Glossary of terms
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A-B | C-D | E-F | G-H | I-K | L-N | O-Q | R-T | U-Z | [Sources]

obduct: to push (one crustal plate of the earth) on top of another. –obduction: the process of obducting; thrusting of a crustal plate upwards over the margin of an adjacent plate. [AHDOS]

obsequent: flowing in a direction opposite to the dip or tilt of the surface strata. [AHDOS]

obsidian: a hard, dark, glassy rock that is formed when rhyolitic lava cools; volcanic glass. [AHDOS]

oceanizatioin: the transformation of continental crust into oceanic crust. [AHDOS]

octahedrite: anatase; a common type of metallic meteorite. [AHDOS]

odontolite: a fossil tooth or bone colored blue by iron phosphate (vivianite). [AHDOS]

Oligocene: the thrid epoch of the Tertiary period of the Cenozoic era, after the Eocene and before the Miocene, during which the first apes appeared and modern mammals became dominant; the rocks formed in this epoch. [AHDOS]

oligoclase: a plagioclase containing more sodium than calcium, occurring in light gray, yellow, or greenish crystals. [AHDOS]

olivenite: a mineral, an arsenate of copper, usually occurring in olive-green crystals or masses. [AHDOS]

olivine: a greenish or yellow mineral, a silicate of magnesium and iron found in mafic and ultramafic igneous rock. [AHDOS]

oolite: a rock, usually limestone, composed of rounded concretions of calcium carbonate resembling the roe of fish; any of the rounded concretions of which his rock is composed, each having a concentric layered structure. [AHDOS]

ooze: a soft mud or slime, especially at the bottom of a pond or river or on the ocean bottom. [AHDOS]

opal: a mineral, an amorphous form of hydrous silica, somewhat like quartz, found in many varieties and colors, certain of which reflect light with an iridescent play of color. Black opals show brilliant colored lights against a dark background; some are so dark as to seem almost black. Milk opals are mily white with rather pale lights. Fire opals are similar with more red and sometimes yellow flashes of color. [AHDOS]

opalescence: the quality of reflecting an iridescent play of colors like that of an opal. [AHDOS]

ophiolite: any of a group of mafic and ultramafic igneous rocks, ranging from basalt to peridotite and including those rich in serpentine. [AHDOS]

ophitic: (of igneous rocks) having a texture characterized by crystals of plagioclase embedded in a matrix of augite. Also called diabasic. [AHDOS]

Ordovician: a geological period, the second in the Paleozoic era, after the Cambrian and before the Silurian. The Ordovician is characterized by the appearance of jawless fishes and the development of many trilobites, brachiopods and other invertibrates; the rocks formed in this period. [AHDOS]

ore: a mineral aggregate or rock containing a high enough concentration of one or more economically valuable metals or minerals to make mining it profitable; a natural substance yielding a nonmetallic material, such as sulfur. [AHDOS]

orogenic: having to do with the formation of mountains. [AHDOS]

orogeny: the formation of mountains, as by the folding of the earth's crust. [AHDOS]

orpiment: a bright-yellow mineral, arsenic trisulfide, found in soft, foliated masses or prepared synthetically as ayellow powder, uased as a pigment. [AHDOS]

outer core: the thrid of the four layers of the earth, lying betweent he mantle and the inner core. [AHDOS]

outlier: a part of a stratified formation left detached through the removal of surrounding parts by denudation. An outlier is surrounded by rocks older than itself. [AHDOS]

outwash plain: a broad, flat plain created by the deposition of sediment carried by the meltwater of a glacier. [AHDOS]

overburden: clay, rock, etc., which has to be removed to get to a deposit of ore. [AHDOS]

overthrust fault: a fault in which one section of crust has ridden up over another. [AHDOS]

ozocerite: a wax-like fossil resin of brownish-yelolow color and aromatic odor, consisting of a mixture of natural hydrocarbons, and sometimes occurring in sandstones. [AHDOS]

pahoehoe: a type of basaltic lava that has hardened into a smooth, undulating, shiny surface. [AHDOS]

paleobiochemistry: a branch of paleontology that deals with the biochemical constituents of fossil organisms. [AHDOS]

paleobotany: the branch of paleontology dealing with fossil plants. [AHDOS]

Paleocene: the earliest epoch of t he Tertiary period, before the Eocene, during which shallow inland seas drained and the first primates appeared. [AHDOS]

paleochronology: the dating of fossil animals and plants, as by counting the ridges on fossil shells and corals. [AHDOS]

paleoclimate: a prehistoric climate. [AHDOS]

paleoecology: the study of the relationship of living things to environment and each other in prehistoric times; the ecology of prehistoric life. [AHDOS]

paleofauna: the fossil fauna of a geological formation or period. [AHDOS]

paleoflora: the fossil flora of a geological formation or period. [AHDOS]

Paleogene: the first of two periods of the Cenozoic era, consisting of the Paleocene, Eocene and Oligocene epochs. [AHDOS]

paleogeography: the geography of prehistoric or geological time. [AHDOS]

paleontology: the science of the forms of life existing in prehistoric time, as represented by fossil animals and plants. [AHDOS]

paleotemperature: the temperature of oceans and seas in prehistoric times, obtained by measuring or analyzing the chemical components of fossil sediments. [AHDOS]

Paleozoic: the era after the Proerozoic and before the Mesozoic, characterized by the development of the first fishes, land plants, amphibians, reptiles, insects, and forests of fernlike trees; the rocks formed in this era. [AHDOS]

paleozoology: the branch of paleontology dealing with fossil animals. [AHDOS]

palisade: a line of high, steep cliffs. [AHDOS]

paludal: of or having to do with a marsh; marshy. [AHDOS]

pan: a natural hollow or depression in the ground, especially one in which water stands temporarily and evaporates, leaving a crust of salt. [AHDOS]

Pangea: a hypothetical supercontinent that included all the land masses of the earth before the Triassic period, when continental droft began with the breaking away of the northern group (Laurasia) from the southern group (Gondwana). [AHDOS]

pan ice: blocks or pieces of ice formed along the shore, and afterwards loosened and driven by winds and currents. [AHDOS]

paragenesis: the occurrence together of several related minerals, from whose textures a sequence of formation may be determined. [AHDOS]

parasite cone: a cinder cone on the side of a volcano. [AHDOS]

parting: a layer of rock or clay lying between two beds of different formations; a surface along which a rock is naturally separable into layers; the breaking of a mineral along parallel planes of weakness, resulting from strain or twinnig. [AHDOS]

peat: a kind of heavy turf made up of unconsolidated, partly decomposed vegetable matter, such as a sphagnum moss, found in peat bogs. [AHDOS]

peat bog: a bog in whihc peat has formed and accumulated through partial decomposition of mosses, sedges, trees, etc. [AHDOS]

pedogenesis: the manner in which soil originates and develops. [AHDOS]

pedology: the science dealing witht eh origin, classification, and utilization of soils. [AHDOS]

pegmatite: a very coarse-grained igneous rock occurring in sills and dikes, and usually containing crystals of the common minerals found in granite, but sometimes containing rare minerals rich in such elements as lithium, uranium, and tantalum. [AHDOS]

pelite: a rock composed especially of particles of mud and clay. [AHDOS]

peneplain: a formerly mountainous or hilly area reduced nearly to a plain by prolonged erosion. [AHDOS]

Pennsylvanian: the sixth period of teh Paleozoic era, after the Mississippian and before the Permian, characterized by coal-m oil-, and gas-bearing deposits and cyclic sedimentation; Upper Carboniferous. [AHDOS]

period: one of the subdivisions of an era. A period is divided itno epochs. [AHDOS]

perlite: a form of hydrated obsidian or other vitreous rock broken up by minute spherical cracks. [AHDOS]

Permian: the last period of the Paleozoic, after the Pennsylvanian and before the Triassic. It was characterized by the end of the trilobites, the spread of the reptiles, and widespread basaltic vulanism; rocks formed in this period. [AHDOS]

permineralization: a process of fossil petrification in which minerals replace bones, shells, or woody tissue without changing the original shape of the fossil. [AHDOS]

petalite: a rare mineral, a silicate of aluminum and lithiium, occurring in pegmatites in white masses, often tinged with black, gray, red, or green. [AHDOS]

petrifaction or petrification: the process of turning organic materials into rock by the replacement of the organic matter with minerals. [AHDOS]

petrify: to turn into rock by petrification; change (organic material) into rock. [AHDOS]

petrogenesis: the origin of rocks, especially as a subject of scientific study. [AHDOS]

Panerozoic: of or having to do with the eon comprising the Paleooic, Mesozoic, and Cenozoic eras and the phenomena of that age. [AHDOS]

pheoncryst: any of a large or conspicuous crystals in a porphyritic rock. [AHDOS]

phosphorite: a rock consisting chiefly of calcium phosphate. It is a primary source of phosphorus for agriculture. [AHDOS]

phyllite: a low-grade metamorphic rock, intermediate between shale and schist and consisting chiefly of mica or chlorite. [AHDOS]

piedmont: lying or formed at the base of a mountain range; a piedmont area or regionl. [AHDOS]

pigeonite: a variety of clinopyroxene similar to augite but with less calcium, occurring in basalt, gabbro, and other rocks. [AHDOS]

pillow lava: submarine lava, usually of a basaltic kind, found in the form of round, closely packed masses. [AHDOS]

pinnacle: a high peak or point of rock. [AHDOS]

placer: a deposit of sand, gravel, or eath in the bed of a stream, containing particles of gold or other valuable minerals with high specific gravities. [AHDOS]

plagioclase: one of the two major types of feldspar, containing sodium or calcium (usually both) and having its two prominent cleavage directions oblique to one another. [AHDOS]

plain: a comparatively flat stretch of land; tract of level or nearly level land; any broad, level expanse, such as part of the sea floor or a lunar sea. [AHDOS]

plate techtonics: a theory that the earth's crust is divided into a series of vast, platelike parts that move or drift as distinct masses. [AHDOS]

Pleistocene: the epoch before the present period; the ice age; of or having to do with the Pleistocene or its deposits. [AHDOS]

Pliocene: the latest epoch of the Tertiary period, during which the first manlike apes appeared. The Pliocene was also marked by the rising of mountains in western America, and the migration of mammals between continents; the rocks formed in this epoch. [AHDOS]

plug: a cylindrical mass of igneous rock formed in the throat or conduit of a vlocano. [AHDOS]

pluton: a body of plutonic rock. [AHDOS]

plutonic: having to do with a class of intrusive igneous rocks that have solidified far below the earth's surface; or or having to do with the theory that the present condition of the earth's crust is mainly due to igneous action. [AHDOS]

pluvial: of, characterized by, or resulting from prolonged rainfall; a period of prolonged rainfall. [AHDOS]

pneumatolysis: the alteration of rocks and formation of minerals by the chemical action of vapors given off from igneous magmas. [AHDOS]

pocket: a small mass of ore, or the cavity containing it; a natural underground cavity containing water. [AHDOS]

polybasite: a blackish mineral with a metallic luster, consisting essentially of silver, sulfur, and antimony. It is a valuable silver ore. [AHDOS]

polyhalite: a mineral consisting essentially of a hydrated sulfate or calcium, magnesium, and potassium, occurring usaually in fibrous masses, and of a brick-red color caused by the presence of hematite. It is an ore of potassium. [AHDOS]

porphyritic: of, having to do with, containing, or resembling porphyry; of the nature of structure characteristic of prophyry; containing distinct crystals embedded in a compact groundmass. [AHDOS]

porphyroid: a rock resembling porphyry or of porphyritic structure, especially a sedimentary or igneous rock that has been altred by some metamorphic agency and has taken on a more or less perfectly developed porphyritic structure. [AHDOS]

porphyry: an igneous rock in which coarse euhedral crystals are scattered through a groundmass of finer-grained minerals or a glassy matrix. [AHDOS]

postglacial: coming after the galcial period or ice age; a postglacial period or deposit. [AHDOS]

potash feldspar: a fledspar having a high potassium content. [AHDOS]

Precambrian: the earliest era in geological time, emcompassing all the time before the Cambrian period; rocks formed in the Precambrian. [AHDOS]

prehnite: a mineral consisiting of a hydrous silicate of aluminum and calcium, occurring in crystalline aggregates, and usually of a pale-green color. [AHDOS]

pressure ridge: a ridge of ice in arctic waters caused by lateral pressure. [AHDOS]

primary: (of ricks) formed directly from magma; formed from the same time as the enclosing rock. [AHDOS]

primary wave: an earthquake wave traveling through solids faster than the secondary wave, and causing rocks to vibrate parallel witht he wave. [AHDOS]

propylite: a volcanic rock with triclinic feldspars greatly altered by hydrothermal action to carbonate, chlorite and other minerals. Propylite is an andesitic rock, and it occurs in regions of silver deposits. [AHDOS]

Proterozoic: the later of two divisions of the Precambrian time, during which sponges, sea worms, and other forms of sea life appeared; the rocks formed during this time. [AHDOS]

proustite: a mineral consisting of a sulfide of arsenic and silver, occurring in crystals or masses of a red color. [AHDOS]

psephite: any coarse fragmental sedimentary rock. [AHDOS]

pseudomorph: a mineral which has the outward form of another mineral. [AHDOS]

psilomelane: any mixture of black manganese oxide minerals. [AHDOS]

pumice: a light, poruous, glassy lava. Pumice is used, especially when powdered, for cleaning, smoothing, and polishing. [AHDOS]

pyrite: a common yellow mineral with a metallic luster, a compound of iron and sulfur, which resembles and is often mistaken for gold; fool's gold. It is used to make sulfuric acid. [AHDOS]

pyroclastic: composed chiefly of fragments of volcanic origin. [AHDOS]

pyrogenic: produced by fire. [AHDOS]

pyrolusite: native manganese dioxide, used as a source of manganese and in making various chemcials, such as chlorine and oxygen. [AHDOS]

pyromorphite: a mineral consisting of chloride and phosphate of lead, occurring both in crystals and masses that are green, yellow, brown, or whitish in color. [AHDOS]

pyrophyllite: a hydrous silicate of aluminum found in metamorphic rock. It resembles talc and is usually whitish or greenish in color. [AHDOS]

pyroxene: any of a group of silicate minerals, usually calcium, magnesium and iron silicate, often found in igneous rocks. [AHDOS]

pyroxenite: a rock composed in large part, of pyroxene. It is an ultramafic igneous rock. [AHDOS]

pyrrhotite: a native iron sulfide having a reddish-brown to bronze color and a metallic luster, occurring in crystals and masses. It often contains nickel and is usually slightly magnetic. [AHDOS]

Q scale: a measure of duration of vibrations in the surface of the earth. [AHDOS]

quaquaversal: dipping away in all directions from a center, as the strata of a domed structure. [AHDOS]

quartz: a very hard mineral composed of silica and found in many different types of rocks, such as sandstone and granite. Quartzes vary according to the size and purity of their crystals. those with submicroscopic crystals are divided into fibrous varieties, or chalcedonies, and granular varieties. Crystals of pure quartz are coarse, colorless, and transparetn, and quartz in this form is called rock crystal. Impure colored varieties of quartz include flint, agate, and amethyst. [AHDOS]

quartz glass: a clear, vitreous solid formed from pure quartz that has been melted, characterized by an ability to withstand large and quick temperature changes, chemical inertness, high melting point and a special transparency to infrared, visible and ultraviolet radiations. [AHDOS]

quartzite: a granular rock composed essentially of quartz, and formed by the metamorphism of sandstone. [AHDOS]

Quaternary: the present geological period, the later of the two periods making up the Cenozoic era, and including the Pleistocene and Recent epochs; the deposits made in this period. [AHDOS]

quicksand: a shifting mass of very deep, soft, wet sand that yields easily to pressure and will not hold up a heavy object but slowly engulfs it. [AHDOS]