Glossary

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Mafic Magma - 

Magma that is relative poor in silica but rich in calcium, magnesium, and iron content. This type of magma solidifies to form rocks relatively rich in calcium, magnesium, and iron but poor in silica.

Magma - 

Molten rock originating from the Earth's interior.

Magma Plume - 

A rising vertical mass of magma originating from the mantle.

Magnetic Declination - 

The horizontal angle between true north and magnetic north or true south and magnetic south.

Magnetic Field - 

The space influence by magnetic force. The Earth's magnetic field is believed to be generated by the planet's core.

Magnetic North - 

Location in the Northern Hemisphere where the lines of force from Earth's magnetic field are vertical. This point on the Earth gradual changes its position with time.

Magnetic Reversal - 

A change in the polarity of the Earth's magnetic field. In the past 4 million years there have been nine reversals.

Magnetic South - 

Location in the Southern Hemisphere where the lines of force from Earth's magnetic field are vertical. This point on the Earth gradual changes its position with time.

Magnetosphere - 

Zone that surrounds 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.

Mangrove - 

Treed wetlands located on the coastlines in warm tropical climates.

Mantle - 

Layer of the Earth's interior composed of mostly solid rock that extends from the base of crust to a depth of about 2,900 kilometres.

Map Projection - 

Cartographic process used to represent the Earth's three-dimensional surface onto a two-dimension map. This process creates some type of distortion artifact on the map.

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.

Maps - 

An abstraction of the real world that is used to depict, analyze, store, and communicate spatially organized information about physical and cultural phenomena.

Marble - 

Metamorphic rock created by the recrystallization of calcite and/or dolomite.

March Equinox - 

One of two days during a year when the declination of the Sun 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.

Marine - 

With reference to ocean environments and processes.

Maritime Effect - 

The effect that large ocean bodies have on the climate of locations or regions. This effect results in a lower range in surface air temperature at both daily and annual scales. Also see Continental Effect.

Maritime Polar Air Mass - 

Air mass that forms over extensive ocean areas of the middle to high latitudes. Around North America, these air mass system form over the Atlantic and Pacific oceans at the 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.

Maritime Tropical Air Mass - 

Air mass that forms over extensive ocean areas of the low latitudes. Around North America, these system 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 can normally stable during the whole year if they have 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.

Mass Balance - 

The relative balance between the input and output of material within a system.

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 - 

General term that describes the downslope movement of sediment, soil, and rock material.

Mass Wasting - 

General term that describes the downslope movement of sediment, soil, and rock material.

mb - 

A unit measurements for quantifying force. Used to measure atmospheric pressure. Equivalent to 1000 dynes per square centimetre.

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 countries in Europe. Also see less 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.

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 - 

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 - 

Sinuous shaped stream channel. Usually found in streams flowing over a very shallow elevation grade.

Medial Moraine - 

Deposit of material found down the center of a glacier. Created when two glacier and their lateral moraines merge.

Median - 

Statistical measure of central tendency in a set of data. The median is the value halfway through a data set where the values have been ordered from lowest to highest. In an even data set, the median is the average of the two halfway values.

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.

Mercalli Scale - 

A scale for rating the power of an earthquake.

Mercator Projection - 

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 seem larger than they should be. Specifically designed for nautical navigation.

Mercury Barometer -  Type of barometer that measures changes in atmospheric pressure by the height of a column of mercury in a U-shaped tube which has one end sealed and the other end immersed in an open container of mercury. The force of the pressure exerted by the atmosphere on the mercury in the open container pushes mercury 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 -  Movement of wind or ocean waters in a direction that is roughly perpendicular to the lines of latitude.
Meridional Transport -  Transport of atmospheric and oceanic energy 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.
Mesocyclone -  A cylinder of cyclonically flowing air that form vertically in a severe thunderstorm. They measure about 3 to 10 kilometers across. About 50% of them spawn tornadoes.
Mesopause -  Thin boundary layer found between the mesosphere and the thermosphere. It is usually found at an average altitude of 80 kilometers. Coldest temperatures in the atmosphere are found at the mesopause.
Mesoscale Convective Complex -  A cluster of thunderstorms covering an area of 100,000 kilometers or more. Convective circulation within this system encourages the growth of new thunderstorms for up to 18 hours.
Mesosphere -  Atmospheric layer found between the stratosphere and the thermosphere. Usually located at an average altitude of 50 to 80 kilometers above the Earth's surface. Air temperature within the mesosphere decreases with increasing altitude.
Mesozoic -  Geologic era that occurred from 245 to 65 million years ago.
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 -  Process that creates metamorphic rocks.
Metasomatic Metamorphism -  Form of metamorphism that causes the chemical replacement of elements in rock minerals when gases and liquids permeate into bedrock.
Meteor -  A body of matter that enters the Earth's atmosphere from space. While traveling through the atmosphere, these objects begin to burn because of friction and are sometimes seen as luminous streaks in the sky by ground observers. Many of these objects burn up completely and never reach the Earth's surface.
Meteorology -  The scientific study of the atmosphere and its associated phenomena.
Methane -  Methane is very strong greenhouse gas found in the 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. Chemical formula for methane is CH4.
Mica -  Silicate mineral that exhibits a platy crystal structure and perfect cleavage. Common forms of mica are biotite and muscovite.
Microwave Radiation -  Form of electromagnetic radiation with a wavelength between 0.1 to 100 centimeters.
Mid-Latitude Cyclone -  Cyclonic storm that forms primarily in the middle latitudes. Its formation is triggered by the development of troughs in the polar jet stream. These storms also contain warm, cold and occluded fronts. Atmospheric pressure in their center can get as low as 970 millibars. Also called wave cyclones or frontal cyclones.
Mid-Oceanic Ridge -  Chain of submarine mountains 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.
Migration -  Movement of organisms in an intentional way between two points in space. Many migrations are seasonal.
Milankovitch Theory -  Theory proposed by Milutin Milankovitch that suggests that changes in the Earth's climate are cause by variations in solar radiation received at the Earth's surface. These variations are due to cyclical changes in the geometric relationship between the Earth and the Sun.
Military Grid Reference System -  A simplified subset of the Universal Transverse Mercator (UTM) Grid System. This rectangular coordinate system used to find location of points on the Earth's surface. Based on the Universal Transverse Mercator projection system.
Milky Way Galaxy -  Aggregation of about 400 billion stars in a flattened, disk-shaped structure in space. Our solar system is found in this structure.
Miller Cylindrical Projection -  Map projection 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 increases as one moves towards the poles. Very popular projection used in world maps.
Millibar -  A unit measurements for quantifying force. Used to measure atmospheric pressure. Equivalent to 1000 dynes per square centimetre.
Mineral -  Component of rocks. A naturally occurring inorganic solid with a crystalline structure and a specific chemical composition. Over 2,000 types of minerals have been classified.
Mineralization -  Decomposition of organic matter into its inorganic elemental components.
Mississippian -  Geologic period that occurred roughly 320 to 360 million years ago. During this period, insects undergo major speciation and ferns first appear. Trees become a dominant plant form on continents.
Mistral -  Term used to describe a katabatic wind in southern France.
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.
Mode -  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.
Model -  (1) Generalization of reality. (2) System describing how a phenomenon functions. (3) Mathematical representation of a system from which predictions or inferences can be made.
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.
Molecule -  Minute particle that consists of connected atoms of one or many elements.
Mollweide Projection -  Map projection system that tries to present more accurate representations of area. Distortion is mainly manifested in terms of map direction and distance.
Monocline -  A fold in layered rock that creates a slight bend.
Monsoon -  A regional scale wind system that predictably change 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 with precipitation.
Montmorillonite -  A type of clay that has a large capacity to shrink and expand with wetting and drying.
Montreal Protocol -  Treaty signed in 1987 by 24 nations to cut the emissions of chlorofluorocarbons (CFCs) into the atmosphere. Since 1987 the treaty has been amended to quicken the reduction in CFC production and use.
Moraine -  A hill of glacial till deposited directly by a glacier.
More Developed Country -  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 countries in Europe. Also see less developed country.
Moss -  About 9,500 species of plants that belong to the division bryophyta. These low growing plants are common in moist habitats.
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 wind. -  A wind common to mountainous regions that involves heavy cold air flowing along the ground from high to low elevations because of gravity. Also called katabatic wind.
Mouth -  End of a stream. 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
MSS -  Remote sensing device found on Landsat satellites that acquires images in four spectral bands from visible to reflected infrared.
Mudflow -  Form of mass movement where fine textured sediments and soil mix with water to create a liquid flow.
Mudstone -  Fine grained sedimentary rock composed of lithified silt and clay particles.
Multispectral Scanner -  Remote sensing device found on Landsat satellites that acquires images in four spectral bands from visible to reflected infrared.
Muscovite -  Rock forming mineral of the mica group.
Muskeg -  Poorly drained marshes or swamps found overlying permafrost.