Earth systems and geophysics

Magnitude of completeness

The magnitude of completeness (Mc) is the lowest magnitude at which 100% of earthquakes in a region are reliably detected by a seismic network; below it, smaller quakes are missed.

Mantle plume

A mantle plume is a buoyant upwelling of hot, low-viscosity rock rising through Earth's mantle, proposed in the 1970s to explain hotspot volcanism; the classic example is the Hawaiian chain.

Mantle transition zone

The mantle transition zone (MTZ) is the layer of Earth's mantle between about 410 and 660 km depth, separating the upper mantle from the lower mantle.

Map

A map is a symbolized representation of geographic reality showing selected features for situations where spatial relationships matter most; making and studying maps is called cartography.

Martin Waldseemüller, Latinized as Martinus Ilacomilus, was a German humanist and cartographer whose 1507 Universalis Cosmographia was the first map to name "America" and depict the New World as a separate continent.

Membrane polarization

Membrane polarization is a mechanism of induced polarization in rocks and soils, in which clay surfaces and narrow pores form ion-selective zones that build up charge-storing ion concentration gradients.

Mercator projection

The Mercator projection, also known as the Mercator chart, is a cylindrical map projection presented by Gerardus Mercator in 1569, in which lines of constant bearing are straight.

Metal factor (geophysics)

The metal factor (MF) is an induced polarization parameter in geophysics, defined as percent frequency effect or chargeability divided by resistivity, with units of conductivity.

Microseismic monitoring

Microseismic monitoring is a passive geophysics method that records very small earthquakes from hydraulic fracturing and fluid flow, used to image subsurface fractures and stress changes.

Migration velocity analysis

Migration velocity analysis (MVA) is a seismic imaging method that estimates the subsurface velocity model by repeatedly migrating data, measuring kinematic errors in prestack images, and tomographically inverting them into velocity updates.

Oct 22, 2026

Missing xenon

Missing xenon, also called the missing xenon paradox or missing Xe problem, is the shortfall of over 90% of Earth's expected atmospheric xenon relative to chondritic meteorites.

Mollisol

A Mollisol is a soil order in USDA Soil Taxonomy defined by a thick, dark, humus-rich surface horizon, making it among the most fertile and extensively farmed soils in the world.

Mollisols

Mollisols, from the Latin mollis meaning soft, are a USDA soil order with a thick, dark, humus-rich surface horizon, formed mainly under grassland and counted among the world's most productive agricultural soils.

Moment tensor inversion

Moment tensor inversion is a seismological technique that estimates the forces at an earthquake source by fitting synthetic seismograms to waveforms, yielding fault orientation, seismic moment, and moment magnitude.

Mountain

A mountain is a landform rising prominently above its surroundings to a peak or summit, typically at least 1,000 feet, though no globally accepted definition of a mountain exists.

Multi-physics coupling

Multi-physics coupling is the joint simulation of interacting thermal, hydraulic, mechanical, and chemical processes in geologic media, applied to nuclear waste disposal, geothermal energy, and CO2 storage.

Multichannel analysis of surface waves

Multichannel analysis of surface waves (MASW) is a seismic method, introduced by Park, Miller, and Xia in 1999, that estimates near-surface shear-wave velocity from Rayleigh-wave dispersion and is widely used in geotechnical engineering.

Muon tomography

Muon tomography, also called muography, is an imaging technique that uses naturally occurring cosmic-ray muons to see inside large opaque objects such as volcanoes, pyramids, mines, and nuclear reactors.

Natural disaster

A natural disaster is a serious disruption of a community caused by a hazardous natural event, such as an earthquake or flood, producing major human and economic losses.

Natural resource

A natural resource is any biological, mineral, or aesthetic asset afforded by nature without human intervention that can be used for benefit; classifications include renewable, non-renewable, and nondepletable.

New Madrid seismic zone

The New Madrid seismic zone, also called the New Madrid fault, is the most seismically active area in the central and eastern United States, producing the great 1811–1812 earthquakes.

Next Generation Gravity Mission

The Next Generation Gravity Mission (NGGM) is a European Space Agency mission, launching around 2032, that will measure changes in Earth's gravity field to track water and ice movement.

Normal moveout

Normal moveout (NMO) is a seismic data processing correction that time-shifts each trace in a CMP gather by an offset-dependent amount, flattening reflections for stacking.

North

North is one of the four cardinal directions: true north toward the geographic North Pole, magnetic north where a compass points, and grid north on maps.

Oasis

An oasis is an area made fertile by freshwater from springs or wells in an otherwise arid region, supporting agriculture, settlement, and trade, such as Egypt's Nile valley.

Ocean bottom node (seismic)

An ocean bottom node is an autonomous seismic receiver placed on the seabed to record air-gun energy, offering wider azimuth and cleaner data than towed streamers.

Ocean-bottom node

An ocean-bottom node (OBN) is a cable-free seismic receiver placed on the seafloor that records data autonomously on battery power and is retrieved for download, offering wide-azimuth imaging advantages.

Offset tracking

Offset tracking, also called pixel-offset tracking, is a synthetic aperture radar technique that measures surface displacement, used for glacier flow and large earthquake deformation where InSAR fails.

Optically pumped magnetometer

An optically pumped magnetometer (OPM) is a magnetic field sensor that reads the Larmor precession of laser-pumped alkali vapor atoms, reaching femtotesla sensitivity comparable to SQUIDs.