temperature is recorded. Compare with a minimum thermometer.
MDC - See More Developed Country.
Mean - Statistical measure of central tendency in a set of data. The mean is calculated by adding all of the data values and dividing this quantity by the total number of data values. Also called the average. Compare with median and mode.
Mean Daily Temperature - The average temperature for one day (24 hours), normally from midnight to midnight. Two techniques are commonly used for this calculation: [(Maximum daily temperature + Minimum daily temperature)/2], and the average of the 24 hourly temperatures over the length of a day.
Mean Sea Level - The average height of the ocean surface as determined from the mean of all tidal levels recorded at hourly intervals.
Mean Solar Day - The time it takes to complete one Earth rotation relative to the position of the Sun (for example, from midnight to midnight). This measurement takes 24 hours and is longer than a sidereal day because it includes the effect of the Earth's movement (Earth revolution) around the Sun.
Meander - A sinuous-shaped stream channel. Usually found in streams flowing over a very shallow elevation grade.
Meander Scar - A crescent-shaped depression in the landscape created in the past by the erosional activity of a meandering stream. Often associated with an oxbow lake.
Meander Terrace - An elevated platform in the landscape formed when a meandering stream channel eroded down into the floodplain.
Meandering Valley - A stream valley that zigzags laterally along its length through a landscape and contains within it a meandering stream. The valley's sinuous course it the result of erosional effects of the meandering stream.
Mechanical Weathering - See physical weathering.
Medial Moraine - A significant deposit of eroded sediment found down the center of a glacier. Medial moraines are created when two glaciers and their lateral moraines merge.
Median - A statistical measure of central tendency in a set of data. The median is the value halfway through a data set when the values are ordered from lowest to highest. In an even data set, the median is the average of the two halfway values. Compare with a mean and a mode.
Medieval Warm Period - A period of relatively warmer climate during the Holocene that occurred in northern Europe, the North Atlantic Ocean, southern Greenland, and Iceland from about 900 to 1200 CE.
Mediterranean Climate - Is a mid-latitude climate found at a latitude of about 30 to 40° on the western coasts of the continents. This climate is characterized by having dry, hot to warm summers because of the regional dominance of the Subtropical High Pressure Zone and continental tropical air masses. In winter, temperatures cool and precipitation increases due to the seasonal migration of maritime polar air masses and associated mid-latitude cyclones. The vegetation here is highly adapted to withstand summer drought.
Mediterranean Front - A front that forms in the winter months over the Mediterranean that is associated with mid-latitude cyclone activity, strong winds, cloud cover, and precipitation. Air masses associated with this front include Continental Tropical air from northern Africa, Maritime Polar air from the Atlantic, and Continental Polar air from northern Europe.
Mediterranean Scrubland - See Chaparral.
Megatherm - A plant that requires very warm temperatures (no month has a mean temperature less than 18°C) and abundant moisture (evapotranspiration is never greater than additions from rainfall) to maintain normal growth.
Megathermal Climate - A tropical or subtropical climate where, over the year, no month has a mean temperature below 18°C.
Melting - The physical process of a solid becoming a liquid. For water, this process requires approximately 80 calories of heat energy for each gram converted.
Meltwater - Water produced from the melting of snow and/or glacial ice.
Meltwater Channel - A channeled flow of water that results from the melting of a glacier or ice sheet.
Meltwater Stream - A meltwater-created stream that is found at the base of a glacier. Also called a marginal channel.
Mercalli Scale - A standard scale used to determine the power of an earthquake. This scale is based on the amount of visible damage inflicted on landscape features and human-built structures. This scale ranges from 1 (not felt) to 12 (complete destruction).
Mercator Map Projection - A type of Cylindrical Map Projection system that presents true compass direction. Distortion is manifested in terms of area. Area distortion makes continents in the middle and high latitudes appear larger than they should. The Mercator Map Projection was specifically designed for nautical navigation. This map-making technique was first employed by the Flemish cartographer Gérardus Mercator in 1569. Mercator's projection system quickly became the standard for maritime mapping in the 16th, 17th, and 18th centuries.
Mercury - A chemical element that has an atomic number of 80 and is represented chemically with the symbol Hg. Mercury lacks a crystal structure in its natural form and, therefore, is not geologically defined as a mineral but a mineraloid. Deposits of mercury are found throughout the world, mostly as mercuric sulfide (cinnabar)
Mercury Barometer - A type of barometer that measures changes in atmospheric pressure by the height of a column of mercury in a U-shaped tube that has one end sealed and the other end immersed in an open container of mercury. The force of atmospheric pressure acting on the mercury in the open container pushes it up the other end of the tube. The height of this level is then used as a measure of atmospheric pressure relative to the surface level of the mercury in the container.
Meridian - A circular arc that meets at the poles and connects all places of the same longitude.
Meridional - The movement of things like wind or ocean waters in a direction that is roughly parallel to the lines of longitude.
Meridional Circulation - A term used in meteorology to describe atmospheric flow in a direction that is roughly parallel to the lines of longitude.
Meridional Heat Transport - The global transport of heat energy stored in the atmosphere and oceans from the equator to the poles.
Mesa - A flat-topped hill that rises sharply above the surrounding landscape. The top of this hill is usually capped by a rock formation that is more resistant to weathering and erosion.
Mesic Layer - A near-surface layer in soil dominated by moderately decomposed organic matter. Compare with fibric and humic layers.
Mesocyclone - A cylinder of cyclonically flowing air that forms vertically inside a severe thunderstorm. Mesocyclones measure between 3 and 10 kilometers (1.9 and 6.2 miles) across. About 50% of these weather events spawn tornadoes.
Mesopause - A thin boundary layer found between the mesosphere and the thermosphere. The mesopause is found at an average altitude of 80 kilometers (50 miles). The coldest temperatures in the atmosphere are found in the mesopause.
Mesopelagic Zone - The layer within the ocean from a depth of roughly 200 meters to 1,000 meters (656 feet to 3,281 feet), which is part of the pelagic zone. Very little sunlight reaches this zone in the ocean. Also see (in order of depth in a water body) pelagic zone, photic zone, aphotic zone, bathyal zone, abyssal zone, hadal zone, demersal zone, and benthic zone.
Mesophyte - A plant species that has moderate water requirements for growth.
Mesoscale Convective Complex - A cluster of thunderstorms covering an area of up to 100,000 square kilometers (38,600 square miles) or more. Convective circulation associated with this system encourages the growth of new thunderstorms for up to 18 hours.
Mesosphere - Atmospheric layer found between the stratosphere and the thermosphere. The mesosphere is located at an average altitude of 50 to 80 kilometers (31 to 50 miles) above the Earth's surface. Air temperature within the mesosphere decreases with increasing altitude.
Mesotherm - A plant that requires an annual mean temperature between 0 and 14°C to maintain normal growth.
Mesothermal Climate - A mild climate that is found in our planet's temperate zones. Such climates experience cyclical fluctuations in temperature and sometimes in precipitation with the seasons. According to the Köppen Climate Classification system, this climate would be classified as C: Moist Mid-latitude Climates with Mild Winters.
Mesotrophic - A water body with a moderate supply of nutrients in its waters. Compare with eutrophic and oligotrophic.
Mesotrophic Lake - A lake with a moderate nutrient supply. Compare with an eutrophic lake and an oligotrophic lake.
Mesozoic - Geologic era that occurred during the Phanerozoic Eon from 251 to 65.5 million years ago. This era contains the geologic periods: Triassic, Jurassic, and Cretaceous. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.
Metabolism - Describes all of the life-sustaining enzymatic-catalyzed chemical reactions that occur in the cells of an organism.
Metamorphic Rock - A rock that forms from the recrystallization of igneous, sedimentary, or other metamorphic rocks through pressure increase, temperature rise, or chemical alteration.
Metamorphism - A process that creates metamorphic rocks by way of pressure increase, temperature rise, or chemical alteration.
Metasomatic Metamorphism - A form of metamorphism that causes the chemical replacement of elements in the minerals that make up a rock. Occurs when gases and liquids permeate into bedrock.
Metastable Equilibrium - A situation where a stable equilibrium is maintained until a threshold is passed. Once the threshold is crossed, the system state moves to a completely new equilibrium.
Meteor - A solid body of matter that enters the Earth's atmosphere from space. While traveling through the atmosphere, these objects begin to burn from friction and are sometimes seen by ground observers as luminous streaks in the sky. Many of these objects burn up completely and never reach the Earth's surface.
Meteorite - A solid body of matter found on the ground surface that originated from outer space and survived travel through Earth's atmosphere.
Meteorite Crater - A depression in the ground surface produced by the violent and explosive impact of a meteorite.
Meteorological Extreme - The highest and the lowest value for a meteorological variable in a specified period of observation.
Meteorological Normal - The calculated average of a measured meteorological variable, like surface air temperature, over a specified period of years. A 30-year period is often used to determine normals.
Meteorology - The scientific study of the atmosphere and its associated phenomena and processes.
Methane - Methane is a very strong greenhouse gas found in our planet's atmosphere. Methane concentrations in the atmosphere have increased by more than 140% since 1750. The primary sources for the additional methane added to the atmosphere (in order of importance) are: rice cultivation, domestic grazing animals, termites, landfills, coal mining, and oil and gas extraction. The chemical formula for methane is CH4.
Methane Clathrate - A substance composed of methane trapped in the frozen crystal structure of water. Significant deposits of methane clathrate have been found in sediments on the ocean floor of our planet. Scientists believe this substance forms when methane gas seeps from faults on the ocean floor. If methane encounters very cold seawater, methane clathrate forms. The chemical description of methane clathrate is (CH4)4(H2O)23. Shown in the image are layers of methane clathrate embedded in some sediment that was recovered from the ocean floor off the coast of Oregon, USA. Also known as methane hydrate, methane ice, fire ice, natural gas hydrate, hydromethane, and gas hydrate. Image Source: Wikimedia Commons. This image is licensed under the Creative Commons Attribution 3.0 Unported license.
Mica - A silicate mineral that exhibits a platy crystal structure and perfect cleavage. Two common forms of mica are biotite and muscovite.
Micelle Fixation - The ability of the negatively charged surface of clay and humus soil particles to hold on to positively charged nutrient atoms and compounds. This ability provides plants with essential nutrients that can be absorbed and used.
Microclimate - The climate conditions associated with a small area that are distinctly different from the local climate that surrounds it. This term may be applied to a space smaller than a centimeter (half an inch).
Microclimatology - The study of atmospheric conditions (climate) around an object at the micro-scale.
Microcracks - Very small cracks in the surface of a rock.
Microfauna - Minute animals occupying an area that can only be seen with the aid of additional optical magnification. Compare with macrofauna.
Microflora - Minute plants occupying an area that can only be seen with the aid of additional optical magnification. Compare with macroflora.
Microfossil - A fossil that requires the use of a microscope to see it. Compare with macrofossil.
Microgranite - A medium-grained igneous rock that is rich in quartz and potassium feldspar. Microgranite contains at least 20 percent quartz by volume. Other minerals common in this rock include muscovite, biotite, and amphibole. Microgranite is equivalent to more coarse-grained granite. Derived from felsic magma.
Micronutrient - A nutritional element required by an organism in relatively very small quantities. Compare with a macronutrient.
Microorganism - An extremely small organism that can only be seen using a microscope.
Microtherm - A plant that requires an annual mean temperature between 0 and 14°C to maintain normal growth. Microtherms are common to Köppen D: Moist mid-latitude climates with cold winters.
Microwave Radiation - A form of electromagnetic radiation with a wavelength between 0.1 and 100 centimeters.
Mid Infrared - Astronomers typically define it as electromagnetic radiation with a wavelength between 5.0 and 25-40 micrometers (µm).
Mid-Atlantic Ridge - A chain of submarine mountains found in the center of the Atlantic Ocean, where oceanic crust is created from rising magma plumes and volcanic activity. Also associated with this feature is plate divergence, which creates a rift zone.
Mid-Latitude Cyclone - A cyclonic storm that forms primarily in the middle latitudes. The formation of these storms is triggered by the development of troughs in the polar jet stream. These storms also contain warm, cold, and occluded fronts. At their center, atmospheric pressure can drop as low as 970 millibars. Also called wave cyclones or frontal cyclones.
Mid-Oceanic Ridge - A chain of submarine mountains where oceanic crust is created from rising magma plumes and volcanic activity. Also associated with this feature is plate divergence that creates a rift zone.
Mie Scattering - One of three types of atmospheric scattering. In this type, the particles responsible for the scattering are larger than the wavelengths of the radiation striking them. Particles responsible for Mie scattering include pollen, dust, smoke, and water droplets. This type of scattering occurs in the atmosphere at an altitude of 0 to 5 km (0 to 3.1 mi), where large particulate matter is common. Mie scattering contributes to the red color of the sky at sunrise and sunset. Also see non-selective scattering and Rayleigh scattering.
Migration - The movement of organisms in an intentional way between two points in space. Many migrations are seasonally timed.
Milankovitch Theory - A theory proposed by Milutin Milankovitch in the 1940s that suggests changes in the Earth's climate may be explained by variations in solar radiation received at the Earth's surface. Further, these variations are due to the combined effects of three different cyclical changes in the geometric relationship between the Earth and the Sun. These three factors include changes in timing (precession) of the equinoxes and solstices, alterations in the tilt of the Earth's rotational axis (obliquity), and variations in the shape of the Earth's orbit around the Sun (eccentricity).
Milky Way Galaxy - An aggregation of about 400 billion stars in a flattened, disk-shaped structure in space. Our Solar System is found in this concentration of stars.
Military Grid Reference System - A simplified subset of the Universal Transverse Mercator (UTM) Grid System. This rectangular coordinate system is commonly used on maps to find the location of points on Earth's surface. Based on the Universal Transverse Mercator map projection system.
Millennium Ecosystem Assessment - Is a United Nations-sponsored study released in 2005 that examined the environmental state of Earth's natural ecosystems. The study involved the work of over 1000 scientists and was done to help political leaders make informed decisions concerning biodiversity, with a focus on ecosystems. The overall conclusion of the report was that human activity has severally degraded the state of the world's ecosystems and that this degradation will spread to other ecosystems unless conservation decisions are made by world leaders. The Millennium Ecosystem Assessment also suggests that ecosystems provide four important “ecosystem services” that positively influence human well-being. See the following website for more information: http://millenniumassessment.org.
Miller Cylindrical Map Projection - A type of Cylindrical Map Projection system that mathematically projects the Earth's surface onto a cylinder that is tangent at the equator. Directions and distances are only true at the equator. Distance, area, and shape distortion increase as one moves towards the poles. Very popular projection used in world maps.
Millibar (mb) - A unit measurements for quantifying force. Used to measure atmospheric pressure. Equivalent to 1,000 dynes per square centimeter.
Mineral - A recognizable chemically unique component part of rocks. A naturally occurring inorganic solid with a crystalline structure and a specific chemical composition. Over 2,000 mineral types have been classified.
Mineral Deposit - A natural concentration of minerals in a mass of rock that makes their extraction relatively easy and economically viable.
Mineralization - Decomposition of organic matter into its inorganic elemental components.
Mineraloid - A naturally occurring substance that appears mineral-like but does not have the property of crystallinity.
Minimum Thermometer - A meteorological thermometer designed to record the minimum temperature over a set time interval, usually 24 hours (midnight to midnight). Liquid-in-glass type of minimum thermometers are normally filled with red-colored alcohol and have a black metal slider that can move up and down through the bore. When the temperature drops, the black metal slider is pushed down the bore by the retreating top surface of the alcohol due to surface tension. When the temperature begins to rise again, the slider is designed not to move, thereby permanently recording the minimum temperature. The slider is reset by positioning the thermometer upside down. Compare with a maximum thermometer.
Miocene - Geologic epoch that occurred from 23.03 to 5.332 million years ago. This epoch had warmer global temperatures than those of the previous Oligocene or the following Pliocene. During this time, global terrestrial flora undergoes a reduction in forest biomes, which are replaced by desert, grassland, and savanna biomes. This change causes evolutionary modifications in the morphology and physiology of plants and animals as they adapt to these new habitats. Mammals and birds were common and well-established. Plate tectonics led to the growth of the Sierra Nevada, Cascade, Himalayas, and Andes (South America) mountain ranges during this epoch. One of two epochs during the Neogene Period. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.
Mirage - A naturally occurring optical illusion caused by light rays bending as they pass through the atmosphere.
Mire - A term used in the United Kingdom to describe a treeless wetland ecosystem dominated by mosses, sedges, and wetland shrubs that decay to produce peat. Synonymous with peatland.
Misfit Stream - A stream that is either too small (underfit stream) or too large (overfit stream) to be accountable for eroding the valley it exists in, or a stream that does not have enough energy to be responsible for the size of the meanders found in its stream channel.
Mississippian - Is a sub-period during the Carboniferous geologic period that occurred roughly from 318 to 359 million years ago. During this time, insects undergo major speciation and ferns first appear. Trees also become a dominant plant form on the continents. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.
Mist - An atmospheric condition where the air contains a dense concentration of minute water droplets or water surrounded particles. Mist exists when visibility near Earth's surface is reduced to no more than 1 kilometer (0.62 miles). Compare with fog.
Mistral - A term used to describe a cold and dry katabatic wind common to southern France.
Mitochondria - The organelle in a cell that oxidizes organic matter to release energy for use in cellular metabolism. Also see cellular respiration.
Mixed Tide - Tides that have a higher high water and lower high water as well as higher low water and lower low water per tidal period.
Mixing Ratio - The ratio between the weight (mass) of water vapor (or some other gas) held in the atmosphere compared to the weight of the dry air in a given volume of air. Usually measured in grams of water vapor (or gas) per kilogram of dry air.
Mobile Belt - A long, narrow area of crust experiencing earthquakes, volcanoes, and mountain building because of tectonic activity.
Mobile Dune - A type of coastal dune that alters its location with changes in wind direction and speed.
Mode - A statistical measure of central tendency in a set of data. The mode is the most frequently occurring value in a data set. Data sets can contain two or more mode values that occur with the same frequency. Compare with a mean and a median.
Model - (1) Generalization of reality. (2) System describing how a phenomenon functions. (3) A mathematical representation of a system from which predictions or inferences can be made.
Moder - A type of humus that has organic matter decomposition between mull and mor.
Mogote - A steep-sided, well-rounded hill made of marble, dolomite, or limestone. These landscape features usually sit isolated on a level alluvial plain.
Moho Discontinuity - The lower boundary of the crust. At this boundary, seismic wave velocities show an increase in speed as they enter the upper mantle.
Moist Adiabatic Lapse Rate - See saturated adiabatic lapse rate.
Moisture Index - (1) The proportion of precipitation used by plants for growth. (2) A monthly calculation made in the Thornthwaite Climate Classification system to determine the proportion of precipitation used for plant growth.
Molasse - A term used to describe sedimentary rocks that were initially deposited ahead of a developing mountain range. Typically, this sedimentary feature contains shales, sandstones, and conglomerates.
Molecule - A minute particle that consists of connected atoms of one or many elements.
Mollisols - Soil order (type) of the United States Natural Resources Conservation Service Soil Classification System. A mollisol soil is typically found in semiarid grassland environments. These soils are rich in organic matter, brown in color, and contain abundant calcium carbonate nodules throughout the profile. Image Source: U.S. Department of Agriculture.
Mollweide Map Projection - A map projection system that tries to present more accurate representations of area. Distortion is mainly manifested in terms of map direction and distance.
Momentum - Abstract concept in physics that models an object's speed of travel along a straight path. Mathematically, it is the product of an object's mass and velocity.
Monadnock - A term used to describe an isolated hill or mountain composed of rock that rises from a level plain or slightly sloping landscape. Synonymous with inselberg.
Monera - Group, at the kingdom level, in the classification of life. Unicellular organisms with prokaryotic cells. See the Encyclopedia of Life for more information on this group of organisms.
http://eol.org/pages/27685799/entries/46149150/overview
Monoclimax - A climax community in a plant succession that is the outcome of only one controlling factor, like climate. Compare with a polyclimax.
Monocline - A fold in layered rock that creates a slight bend. Compare with anticline, syncline, overturned fold, and recumbent fold.
Monolith - A vertical soil section mounted for display.
Monsoon - A regional-scale wind system that predictably changes direction with the passing of the seasons. Monsoon winds blow from land to sea in the winter, and from sea to land in the summer. Summer monsoons are often accompanied by precipitation.
Monthly Mean Temperature - The average surface temperature as recorded at a meteorological station for a specific month. Usually, it is calculated by summing the daily mean temperatures for a particular month and dividing the result by the number of days in that month.
Monthly Temperature Range - The difference between the warmest and coldest daily mean temperatures recorded at a meteorological station during the course of a specific month (midnight to midnight).
Montmorillonite - A type of clay composed of phyllosilicate minerals that has a large capacity to shrink and expand with changes in moisture content. Often forms from weathering of volcanic ash.
Montreal Protocol - A United Nations international treaty first signed in 1987 by 24 nations in Montreal, Canada, to cut the emissions of chlorofluorocarbons (CFCs) into the atmosphere. This treaty is an outcome of the Vienna Convention for the Protection of the Ozone Layer. Since 1987, the treaty has been amended to quicken the reduction in CFC production and use. The United Nations Environment Programme - Ozone Secretariat administers both the Vienna Convention and the Montreal Protocol. More information is available at the following website: http://montreal-protocol.org/.
Moon - (1) A celestial body that has an orbit around a planet or dwarf planet. More commonly called a satellite in the field of Astronomy. (2) The name given to the celestial body that orbits the Earth.
Moorland (moor) - Habitat characterized by low-growing plants like moss, heather, sedges, and/or grasses. Soils in moorland are often waterlogged and acidic. The term is common in the United Kingdom.
Mor - A type of humus that forms under very acidic soil conditions, usually under coniferous trees. Mor usually does not contain mineral particles in the surface horizon of a soil. Compare with mull.
Moraine - A mound or hill of glacial till deposited directly by a glacier.
More Developed Country (MDC) - A highly industrialized country characterized by significant technological development, high per capita income, and low population growth rates. Examples of such countries include the United States, Canada, Japan, and many European countries. Also see Less Developed Country.
Morphological System - This is a system in which we understand the relationships between elements and their attributes in a vague sense, based solely on measured features or correlations. In other words, we understand the form or morphology a system has based on the connections between its elements. We do not understand exactly how the processes work to transfer energy and/or matter through the connections between the system elements.
Morphometry - The measurement of shape. Measurements are then statistically or mathematically manipulated to reveal inherent properties.
Mosaic - A group of overlapping images produced by aerial photography or other forms of remote sensing. This technique is used when a single image does not cover the area of interest.
Moss - About 9,500 species of plants that belong to the division Bryophyta. These low-growing plants are common in moist habitats. See the Encyclopedia of Life for more information on this group of organisms.
http://eol.org/pages/3768/overview
Mottled Zone - (1) Can refer to a layer in the profile of some soils that is defined by spots or patches of different colors or shades of a color. (2) A soil profile zone of red patches from iron oxidation in a bleached kaolinite-rich soil.
Moulin - A vertical tube up to 10 meters (30 feet) wide that moves glacial meltwater from the surface of a glacier into it. Moulins can be hundreds of meters deep and reach the bottom of the glacier. Often found in the vicinity of transverse crevasses. Shown is an image of a moulin on the Athabasca Glacier in Canada. Also called a glacier mill. Image Source: Wikimedia Commons, photographed by China Crisis. This image is licensed under the Creative Commons Attribution 3.0 Unported license.
Mountain - An area where the Earth's surface rises abruptly to elevations significantly higher than the adjacent landscape.
Mountain Breeze - Local thermal circulation pattern found in areas of topographic relief. In this circulation system, surface winds blow from areas of higher elevation to valley bottoms during the night.
Mountain Building - A term that describes the processes that create mountains. Mountains form when the Earth's crust is deformed by compressive forces. The most common mechanisms of mountain building are tectonic plate collisions and volcanism.
Mountain Chain - A long linear sequence of mountains, usually part of a much larger mountain range and often formed by folding.
Mountain Meteorology - A subfield of meteorology that examines the weather of mountain environments.
Mountain Range - An area consisting of a number of mountains that are geologically related in terms of their formation, structure, and rock composition. Also called a mountain belt.
Mouth - The endpoint of a stream. The point at which a stream enters a lake, sea, or ocean.
Movement - A term used in geography that deals with the migration, transport, communication, and interaction of natural and human-made phenomena across the spatial dimension.
Mud - A gooey mixture of clay and/or silt that contains a high water content. Can form after heavy rainfall or be associated with water inundation from streams, lakes, or oceans.
Mud Volcano - A volcanic mound of mud created by a geyser venting slurries of mud, water, and gas. The largest mud volcano on our planet is Indonesia's Lusi, which rises 700 meters (2,300 feet) above sea level and spans about 10 kilometers (6 miles). Most mud volcanoes are much smaller than this. Shown is an image of some small mud volcanoes located in Gobustan National Park, Azerbaijan. Smaller mud volcanoes are often called mud pots. Image Source: Wikimedia Commons, photographed by Nick Taylor. This image is licensed under the Creative Commons Attribution 2.0 Generic license.
Mud flats (or Mudflats) - A flat area of clay, silt, and sand deposits devoid of vegetation found in an estuary that is repeatedly submerged and exposed to the atmosphere by varying tidal levels. Also called tidal flats.
Mudflow - A form of mass movement where fine-textured sediments and soil mix with water to create a liquid flow.
Mudslide - A landslide where the material being moved rapidly downslope is mainly clay and silt.
Mudstone - A fine-grained sedimentary rock composed of lithified clay and silt particles.
Mulch - A layer of material applied to cover the surface of a soil. Mulch is usually organic and used to improve soil quality. Mulch can be used to conserve soil moisture, add nutrients, reduce weed growth, increase soil temperature, and to limit frost penetration below the soil surface.
Mull - A type of humus that forms under hardwood deciduous trees. Mull has a granular texture because it is mixed with mineral particles from the soil's surface horizons. Compare with a mor.
Multifinality - An idea in systems theory that suggests that in some systems, an initial state or cause can have many different trajectories producing numerous distinct end states. Compare with equifinality.
Multispectral Scanner (MSS) - A remote sensing device found on the first five Landsat satellites. This device acquires images in four or five wavelength bands in the spectral range from 0.5 to 1.1 µm.
Munsell Color System - An organization of colors that varies based on three properties: hue (basic color), chroma (intensity of the color), and value (tone). This system was developed by art professor A.H. Munsell around 1910. The Munsell Color System is used by soil scientists to classify soil color.
Muscovite - A rock-forming mineral of the mica group.
Muskeg - Poorly drained marsh or swamp found overlying permafrost.
Mutation - A change in the structure of a gene or chromosome of an organism caused by a metabolic error in copying or an environmental factor.
Mutualism - Interspecific biotic interaction where both species experience an increase in their fitness after interacting with the other species. Mutualistic interactions between species can be of two types: symbiotic or non-symbiotic.
Mycorrhiza (Mycorrhizae plural) - The mutualistic association of a fungus with the root of a higher plant. In this relationship, the fungus helps the plant in extracting essential nutrients required for growth from the soil. In exchange, the fungus is provided with a habitat and nutrition in the form of carbohydrates.