Glossary Starting with the letter: M


Molten rock within the Earth. Igneous rocks form when magma cools and crystallizes.


The development and movement of magma, and its solidification to form igneous rock.

Magnetic Field

The region of influence of a magnetized body, such as Earth.

Magnetic Pole

Either of two variable points on Earth, close to but not coinciding with the geologic North and South Poles, where the Earth’s magnetic field is most intense and towards which a compass needle points.


The layer within the interior of the Earth that lies between the crust and the core.

Mass Spectrometer

An instrument for separating atoms or molecules according to mass and counting them. Typically used for determining isotopic abundances.

Mass Wasting

The process in which a large amount of loosened soil and rock is transported downslope under the direct influence of gravity.


An isolated, steep-sided, flat-topped landmass rising above the surrounding geography.


The Era of geologic time (from the end of the Paleozoic, 248 million years ago, to the beginning of the Cenozoic Era, 65 million years ago), including the Triassic Period, the Jurassic Period, and the Cretaceous Period. This Era is characterized by the development of flying reptiles, birds, and flowering plants, and the appearance and extinction of dinosaurs.

Metamorphic rocks

Rocks that form as the result of transformation from other rocks. Metamorphic rocks are created when igneous, sedimentary, or metamorphic rocks change in response to extreme temperatures and/or pressures, but do not completely melt. Metamorphic rock types include slate, schist, and gneiss.


A fragment of rock that has reached the Earth’s surface from beyond the Earth’s atmosphere.


The study of Earth’s atmosphere and the motion within the atmosphere. Meteorology includes understanding the aspects of the atmosphere for weather forecasting.


An odorless, colorless, flammable gas (CH4). Methane is an important source of hydrogen and a wide variety of organic compounds, as well as a principle constituent of natural gas.

Mid-ocean Ridge

A nearly continuous undersea mountain chain that marks the location where tectonic plates (pieces of the lithosphere) are diverging or moving apart. Mid-ocean ridges are the locations of creation of new ocean crust.

Milankovitch Cycles

The three cycles related to variations in the Earth’s rotational and orbital characteristics around the sun that are believed to influence the occurrence of ice ages, occurring at 100,000, 41,000, and 22,000 years. Named after Serbian mathematician Milutin Milankovitch.


A naturally-occurring, homogeneous inorganic element or compound having a definite chemical composition and orderly internal structure, crystal form, and characteristic chemical and physical properties.

Mineral Physics

The study of the physical properties of minerals.


The study of minerals, including their formation, occurrence, properties, composition, and classification.

Mohorovicic Discontinuity

(Moho) The boundary between the crust and mantle marked by abrupt increases in seismic velocities. The Moho occurs at about five to ten kilometers beneath the ocean floor and about forty kilometers beneath the continents (although it may reach sixty kilometers or more under some mountain ranges).


The smallest unit of matter into which an element or a compound can be divided and still retain its chemical and physical properties. It consists of a single atom or group of like or different atoms bonded together by chemical forces.


A metallic, lead-gray, hexagonal mineral made of molybdenum and sulfur (MoS2.)


A wind system that influences climatic regions and reverses direction seasonally. The Indian Monsoon brings wind from the southwest or south and brings great annual variation of rainfall to southern Asia and along the coasts of other regions of the Indian Ocean.


An accumulation of poorly sorted glacial sediments deposited beneath or at the margin of a glacier and having a surface form that is unrelated to the underlying bedrock.