Earth and Space Terms
Sedimentary Rocks
Sediment: Loose particles of rock, minerals, or organic material.
Clastic: Sedimentary rocks made from fragments of other rocks.
Detrital Sedimentary Rock: Another name for clastic sedimentary rock.
Biochemical: Sedimentary rocks formed from the remains of living things.
Chemical Sedimentary Rock: Rocks formed from minerals precipitated from water.
Nonclastic: Sedimentary rocks not formed from fragments of other rocks (includes biochemical and chemical types).
Compaction: The process where sediment is squeezed together by the weight of overlying material.
Cementation: The process where minerals precipitate in the spaces between sediment grains, gluing them together.
Lithification: The process of turning sediment into rock, including compaction and cementation.
Beds (Strata): Layers of sedimentary rock.
Bedding Plane: The boundary between two layers of sedimentary rock.
Cross-bedding: Layers within a bed that are inclined to the main bedding planes, indicating current direction.
Graded Bed: A bed where the grain size changes from coarse at the bottom to fine at the top.
Mud Crack: Cracks that form in mud as it dries.
Ripple Mark: Wavy features formed on the surface of sediment by wind or water currents.
Salt Flat: A flat area covered with salt deposits, formed by the evaporation of water.
Evaporate Deposit: A sedimentary rock formed from minerals that precipitate as water evaporates.
Facies: A body of rock with specific characteristics that reflect the environment in which it was formed.
Fissility: The tendency of a rock to split along closely spaced planes.
Fossil: Preserved remains or traces of organisms.
Sorting: The process by which sediment grains are separated by size or density.
Metamorphic Rocks
Metamorphism: The process by which rocks are changed by heat, pressure, or chemical reactions.
Aureole: A zone of altered rock around an igneous intrusion.
Contact Metamorphism: Metamorphism that occurs when rock is heated by contact with magma.
Regional Metamorphism: Metamorphism that occurs over a large area, usually due to tectonic forces.
Foliation: The layering or banding in metamorphic rocks caused by the alignment of minerals.
Nonfoliated: Metamorphic rocks that do not have a layered or banded appearance.
Rock Cleavage: The tendency of a rock to break along specific planes.
Slaty Cleavage: A type of rock cleavage that causes rocks to break into thin, flat slabs.
Schistosity: A type of foliation in which platy minerals are aligned to form a layered structure.
Gneiss: A metamorphic rock with a banded appearance.
Migmatite: A rock that is part igneous and part metamorphic.
Hydrothermal Solution: Hot, chemically active fluids within the Earth.
Index Mineral: A mineral that is characteristic of a particular metamorphic grade.
Shear: A type of stress that causes rocks to slide past each other.
Stress: Force acting on a rock.
Shield: A large area of exposed Precambrian rock.
Streams and Rivers
Stream: A channel that carries flowing water.
Alluvial Fan: A fan-shaped deposit of sediment formed where a stream flows out of a mountain canyon.
Alluvium: Sediment deposited by a stream.
Antecedent Stream: A stream that maintains its course even as the land around it is uplifted.
Back Swamp: A low-lying area on a floodplain that is poorly drained.
Bar: A deposit of sediment in a stream channel.
Base Level: The lowest point to which a stream can erode.
Bed Load: Sediment that is transported along the bottom of a stream.
Braided Stream: A stream with multiple channels that intertwine.
Capacity: The maximum amount of sediment that a stream can carry.
Competence: The largest size of sediment that a stream can carry.
Cut Bank: The eroded bank on the outside of a meander.
Cutoff: A shortcut formed when a meander is cut off.
Delta: A deposit of sediment formed where a river enters a lake or ocean.
Dendritic Pattern: A branching stream pattern.
Discharge: The volume of water that flows past a point in a stream in a given time.
Dissolved Load: Material that is dissolved in stream water.
Distributary: A small stream that branches out from a larger stream in a delta.
Divide: The boundary between two drainage basins.
Drainage Basin: The area of land drained by a stream and its tributaries.
Entrenched Meander: A meander that is cut deep into the ground.
Evapotranspiration: The combined loss of water from a surface by evaporation and transpiration.
Floodplain: A flat area adjacent to a stream that is subject to flooding.
Graded Stream: A stream in which the slope and channel characteristics are adjusted so that the stream can transport its load efficiently.
Gradient: The slope of a stream channel.
Head (Headwaters): The source of a stream.
Headward Erosion: Erosion that occurs at the head of a stream.
Hydrologic Cycle: The continuous movement of water between the oceans, atmosphere, and land.
Infiltration: The process by which water soaks into the ground.
Infiltration Capacity: The maximum rate at which water can infiltrate the ground.
Lag Time: The time between the peak of rainfall and the peak of streamflow.
Laminar Flow: Smooth, layered flow of water.
Local (Temporary) Base Level: A temporary base level, such as a lake or reservoir.
Longitudinal Profile: A graph of the elevation of a stream channel along its length.
Meander: A bend in a stream.
Meander Scar: A mark left on the landscape by a former meander.
Mouth: The point where a stream flows into a larger body of water.
Natural Levee: A raised bank along a stream channel, formed by the deposition of sediment during floods.
Oxbow Lake: A lake formed when a meander is cut off.
Peneplain: A nearly flat, featureless landscape formed by erosion.
Playfair's Law: The law that states that valleys will erode to form a graded profile.
Point Bar: A deposit of sediment on the inside of a meander.
Pothole: A hole eroded in the bedrock by the swirling action of water and sediment.
Radial Pattern: A stream pattern in which streams flow outward from a central point.
Rapids: A section of a stream with fast-moving, turbulent water.
Rectangular Pattern: A stream pattern with many right angles.
Rejuvenated Stream: A stream that has had its base level lowered, causing it to erode more actively.
Runoff: Water that flows over the surface of the ground.
Saltation: The movement of sediment by a series of short jumps.
Settling Velocity: The speed at which a particle falls through a fluid.
Sheetflow: Unconcentrated flow of water over a surface.
Sorting: The process by which sediment grains are separated by size or density.
Stream Piracy: The capture of the headwaters of one stream by another stream.
Superposed Stream: A stream that maintains its course even as the underlying rock is deformed.
Suspended Load: Sediment that is carried within the water column of a stream.
Terrace: A flat, elevated area along a stream valley.
Transpiration: The release of water vapor from plants.
Trellis Pattern: A stream pattern in which tributaries join the main stream at right angles.
Turbulent Flow: Chaotic, swirling flow of water.
Ultimate Base Level: The lowest possible base level, which is sea level.
Waterfall: A cascade of water over a steep drop.
Water Gap: A gap in a ridge or mountain through which a stream flows.
Wind Gap: A gap in a ridge or mountain that is no longer occupied by a stream.
Yazoo Tributary: A tributary that flows parallel to the main stream for some distance before joining it.
Geologic Time
Absolute Date: A date expressed in years.
Angular Unconformity: When layers of rock are tilted, eroded, and then new layers are deposited on top, creating an angle between the old and new layers.
Archean Eon: The eon from 4.0 to 2.5 billion years ago, when the first life appeared.
Cenozoic Era: The most recent era, from 66 million years ago to the present, known as the "Age of Mammals."
Conformable: When layers of rock are deposited continuously without any significant breaks in time.
Correlation: Matching rocks and fossils from different locations to determine their relative ages.
Cross-cutting Relationships Principle: A principle that states that any geologic feature that cuts across other rocks is younger than the rocks it cuts across.
Daughter Product: The new element that is formed from the radioactive decay of a parent element.
Disconformity: An unconformity between parallel layers of sedimentary rock, representing a period of erosion or non-deposition.
Eon: The largest division of geologic time.
Epoch: A subdivision of a period.
Era: A major division of geologic time.
Faunal Succession Principle: A principle that states that fossils appear in a specific order through time.
Half-life: The time it takes for half of the parent atoms in a sample to decay into daughter atoms.
Inclusions: Fragments of one rock type that are enclosed within another rock type.
Index Fossil: A fossil that is used to date rocks because it lived for a short period of time and was geographically widespread.
Mesozoic Era: The era from 252 to 66 million years ago, known as the "Age of Reptiles."
Nonconformity: An unconformity between sedimentary rocks and metamorphic or igneous rocks.
Original Horizontally Principle: A principle that states that sedimentary layers are originally deposited horizontally.
Paleozoic Era: The era from 541 to 252 million years ago, known as the "Age of Fishes."
Parent: The original element that undergoes radioactive decay.
Period: A subdivision of an era.
Phanerozoic Eon: The eon from 541 million years ago to the present, which includes the Paleozoic, Mesozoic, and Cenozoic eras.
Precambrian: The time from the formation of Earth to 541 million years ago, which includes the Archean and Proterozoic eons.
Proterozoic Eon: The eon from 2.5 billion to 541 million years ago, when the first complex life appeared.
Radioactivity: The process by which some atoms spontaneously decay, releasing energy and particles.
Radiocarbon: A radioactive isotope of carbon that is used to date organic materials up to about 50,000 years old.
Radiometric Dating: A method of dating rocks and minerals using the decay of radioactive isotopes.
Relative Dating: Determining the order in which events occurred, without knowing their exact age in years.
Superposition Law: A principle that states that in undisturbed sedimentary rock layers, the oldest layer is on the bottom and the youngest layer is on the top.
Unconformity: A break in the geologic record representing a period of erosion or non-deposition.
Mass Wasting
Angle of Repose: The steepest angle at which a material can remain stable.
Creep: The slow, gradual movement of soil or rock down a slope.
Debris Slide: A rapid movement of a mass of loose rock and soil down a slope.
Earthflow: A slow to rapid movement of a mass of soil down a slope.
Fall: A free fall of rock or soil from a cliff or steep slope.
Flow: A mass movement in which the material moves as a viscous fluid.
Lahar: A mudflow of volcanic ash and water.
Mass Wasting: The downslope movement of rock and soil due to gravity.
Mudflow: A rapid flow of mud and water.
Permafrost: Permanently frozen ground.
Rock Avalanche/Rockslide: A rapid movement of a large mass of rock down a slope.
Slide/Slump: A mass movement in which a block of material moves along a curved surface.
Solifluction: The slow, downslope flow of soil saturated with water in periglacial environments.
Groundwater
Aquiclude: A layer of impermeable rock or sediment that prevents the flow of groundwater.
Aquifer: A layer of permeable rock or sediment that can store and transmit groundwater.
Artesian: Groundwater that is under pressure and can rise to the surface without pumping.
Belt of Soil Moisture: The uppermost zone of soil where water is held by molecular forces.
Capillary Fringe: The zone above the water table where water is drawn up by capillary action.
Cavern: A large underground chamber.
Cone of Depression: A cone-shaped depression in the water table around a pumping well.
Darcy's Law: A law that describes the flow of groundwater through a porous medium.
Drawdown: The lowering of the water table around a pumping well.
Effluent Stream: A stream that gains water from groundwater.
Flowing Artesian Well: An artesian well in which the water flows to the surface without pumping.
Geyser: A hot spring that erupts periodically, sending a column of hot water and steam into the air.
Groundwater: Water that is stored beneath the surface of the Earth.
Head: The height of a water column above a datum.
Hot Spring: A spring that has a water temperature significantly higher than the average air temperature.
Hydraulic Gradient: The slope of the water table.
Influent Stream: A stream that loses water to groundwater.
Karst Topography: A landscape characterized by caves, sinkholes, and underground drainage, formed by the dissolution of limestone.
Nonflowing Artesian Well: An artesian well in which the water does not flow to the surface without pumping.
Perched Water Table: A water table that sits above the main water table due to an impermeable layer.
Permeability: The ability of a material to transmit fluids.
Porosity: The percentage of open space in a material.
Sinkhole (Sink): A depression in the ground caused by the collapse of a cave roof.
Speleothem: A cave formation, such as a stalactite or stalagmite.
Spring: A place where groundwater flows to the surface.
Stalactite: A speleothem that hangs from the ceiling of a cave.
Stalagmite: A speleothem that grows upward from the floor of a cave.
Water Table: The upper surface of the zone of saturation.
Well: A hole dug into the ground to access groundwater.
Zone of Aeration: The zone in the soil above the water table where the pore spaces are filled with air and water.
Zone of Saturation: The zone in the soil below the water table where the pore spaces are filled with water.
Glaciers
Ablation: The loss of ice from a glacier by melting, evaporation, or calving.
Abrasion: The wearing away of rock by the grinding action of sediment carried by a glacier.
Alpine Glacier: A glacier that forms in mountainous areas.
Arete: A sharp ridge that separates two cirques.
Basal Slip: The movement of a glacier by sliding over its bed.
Calving: The breaking off of icebergs from the front of a glacier.
Cirque: A bowl-shaped depression at the head of a glacier.
Col: A low point in a ridge between two cirques.
Crevasse: A crack in the surface of a glacier.
Drift: Sediment deposited by a glacier.
Drumlin: A streamlined hill of glacial till.
End Moraine: A ridge of till deposited at the terminus of a glacier.
Esker: A long, winding ridge of stratified drift deposited by a stream flowing under a glacier.
Fjord: A long, narrow inlet of the sea with steep sides, formed by a glacier.
Firn: Partially compacted and recrystallized snow.
Glacial Erratic: A large rock that has been transported by a glacier and deposited far from its source.
Glacial Striations: Grooves or scratches in bedrock caused by a glacier.
Glacial Trough: A U-shaped valley carved by a glacier.
Glacier: A large mass of ice that moves slowly over land.
Ground Moraine: A layer of till deposited beneath a glacier.
Hanging Valley: A tributary valley that joins a main valley at a higher elevation.
Horn: A sharp peak formed by the intersection of three or more cirques.
Ice Cap: A dome-shaped mass of ice that covers a relatively small area.
Ice-Contact Deposit: A deposit of sediment formed in contact with a glacier.
Ice Sheet: A vast expanse of ice that covers a large area, such as Greenland or Antarctica.
Ice Shelf: A large, floating sheet of ice that is attached to a coastline.
Kame: A mound of stratified drift deposited by a stream flowing on or within a glacier.
Kame Terrace: A terrace-like deposit of stratified drift formed along the side of a valley glacier.
Kettle: A depression in the ground formed by the melting of a buried block of ice.
Lateral Moraine: A ridge of till deposited along the side of a valley glacier.
Medial Moraine: A ridge of till formed by the merging of two lateral moraines.
Outlet Glacier: A glacier that flows out from an ice cap or ice sheet.
Outwash Plain: A flat area in front of a glacier, covered with stratified drift deposited by meltwater streams.
Pater Noster Lakes: A series of small lakes along a glacial trough, resembling beads on a string.
Piedmont Glacier: A large, lobe-shaped glacier formed where a valley glacier spreads out onto a plain.
Plastic Flow: The movement of ice within a glacier by the deformation of ice crystals.
Pleistocene Epoch: The epoch from 2.58 million to 11,700 years ago, characterized by repeated glaciations.
Plucking: The process by which a glacier lifts blocks of rock from its bed.
Pluvial Lake: A lake formed in an arid region during a period of increased precipitation, often associated with glacial periods.
Recessional Moraine: A ridge of till deposited as a glacier retreats.
Roche Moutonnee: A rock formation with a smooth, gently sloping side and a steep, jagged side, formed by glacial erosion.
Rock Flour: Finely ground rock particles produced by glacial abrasion.
Snowline: The elevation above which snow persists year-round.
Stratified Drift: Sediment deposited by meltwater streams, which is sorted and layered.
Surge: A sudden, rapid movement of a glacier.
Tarn: A small lake in a cirque.
Terminal Moraine: The end moraine marking the farthest extent of a glacier.
Till: Unsorted sediment deposited directly by a glacier.
Tillite: A sedimentary rock composed of till.
Truncated Spur: A ridge of land that has been truncated by a glacier.
Valley Glacier: A glacier that flows down a valley.
Valley Train: A long, narrow deposit of stratified drift along a valley, formed by meltwater streams from a glacier.
Zone of Accumulation: The upper part of a glacier where snow accumulates.
Deserts
Abrasion: The wearing away of rock by the impact of windblown sand.
Alluvial Fan: A fan-shaped deposit of sediment formed where a stream flows out of a mountain canyon into a desert.
Bajada: A gently sloping plain formed by the coalescence of alluvial fans.
Barchan Dune: A crescent-shaped dune with its horns pointing downwind.
Barchanoid Dune: A series of barchan dunes aligned perpendicular to the wind.
Bed Load: Sediment that is transported along the bottom of a desert by wind.
Blowout: A depression in the ground caused by the removal of sand by wind.
Cross Beds: Layers within a dune that are inclined to the main bedding planes.
Deflation: The removal of fine-grained sediment by wind.
Desert: A region with very low precipitation.
Desert Pavement: A surface of closely packed rocks that protects the underlying fine-grained sediment from deflation.
Dry Climate: A climate characterized by low precipitation.
Dune: A mound of sand or other loose sediment that is shaped by the wind.
Inselberg: An isolated hill or mountain rising abruptly from a plain.
Interior Drainage: A drainage system in which water does not reach the ocean.
Loess: A deposit of fine-grained, windblown sediment.
Longitudinal Dune: A long, narrow dune that is aligned parallel to the wind.
Parabolic Dune: A U-shaped dune with its horns pointing upwind.
Pediment: A gently sloping, rocky surface at the base of a mountain.
Playa: A flat area in a desert that is periodically flooded by shallow water.
Playa Lake: A temporary lake that forms in a playa.
Rainshadow Desert: A desert that forms on the leeward side of a mountain range.
Saltation: The movement of sand by a series of short jumps.
Slip Face: The steep, downwind side of a dune.
Star Dune: A dune with three or more arms radiating from a central peak.
Steppe: A dry grassland.
Suspended Load: Fine-grained sediment that is carried in the air by wind.
Transverse Dune: A long, ridge-like dune that is aligned perpendicular to the wind.
Ventifact: A rock that has been shaped by windblown sand.
Wash: A dry stream bed that carries water only during periods of rainfall.
Xerophyte: A plant that is adapted to dry conditions.
Coasts
Abrasion: The wearing away of rock by the grinding action of sediment carried by waves.
Barrier Island: A long, narrow island that runs parallel to the coast.
Baymouth Bar: A spit that extends across the mouth of a bay.
Beach Drift: The movement of sediment along the coast by longshore currents.
Beach Nourishment: The process of adding sand to a beach to combat erosion.
Breakwater: A structure built to protect a harbor or coastline from waves.
Ebb Tide: The falling tide.
Emergent Coast: A coast that is rising relative to sea level.
Estuary: A partially enclosed body of water where freshwater from rivers mixes with saltwater.
Fetch: The distance that wind blows over water.
Flood Tide: The rising tide.
Groin: A structure built perpendicular to the coast to trap sand.
Jetty: A structure built to protect the entrance to a harbor or river.
Longshore Current: A current that flows parallel to the coast.
Neap Tide: A tide with a small tidal range.
Sea Arch: An arch-shaped opening eroded through a sea cliff.
Sea Stack: A pillar of rock isolated from the coast by erosion.
Seawall: A wall built to protect the coast from erosion.
Spit: A long, narrow ridge of sand that extends from the coast into a bay or other body of water.
Spring Tide: A tide with a large tidal range.
Submergent Coast: A coast that is sinking relative to sea level.
Surf: The turbulent water of the ocean waves.
Tidal Current: A current caused by the tides.
Tidal Flats: Flat areas along the coast that are submerged at high tide and exposed at low tide.
Tide: The periodic rise and fall of sea level.
Tombolo: A spit that connects an island to the mainland.
Wave-cut Cliff: A cliff that has been eroded by wave action.
Wave-cut Platform: A flat, rocky surface at the base of a wave-cut cliff.
Wave Height: The vertical distance between the crest and trough of a wave.
Wave Length: The horizontal distance between two successive crests or troughs.
Wave of Oscillation: A wave in which the water particles move in a circular or elliptical path, but do not move forward with the wave.
Wave of Translation: A wave that moves forward, transporting water and energy.
Wave Period: The time it takes for one wave to pass a given point.
Wave Refraction: The bending of waves as they approach a shoreline at an angle.
Geologic Structures
Anticline: An upward fold in rock layers, shaped like an arch.
Basin: A downward fold in rock layers, shaped like a bowl.
Compressional Stress: Stress that squeezes rocks together.
Confining Pressure: Stress that is applied equally in all directions.
Deformation: The process by which rocks change shape or volume in response to stress.
Dip: The angle at which a rock layer is inclined from the horizontal.
Dip-slip Fault: A fault in which the movement is primarily vertical.
Dome: A circular or elliptical upwarp in rock layers.
Elastic Deformation: A temporary change in the shape of a rock that is recovered when the stress is removed.
Fault: A fracture in the Earth's crust along which movement has occurred.
Fault-block Mountain: A mountain range formed by faulting.
Fault Scarp: A cliff or steep slope formed by faulting.
Fold: A bend in rock layers.
Graben: A down-dropped block of rock between two faults.
Hogback: A sharp-crested ridge formed by steeply dipping rock layers.
Horst: An uplifted block of rock between two faults.
Joint: A fracture in rock along which no movement has occurred.
Klippe: A remnant of a thrust sheet that is isolated on the surface.
Monocline: A step-like bend in rock layers.
Normal Fault: A fault in which the hanging wall moves down relative to the footwall.
Oblique-slip Fault: A fault in which the movement is both vertical and horizontal.
Plastic Deformation: A permanent change in the shape of a rock that occurs when the stress exceeds the elastic limit.
Reverse Fault: A fault in which the hanging wall moves up relative to the footwall.
Rock Structure: The arrangement of rocks in the Earth's crust.
Shear: Stress that causes rocks to slide past each other.
Stress: Force acting on a rock.
Strike: The direction of a horizontal line on a rock layer.
Strike-slip Fault: A fault in which the movement is primarily horizontal.
Syncline: A downward fold in rock layers, shaped like a trough.
Tensional Stress: Stress that pulls rocks apart.
Thrust Fault: A reverse fault with a low angle of dip.
Transform Fault: A fault in which the movement is primarily horizontal and parallel to the fault.
Earthquakes
Aftershock: A smaller earthquake that follows a larger earthquake.
Benioff Zone: A zone of earthquakes that dip down into the mantle at subduction zones.
Body Wave: An earthquake wave that travels through the Earth's interior.
Earthquake: A sudden release of energy in the Earth's crust that creates seismic waves.
Elastic Rebound: The theory that earthquakes occur when rocks that are deformed by stress suddenly snap back to their original shape.
Epicenter: The point on the Earth's surface directly above the focus of an earthquake.
Fault: A fracture in the Earth's crust along which movement has occurred.
Focus: The point within the Earth where an earthquake originates.
Foreshock: A small earthquake that precedes a larger earthquake.
Inertia: The tendency of an object to resist changes in motion.
Intensity: A measure of the effects of an earthquake at a particular location.
Long (L) Waves: Surface waves that travel along the Earth's surface and cause most of the damage during an earthquake.
Magnitude: A measure of the amount of energy released by an earthquake.
Mercalli Intensity Scale: A scale that measures the intensity of an earthquake based on its effects on people and structures.
Primary (P) Waves: The fastest body waves that travel through solids, liquids, and gases.
Richter Scale: A scale that measures the magnitude of an earthquake based on the amplitude of seismic waves.
Secondary (S) Waves: Body waves that travel through solids only and are slower than P waves.
Seismic Sea Waves: Tsunamis, which are large ocean waves caused by underwater earthquakes or other disturbances.
Seismogram: A record of earthquake waves.
Seismograph: An instrument that detects and records earthquake waves.
Seismology: The study of earthquakes.
Surface Wave: An earthquake wave that travels along the Earth's surface.
Tsunami: A large ocean wave caused by an underwater earthquake or other disturbance.
Earth's Interior
Asthenosphere: The partially molten layer of the mantle below the lithosphere.
Crust: The outermost layer of the Earth.
Discontinuity: A boundary between two layers of the Earth's interior.
Inner Core: The solid, innermost layer of the Earth.
Lithosphere: The rigid outer layer of the Earth, consisting of the crust and the uppermost part of the mantle.
Low-velocity Zone: A layer in the mantle where seismic waves travel slower than in the surrounding layers.
Mantle: The layer of the Earth between the crust and the core.
Mohorovicic Discontinuity: The boundary between the crust and the mantle.
Outer Core: The liquid layer of the Earth's core.
Shadow Zone: An area where seismic waves cannot travel due to the Earth's core.
Plate Tectonics
Continental Drift: The theory that the continents have moved over time.
Convergent Boundary: A boundary where two plates collide.
Curie Point: The temperature at which a magnetic material loses its magnetization.
Deep-ocean Trench: A long, narrow depression on the ocean floor formed at a subduction zone.
Divergent Boundary: A boundary where two plates move apart.
Hotspot: A place where magma rises from deep within the Earth.
Island Arc: A chain of volcanic islands formed at a subduction zone.
Magnetometer: An instrument that measures the strength and direction of a magnetic field.
Normal Polarity: The present state of the Earth's magnetic field, in which the north magnetic pole is near the north geographic pole.
Paleomagnetism: The study of the Earth's magnetic field in the past.
Pangaea: A supercontinent that existed about 250 million years ago.
Plate: A large, moving piece of the Earth's lithosphere.
Polar Wandering: The apparent movement of the Earth's magnetic poles over time.
Reverse Polarity: A state of the Earth's magnetic field in which the north magnetic pole is near the south geographic pole.
Rift (Rift Valley): A valley formed by the spreading of two plates.
Sea-floor Spreading: The process by which new oceanic crust is formed at mid-ocean ridges and moves away from the ridge.
Subduction Zone: A zone where one plate is forced beneath another plate.
Transform Boundary: A boundary where two plates slide past each other.
Volcanic Arc: A chain of volcanoes formed at a subduction zone.
Oceanography
Abyssal Plain: A flat, deep area of the ocean floor.
Atoll: A ring-shaped coral reef that encloses a lagoon.
Biogenous Sediment: Sediment that is derived from living organisms.
Continental Margin: The zone of the ocean floor that separates the continents from the deep-ocean basin.
Continental Rise: A gentle slope at the base of the continental slope.
Continental Shelf: A shallow, gently sloping area of the ocean floor that extends from the coast to the continental slope.
Continental Slope: A steep slope that connects the continental shelf to the deep-ocean basin.
Deep-ocean Basin: The part of the ocean floor that lies beyond the continental margin.
Echo Sounder: A device that measures the depth of the ocean by sending sound waves to the ocean floor and measuring the time it takes for the echoes to return.
Graded Bedding: A type of bedding in which the grain size of the sediment changes from coarse at the bottom to fine at the top.
Guyot: A flat-topped seamount.
Hot Spot: A place where magma rises from deep within the Earth and creates volcanoes on the ocean floor.
Hydrogenous Sediment: Sediment that is formed from minerals that precipitate from seawater.
Manganese Nodule: A rounded lump of manganese and other metals found on the ocean floor.
Mid-ocean Ridge: A long, underwater mountain range formed at a divergent plate boundary.
Ophiolite Complex: A section of oceanic crust that has been uplifted and exposed on land.
Rift Zone: A zone where the Earth's crust is being pulled apart, such as at a mid-ocean ridge.
Seamount: A submarine volcano.
Sheeted Dike: A series of vertical dikes intruded next to each other, often found in ophiolite complexes.
Submarine Canyon: A deep canyon cut into the continental slope by turbidity currents.
Terrigenous Sediment: Sediment that is derived from the weathering of rocks on land.
Trench: A long, narrow depression on the ocean floor formed at a subduction zone.
Turbidite: A sedimentary rock formed from a turbidity current.
Turbidity Current: A dense current of sediment-laden water that flows down the continental slope.
Wilson Cycle: The cycle of opening and closing of ocean basins.
Orogenesis and Related Terms:
Accretionary Wedge: A mass of sediment that has been scraped off a subducting plate and added to the overriding plate.
Fault-block Mountains: Mountains formed by the movement of blocks of rock along faults.
Isostasy: The state of gravitational equilibrium between the Earth's crust and mantle.
Isostatic Adjustment: The vertical movement of the Earth's crust to maintain isostatic equilibrium.
Orogenesis: The process of mountain building.
Passive Margin: A continental margin that is not tectonically active.
Terrane: A fragment of crust that has been accreted to a continent.
Economic Geology:
Cap Rock: An impermeable layer of rock that prevents oil and gas from escaping.
Disseminated Deposit: A mineral deposit in which the ore minerals are scattered throughout a large volume of rock.
Fossil Fuel: A fuel formed from the remains of ancient organisms, such as coal, oil, and natural gas.
Geothermal Energy: Energy derived from the heat of the Earth.
Hydrothermal Solution: A hot, chemically active fluid within the Earth.
Mineral Resource: A naturally occurring substance that can be used by humans.
Nonmetallic Mineral Resource: A mineral resource that is not a metal, such as sand, gravel, and limestone.
Nonrenewable Resource: A resource that cannot be replenished at a rate comparable to its rate of use.
Nuclear Fission: The splitting of an atom's nucleus, which releases energy.
Oil Trap: A geologic structure that prevents oil and gas from escaping.
Ore: A rock that contains minerals in economically valuable concentrations.
Pegmatite: A very coarse-grained igneous rock.
Placer: A deposit of valuable minerals in a stream or riverbed.
Renewable Resource: A resource that can be replenished at a rate comparable to its rate of use.
Reserve: A known quantity of a resource that can be economically extracted.
Reservoir Rock: A rock that can store oil and gas.
Secondary Enrichment: The process by which minerals are concentrated in a deposit by hydrothermal solutions.
Vein Deposit: A mineral deposit in which the ore minerals are concentrated along fractures in the rock.
Space:
Asteroid: A small, rocky object that orbits the Sun.
Cassini Gap: A gap in the rings of Saturn.
Coma: The fuzzy atmosphere surrounding the nucleus of a comet.
Comet: A small, icy object that orbits the Sun and has a tail when it gets close to the Sun.
Greenhouse Effect: The trapping of heat by gases in the atmosphere.
Iron Meteorite: A meteorite made mostly of iron.
Joivan Planet: A gas giant planet, like Jupiter.
Lunar Breccia: A rock formed from fragments of other lunar rocks.
Lunar Regolith: The layer of loose rock and dust on the Moon's surface.
Maria: Dark, smooth plains on the Moon.
Meteorite: A meteoroid that has entered the Earth's atmosphere and landed on the Earth's surface.
Meteoroid: A small, rocky object in space.
Meteor Shower: A display of meteors that occur when the Earth passes through a stream of meteoroids.
Micrometeorite: A tiny meteoroid.
Nebular Hypothesis: The theory that the solar system formed from a cloud of gas and dust.
Occultation: The obscuration of one celestial body by another.
Ray: A bright streak extending from an impact crater on the Moon.
Stony Meteorite: A meteorite made mostly of rock.
Stony-iron Meteorite: A meteorite made of both rock and iron.
Terrestrial Planet: A rocky planet, like Earth.