Earth: Inside and Out Glossary

abyssal ocean The deep ocean that lies in water depths of 4,000 meters or deeper.

Acasta gneiss The oldest rock dated on Earth, found in the Northwest Territories, Canada.

accretion The process by which solid bodies gather together to form a continent or planet.

aerobic Growing or thriving only in the presence of oxygen.

aerosols Fine particles or liquid droplets suspended in the atmosphere, some of which are byproducts of industrial pollution.

air mass A large, widespread body of air that has the same properties of humidity, temperature, and density (with only slight variations) throughout.

albedo The proportion of incoming solar radiation reflected back into space either by the Earth’s surface or by particles in the atmosphere.

amino acid An organic compound containing an amino (-NH2) and a carboxyl (-COOH) that link together to form proteins.

ammonia (NH3) A colorless, pungent gas, used to manufacture fertilizers and a wide variety of nitrogen-containing inorganic and organic chemicals.

ammonium The chemical ion (NH4+) derived from ammonia that does not appear in a free state, but forms salts and compounds analogous to those of the alkali metals.

anaerobic The condition of the absence of free oxygen.

analog model A physical model that reproduces aspects of a specific phenomenon.

andesite A dark-colored, fine-grained volcanic rock that is intermediate in composition (with respect to silica, magnesium and iron) between rhyolite (high silica, low magnesium and iron) and basalt (low silica, high magnesium and iron).

ankylosaurus A genus of squat, quadrupedal, armored dinosaurs of the Cretaceous Period.

anorthite A calcium-oxide-rich plagioclase feldspar mineral, typically white, that occurs in igneous rocks.

anorthosite An intrusive igneous rock that consists chiefly of the mineral plagioclase feldspar.

Archean The interval in Earth’s history between 3.8 billion and 2.5 billion years before present.

atmosphere The envelope of gases surrounding a planet, e.g. Earth’s atmosphere consists predominantly of eighty percent nitrogen and twenty percent oxygen.

atmospheric circulation Movement within the atmosphere caused by differences in air pressure.

atom The smallest unit of an element that can still retain all of the characteristics and properties of that element, an atom consists of a dense, central, positively-charged nucleus surrounded by a cloud of electrons.

atomic weight The total number of protons and neutrons in an atom. Also, the mass of an atom relative to the mass of the carbon atom having six protons and six neutrons, which is taken as 12.

bar A unit of pressure equal to the weight of the atmosphere at one hundred meters above mean sea level.

basalt A dark-colored, fine-grained volcanic rock that contains more iron and magnesium and less silica than andesite (rhyolite, andesite and basalt span the compositional spectrum of common volcanic rocks).

bedrock The solid rock that underlies soil, sand, gravel, or other loose material.

bicarbonate A salt containing a cation (any positively-charged ion) and the radical HCO3, e.g., NaHCO3.

bioherm A mound-like or circumscribed mass of rock built almost exclusively of sedentary marine organisms that is embedded in a rock of different character.

biomass The mass of living material.

biosphere All living organizsms on Earth, including in its atmostphere, its waters, and in the solid Earth.

black smoker A chimney-like structure made primarily of sulfide minerals that forms around hydrothermal vents on the ocean floor.

blue-yellow light The most intense, dominant light from the solar spectrum, it is absorbed by plants’ chlorophyll to make carbohydrates.

body wave A seismic wave that passes through the Earth, traveling outward from an earthquake focus (the point of first release of energy that causes an earthquake).

bottom water The deepest (and usually coldest) layer of the oceans consisting of the water that is in contact with the ocean floor.

butte An isolated desert hill—usually flat-topped and steep-sided, smaller than a mesa.

calcium carbonate (CaCO3) A colorless or white crystalline compound occurring naturally as the minerals calcite or aragonite in rocks like chalk, limestone, and marble.

carbon-14 dating A method of determining the approximate age in years of a carbon-bearing object by measuring the decay of radioactive carbon-14.

carbonate Any mineral compound that contains the anion (negatively charged molecule) CO3-2.

carbonate sedimentation A process by which carbonate sediment is deposited.

carbon cycle The combined processes by which carbon as a component of various compounds cycles between its major reservoirs: the atmosphere, oceans, living organisms, and solid Earth. The processes include photosynthesis, decomposition, respiration, sedimentation, lithification, burial, uplift, erosion, and volcanism.

carbon dioxide (CO2) An odorless, colorless, incombustible gas that is 1.5 times as dense as air and is formed during respiration, combustion, and organic decomposition. It is one of the principle greenhouse gases in Earth’s atmosphere (after water vapor).

carbon monoxide (CO) A colorless, almost odorless, poisonous, and flammable gas. It is a pollutant formed by the incomplete combustion of carbon-containing fuel.

carbonic acid (H2CO3) A weak acid resulting from the solution of carbon dioxide in rain or groundwater.  

catastrophism The concept that major features in the Earth’s crust such as mountains, valleys and oceans, have been produced by a few great catastrophic events, such as the Great Flood.

chemical equilibrium The state in which forward and reverse chemical reactions occur at equal rates so that the concentration of the reactants and products does not change with time.

chemical weathering The process of breaking down rocks or minerals at or near the Earth’s surface by chemical processes, including hydrolysis, hydration, ion exchange, and oxidation.

chlorofluorocarbon Synthetic chemical compounds used in refrigeration, solvents, and styrofoam manufacture. These compounds break down in the upper atmosphere and release chlorine atoms which destroy ozone.

chlorophyll A group of green, light-collecting pigments found in green plants, algae, and some bacteria that in the presence of sunlight convert CO2and H2O into carbohydrates.

climate Average weather conditions of a region, including temperature, precipitation, and winds.

climatology The scientific study of climate.

cobble Any rock fragment larger than a pebble and smaller than a boulder, especially one that has been naturally rounded.

continental crust The part of the Earth’s crust that comprises the continents and is typically ~45 km thick. Together with the oceanic crust it makes up the outermost shell of the solid Earth.

continental drift The slow, lateral movements of continents across the surface of the Earth.

convection The process by which hot, less dense material rises upward and is replaced by cold, more dense, downward-flowing material.

convection cells The currents that are set up by convection.

core The spherical mass, largely of metallic iron and nickel, at the center of the Earth. The outer core extends from 2,900 kilometers to 5,100 kilometers from Earth’s surface and is molten. The inner core, from 5,100 kilometers to the center of Earth at 6,400 kilometers, is solid.

core-mantle boundary The separation between the liquid metal of the outer core and the solid rock of the lower mantle (also called the Gutenberg Discontinuity).

Coriolis force An affect that causes any body that moves freely with respect to the solid Earth to veer to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

cosmic radiation High-energy, subatomic particles from outer space, which bombard Earth’s atmosphere. Most cosmic radiation is absorbed in Earth’s upper atmosphere.

country rock The older, preexisting rock that encloses or is traversed by an igneous intrusion or a mineral deposit.

crevasse A deep, almost vertical, crack or split in the upper part of a glacier.

crossbeds Layers that are inclined with respect to a thicker layer within which they occur.

crust The outermost and thinnest of the solid Earth’s layers, which consists of rocky material that is less dense than the rocks of the mantle below.  

crustal thickening The thickening of the continental crust when two continents collide to create mountains.

cryptoendoliths Microscopic organisms that grow within rocks.

crystal Any homogeneous solid with a regularly repeating atomic arrangement that may be expressed by plane faces; and a characteristic composition.

crystal fractionation The separation of crystals of one composition from magma of another composition, thus causing the residual magma to change composisiton.

D” (D double prime) The name for the lowermost 100–200 kilometers of the mantle where physical and chemical properties change rapidly.

debris flow The down-slope movement of a mass of unconsolidated rock, sand, and dirt, more than half of which is coarser than sand.  

decadal On the time scale of decades, or tens of years.

deep water The layer of the ocean between the intermediate water and the bottom water.

deglaciation The uncovering of a landmass by retreat or melting of glacial ice.

dendroclimatology Measurement of time intervals by counting the annual rings of trees; applicable to the last 3,000–4,000 years.

denudation The progressive lowering of the Earth’s surface by erosion, weathering, mass wasting, and transportation.

deposition The act of laying down rock-forming matter (sediment) by a natural process, such as wind, water, or ice.

deuterium An isotope of hydrogen with one proton and one neutron in the nucleus, giving it an atomic weight of 2.

differentiation The process by which a magma changes composition, for example, by crystal fractionation. In reference to planets, the process of forming concentric layers of different composition, usually by sinking of dense material and floating of light material.

dipole Two poles of opposite charge but equal magnitude.

discontinuity A surface at which seismic wave velocities abruptly change due to changes in physical properties of the material the waves are traveling through (such as density or composition).

ductility The ability of a material to irreversibly deform without rupture.

earthquake A sudden motion or trembling of the Earth’s crust caused by the passage of seismic waves radiated from a fault along which sudden movement has occurred.

electron Negatively charged atomic particle.

electron microscope A type of microscope that uses electrons rather than visible light to produce magnified images.

element A substance composed of atoms having an identical number of protons in each nucleus. An element cannot be separated into simpler substances by chemical means. A trace element is one present in only small quantities, less than 0.1 percent by weight.

elevation The height of an object above a particular reference level, usually sea level.

elliptical orbit A planetary orbit that traces an elliptical course.

El Niño A climactic event that generally occurs every four to twelve years, in which warming of the ocean surface off the western coast of South America disrupts the normal pattern of the upwelling of cold, nutrient-rich water. It causes fish and plankton to die and affects weather over much of the Pacific Ocean and around the world.

El Niño Southern Oscillation (ENSO) A global set of climatic conditions made up of two components: an oceanographic one called El Niño, and an atmospheric one, the Southern Oscillation.

erosion The complex group of related process by which rock is broken down physically and chemically and the products are moved. Agents of erosion include water, wind, and ice, as well as biological processes.

eukaryote A cell that possesses a defined nucleus surrounded by a membrane. Protists, fungi, plants, and animals are eukaryotes.

exobiology The branch of biology that deals with of the origin and distribution of possible life on other planetary bodies.

exsolution The process by which one distinct phase of matter separates from another.  

extraterrestrial Occurring or originating away from Earth and its atmosphere.

fault A rock fracture or fracture zone along which there has been movement.

feldspar A group of rock-forming minerals that contain aluminum and silica and varying amounts of sodium, potassium, and calcium. Feldspar is the most abundant mineral group and makes up sixty percent of the Earth’s crust. Feldspar is typically white, pink, or clear.

filamentous bacteria Bacteria comprised of long threadlike microorganisms, often in interwoven colonies.

fission The process, either spontaneous or induced, of the splitting of a nucleus into two or more large fragments of comparable mass plus some neutrons. It is accompanied by the release of vast amounts of energy.

folding The bending of layers of rock, usually due to compression.

foraminifera Microscopic, hard-shelled, marine to brackish (slightly salty water) single-celled organisms. Foraminifera create shells of calcium carbonate or make a shell by gluing tiny particles together; the composition of the shells are highly sensitive to temperature change.

fossil Any remnant or trace of an organism of a past geologic or prehistoric age, such as a skeleton or the imprint of a leaf, embedded and preserved in the Earth’s crust.

fossil fuels A general term for any hydrocarbon deposit, such as oil, coal, or natural gas, derived from organic matter of a previous geologic time and used for fuel.

fossiliferous Containing fossils.

geochemistry The study of the distribution and amounts of chemical elements in the various systems that comprise the Earth.

geodynamics The study of processes, such as the movement of material, in the Earth’s interior.

geologic time The period of time covering the physical formation and development of Earth, as recorded within the succession of rocks.

geology The study of Earth, its history, its composition, its structure, and the dynamic processes that shape it.

geomagnetism The study of the magnetic activity of Earth and its atmosphere.

geomorphology The study of landforms and their origin on the surface of Earth and other planets.

geophysics The study of physical properties of Earth.

GHGs See greenhouse gases

gigapascal A billion pascals; a pascal is a unit of pressure (force per unit area).

glacial period See ice age.


glacier A large mass of ice, air, water, and rock debris formed at least partially on land which flows by internal deformation in response to gravity. Glaciers include small valley glaciers, ice streams, ice caps, and ice sheets.

granite An intrusive igneous rock, usually light-colored. Granites commonly contain high amounts of quartz and feldspar. Micas, such as muscovite and biotite, may also be present. The extrusive igneous counterpart to a granite is a rhyolite.

greenhouse gases Gases, primarily water vapor, carbon dioxide, and methane, that increase global temperatures by absorbing outgoing radiation emitted by the Earth’s surface. Also called GHGs.

guyot A submerged, flat-topped volcanic mountain formed in deep oceans.

gyre The large, roughly circular current that is the main feature of wind-driven surface circulation found in most major ocean basins.

half-life The time required for half of the initial number of atoms of a radioactive parent element (such as 14carbon ) to change into atoms of its daughter element (14 nitrogen).

headward erosion The process by which the higher, originating end of a river wears away the rock around it, thereby lengthening tributary streams.

Holocene epoch The most recent geologic epoch of the Quaternary Period extending from the end of the Pleistocene (11,000 years ago) to the present.

hotspot A fixed point on the Earth’s surface defined by long-lived volcanism.  

hydrosphere All of Earth’s water, including the oceans, lakes, streams, water underground, and all the snow and ice, including glaciers.  

hydrothermal fluids Hot brines either given off by cooling magmas, or produced by reactions between hot rock and circulating water, that concentrate minerals in solutions.  

ice age (glacial period, glacial epoch) Recurring periods in Earth’s history when the climate was colder and glaciers expanded to cover larger areas of the Earth’s surface. The most recent ice age occurred during the Pleistocene epoch.

ice core samples Samples of layered ice from glaciers which may contain dust, chemicals, and gases that have been deposited with snow over hundreds of thousands of years. These layers reveal past climate characteristics and many of their potential causes.

ice sheet A large mass of ice thick enough to cover the topography under it. Ice sheets are large enough to deform and move with gravity.

igneous Rock formed by solidification from a molten or partially molten state.

inclusion Any solid, liquid, or gaseous foreign substance trapped inside a mineral or rock. Also refers to a fragment of an older rock embedded within an igneous rock.

infrared Electromagnetic radiation lying in the infrared spectrum with wavelengths greater than red light (longer than the longest visible wavelengths).

inner core The central or innermost part of the Earth’s core, extending from a depth of 5,100 kilometers to the center of Earth at 6,400 kilometers. It is believed to be solid, as opposed to the outer core, which is liquid.

insolation The solar radiation falling on Earth’s surface or its atmosphere. A contraction of “incoming solar radiation.”

interdune valleys Valleys or depressions between sand dunes.

interglaciation (interglacial period) The time between glaciations when Earth’s climate is warmer and the ice sheets have withdrawn, or retreated, from large areas of the continents. The present time is part of an interglacial period.

intermediate water Layer of ocean water above the deep water and immediately below the mixed layer of water at the ocean surface.

ion A positively or negatively electronically-charged atom or molecule.

island arc A curving group of volcanic islands parallel to a deep-sea trench. Island arcs and the deep-sea trench mark the location where oceanic crust is being subducted under oceanic crust. The Aleutian Islands are an island arc.

isostasy The mechanism whereby areas of the crust rise or subside until the mass of their topography is buoyantly supported or compensated by the thickness of crust below, which “floats” on the denser mantle below.  


Jacutophyton A stromatolite with a central conical structure that has branches coming off all around it.

lahar A fast-moving mudflow of unconsolidated volcanic ash, dust, breccia, and boulders mixed with rain, melting ice, or the water of a lake displaced by a lava flow.  

landslide Any perceptible downslope movement of a mass of bedrock or unconsolidated rock, sand and dirt, or a mixture between the two.

La Niña The movement of colder-than-normal surface waters across the Pacific towards Indonesia that sometimes follows in the year after an El Niño event.

latitude Imaginary lines that allow measurement of position north or south of the equator (“horizontal”). Latitude is measured in degrees (at the equator one degree = 60 nautical miles, or 111 kilometers). The equator is at a latitude of 0° and the poles lie at latitudes of 90° north (North Pole) or 90° south (South Pole).

lava Molten rock that erupts onto the Earth’s surface through a volcanic vent or fissure.

lava dome A dome-shaped mass of sticky, gas-poor lava erupted from a volcanic vent, often following a major eruption.

Lehmann Discontinuity The boundary between the Earth’s solid inner core and liquid outer core. Named for the Danish seismologist Inge Lehmann.

limestone A common sedimentary rock consisting mostly of calcium carbonate, predominantly formed from the skeletons of marine organisms, calcareous sand, microorganisms, shell fragments, and coral.

lithosphere The solid, outermost shell of the Earth (~100 km thick), where rocks are more rigid than those below. The lithosphere is made up of the uppermost mantle and the crust.

lodestone A piece of naturally occurring magnetic iron oxide (Fe3O4, the mineral magnetite).

longitude Imaginary lines that wrap around Earth intersecting at the north and south geographic poles. Lines of longitude are numbered from 0° (Greenwich Meridian, passing through London, England) to 180°. Longitudes are designated east if they fall east of the Greenwich Meridian, and west if they fall west of the Greenwich Meridian.

lower mantle The part of the mantle that lies below a depth of 1,000 kilometers. In this layer, seismic velocity increases slowly with depth.

Lunar Highlands Light-colored regions of the Moon that are mostly composed of the rock anorthosite.

Lunar Magma Ocean A sea of molten rock that existed shortly after the Moon formed which covered the entire Moon and extended down to depths of several hundred kilometers.

Lunar Mare Dark-colored, low-lying regions of the Moon comprised mostly of basalt.

magmatism The development and movement of magma, and its solidification to form igneous rock.

magma Molten rock within the Earth. Igneous rocks form when magma cools and crystallizes.

magnetic field The region of influence of a magnetized body, such as Earth.

magnetic pole Either of two variable points on Earth, close to but not coinciding with the geologic North and South Poles, where the Earth’s magnetic field is most intense and towards which a compass needle points.

mantle The layer within the interior of the Earth that lies between the crust and the core.

mass spectrometer An instrument for separating atoms or molecules according to mass and counting them. Typically used for determining isotopic abundances.

mass wasting The process in which a large amount of loosened soil and rock is transported downslope under the direct influence of gravity.

mesa An isolated, steep-sided, flat-topped landmass rising above the surrounding geography.

Mesozoic The Era of geologic time (from the end of the Paleozoic, 248 million years ago, to the beginning of the Cenozoic Era, 65 million years ago), including the Triassic Period, the Jurassic Period, and the Cretaceous Period. This Era is characterized by the development of flying reptiles, birds, and flowering plants, and the appearance and extinction of dinosaurs.

metamorphic rocks Rocks that form as the result of transformation from other rocks. Metamorphic rocks are created when igneous, sedimentary, or metamorphic rocks change in response to extreme temperatures and/or pressures, but do not completely melt. Metamorphic rock types include slate, schist, and gneiss.

meteorite A fragment of rock that has reached the Earth’s surface from beyond the Earth’s atmosphere.

meteorology The study of Earth’s atmosphere and the motion within the atmosphere. Meteorology includes understanding the aspects of the atmosphere for weather forecasting.

methane An odorless, colorless, flammable gas (CH4). Methane is an important source of hydrogen and a wide variety of organic compounds, as well as a principle constituent of natural gas.

mid-ocean ridge A nearly continuous undersea mountain chain that marks the location where tectonic plates (pieces of the lithosphere) are diverging or moving apart. Mid-ocean ridges are the locations of creation of new ocean crust.

Milankovitch cycles The three cycles related to variations in the Earth’s rotational and orbital characteristics around the sun that are believed to influence the occurrence of ice ages, occurring at 100,000, 41,000, and 22,000 years. Named after Serbian mathematician Milutin Milankovitch.

mineral A naturally-occurring, homogeneous inorganic element or compound having a definite chemical composition and orderly internal structure, crystal form, and characteristic chemical and physical properties.

mineralogy The study of minerals, including their formation, occurrence, properties, composition, and classification.

mineral physics The study of the physical properties of minerals.

Mohorovicic discontinuity (Moho) The boundary between the crust and mantle marked by abrupt increases in seismic velocities. The Moho occurs at about five to ten kilometers beneath the ocean floor and about forty kilometers beneath the continents (although it may reach sixty kilometers or more under some mountain ranges).

molecule The smallest unit of matter into which an element or a compound can be divided and still retain its chemical and physical properties. It consists of a single atom or group of like or different atoms bonded together by chemical forces.

molybdenite A metallic, lead-gray, hexagonal mineral made of molybdenum and sulfur (MoS2.

monsoon A wind system that influences climatic regions and reverses direction seasonally. The Indian Monsoon brings wind from the southwest or south and brings great annual variation of rainfall to southern Asia and along the coasts of other regions of the Indian Ocean.


neutron An electrically neutral atomic particle that is stable when bound in an atomic nucleus. Neutrons are present in all known nuclei except the lightest isotope of hydrogen.

nitrous oxide A colorless gas or liquid (N2O), soluble in water or alcohol. It is a product of the combustion of fossil fuel.

Northern Hemisphere The half of the planet north of the equator.

nuclear fusion A nuclear reaction in which two light nuclei combine, at extremely high temperatures, to form a heavier nuclei and simultaneously release a vast amount of energy.

ocean basin The area of the seafloor from the base of the continental margin (generally at the beginning of the continental rise) to the mid-ocean ridge.

oceanic crust Part of the crust that forms the upper part of the rigid lithosphere, the outer layer of the Earth. Oceanic crust is formed at mid-ocean ridges and is denser and typically thinner than continental crust. Oceans are underlain by oceanic crust.

oceanic trench A narrow, steep-sided, elongated depression of the deep-sea floor.

oceanography The study of the ocean, including the physical properties of the ocean such as the currents and waves (physical oceanography), the chemistry of the ocean (chemical oceanography), the geology of the seafloor (marine geology), and the organisms that carve their niche within the ocean realm (marine biology and marine ecology).

outer core The upper or outermost part of the Earth’s core extending from 2,900 to 5,100 kilometers below the surface. It is believed to be liquid because it significantly reduces P-wave (compressed waves) velocities and does not transmit S-waves (shear waves).

Oviraptor A primitive theropod dinosaur that developed from the carnivorous raptor theropods during the Late Cretaceous Period. The Oviraptor’s skull was light in weight and its bones thin, much like modern birds. It had a short, thick, parrot-like beak and ran on its hind legs.

oxidation A process in which a chemical element loses electrons.

ozone A chemically active bluish gas that is made of molecules of three oxygen atoms (O3). In the stratosphere, ozone acts as a protective barrier for Earth’s surface by absorbing much of the potentially damaging ultraviolet radiation that comes from the Sun. In the troposphere, ozone acts as a harmful pollutant.

ozone layer A layer in the stratosphere that contains about ninety percent of Earth’s ozone. The ozone layer occurs approximately twenty-five kilometers (sixteen miles) above the surface of Earth.

Pacific Rim The countries and landmasses surrounding the Pacific Ocean.

paleoclimate The past climate.

paleontology The study of the plants and animals of the past.

paleoseismology The study of past earthquakes.

paleothermometer The method or substance used to determine temperatures at a given time in the past.

patina The colored film or thin layer on the surface of a rock produced by chemical weathering.

peridotite A coarse-grained igneous or metamorphic rock composed primarily of olivine, with or without other minerals such as pyroxenes, amphiboles, or micas.

permafrost Permanently frozen soil or subsoil, occurring throughout the polar regions and locally in perennially frigid areas. Its thickness ranges from thirty centimeters to over 1,000 meters and underlies approximately one-fifth of Earth’s land area.

perovskite The mineral CaTiO3. At high pressure the mineral pyroxene (MgSiO3) transforms to a form having the same structure, or arrangement of atoms, as perovskite. This MgSiO3 perovskite makes up most of the lower mantle.

photic zone The layer in a body of water or aquatic system that is penetrated by sunlight sufficient for photosynthesis to occur, extending as much as 150 meters below the surface.

photolysis Chemical decomposition of a substance into simpler units as a result of its absorbing light or other radiant energy.

photosynthesis The chemical process in which green plants (and blue-green algae) make carbohydrates from carbon dioxide and water using sunlight or light as an energy source. Most forms of photosynthesis release oxygen as a byproduct, the chief source of atmospheric oxygen.

physical weathering The processes that mechanically break up rocks into fragments, such as the movement of water, wind and ice, and frost action. Physical weathering is also known as mechanical weathering.

physics The study of matter, energy, motion, force, and their interrelationships.

planetesimal Any of innumerable small bodies or satellites that are the precursors of a planet during the early stages of the solar system. A planetesimal can also be the fragmented result of a cataclysmic collision between a planet and another celestial body.

plasma An electronically neutral, highly ionized gas composed of ions, electrons, and neutral particles. It is a state of matter distinct from solids, liquids, and normal gases that exists in extreme heat.

plate One of several large, mobile pieces of the Earth’s lithosphere adjoining other plates along zones of seismic activity.

plateau A relatively elevated, comparatively level expanse of land with at least one abruptly steep side. It is higher than a plain and more expansive than a mesa.

plate spreading The separation of two lithospheric plates, as occurs at the mid-ocean ridge.

plate tectonics The theory and study of plate formation, movement, interaction and destruction; the attempt to explain seismicity, volcanism, mountain-building, and paleomagnetic evidence in terms of plate motions.  

Pleistocene The earlier of the two epochs of the Quaternary Period, ending around 11,000 years ago and characterized by the alternate appearance and recession of northern glaciation and the appearance of progenitors of human beings.

plumb bob A pointed, tapering weight attached to a plumb line, used to measure the verticality of objects.

plume A narrow, focused upwelling of unusually hot solid rock moving upward through the Earth’s mantle.

Precambrian The largest division (ninety percent) of geologic time, from 4.55 to 0.55 billion years ago. It precedes the Cambrian Period and is subdivided into the Hadean, Archean and Proterozoic Eons.  


primordial Happening first in a sequence of time. Belonging to or remnant of the first stage of development, as in the formation of the Earth.

Proterozoic Era The more recent of three divisions of the Precambrian, from 2.5 billion to 543 million years ago.

protons A stable, positively-charged atomic particle found in the nuclei of matter.

protosun A contracting gas cloud which was the earliest stage of the Sun’s formation. This stage took place before the temperature and pressure of the interior became so high as to cause thermonuclear reactions.

pumice Light-colored, porous, glassy fragments of lava typically with the composition of rhyolite.

P-wave Primary or compressional seismic wave. It is the fastest of the seismic waves, traveling 5.5–7.2 kilometers per second in the crust and 7.8–8.5 kilometers per second in the upper mantle.

pyrite A common, brass yellowish-white, metallic, cubic mineral made of iron and sulfur (FeS2). It is used to produce sulfur dioxide for sulfuric acid. Popularly known as “fool’s gold,” pyrite has metallic luster and has been mistaken for gold, which is more yellow, softer, and heavier than pyrite.

pyroclast A molten fragment of pumice and ash ejected during a volcanic eruption.

pyroclastic flow A high speed avalanche of hot volcanic ash, rock fragments, and gas.

quartz A typically clear, hard mineral composed of silicon and oxygen. It is common in igneous, metamorphic, and sedimentary rocks, including sandstone and granite.

radioactivity The spontaneous emission of energetic particles and/or radiation (including alpha particles, nucleons, electrons, and gamma rays) from either unstable atomic nuclei or as the consequence of a nuclear reaction.

radiocarbon dating A dating method used to quantitatively measure the age of organic matter (such as bone, shell, or wood). The method can by applied to materials formed within the last 50,000 years or so.

radiometric dating Determining the absolute age of rocks by measuring the decay of naturally occurring radioactive isotopes like carbon-14 or long-life isotopes of potassium, thorium, and uranium.

rhyolite A fine-grained volcanic igneous rock usually light in color. Rhyolites commonly contain high amounts of silica, and low amounts of iron and magenesium. The fine-grained nature is a result of rapid cooling at the surface of Earth.

Richter scale An exponential scale ranging from 1 to 9 that measures the amount of energy released during an earthquake.

Ring of Fire The name of the extensive area of volcanic and seismic activity that roughly coincides with the borders of the Pacific Ocean.

rock Any naturally formed aggregate of one or more minerals, such as granite, shale, or marble.

rockfall The free falling of detached bodies of bedrock from a cliff or steep slope.  

salinity The amount of dissolved salts in water.

sand slurry A thin mixture of sand and a liquid, usually water.

sandstone A clastic, sedimentary rock formed by the consolidation and compaction of sand (primarily quartz) in a matrix of silt or clay and held together by a natural cementing material, such as silica, iron oxide, or calcium carbonate.

seafloor spreading The movement of plates and the formation of new ocean crust at divergent plate boundaries, as at mid-ocean ridges.

seamount An elevation of the seafloor, 1,000 meters or higher, having either a flat-topped or peaked summit below the surface of water, usually volcanic in origin.

sediment Unconsolidated particles, ranging from clay-size to boulders produced by the breakdown of rocks that may be carried by natural agents (wind, water, and ice) and eventually deposited to form sedimentary deposits. Organisms and chemical precipitation can also produce sediment.

sedimentary rock A rock formed by the consolidation or cementation of sediment particles, or chemically precipitated at the depositional site.

sedimentation The process of sediment accumulation.

seismic tomography An imaging technique using speeds of seismic waves to infer the three-dimensional internal density structure of the Earth.

seismic wave Elastic vibration that travels through the Earth caused by an earthquake or a manmade explosion.

seismogram The record of seismic waves on a seismograph after they have traveled through the Earth and arrived at a given seismic station. This record can be used to determine the location and strength of an earthquake.

seismology The study of earthquakes and the mechanical properties of the Earth.

shale A fine-grained, finely laminated, sedimentary rock, formed by the compaction of clay, silt, or mud. Its laminated structure makes it fissile, or easily split along close-spaced planes, especially on weathered surfaces.

sheetwash Erosion of the ground surface by thin sheets of rainwater. Also known as “sheet erosion.”

shock wave A large amplitude compressional wave formed by an explosion or by supersonic motion, such as breaking of rock and movement along a fault within the Earth.

short-wavelength visible radiation See visible light.

silica The compound silicon dioxide (SiO2). Silica is an important component of many rocks and minerals. It can be found in several forms, including quartz and opals.

silicate Any mineral with a crystal structure containing silicon and oxygen (SiO4) tetrahedra either isolated or joined through one or more oxygen atoms to form groups or three-dimensional structures with metallic elements.

silt A sedimentary rock fragment or mineral particle that is finer than sand but coarser than clay.

siltstone A fine-grained rock of consolidated silt with the texture and composition of shale, but lacking its fine lamination or fissility (ability to be easily split).

slump block A large piece of rock that has broken off from the bedrock but has not yet shattered and broken. In the Grand Canyon, some slump blocks are as large as 2,000 meters long and 1,000 meters thick.

solar nebula The cloud of interstellar gas and dust out of which the Sun and planets of the Earth’s solar system were formed, roughly 4.5 billion years ago.

solar radiation Energy from the Sun in the form of electromagnetic waves.

Southern Hemisphere The half of the planet south of the equator.

spectroscope An instrument that separates light by its wavelength. It is used to observe the light spectrum.

spiral galaxy A system of roughly one hundred million stars arranged as a disk with a nucleus of older stars and spiral arms consisting mainly of dust, gas, and young stars.

spreading center A linear region on the seafloor from which adjacent crustal plates are moving apart and along which magma rises to form new oceanic lithosphere.

statistical model A model or simulation based on a sequence of past observations.

strata Layers of sedimentary rocks which might contain differences in texture, color, fossil content, or material type.

stratification Deposition of sediment in layers or strata.

stratosphere The layer of the atmosphere above the troposphere. The stratosphere extends from approximately ten to fifty kilometers above Earth’s surface.

stromatolite A widely distributed sedimentary structure consisting of laminated carbonate or silicate rocks. It is produced over geologic time by the trapping, binding, or precipitating of sediment by groups of microorganisms, especially blue-green algae, in shallow, warm waters.

subcontinent A large, relatively distinct landmass, such as India, which is part of a continent but geographically considered an independent entity.

subduction The process of one tectonic plate moving beneath another tectonic plate at a convergent margin (where two plates collide). If continental lithosphere and oceanic lithosphere converge, the less dense continental lithosphere rides over the oceanic lithosphere and the denser oceanic lithosphere is subducted.

subduction zone A long, narrow belt where subduction occurs, usually marked by island arcs such as the Aleutians, or volcanically active mountain chains such as the Andes.

sulfate A negatively charged molecule containing sulfur and oxygen  (SO4-2). Major sources of sulfates are fossil fuel burning and volcanic activity.

sulfide chimneys A tower or chimney composed primarily of sulfide minerals and built up on the deep seafloor by mineral deposits precipitated from hydrothermal fluid ejected from a hydrothermal vent. The hydrothermal fluid contains hydrogen sulfide and high concentrations of metals such as iron, copper, and zinc, which can build a sulfide chimney around the vent as high as forty-five meters.

superposition (law of superposition) A general law upon which all geologic chronology is based that states that in undisturbed, stratified sedimentary rocks (or of extrusive, igneous rocks) the lowest layers were deposited the earliest and are the oldest, and the top layers were deposited later and are therefore younger.  

S-wave A secondary or shear seismic wave. It does not travel through liquids and therefore cannot penetrate the outer core of the Earth. It travels 3.0–4.0 kilometers per second in the crust and 4.4–4.6 kilometers per second in the upper mantle.

talus Accumulation of fragmented rock debris, usually coarse, angular, and of different sizes, found at the base of valley slopes and walls and produced by falling, rolling, or sliding.

tectonic Relating to the forces and the movements of Earth and its lithosphere. Earthquakes, volcanoes, and mountain building are related to tectonic activity.

terrestrial Relating to land or the planet Earth. Refers to the land above sea level.

thermocline A thin region of rapid temperature change separating the warm waters of the upper ocean from the cold waters of the abyssal ocean.

thermohaline circulation Circulation or movement of ocean water masses resulting from density differences caused by variation of temperature and salinity.

tidal basin A body of water, natural or manmade, subject to tidal action.

tilted laminations Thin layers of rock or sediment oriented at an angle from the horizontal.

topography Surface relief of the land. Topography is usually measured in meters above sea level.

trade winds Part of one of the three major circulation cells in each hemisphere, the trade winds exist from approximately 0° to 30° north or south latitude. Within the regions of the trade winds, prevailing winds blow easterly, or toward the west.

transition zone A region within the mantle that separates the upper and the lower mantle that is characterized by a rapid increase in seismic wave velocities. The depth of the transition zone varies from 410 to 1,000 kilometers beneath the Earth’s surface.

tributary A stream that flows into or joins a larger stream or another body of water.

troposphere The lowest layer of Earth’s atmosphere in contact with Earth’s surface. Most weather occurs within the troposphere. The troposphere extends from Earth’s surface to elevations of approximately 10–15 kilometers.

tsunami Enormous ocean wave produced by an underwater earthquake, landslide or volcanic eruption.

ultraviolet radiation (UV) A portion of the electromagnetic spectrum. Ultraviolet radiation has a shorter wavelength than visible light.

unconformity An erosional surface representating a gap in the geologic record between rock layers of different ages indicating that deposition was not continuous.

Uniformitarianism The principle that the same geological processes and natural laws that modify the Earth’s crust operating today have operated in the same manner or way throughout geologic time.

upper mantle The part of the mantle which lies above the transition zone. It is presumed to be composed of peridotite.

upwelling In the solid earth, the act of warm mantle rising upwards relative to surrounding, cooler mantle. In the ocean, the rising of deeper waters to replace migrating surface waters. Upwelling may bring waters rich in nutrients to the surface, resulting in a region where ocean productivity is high.

viscosity The property of a substance which determines the amount of its internal resistance to flow. The opposite of fluidity.

visible light The portion of the electromagnetic spectrum with wavelengths shorter than infrared radiation but longer than ultraviolet radiation. Visible light includes wavelengths that include all the colors we see: violet, blue, green, yellow, orange, and red light from the Sun.

volatiles Elements and compounds that evaporate readily under normal surface conditions.

volcanism Any of the processes by which magma and its associated gases rise to the crust from the Earth’s interior and are discharged onto the surface and into the atmosphere.

volcano A vent or fissure in the Earth’s surface through which molten lava, ash, and gases are ejected. It is also the name for the structure, usually conical, formed by the materials ejected from the vent or fissure.

volcanology The study of the causes and phenomena associated with volcanoes and volcanism.

X-ray Electromagnetic radiation of short wavelength. The wavelengths of X-rays are shorter than those of ultraviolet radiation (which are of shorter wavelength than visible light).

zircon A durable mineral (ZrSiO4) that occurs in tetragonal prisms, has various colors, and is found in many different rock types.
