airfall - shower-like falling of pyroclastic fragments from an eruption cloud.

annular disk - a thin, flat, circular object that is shaped like or forms a ring.

basalt - a dark-colored, fine-grained volcanic rock composed of plagioclase (over 50%) and pyroxene. Olivine may or may not be present.

batholith - large body of intrusive igneous rock exposed over an area of at least 100 square kilometers.

caldera - a large, more or less circular depression or basin associated with a volcanic vent. Calderas are believed to result from subsidence or collapse and may or may not be related to explosive eruptions.

corona (pl. coronae) - oval-shaped feature on the surface of Venus or Miranda. Also used to describe the bright, irregular-shaped envelope of ionized gas around the Sun.

dacite - a fine-grained extrusive rock composed mostly of plagioclase feldspar, with amphibole and biotite.

ejecta - the deposit surrounding an impact crater composed of material thrown from the crater during its formation.

graben - an elongate crustal depression bounded by normal faults on its long sides.

groundwater sapping - erosion on the surface of a planet resulting from the seepage of groundwater (subsurface water) onto the surface.

impact melt - molten material produced by fusion of target rock during a meteorite impact, usually found in and around the resulting crater.

mantle - the interior zone of a planet or satellite below the crust and above the core that behaves plastically.

metamorphic rocks - rocks formed from preexisting rocks within a planet's crust by changes in temperature and pressure and by chemical action of fluids.

outflow channel - a valley on Mars consisting of irregular networks, thought to have been formed by the sudden release of water.

rhyolite - a fine-grained volcanic rock composed of quartz, potassium feldspar, and plagioclase. Rhyolite is the extrusive equivalent of a granite.

scarps - steep slopes produced by erosion or faulting.

stratovolcano - a steep-sided volcano built up of alternating layers of ash and lava flows.

subduction - subsidence of the leading edge of a lithospheric plate into the mantle.

tectonism - the process of deformation of planetary materials, for example, faulting of the crust.

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