Vents in the surface of the Earth through which magma and associated gases erupt; also, the forms or structures, usually conical, that are produced by the erupted material. [Glossary of Geology, 4th ed.]
This category is also used for volcanos.
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Aeromagnetic expression of buried basaltic volcanoes near Yucca Mountain, Nevada

Mathematical modeling and analysis of aeromagnetic anomaly data (gridded data provide in another report).

High resolution, low altitude aeromagnetic and electromagnetic survey of Mount Rainier

Flight-line and gridded data, with description of methodology and interpretation