Glossary

A

Aa A type of lava flow that has a jagged, blocky surface.

Abrasion The grinding and scraping of a rock surface by the friction and impact of rock particles carried by water, wind, or ice.

Active continental margin Usually narrow and consisting of highly deformed sediments, they occur where oceanic lithosphere is being subducted beneath the margin of a continent.

Angular unconformity An unconformity in which the older strata dip at an angle different from that of the younger beds.

Anticline A fold in sedimentary strata that resembles an arch.

Asthenosphere A subdivision of the mantle situated below the lithosphere. This zone of weak material exists below a depth of about 100 km and in some regions extends as deep as 700 kilometers. The rock within this zone is easily deformed.

Atmosphere The gaseous portion of a planet; the planet’s envelope of air. One of the traditional subdivisions of Earth’s physical environment.

B

Basalt An aphanitic igneous rock of mafic composition.

Basaltic Term used to describe igneous rocks that contain abundant dark (ferromagnesian) minerals and about 50 percent silica.

Batholith A large mass of igneous rock that formed when magma was emplaced at depth, crystallized, and was subsequently exposed by erosion.

Bedding plane A nearly flat surface separating two beds of sedimentary rock. Each bedding plane marks the end of one deposit and the beginning of another having different characteristics.

Bedrock A general term for the rock that underlies soil or other unconsolidated surface materials.

Breccia A sedimentary rock composed of angular fragments that were lithified.

C

Caldera A large depression typically caused by collapse of the summit area of a volcano following a violent eruption.

Cap rock A necessary part of an oil trap, the cap rock is impermeable and hence keeps upwardly mobile oil and gas from escaping at the surface.

Capacity The total amount of sediment a stream is able to transport.

Carrier beds are rock layers that are very permeable and transmit fluids.

Cavern A naturally formed underground chamber or series of chambers most commonly produced by solution activity in limestone.

Chemical weathering The processes by which the internal structure of a mineral is altered by the removal and/or addition of elements.

Cinder cone A rather small volcano built primarily of pyroclastics ejected from a single vent.

Cleavage The tendency of a mineral to break along planes of weak bonding.

Compaction A type of lithification in which the weight of overlying material compresses more deeply buried sediment. It is most important in fine-grained sedimentary rocks such as shale.

Composite cone A volcano composed of both lava flows and pyroclastic material.

Conduit A pipelike opening through which magma moves toward Earth’s surface. It terminates at a surface opening called a vent.

Continental slope The steep gradient that leads to the deep ocean floor and marks the seaward edge of the continental shelf.

Continental volcanic arc Mountains formed in part by igneous activity associated with the subduction of oceanic lithosphere beneath a continent. Examples include the Andes and the Cascades.

Convergent plate boundary A boundary in which two plates move together, resulting in oceanic lithosphere being thrust beneath an overriding plate, eventually to be reabsorbed into the mantle. It can also involve the collision of two continental plates to create a mountain system.

Core Located beneath the mantle, it is Earth’s innermost layer. The core is divided into an outer core and an inner core.

Crater The depression at the summit of a volcano, or that which is produced by a meteorite impact.

Craton That part of the continental crust that has attained stability; that is, it has not been affected by significant tectonic activity during the Phanerozoic eon. It consists of the shield and stable platform.

Creep The slow downhill movement of soil and regolith.

Cross-bedding Structure in which relatively thin layers are inclined at an angle to the main bedding. Formed by currents of wind or water.

Cross-cutting A principle of relative dating. A rock or fault is younger than any rock (or fault) through which it cuts.

Crust The very thin, outermost layer of Earth.

D

Dark silicate Silicate minerals containing ions of iron and/or magnesium in their structure. They are dark in color and have a higher specific gravity than nonferromagnesian silicates.

Debris flow A relatively rapid type of mass wasting that involves a flow of soil and regolith containing a large amount of water; also called mudflows.

Deep-ocean basin The portion of seafloor that lies between the continental margin and the oceanic ridge system. This region comprises almost 30 percent of Earth’s surface.

Deflation The lifting and removal of loose material by wind.

Deformation General term for the processes of folding, faulting, shearing, compression, or extension of rocks as the result of various natural forces.

Density The weight per unit volume of a particular material.

Desert One of the two types of dry climate; the drier of the dry climates.

Dike A tabular-shaped intrusive igneous feature that cuts through the surrounding rock.

Disconformity A type of unconformity in which the beds above and below are parallel.

Discordant A term used to describe plutons that cut across existing rock structures, such as bedding planes.

Distributary A section of a stream that leaves the main flow.

Divergent plate boundary A boundary in which two plates move apart, resulting in upwelling of material from the mantle to create new seafloor.

Dry climate A climate in which the yearly precipitation is less than the potential loss of water by evaporation.

E

Earthquake The vibration of Earth produced by the rapid release of energy.

Elastic deformation Nonpermanent deformation in which rock returns to its original shape when the stress is released.

Erosion The incorporation and transportation of material by a mobile agent, such as water, wind, or ice.

Evaporite A sedimentary rock formed of material deposited from solution by evaporation of water.

External process Process such as weathering, mass wasting, or erosion that is powered by the Sun and transforms solid rock into sediment.

Extrusive Igneous activity that occurs at Earth’s surface.

F

Fault A break in a rock mass along which movement has occurred.

Felsic A term derived from feldspar and silica (quartz). It is a term used to describe granitic igneous rocks.

Fissure A crack in rock along which there is a distinct separation.

Fissure eruption An eruption in which lava is extruded from narrow fractures or cracks in the crust.

Flood The overflow of a stream channel that occurs when discharge exceeds the channel’s capacity. The most common and destructive geologic hazard.

Flow A type of movement common to mass-wasting processes in which water-saturated material moves downslope as a viscous fluid.

Fossil The remains or traces of organisms preserved from the geologic past.

Fracture (rock) Any break or rupture in rock along which no appreciable movement has taken place.

Frost wedging The mechanical breakup of rock caused by the expansion of freezing water in cracks and crevices.

G

Geology The science that examines Earth, its form and composition, and the changes it has undergone and is undergoing.

Geosphere The solid Earth; one of Earth’s four basic spheres.

Glacier A thick mass of ice originating on land from the compaction and recrystallization of snow. The ice shows evidence of past or present flow.

Glassy texture A term used to describe the texture of certain igneous rocks, such as obsidian, that contain no crystals.

Granitic Igneous rocks composed mainly of light-colored silicates (quartz and feldspar) are said to have this composition.

Groundwater Water in the zone of saturation.

H

Hardness A mineral’s resistance to scratching and abrasion.

Hydrologic cycle The unending circulation of Earth’s water supply. The cycle is powered by energy from the Sun and is characterized by continuous exchanges of water among the oceans, the atmosphere, and the continents.

Hydrosphere The water portion of our planet; one of the traditional subdivisions of Earth’s physical environment.

Hydrothermal metamorphism Chemical alterations that occur as hot, ion-rich water circulates through fractures in rock.

I

Ice sheet A very large, thick mass of glacial ice flowing outward in all directions from one or more accumulation centers.

Igneous rock A rock formed by the crystallization of molten magma.

Inclusion A piece of one rock unit contained within another; inclusions are used in relative dating. The rock mass adjacent to the one containing the inclusion must have been there first in order to provide the fragment.

Infiltration The movement of surface water into rock or soil through crack and pore spaces.

Inner core The solid, innermost layer of Earth, about 1216 km in radius.

Intensity (earthquake) A measure of the degree of earthquake shaking at a given locale based on the amount of damage.

Interface A common boundary where different parts of a system interact.

Intermediate Compositional category for igneous rocks found near the middle of Bowen’s reaction series, mainly amphibole and the intermediate plagioclase feldspars.

Intrusive rock Igneous rock that formed below Earth’s surface.

J

Joint A fracture in rock along which there has been no movement.

L

Laccolith A massive, concordant igneous body intruded between preexisting strata.

Lahar Mudflows on the slopes of volcanoes that result when unstable layers of ash and debris become saturated and flow downslope, usually following stream channels.

Laminar flow The movement of water particles in straight-line paths that are parallel to the channel. The water particles move downstream without mixing.

Lava Magma that reaches Earth’s surface.

Lava dome A bulbous mass associated with an old-age volcano, produced when thick lava is slowly squeezed from the vent. Lava domes may act as plugs to deflect subsequent gaseous eruptions.

Law of superposition In any undeformed sequence of sedimentary rocks or surface deposited igneous materials, each layer is older than the one above it.

Light silicate Silicate minerals that lack iron and/or magnesium. They are generally lighter in color and have lower specific gravities than dark silicates.

Lithification The process, generally by cementation and/or compaction, of converting sediments to solid rock.

Lithosphere The rigid outer layer of Earth, including the crust and upper mantle.

Lower mantle The part of the mantle that extends from a depth of 660 km to the top of the core, at a depth of 2900 km.

Luster The appearance or quality of light reflected from the surface of a mineral.

M

Mafic Because basaltic rocks contain a high percentage of ferromagnesian minerals, they are also called mafic (from magnesium and ferrum, the Latin name for iron).

Magma A body of molten rock found at depth, including any dissolved gases and crystals.

Magnitude (earthquake) The total amount of energy released during an earthquake.

Mantle The 2885-km thick layer of Earth located below the crust.

Mass wasting The downslope movement of rock, regolith, and soil under the direct influence of gravity.

Maturity is the degree to which petroleum generation has occurred in a source rock.

Mechanical weathering The physical disintegration of rock, resulting in smaller fragments.

Metamorphic rock Rock formed by the alteration of preexisting rock deep within Earth (but still in the solid state) by heat, pressure, and/or chemically active fluids.

Metamorphism The changes in mineral composition and texture of a rock subjected to high temperature and pressure within Earth.

Migration is the vertical and lateral flow of the petroleum from the source rock.

Mineral A naturally occurring, inorganic crystalline material with a unique chemical structure.

Mineral resource All discovered and undiscovered deposits of a useful mineral that can be extracted now or at some time in the future.

Mohs scale A series of 10 minerals used as a standard in determining hardness.

N

Nebular theory A model for the origin of the solar system that assumes a rotating nebula of dust and gases that contracted to produce the Sun and planets.

Nonconformity An unconformity in which older metamorphic or intrusive igneous rocks are overlain by younger sedimentary strata.

Nonmetallic mineral resource Mineral resource that is not a fuel or processed for the metals it contains.

Numerical date Date that specifies the actual number of years that have passed since an event occurred.

O

Oceanic ridge A continuous mountainous ridge on the floor of all the major ocean basins and varying in width from 500 to 5000 km. The rifts at the crests of these ridges represent divergent plate boundaries.

Offshore zone The relatively flat submerged zone that extends from the breaker line to the edge of the continental shelf.

Oil field brine is very salty water that shared the pores with the oil.

Oil-water contact is the boundary between the oil and water reservoir.

Oil trap A geologic structure that allows for significant amounts of oil and gas to accumulate.

Ore Usually a useful metallic mineral that can be mined at a profit. The term is also applied to certain nonmetallic minerals such as fluorite and sulfur.

Organic sedimentary rock Sedimentary rock composed of organic carbon from the remains of plants that died and accumulated on the floor of a swamp. Coal is the primary example.

Outer core A layer beneath the mantle about 2270 km thick that has the properties of a liquid.

Outlet glacier A tongue of ice normally flowing rapidly outward from an ice cap or ice sheet, usually through mountainous terrain to the sea.

P

Pahoehoe A lava flow with a smooth-to-ropy surface.

Paleoclimatology The study of ancient climates; the study of climate and climate change prior to the period of instrumental records using proxy data.

Paleontology The systematic study of fossils and the history of life on Earth.

Pangaea The proposed supercontinent that 200 million years ago began to break apart and form the present landmasses.

Parent rock The rock from which a metamorphic rock formed.

Partial melting The process by which most igneous rocks melt. Because individual minerals have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees. If the liquid is squeezed out after some melting has occurred, a melt with a higher silica content results.

Pegmatite A very coarse-grained igneous rock (typically granite) commonly found as a dike associated with a large mass of plutonic rock that has smaller crystals. Crystallization in a waterrich environment is believed to be responsible for the very large crystals.

Pegmatitic texture A texture of igneous rocks in which the interlocking crystals are all larger

than one centimeter in diameter.

Peridotite An igneous rock of ultramafic composition thought to be abundant in the upper mantle.

Permeability is a measure of the ease with which a fluid can flow through a rock.

Phaneritic texture An igneous rock texture in which the crystals are roughly equal in size and large enough so that individual minerals can be identified with the unaided eye.

Physical geology A major division of geology that examines the materials of Earth and seeks to understand the processes and forces acting upon Earth’s surface from below.

Pillow lava Basaltic lava that solidifies in an underwater environment and develops a structure that resembles a pile of pillows.

Pipe A vertical conduit through which magmatic materials have passed.

Plate tectonics The theory that proposes Earth’s outer shell consists of individual plates, which interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself.

Porosity is the volume of open spaces in rock or soil.

Porphyritic texture An igneous rock texture characterized by two distinctively different crystal sizes. The larger crystals are called phenocrysts, and the matrix of smaller crystals is termed the groundmass.

Pyroclastic flow A highly heated mixture, largely of ash and pumice fragments, travelling down the flanks of a volcano or along the surface of the ground.

R

Rainshadow desert A dry area on the lee side of a mountain range; many middle-latitude deserts are of this type.

Regolith The layer of rock and mineral fragments that nearly everywhere covers Earth’s land surface.

Relative dating Rocks are placed in their proper sequence or order; only the chronological order of events is determined.

Reservoir rock is a rock that can both store and transmit fluids.

River A general term for a stream that carries a substantial amount of water and has numerous tributaries.

Rock A consolidated mixture of minerals.

Rock cycle A model that illustrates the origin of the three basic rock types and the interrelatedness of Earth’s materials and processes.

Rockslide The rapid slide of a mass of rock downslope along planes of weakness.

Runoff Water that flows over the land rather than infiltrating into the ground.

S

Saturation is the relative amount of the water and oil or gas sharing the pores of the reservoir which vary from reservoir to reservoir.

Seafloor spreading The hypothesis first proposed in the 1960s by Harry Hess, suggesting that new oceanic crust is produced at the crests of mid-ocean ridges, which are the sites of divergence.

Sediment Unconsolidated particles created by the weathering and erosion of rock, by chemical precipitations from solution in water, or from the secretions of organisms, and transported by water, wind, or glaciers.

Sedimentary rock Rock formed from the weathered products of preexisting rocks that have been transported, deposited, and lithified.

Seismic contractor is a company that owns and operates the seismic equipment and runs the seismic survey

Sheeting A mechanical weathering process characterized by the splitting off of slablike sheets of rock.

Shield A large, relatively flat expanse of ancient metamorphic rock within the stable continental interior.

Shield volcano A broad, gently sloping volcano built from fluid basaltic lavas.

Silicate Any one of numerous minerals that have the siliconoxygen tetrahedron as their basic structure.

Slide A movement common to mass-wasting processes in which the material moving downslope remains fairly coherent and moves along a well-defined surface.

Slip face The steep, leeward surface of a sand dune that maintains a slope of about 34 degrees.

Slump The downward slipping of a mass of rock or unconsolidated material moving as a unit along a curved surface.

Snowfield An area where snow persists year-round.

Source rock is a rock that can generate natural gas and/or crude oil.

Soil A combination of mineral and organic matter, water, and air; that portion of the regolith that supports plant growth.

Solar nebula The cloud of interstellar gas and/or dust from which the bodies of our solar system formed.

Steppe One of the two types of dry climate. A marginal and more humid variant of the desert that separates the desert from bordering humid climates.

Strata Parallel layers of sedimentary rock.

Stream A general term to denote the flow of water within any natural channel. Thus, a small creek and a large river are both streams.

Subduction The process of thrusting oceanic lithosphere into the mantle along a convergent zone.

Superposition, law of In any undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below.

Surface soil The upper portion of a soil profile consisting of the O and A horizons.

T

Tectonics The study of the large-scale processes that collectively deform Earth’s crust.

Terrace A flat, benchlike structure produced by a stream that was left elevated as the stream cut downward.

Texture The size, shape, and distribution of the particles that collectively constitute a rock.

Tight (or tight sands) is a very low or no permeability sandstone or limestone.

U

Unconformity A surface that represents a break in the rock record; caused by erosion or nondeposition.

V

Vesicular texture A term applied to aphanitic igneous rocks that contain many small cavities, called vesicles.

Viscosity A measure of a fluid’s resistance to flow.

Volatiles Gaseous components of magma dissolved in the melt. Volatiles will readily vaporize (form a gas) at surface pressures.

Volcanic Pertaining to the activities, structures, or rock types of a volcano.

Volcanic neck An isolated, steepsided, erosional remnant consisting of lava that once occupied the vent of a volcano.

Volcano A mountain formed from lava and/or pyroclastics.

W

Water table The upper level of the saturated zone of groundwater.

Weathering The disintegration and decomposition of rock at or near the surface of the Earth.

Wetting fluid is the fluid that occupies the outside of the pore and is in contact with the rock surface.

Z

Zone of fracture The upper portion of a glacier consisting of brittle ice.

Zone of saturation Zone where all open spaces in sediment and rock are completely filled with water.