by Jeff Medkeff
Angle of Repose - the steepest slope at which a sediment will rest without sliding down. For ball bearings, the angle of repose is very shallow; for small angular rocks with a lot of sharp edges, the angle of repose will be considerably steeper. When a material exceeds its angle of repose, slumping occurs, which accounts for piles of debris at the bottom of crater walls, etc.
B
basalt - a dark, mafic, igneous rock, usually fine grained, composed of pyroxene and plagioclase. Basalts fill the moon's maria and have some important compositional differences from Earth basalts.
basement - the oldest rocks in a given area.
basin - an impact feature of great size, often covering several percent or more of the surface of a body and giving rise to formations globally. On the moon, the conspicuous basins are found mostly on the nearside, and have filled to varying degrees with mare basalts. Basin rims form long circular mountain chains.
C
Cayley Formation - a deposit covering the floors of several craters in the central nearside, relatively flat but with gently rolling hills and ridges when seen under low light. Once thought to be mare flooding, but Apollo 16 showed it is in fact basin ejecta. Is is possible it is a result of a fluid-like flow of the Imbrian impact.
central peak - a raised peak in the center of an impact crater. The details of central peak formation are not agreed upon.
colongitude - the selenographic longitude of the sunrise terminator, measured from the central meridian, increasing westward from 0 degrees to 360 degrees.
consolidated - 1. a sediment that has a mineral cement or matrix that holds its grains together - rocks. 2. a ground-based photographic lunar atlas of extraordinary quality.
crater - a depression, often surrounded by a ring-shaped wall, and subject to a wide range of secondary modification. Almost all of the craters on the moon are impact craters, formed by the collision of a solid body with the moon. A few craters - generally quite small ones - are endogenic, typically of volcanic origin.
D
differentiated - basically, it means separated. A "differentiated pyroclastic flow" is a bed of volcanic ash which is not mixed in with its underlying material.
dome - an endogenic feature of positive relief (it sticks out from the surface), generally with shallow sides. It is formed by pressure from below.
E
ejecta - material thrown away from an impact crater (or volcano) that lands on the surrounding terrain.
ejecta blanket - a layer of material thrown away from an impact crater (or volcano) that covers the terrain in the immediate vicinity of the source.
endogenic - of internal origin. A volcano is endogenic, while an impact crater is exogenic, in origin.
F
fault - a crack in the lunar crust across which there has been movement.
fumarole - a small vent from which gasses may emerge (but no lava, other than possibly a light spray).
G
graben - a sunken area between two roughly parallel faults. The faults converge toward one another below the surface, so that they look like the letter "V" in cross section. Graben are the result of tension in the crust.
impact crater - a crater formed by the collision solid bodies.
lava tube - a sinuous tunnel formed when the surface of a lava flow cools and solidifies, and the lava passing through it drains away. Some of the lunar rilles are lava tubes.
laminar flow- a flow in which the movement of particles is parallel and straight.
lunar geology - the study of the moon's rocks, stratigraphy, etc. The term has been criticized by people more interested in English semantics than in Greek etymology or science. We use the term "geology" here on the following grounds: (1) The principles of geology on Earth are applicable to all other solid bodies; (2) It eliminates the need for a new, unique term for every new planet or satellite observed at geologically useful scales, the number of which now exceeds 20; (3) the Greek eymology includes the meanings of "land" and "ground;" (4) Three decades of usage by the planetary science community and by the majority of amateur astronomers. This list is taken from Wilhelms, p. VII, because the author of these pages concurs strongly with the reasoning laid out in that publication.
maar - a volcanic crater with no cone, probably formed by an eruption of gas or gas and ash, but without significant flows of lava.
mafic - a magnesium ferric rock. Basalt is mafic.
pyroclast, pyroclastic - strictly speaking, a pyroclast is a rock made up of fragments of volcanic rock thrown around by volcanic eruptions, usually violent eruptions. The term refers to volcanically-formed clasts and volcanic ash on the moon.
ray - a streaky and linear (or sometimes ropy) albedo feature that are arranged radially around some lunar craters. They are probably the result of several combined actions, including impact ground surge depositing fine ejecta on the surface, and small secondary cratering and consequent gardening of the surface around a ray. See also sunrise/sunset ray.
relief - a local or regional difference in elevation. Relief features therefore stick out of, or are holes in, the surrounding terrain.
rift valley - a fault trough formed through tension. The Alpine Valley is the premier example.
subsidence - a gentle movement of a broad area of crust that does not experience appreciable deformation.
sunrise/sunset ray - a peculiar streak of lighting caused by a gap in a crater wall or mountain range when the illumination angle is low. Most of them were noted in the 19th century, but their observation has been enjoying a renaissance in the late 20th and early 21st centuries. These are not geologic features, and are very common.
talus - a deposit of large rocks, usually at the base of a slope or cliff.
tuff - a rock composed of pyroclastic fragments and ash. See also welded tuff.
turbulent flow - a flow in which the particle movement is not straight or parallel, but chaotic, eddying, etc.
unconsolidated - sediment with no mineral cement or matrix holding its grains together. Dust, sand.
unit - an area of similar composition or morphology.
uplift - the process of gentle increase in the elevation of a unit of terrain.
volcanic ash - a sediment of fine rock fragments, usually considered to be smaller than some arbitrary size (typically less than half a centimeter), which are formed when volcanic gasses produce a fine spray of magma. Volcanic ash that cools quickly, thus preventing the formation of crystals, produces volcanic glass.
welded tuff - a tuff in which the particles were melted together slightly by their own heat. See also tuff.
part of Jeff Medkeff's Notes on Lunar Features
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