a'a (AH-ah) - A type of basalt lava flow characterized by an extremely rough clinkery top and a dense interior. A'a flows are usually 110 meters thick, have flow-front velocities between 10 and 2000 meters/hour, and are associated with high-fountaining high-effusion-rate eruptions.
blackbody temperature - The temperature of an object if it is reradiating all the thermal energy that has been added to it. If an object is not a blackbody radiator, it will not reradiate all the excess heat and the leftover will go toward increasing its temperature.
bomb sag - A depression left in finer-grained pyroclastic layers by the impact of a larger bomb or block.
caldera (call-DAIR-uh) - A depression in the summit of a volcano (usually greater than 1 kilometer across) caused by subsidence into a magma chamber.
clast - A fragment of rock that has been transported, either by volcanic or sedimentary processes.
ejecta blanket - The material deposited around the rim of a crater formed by the impact of a meteorite or comet on the surface of a solid planet. Ejecta may be thrown to several times the radius of the crater, depending on the planet where the crater forms. High atmospheric pressure and high surface gravity tend to concentrate the ejecta close to the crater rim, and vacuum conditions and low gravity promote a widely distributed ejecta blanket.
en echelon fissures (on ESH-uh-lon FISH-ures) - Fissures that are parallel in trend to each other, but offset to either the left or right.
graben (GRAH-bun) - A depression formed by the dropping of a central block between two parallel inward-sloping normal faults. Graben indicate a zone of horizontal tension.
impact crater - A crater formed by the collision of a meteoroid and the surface of a planet. The floor of an impact crater is always lower than the level of the terrain in which the crater is excavated. Impact craters can have any size, from micrometers to hundreds of kilometers in diameter.
incidence angle - For radar imaging, the angle (measured from vertical) between an object and a satellite that is observing the object. An incidence angle of 0° is called a "nadir" (NAY-deer) view.
littoral (LIT-uh-ral) cone - A pyroclastic cone formed of fragments of a lava flow thrown out by steam explosions that take place when the lava flow enters the ocean.
mare (MAH-ray) (pl. maria; MAH-ree-ah) - The dark areas of the Moon, which consist of basalt lava flows erupted into depressions (typically old large impact craters called "basins") a few tens to hundreds of millions of years after the impact event.
pahoehoe (pa-HO-ee-HO-ee) - A type of basalt lava flow characterized by a smooth glassy skin, and constructed of innumerable "flow units" called "toes." Pahoehoe flows advance at rates between 1 and 10 meters/hour, and are associated with low-effusion-rate eruptions with little to no fountaining.
phreatomagmatic (free-AT-oh-mag-MAT-ic) - A style of volcanic eruption that takes place when the rising magma contacts shallow groundwater, or erupts under shallow oceanwater or lakes. The water and magma mix and generate strong steam explosions that blast apart the magma (and any preexisting rock) into fine-grained material. The resulting deposits form distinctive structures called tuff rings or tuff cones.
radar backscatter - The radar energy that is reflected back to a detector. The amount of backscatter is high if the surface is perpendicular to the path of the radar wave, so unless it is perfectly oriented, a smooth surface will bounce the radar energy off in some other direction and appear to be dark. A rough surface, on the other hand, has lots of small surfaces in all orientations. If enough of them happen to be oriented perpendicular to the radar path, the surface will reflect enough energy to appear bright.
rift - In general a rift is a fracture or crack in the Earth's surface caused by extension. On some volcanos subsurface intrusions are concentrated in certain directions and this causes tension at the surface and also means that there will be more eruptions in these "rift zones."
sapping - A process of erosion where water leaks to the surface through the pores of rocks. As the water flows away it slowly removes material to form valleys and channel networks.
shield volcano - The type of volcano that forms when the erupting lava has a low viscosity such as basalt. Such lava cannot construct steep slopes, so shield volcanos are characterized by broad gentle slopes.
sinuous rille (SIN-you-us RILL) - Narrow, winding, ditchlike channels found on the Moon and Mars. Sinuous rilles are believed to be tubes and channels that carried flowing lava.
spectroradiometer (SPEC-tro-RAY-dee-om-it-er) - A device that measures the amount of reflected or radiated energy from a surface in two or more wavelengths.
specular (SPEC-you-ler) - A surface that is smooth at the wavelength of the energy falling on it, permitting a mirrorlike reflection of the incident energy.
strombolian (strom-BOH-lee-un) - A style of eruption characterized by repeated relatively small explosions, and associated with basaltic and andesitic volcanos. Strombolian eruptions form large cinder cones.
subangular - Describes a fragment or clast that shows evidence of having been broken off a larger piece, but the original sharp edges and points have been abraded a little.
subrounded - Describes a fragment or clast that broke off a larger piece, but has undergone enough abrasion since then that the edges and points are almost unrecognizable.
wrinkle ridge - A sinuous ridge common on the Moon and Mars that may be a few kilometers in width and hundreds of kilometers in length. Wrinkle ridges are most likely tectonic features formed by horizontal compression.