Geology is the study of the solid materials of planet, moon, or asteroid including rocks and other surface materials and processes that give rise to them but exclusive of any atmosphere or hydrosphere.
This important science includes examination and analysis of the structure, composition, and history of a terrestrial body's surface and interior; understanding, classification and analysis of various naturally occurring rock types; understanding geochemistry, tectonic forces, erosion, glaciations, and volcanism; geophysics; mapping the geological history of the body in question, and so on. A sub-division of planetology.
- Age - Text by Mark Ryherd
A historical or geological unit of time, shorter than eon, era or epoch.
- Asthenosphere - Text by M. Alan Kazlev
In an Earth-like planetary body, the zone of the upper mantle that exhibits plastic properties. This subsurface layer that is more plastic than adjacent layers because the combination of pressure and temperature places it near (or slightly above) the melting point. Asthenospheric movements may disrupt the planet's surface In an Earth-type planet, it is located below the lithosphere. On Old Earth the asthenosphere is between about 100 and 250 kilometers deep, but on other worlds this figure can vary fairly markedly.
- Basalt - Text by M. Alan Kazlev
Hard, dark igneous rock, composed of 45 to 54 percent silica (SiO2) and often rich in iron and magnesium. Basalt is the most common type of rock in the crusts of many terrestrial worlds (especially Selenian, Arean, Venusian, and Gaian worlds.
- Basin - Text by M. Alan Kazlev
Large impact crater on a planet, usually several hundred kilometres across, flooded with basaltic lava and surrounded by concentric rings of faulted cliffs.
- Breccia - Text by M. Alan Kazlev
A type of rock that is composed of rough, angular pieces of broken-up, older rocks cemented together. Impact craters produce breccia.
Brecciated meteorite: A meteorite formed from cemented fragments of one or more meteorite types.
- Catena - Text by M. Alan Kazlev
A chain of craters on the surface of a moon or planet.
- Cavus - Text by M. Alan Kazlev
Hollows, irregular steep-sided depressions, usually in arrays or clusters, on the surface of a moon or planet.
- Chaos (planetology) - Text by M. Alan Kazlev
A distinctive area of broken terrain on the surface of a moon or planet.
- Chasma - Text by M. Alan Kazlev
A deep, elongated, steep-sided depression on the surface of a moon or planet.
- Colles - Text by M. Alan Kazlev
Small hills or knobs on the surface of a moon or planet.
- Continent (geography) - Text by Stephen Inniss
Originally, one of six or seven very large contiguous landmass on Old Earth, separated or largely separated from other continents by ocean. Some Gaian class worlds, some terraformed worlds, and some extremely large habitats such as Banks Orbitals may also have land masses that are regarded by their inhabitants as continents.
- Continent (geology) - Text by Stephen Inniss
On Old Earth, or on Gaian style worlds exhibiting similar patterns of plate tectonics, a large platform of metamorphic rock and largely granitic igneous rock, covered over much of its area by relatively thin layers of sedimentary rocks.
- Continental Drift - Text by M. Alan Kazlev
The slow movement of crustal plates (usually bearing continents or terranes) on a tectonically active eoarean, eogaian, or gaian world. The plates float over the molten asthenosphere.
- Continental Shield - Text by M. Alan Kazlev
A stable, ancient region, usually flat to slightly convex, in a continent on a world that experiences continental drift; the core from which the continent has grown over time.
- Crater - Text by M. Alan Kazlev
A circular depression caused by an impact with another celestial body, or a volcanic eruption. One of the most common features on asteroid, planetoid, and terrestrial class bodies with hard rocky crusts or surfaces.
- Crystal - Text by Stephen Inniss; original by M. Alan Kazlev
In materials science and solid state physics, any solid material, whether natural or artificial, inorganic or organic, in which the molecules, ions, or atoms are arranged in an orderly repeating pattern that extends in all three spatial directions. An early benefit of nanoscale technologies was improved manipulation of crystal size and type in bulk materials; the various substances termed 'diamondoid' are but one example of this. Colloquially, a crystal is any object that exhibits a well defined geometric shape as a result of its internal order. Such objects are often aesthetically pleasing, and may have artistic and cultural value.
- Depressio - Text by Steve Bowers
A lowland or depression on a planetary surface, generally well below datum level (plural: depressiones)
- Dike, magmatic - Text by M. Alan Kazlev
On Terrestrial Class worlds with active geology, a vertical or near-vertical sheet-like intrusion of magma that forces its way through a layer of fractured rock. The magma flows through cracks in the rock and later cools and solidifies into a sheet of igneous rock. Magmatic intrusions that are closer to horizontal or run along sedimentary bedding planes are called sills.
- Ejecta - Text by M. Alan Kazlev
In geology or planetology, material thrown from a volcanic eruption or as a result of a bolide impact crater, or exploding star. In virch or computational environments, virtual and informational ejecta resulting from catastrophic collapse of a large simm or evolving digital structure.
- Eon - Text by M. Alan Kazlev
Two or more geological Eras. The Eon is the largest division of geologic time, lasting many hundreds or even several thousands of millions of years, and is defined by particular planetological, geological or biological processes.
- Epoch (Geology) - Text by M. Alan Kazlev and Mark Ryherd
A division of geological time, lasting several million years or less. Epochs are grouped into periods, and divided into ages.
- Era - Text by M. Alan Kazlev
 Two or more geological periods. An era may be hundreds of millions of years in duration, and is defined by particular geological or biological processes.
 An extended historical, or even galactic, period of time, that is characterized by particular historical, astronomical, or even cosmological events.
- Eruption Cloud - Text by M. Alan Kazlev
Also, known as ash cloud. The cloud of ash, gas and rock fragments that forms after some volcanic eruptions.
- Eruptive Center - Text by M. Alan Kazlev
Active volcanic centers on Ionian type worlds.
- Eruptive Vent - Text by M. Alan Kazlev
A vent from which volcanic material is released.
- Farrum - Text by M. Alan Kazlev
Pancake-like structure, or a row of such structures, on the surface of a planet or a moon (pl. farra).
- Flexus - Text by M. Alan Kazlev
A very low curvilinear ridge with a scalloped pattern, on the surface of a planet or a moon.
- Fluctus - Text by M. Alan Kazlev
Flow terrain on the surface of a planet or a moon.
- Fold Mountains - Text by M. Alan Kazlev
On Gaian Type Worlds with active plate tectonics, the type of mountain range formed by two continental plates colliding. The colliding crust is compressed and pushed upwards (or provolved), forming mountains.
- Fossa - Text by M. Alan Kazlev
Long, narrow, shallow depressions on the surface of a planet.
- Freelance Prospector - Text by M. Alan Kazlev
Individualist miners or artifact fossickers away from the developed worlds
- Fumarole - Text by M. Alan Kazlev
A vent from which volcanic gases such as sulphur vapor, escape. Fumaroles can occur along small cracks or long fissures on tectonically active Terrestrial class worlds.
- Geosphere - Text by M. Alan Kazlev
In a Terrestrial Class planet, the rocky mass of the world, everything apart from atmosphere and ocean. In the case of Gaian Type terrestrial planets includes continental and oceanic crust as well as the various layers of the planet's interior. The interior of the Terrestrial Class in general and the Gaian Types in particular is layered both chemically and mechanically. While some terrestrial class worlds have a static geosphere, in almost all Gaian Types worlds and some other terrestrial types the geosphere is active, with the crust is in a constant state of motion that gives rise to movement of the continents. The study of the geosphere is known as geology.
- Graben - Text by M. Alan Kazlev
A valley between two faults. A rift-valley.
- Granite - Text by M. Alan Kazlev
Igneous rock that is light-colored and coarse grained. Granite has a modest density and high silica content, and is characterized by the minerals orthoclase and quartz with some plagioclase feldspar and iron-magnesium minerals. It is formed in association with differentiation processes where, being low in density, it tends to float to the surfaces of planets that have had extensive melting in the outer layers. Granite is found primarily on Gaian Type worlds, where it underlies much of the continental crust.
- Hydrology - Text by M. Alan Kazlev
The science and engineering application of water or other fluids. The study of liquid water (or other liquids, e.g. ammonia, methane, etc. in exotic environments), including its physical and chemical properties, geographical distribution, fluid dynamics, interaction with the surrounding landforms (rivers, flooding, erosion, etc.), engineering for the creation and maintenance of rivers, canals, lakes, dams, seas and oceans, and the water cycle in ecology and planetology, and the part that water plays in making terraformed planets and megastructures habitable to biological organisms.
- Ice Age - Text by Steve Bowers
Any period of prolonged and widespread glaciation on a terrestrial world
- Igneous Intrusion - Text by M. Alan Kazlev
Also called a laccolith or a plutonic formation; a geological formation in which magma is trapped beneath the surface of the Earth and pushes the rock located above it into a dome shape, with a flat base and a convex upper surface. Once the fractured sedimentary rock is eroded, the underlying magma is exposed.
- Igneous Rock - Text by M. Alan Kazlev
Formed from cooling of molten rock. Some igneous rocks include granite, obsidian (volcanic glass), basalt, and andesite porphyry.
- Impact Basin - Text by M. Alan Kazlev
Generally, an impact crater that has a rim diameter greater than 300 kilometres. Such large catastrophic impacts produce faulting and other crust deformations. Material ejected from impact basins is distributed over wide areas.
- Impact Crater - Text by M. Alan Kazlev
Result of a bolide impact upon a Terrestrial Class planet or moon; as distinct from a volcanic crater.
- Impact Melt - Text by M. Alan Kazlev
When an object like an asteroid, comet, or meteorite strikes a Terrestrial Class planet or moon, forming a crater, the energy released at impact causes extremely high temperatures which can melt some surrounding rock, forming pools or sheets of lava at the bottom of the crater. Such impact melt varies greatly in texture, but is uniform in its composition, made mostly of the planet's or moon's rocks, but also containing a bit of the impactor material.
- Labes - Text by M. Alan Kazlev
On a Terrestrial Class world, a Landslide geological feature.
- Labyrinthus - Text by M. Alan Kazlev
On a Terrestrial Class world, a complex of intersecting valleys (pl. labyrinthi).
- Laccolith - Text by M. Alan Kazlev
A geological formation found on actively sedimentary and usually Gaian Type worlds; in which magma trapped beneath the surface pushes the rock located above it into a dome shape.
- Lahar - Text by M. Alan Kazlev
On a Gaian Type world; a mudflow or debris flow; a moving mixture of rock, water, and other debris that falls down the slopes of a volcano and/or a river valley.
- Lava Bomb - Text by M. Alan Kazlev
A chunk of viscous lava that is ejected from a volcano on the surface of a Terrestrial Class world. Since they are still viscous and semi-molten when ejected, they acquire distinctive rounded, aerodynamic shapes. Type of bombs include: bread crust bombs, ribbon bombs, spindle bombs, spheroidal bombs, and "cow-dung" bombs. Collecting and trade in lava bombs is quite popular among some geoaesthetic clades, as they are prized for varied natural shapes.
- Lava Tube - Text by M. Alan Kazlev
Occurring occasionally on geologically active Gaian Type worlds, a tube-like, underground conduit formed by flowing lava. The flowing lava crusts over at the edges and drains out onto lower ground - what is left is a smooth, tube-like tunnel with hanging lava stalactites and a flat floor.
- Linea - Text by M. Alan Kazlev
On a Terrestrial Class world, any dark or bright elongate marking, may be curved or straight (pl. lineae).
- Lithosphere - Text by M. Alan Kazlev
The solid, rocky, outer part of a Gaian Type planet consisting of the crust and the upper mantle; the solid rocky layer in a partially molten or liquid planet.
- Mensa - Text by M. Alan Kazlev
On Terrestrial Class World, a flat-topped prominence with cliff-like edges (pl. mensae).
- Mineral - Text by M. Alan Kazlev
Any naturally-occurring solid of definite chemical composition whose atoms usually form a regular pattern. Artificial or entirely biogenic solids, or amorphous materials like glass, are not considered minerals. Common examples of minerals on the surface of a Gaian type planet are quartz, various feldspars, calcite, or olivine. In the outer reaches of a solar system ice is one of the most abundant minerals on the surface. Rocks may be composed of one or many different kinds of minerals.
- Mons - Text by M. Alan Kazlev
On Terrestrial Class World, a mountain or similar geographic feature (pl. montes).
- Oceanus - Text by M. Alan Kazlev
On a terrestrial class planet or moon, a very large dark area.
- Palus - Text by M. Alan Kazlev
On a Selenian type planet or moon, a small plain (literally : "swamp") (plural : paludes ).
- Patera - Text by M. Alan Kazlev
On a terrestrial class planet or moon, an irregular crater, or a complex one with scalloped edges (plural : paterae).
- Period - Text by M. Alan Kazlev and Mark Ryherd
In geology, originally the basic unit of geological time in which a single type of rock system is formed. In history, a long span of time characterized by a particular set of political, cultural, military, or technological traits.
- Planitia - Text by M. Alan Kazlev
On a terrestrial class planet or moon, a low plain (plural:planitiae).
- Planum - Text by M. Alan Kazlev
On a terrestrial class planet or moon, a plateau or high plain (plural: plana).
- Promontorium - Text by M. Alan Kazlev
On a terrestrial class planet or moon, a feature superfically resembling a "cape"; headland. Plural: promontoria. Abbreviation: PR.
- Regio - Text by M. Alan Kazlev
On a terrestrial class planet or moon, a large area marked by reflectivity or color distinctions from adjacent areas, or a broad geographic region (plural : regiones).
- Regolith - Text by M. Alan Kazlev
A powdery soil layer on the surface of Asteroidal, Planetoidal, Mercurian, or Selenian and other airless bodies caused by meteorite bombardment.
- Rupes - Text by M. Alan Kazlev
On a terrestrial class planet or moon, a scarp.
- Scopulus - Text by M. Alan Kazlev
On a terrestrial class planet or moon, a lobate or irregular scarp.
- Sedimentary Rock - Text by M. Alan Kazlev
Rock formed from accumulated deposition of sediments. Most often pertain to Gaian Type worlds.
- Sill, magmatic - Text by Stephen Inniss
On terrestrial class worlds with an active geology, a tabular sheet of rock resulting from the intrusion of magma between layers of sedimentary rock, older beds of volcanic rock, or along the foliation planes of metamorphic rock. Unlike a dike, a sill is initially close to horizontal, though later events may turn the rock layers up at an angle. As on Old Earth, such intrusions may carry important ore deposits of rare elements.
- Sinus - Text by M. Alan Kazlev
On a terrestrial class planet or moon, a "bay"-like feature - i.e. a small plain.
- Sulcus - Text by M. Alan Kazlev
On a terrestrial class planet or moon, subparallel furrows and ridges.
- Tectonics - Text by M. Alan Kazlev
Disruption of planetary or satellite surfaces by large-scale mass movements, such as faulting.
- Terra - Text by M. Alan Kazlev
On a terrestrial class planet or moon, an extensive (usually raised, e.g. above a plain or ocean) land mass.
- Tessera - Text by M. Alan Kazlev
On a terrestrial class planet or moon, tile-like, polygonal terrain.
- Tholus - Text by M. Alan Kazlev
On a terrestrial class planet or moon, a small dome shaped mountain or hill (plural : tholi).
- Triassic Period - Text by M. Alan Kazlev
The first period of the Mesozoic Era, Old Earth, lasting from 251 to 199 million years ago.
- Ultrabasic Rock - Text by M. Alan Kazlev
A rock of high density, low silica content, and high iron content, often derived from the upper mantle of a terrestrial class planet or satellite.
- Undae - Text by M. Alan Kazlev
On a terrestrial class planet, wind-blown dust, sand or pebble dunes.
- Vallis - Text by M. Alan Kazlev
On a terrestrial class planet, a valley.(Plural: valles.)
- Vastitas - Text by M. Alan Kazlev
On a terrestrial class planet, an extensive plain. (Plural: vastitates)
Text by M. Alan Kazlev
from an original by Robert J. Hall
Initially published on 28 October 2001.