Lunar and Planetary Institute


Volcanoes on Mars


Definitions provided here are summarized from Bates and Jackson (1987).

Albedo - The ratio of the amount of light reflected by a surface to the amount of light incident upon it.

Ash - Fine-grained material produced by a pyroclastic eruption. An ash particle is defined to have a diameter of less than 2 mm. See pyroclastic.

Basalt - A general term for dark-colored igneous rocks composed of minerals that are relatively rich in iron and magnesium.

Caldera - A large, basin-shaped volcanic depression that is more or less circular in form. Most calderas are produced by collapse of the roof of a magma chamber due to removal of magma by voluminous eruptions or subterranean withdrawal of the magma, although some calderas may be formed by explosive removal of the upper part of a volcano. See magma.

Cinder cone - A conical hill formed by the accumulation of pyroclastic fragments that fall to the ground in an essentially solid condition. See pyroclastic.

Eruption - The ejection of volcanic materials (lavas, pyroclasts, and volcanic gases) onto the surface, either from a central vent or from a fissure or group of fissures. See lava, pyroclastic.

Graben - An elongate, relatively depressed crustal unit or block that is bounded by faults on its long sides.

Igneous - Used to describe a rock or mineral that solidified from molten or partly molten material, i.e., from a magma. See magma.

Lava - A general term for molten rock that is extruded onto the surface.

Magma - Molten rock within the crust of a planet that is capable of intrusion into adjacent crustal rocks or extrusion onto the surface. Igneous rocks are derived from magma through solidification and related processes or through eruption of the magma at the surface. See igneous.

Phreatic eruption - A volcanic eruption or explosion of steam, mud, or other material that is not incandescent. This form of eruption is caused by the heating and consequent expansion of ground water due to an adjacent igneous heat source. See igneous.

Pseudocrater - A generally circular crater produced by a phreatic eruption resulting from emplacement of a lava flow over wet ground. See phreatic eruption.

Pyroclastic - Pertaining to a clastic (broken and fragmented) rock material formed by volcanic explosion or aerial expulsion from a volcanic vent.

Scarp - A relatively straight, clifflike face or slope of considerable linear extent, breaking the general continuity of the land by separating surfaces lying at different levels.

Shield volcano - A volcano in the shape of a flattened dome, broad and low, built by flows of very fluid lava.

Volcano - A vent in the planetary surface through which magma and associated gases and ash erupt; also, the form or structure produced by the erupted materials. See ash, eruption, magma.


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