achondrites Stony meteorites devoid of chondrules, showing they have been modified since the beginning of the solar system.
AGB star An asymptotic giant branch (AGB) star is a low to intermediate mass star in a late stage of its stellar evolution and appears as a red giant.
albedo The proportion of light reflected from a planet. High albedo means high reflectivity.
albite One endmember of the feldspar group of minerals, NaAlSi3O8. Albite is a primary constituent of granites and the continental crust.
alpha decay One of the three principal forms of radioactive decay. It occurs only in heavy elements such as U and Th. Alpha decay is the emission of the nucleus of a helium atom (i.e., a package of two protons and two neutrons), causing the number of nuclides of the remaining isotope to decrease by four. For example, the decay of 238U to 206Pb includes eight alpha decays.
alpha particle A positively charged particle with an atomic mass of 4 and a charge of +2 (a helium nucleus) produced by alpha decay.
amino acid A complex organic molecule consisting of a central carbon whose four bonds are connected to an “amino” group (NH2), a carboxyl group (COOH), a hydrogen atom, and a side chain that is called an R group; the building blocks of all proteins are the twenty-two amino acids of life.
amphiboles A silicate group of minerals (inosilicates) characterized by double chains of silica (SiO4) tetrahedra. All amphiboles contain a water molecule and are a primary means by which water is transported in plates.
amphibolite A metamorphic rock with amphibole as a main component.
anaerobic Related to the absence of oxygen. Anaerobic bacteria thrive in the absence of oxygen.
anorthite The Ca-rich endmember of the plagioclase minerals; CaAl2Si2O8 forms complete solid solution with albite.
asteroids Small rocky objects that orbit our sun, mostly in the asteroid belt between Mars and Jupiter.
atmophile “Gas-loving” elements and molecules that are volatile and tend to occur as gas or liquid molecules under the conditions on Earth; noble gases (such as helium, neon, and argon), water, carbon dioxide, and nitrogen are common atmophiles; they are overwhelmingly concentrated in the ocean and atmosphere.
atom The basic chemical unit of matter on planets; consists of a dense core of neutrons and protons orbited by a cloud of electrons.
autotrophs Organisms that produce their own organic carbon making use of an external energy source.
banded iron formations Fe-rich rocks, with the acronym BIFs, that occur largely between 2,500 Ma and 1,800 Ma and indicate an important transition in the oxygenation of Earth’s surface.
basalt An igneous volcanic rock, derived by melting of the mantle that makes up much of Earth’s oceanic crust and an important part of the deep continental crust. It consists mainly of the minerals olivine, pyroxene, and plagioclase. The plutonic equivalent, which has a similar chemical composition, is named gabbro.
Benioff zone Dipping plane of earthquakes located along a subduction zone; Benioff zones mark the boundary between the descending ocean crust and the overlying mantle.
bentic foraminifera Tiny-shelled organisms that inhabit the lowest levels of the seafloor.
beta decay One of the three major forms of radioactive decay, where a neutron emits an electron and is converted to a proton.
Big Bang The name given to the event that is the origin of the universe.
bilayer A double layer of molecules that makes up cell membranes and is characterized by hydrophilic (water-loving) ends on the outside and hydrophobic (water-hating) ends on the inside.
binomial nomenclature A Latin nomenclature used to name different species, where the first word is the genus and the second the species. E.g., “Homo (genus) sapiens (species).
biomarkers Complex organic molecules that do not break down easily and would be the result only of life.
blackbody A body or surface that reflects no light. It has an albedo of zero.
blackbody radiation While a blackbody reflects no light, it has a temperature, and that temperature emits characteristic wavelengths of radiation, which are called blackbody radiation. The fact that the universe has a uniform blackbody radiation is one of the lines of evidence for the Big Bang.
black hole An ultradense region where gravity is so intense that even light is unable to escape. Black holes are believed to occupy the central regions of many galaxies.
black smokers High-temperature chimneys in the ocean that can reach the heights of ten-story buildings and belch fluids at temperatures as hot as 400°C. They are sustained by volcanic heat at ocean ridges.
calcite (or calcium carbonate) A mineral consisting of the elements calcium, carbon, and oxygen; a primary constituent of limestones and marbles, and the major inorganic carbon reservoir. Most of Earth’s surface CO2 is locked up in carbonate minerals.
cap carbonates Thick sequences of calcium carbonate (limestones) overlying the glacial deposits that are the evidence for “snowball Earth” episodes.
capture hypothesis An hypothesis proposing that the moon accreted in an orbit similar to Earth, from which it was passively captured to enter orbit around Earth.
carbohydrates Organic molecules that have chemical formulae in the same proportions as carbon matched with one water molecule (CH2O). For example, glucose. C6H12O6, could also be written as 6CH2O. Carbohydrates are a prime energy source for all cells.
carbonaceous chondrites A type of chondrite containing many mineral phases, including organic molecules, that would be destroyed by even moderate heating.
carbon capture and sequestration (CCS) Phrase descriptive of one solution to rising CO2 in the atmosphere, i.e., to remove the carbon and sequester it.
carbon cycle The geochemical cycle that links the carbon in organic molecules, the atmosphere, the oceans, and limestone (CaCO3). The carbon cycle involves Earth’s interior, the atmosphere, the oceans, and life in a complex process that sustains Earth’s climate and permits life.
carbon dioxide A trace gas present in the atmosphere and dissolved in the ocean; because of its strong greenhouse effect, it plays an important role in climate; chemical formula CO2.
chalcophile Chemical elements with affinity to bond with sulfur (“sulfur-loving”); silver, bismuth, mercury, and zinc are common chalcophile elements.
chemoautotrophs Microorganisms that obtain their energy by chemical reactions rather than from the sun.
chondrites Stony meteorites that contain chondrules and are believed to be preserved objects of earliest solar system history.
chondrules Round grains found in meteorites. Meteorites that contain chondrules are called chondrites. The chondrules reflect the time of nebular condensation and subsequent processes of planetary differentiation as they have been preserved in the almost perfect vacuum of space.
codon The three distinct bases defined by the genetic code DNA that are used to specify which amino acid should be placed in a protein.
comets Small icy objects that orbit the sun. Most comets reside in the outer solar system.
convection cells The simplest kind of convection, where hot material ascends at one location, flows laterally, cools off, and descends; the zones of ascent and descent define the cell.
convergent margins Locations where two plates converge, one plate descending beneath the other and returning to the mantle, in a process called subduction. A roughly equivalent term is subduction zone.
core The central portion of Earth’s interior consisting of molten iron and nickel and some unknown light element. It underlies Earth’s mantle.
cosmogenic radionuclides Radioactive isotopes that do not have long half-lives and are produced continually by cosmic radiation in Earth’s atmosphere. Many of them, such as 14C, are used to provide timescales for young events on Earth.
crust The layer above the Mohorivic discontinuity and below the ocean and atmosphere. Oceanic and continental crust has distinct chemical compositions and densities.
cyanobacteria Bacteria that carry out oxygenic photosynthesis at shallow water levels; they played a key role in the progressive oxygenation of Earth’s surface.
daughter isotope The isotope that is the product of decay of its radioactive parent isotope. For example, when 87Rb decays to 87Sr, 87Rb is the parent and 87Sr the daughter.
density The ratio of an object’s mass to its volume, e.g., kg/m3.
depth of compensation Term referring to the depth at which the pressures beneath two columns of material are the same.
D/H ratio The ratio of 2H/1H, or deuterium to hydrogen; these two isotopes have 100% mass difference and can be easily fractionated on the basis of their mass difference at low temperatures, even though they are the same element. The per mil variations in this ratio are referred to as δD.
differential volatility Term expressing the fact that at the same temperature different molecules may be solid, liquid, or gas, with the most volatile molecule being gaseous and the least volatile solid.
diopside A pyroxene mineral with the mineral formula CaMgSi2O6; a primary constituent of igneous rocks and also a reaction product, when at high temperatures CaCO3 reacts with silicates and breaks down, releasing CO2.
dolomite A mineral and also the name of a sedimentary rock consisting primarily of a carbonate mineral (akin to calcite) in which half of the metal atoms are magnesium and half are calcium, CaMg(CO3)2.
eclogite A metamorphic, garnet-bearing rock with a density of 3.35 gm/cm3; basalt converts to eclogite at high pressures. Eclogite in the down-going slab causes the slab to be heavy and sink into the mantle.
electron One of the basic units of the atom; characterized by a very small mass (compared to the proton and neutron) and by a negative electrical charge.
electron capture One of the three forms of radioactive decay, when a neutron captures an electron and becomes a proton.
element The chemist’s subdivision of atoms; based on number of protons in the atom’s nucleus. The same element can have many isotopes if the number of neutrons varies.
endosymbiosis The biological process by which partnership between simple cells leads to codependency and ultimately larger and more complex cells.
eukaryotes Refers both to the single-celled organisms that have a nucleus and organelles and also to multicellular organisms where many different cells have become differentiated and specialized. Human beings are eukaryotes.
eutectic The minimum melting point and composition in equilibrium with a fixed number of solid phases, where the liquid composition remains fixed as long as all the solid phases remain present.
eutectic phase diagram A phase diagram that illustrates melting of solid phases that do not form a solid solution, and a liquid phase where all the molecules are fully miscible. The horizontal axis represents the proportion of each mineral in the mixture. Each pure mineral plots at one end of the horizontal axis, and melts at a single temperature. All other compositions along the axis are mixtures of the two minerals. Temperature plots along the vertical axis, increasing upward.
extinct radionuclides Radionuclides with short half-lives so that the parent isotope no longer exists; measurement of the daughter products of extinct radionuclides constrains events in the early history of the solar system.
fayalite Endmember of the olivine solid-solution series with a chemical composition of Fe2SiO4; forsterite is the other endmember.
feldspar A group of silicate minerals made up of a three-dimensional framework of both Si and Al tetrahedra, combined with some combination of K, Ca, and Na. Endmembers are orthoclase (KAlSi3O8), albite (NaAlSi3O8), and anorthite (CaAl2Si2O8). Feldspars are the most abundant minerals in Earth’s crust.
felsic Rocks rich in feldspar and quartz (SiO2, silica) that form most of the upper continental crust. Granites are examples of felsic rocks.
fission The breakup of a nucleus into two major fragments; in some cases the breakup is spontaneous, in others it is induced by the impact of a neutron. Fission occurs only for very large mass isotopes, when repulsive forces become strong enough for the nucleus to break apart.
fission hypothesis The hypothesis proposing that the moon formed by fission from Earth after Earth’s core had formed, because of a very fast spin of the early Earth.
forsterite Endmember of the olivine solid-solution series with a chemical composition of Mg2SiO4; fayalite is the other endmember. Forsteritic olivine is the mineral that makes up more than half of the upper mantle.
freezing point depression The decrease in the melting temperature of a substance induced by the addition of another compound, as long as the two compounds are miscible in the liquid state.
fusion The merger of two nuclides into a single nuclide. Fusion is the process that produces stellar energy.
gabbro A mafic igneous rock; the plutonic equivalent of basalt, consisting largely of the minerals plagioclase, pyroxene, and olivine.
giant impact hypothesis The hypothesis proposing that another planet about the size of Mars had a grazing impact with Earth, ejecting large amounts of material into space around the joined planets, which condensed to form the moon.
granite An igneous rock that makes up much of the Earth’s continental crust; granite consists mainly of the minerals quartz and feldspar.
greenschist A mafic volcanic rock such as basalt or gabbro that has been metamorphosed in the presence of water and contains water-bearing (hydrous) minerals. Greenschists are very common in the ocean crust after it has interacted with seawater at ocean-ridge hydrothermal systems.
Gutenberg discontinuity The discontinuity in density and seismic velocity that occurs at the core/mantle boundary.
half-life The time for half a population of radioactive isotopes to decay. E.g., after ten half-lives, 99.9% of the radioactive isotopes have decayed.
halides A nonsilicate mineral group consisted of a halide ion bearing a negative charge (F–, Cl–, Br–, I–) and an element such as Na, K, Ag, etc., to form a binary compound (e.g., NaCl).
heterogeneous accretion Hypothesis for the formation of Earth’s layers as a sequence of accretion during Earth’s formation. First, metals would have gathered to form the core, then silicates were added on top of the core, and finally the gas and water formed the ocean and atmosphere on the surface. The hypothesis is discredited for the solid Earth, but may have merit for the volatile budget.
heterotrophs Organisms that eat organic carbon for their energy and carbon compounds, in contrast to autotrophs.
homogeneous accretion Hypothesis that Earth accretes through undifferentiated objects that are more or less homogeneous in composition. Then separation of Earth into layers would occur subsequently, the core and mantle separating by immiscibility, the crust forming by partial melting, and ocean and atmosphere by degassing. The alternative (and largely accepted) model to heterogeneous accretion.
horizontal gene transfer (HGT) The change in DNA characterized by gene sharing among organisms, leading to lateral changes between different branches of the Tree of Life, rather than linear changes through mutations passed through generations along a single branch of the tree. HGT appears to be common particularly in the bacterial realm.
hot spot A region of excess volcanism and mantle melting that can occur in the middle of a plate as well as occasionally at plate margins. Predominant hypothesis for hot spots is that they are caused by upwelling mantle plumes. Prominent examples include Hawaii, Iceland, and Yellowstone.
human energy revolution The latest energy revolution on Earth, permitting a hundred times more energy production for use by a single species than can be obtained by biological metabolism.
hydrocarbon An organic molecule with a carbon atom connected to hydrogen atoms; methane (CH4) is the simplest hydrocarbon, and oil and gas are made up of a complex assemblage of hydrocarbons.
hydrogen The first element of the periodic table, but a term also used for the dominant chemical compound in the early solar nebula; now nearly absent from the atmosphere; chemical formula H2.
hydrothermal vents Locations along spreading axes where seawater circulating through newly created hot basalts spews back into the sea with focused, high-temperature flow. Underwater hot springs.
igneous rock A rock that forms from a silicate liquid (i.e., magma). Magmas form by partial melting, change composition by differentiation, and once cooled and solidified are igneous rocks. Cooling at depth, they produce large crystals and form plutonic rocks. Erupting as lava flows, they are usually fine grained and form volcanic rocks.
immiscibility Property of two phases that do not mix together to form a single phase. The term is usually used in reference to two liquids that do not mix but separate by density to form discrete layers. Examples are oil and water, and metal liquids and silicate liquids.
inorganic molecules A general term for molecules that do not have carbon-hydrogen bonds. They constitute the first building blocks for solid planets and in the solid state are called minerals.
intra-plate volcanism Volcanism happening in the middle of plates and generally accepted to be related to a deep hot mantle source. Hot spots are voluminous examples of intra-plate volcanism.
invasive species Nonnative species that have been generally transported by humans from one geographical location to another; also called exotic species.
ion An atom in which the negative charge carried by the orbiting electrons does not match the positive charge of the protons in its nucleus. Ions can be either negatively charged anions, or positively charged cations.
ionic radius The radius of an atom’s ion, determining its size and the geometrical constraints that control how it can combine with other ions to make molecules.
iron meteorites Meteorites consisting of alloys of nickel and iron metal. Fragments of cores of early planetesimals in the solar system.
isobar A term used for the chart of the nuclides, referring to isotopes with constant numbers of nuclear particles. Isotopes along the same isobar are always different elements, because, e.g., an increase in number of neutrons must be accompanied by a corresponding decrease in the number of protons. Electron capture and beta decay produce daughter isotopes along the same isobar as the parent isotope.
isochron A straight line on an isotope ratio diagram indicating that all the materials along the line formed at the same time from a homogeneous reservoir. The isochron then provides the time which is the age of the event. This is a primary method of radioactive dating.
isotone Vertical line on the chart of the nuclides for nuclides with the same neutron number but different proton numbers.
isotope A term that can be used to refer to specific radionuclides, but is generally used to refer to nuclides of the same element that have different masses. Different isotopes of the same element have the same proton number but different neutron numbers.
Jack Hills formation A sedimentary formation located in Australia; the oldest zircon found on Earth (4.4 billion years) comes from this formation.
kimberlite An explosively generated volcanic rock that rises so rapidly from deep in the interior that it contains rock fragments captured at various depths below the surface. Kimberlites bring up fragments of the mantle and are also the source of all natural diamonds.
komatite A very Mg-rich rock that occurs almost exclusively during the Archean. It indicates high temperatures of magma formation in the early Earth.
Kuiper Belt A solar system region located beyond the planets that includes more than 70,000 objects larger than 100 km in size; Pluto is one of the largest of these objects.
late heavy bombardment (LHB) The increased cratering intensity for a short period of time, between 3.9 and 3.8 billion years; also called terminal cataclysm. Evidence comes from the lunar cratering record.
Lehmann discontinuity A change in the Earth’s interior from a liquid (outer core) to a solid stage (inner core), defined by a change in density and in seismic wave velocities.
limestone A sedimentary rock consisting primarily of the mineral calcite. In young rocks most often this calcite was originally manufactured by marine organisms.
lipids Complex organic molecules with low oxygen content able to store high energy content (e.g., fats); they are also essential building blocks of living cells.
liposome A little spherule formed by bilayers that spontaneously make a sphere; the inner and outer surfaces consist of the hydrophilic ends of the molecules. The bilayers’s structure isolates the hydrophobic ends from water.
liquidus The boundary in temperature, pressure and compositional space that separates an all liquid state from the first appearance of a solid
lithophile Rock-loving elements with strong affinity to be in Earth’s silicate reservoirs; silicon, magnesium, oxygen, calcium, aluminum, and titanium are common lithophiles. Lithophile trace elements include the rare earth elements, Rb, Sr, and Hf, for example. Lithophile elements are overwhelmingly in Earth’s mantle and crust.
mafic Mg-Fe rich materials such as basalt and gabbro. Mantle peridotite is ultramafic.
magmaphile Magma-loving elements and molecules that greatly prefer the liquid to the solid during melting. Magmaphiles include lithophiles such as Rb, K, Cs, Sr, rare earth elements, siderophiles such as W, and volatiles such as H20 and CO2.
mantle The major unit of Earth’s interior consisting of solid silicate minerals; it overlies the Earth’s core.
mantle plume Widely accepted hypothesis for the cause of hot spots. Mantle plumes would form at a deep hot boundary layer and rise as a relatively stationary column that stays fixed in location beneath the moving plates.
marble A common metamorphic rock formed when limestones are recrystallized.
mass dependent fractionation The process by which two stable isotopes of the same element are slightly separated during low temperature processes. The variations are so small that they are generally reported as “per mil” (parts per thousand). Common isotope fractionation occurs during the water and carbon cycles and by biological processes.
mass extinction Catastrophe characterized by the disappearance of a large number of species in Earth’s history over very short intervals of time. Mass extinctions are generally used to define the differently named time periods of Earth’s history.
mass independent fractionation Small variations in stable isotopes of a single element that are independent of mass. They can be produced by photochemical reactions, for example. Often abbreviated to MIF, mass independent fractionation of sulfur isotopes has been used to constrain the time of the initial rise of oxygen in the atmosphere.
melting regime The region of Earth’s mantle that is melted to produce the magmas and igneous rocks that appear at the surface. Most commonly used in reference to ocean ridges.
metamorphic rock A rock that is formed by the recrystallization under heat and pressure of a precursor rock. At the surface, metamorphism often involves a gain in volatiles. At depth where pressures and temperatures are high, the metamorphic reactions cause release of volatiles, which can also transport some other elements that prefer the volatile phase.
metamorphism The high-temperature, high-pressure process that causes minerals to break down and crystallize as other mineral phases.
meteorites Fragments of solar system materials that resided in outer space and have impacted Earth and survived to be recovered.
methane A trace constituent of Earth’s atmosphere with a significant greenhouse capacity; chemical formula CH4.
methanogens Microorganisms that derive energy to make organic matter from chemical reactions that produce methane. For example, CO2 + 4H2 = CH4 + 2H2O.
Milankovitch cycles Orbital cycles that cause the changes in the seasonal and latitudinal distribution of the sunlight reaching Earth.
mineral An inorganic, naturally occurring chemical compound with a chemical composition that can be written as a chemical formula, a crystalline form, and fixed physical properties. The solid Earth is made up of minerals.
Mohorovicic discontinuity An abrupt increase in density from about 2.7–3.3 gm/cm3, causing a change in seismic velocity at the base of the crust defining the crust/mantle boundary; also called the “Moho.”
molecule The chemical combination of two or more atoms into a single species.
monomer An organic chemical compound that can combine with like species to make very extended molecules called polymers.
moon A naturally occurring object that orbits a planet.
moraines A distinctive heap of debris transported into place by the advancing ice.
natural gas Methane produced by natural processes that occur in the earth. Most mined natural gas is biogenic, but some is also produced by rock reactions.
Near-Earth objects Asteroids with Earth-crossing orbits; also called NEO.
neutron One of the basic units of the atom, characterized by the absence of an electrical charge. In isolation neutrons decay to a proton plus an electron with a half-life of 10.3 minutes.
neutron capture The incorporation by a nucleus of a passing neutron.
nitrogen A major constituent of the Earth’s atmosphere also present in dissolved form in the ocean; chemical formula N2.
nuclear fission A highly exothermic nuclear reaction in which the heavy nucleus of an atom splits into smaller parts, two lighter nuclei and neutrons and photons in the form of gamma rays.
nucleic acids A complex organic molecule able to make complementary chains and carry out information, communication and memory functions within living cells; DNA and RNA are nucleic acids that together with proteins make up the most important macromolecules.
nucleobase Essential constituents of nucleotides, DNA and RNA. The bases contain the “letters” of the DNA alphabet. For DNA the four nucleobases are cytosine, guanine, adenine, and thymine.
nucleotide An organic molecule that is an essential constituent of RNA and DNA; it requires the joining of the nucleobases with the sugar backbone and phosphate groups.
nuclide The basic physical unit of matter; consists of a dense package of neutrons and protons.
olivine A mineral with the formula (Mg,Fe)2SiO4 that is a solid solution between the magnesium endmember forsterite and the iron end-member fayalite. An important mineral in basalts and gabbros, and the predominant mineral in Earth’s upper mantle.
Oort Cloud The farthest reaches of the solar system, beyond the Kuiper Belt; billions of potential comets reside in vastly distant orbits extending much of the distance to the nearest star. When their orbits are disturbed by passing stars, they can become the comets that intersect the inner solar system, leading to impacts with the planets.
ophiolite A geological section of oceanic crust and underlying upper mantle, uplifted and exposed at the surface.
ore deposits Concentrations of individual metals sufficiently rich to merit commercial recovery.
organic molecules Molecules that contains carbon and hydrogen. They constitute the first building blocks of life.
orthoclase The K-rich endmember of the feldspar group, KAlSi3O8. Orthoclase is a primary constituent of granites.
oxidation state A measure of the valence of atoms. The fewer the number of electrons, the greater the positive charge and the higher the oxidation state.
oxides A nonsilicate mineral group where metals combine with oxygen but not silicon; minerals such as magnetite (Fe3O4) and hematite (Fe2O3) are important oxides.
oxygen A major constituent of Earth’s atmosphere also present in dissolved form in the ocean; necessary to animal life; chemical formula O2.
oxygenic photosynthesis The process of converting CO2 and H2O into organic compounds (commonly abbreviated as CH2O) and producing O2 as the oxidized by-product. Oxygenic photosynthesis has led to the rise of O2 over Earth’s history and is the base of most food chains to the present day.
parent isotope The radioactive isotope that decays, to produce the daughter isotope.
partial melting The major process that forms the Earth’s crust. Materials that have multiple solid phases, such as rocks, melt over a range of temperatures, and therefore can melt partially.
passive upwelling The mantle flow generated at most spreading centers as the mantle rises to fill the gap created by the spreading plates; to be contrasted to dynamically driven active upwelling, which is determined by internal mantle properties.
peak oil The predicted maximum global output of oil. Peak oil is well established for individual fields and well-explored nations. Global peak oil is likely to occur in the coming decade.
peptide bond The bond that combines amino acids and permits the construction of proteins.
peridotite A mantle rock consisting of the mafic minerals olivine and pyroxenes and with a density of about 3.33 gm/cm3. It is the rock that makes up the Earth’s mantle.
phase diagram A two dimensional representation of parameters affecting the identity and proportions of mineral and liquid phases; temperature-composition phase diagrams are commonly used to study the properties of melting mixtures, such as rocks.
photoautotrophs Microorganism able to harvest light energy from the sun to live and produce organic matter
plagioclase A solid solution of the feldspar group with albite and anorthite as endmembers. Plagioclase is the most abundant mineral in basalts, and is also common in granites.
planet A large object that orbits a central star; distinguished from a star in that it is not sufficiently massive to ignite a nuclear fire.
planetesimals The smaller precursors to planets that formed during the process of accretion in the solar nebula. Planetesimals then impacted with each other to form planets.
plasma One of the four states of matter that consists of an electrically conductive ionized gas and free electrical charges.
plate tectonics Accepted theory that Earth’s surface is made up of fixed plates that are continually in motion, created at ridges and destroyed at subduction zones.
plates Large sections of the Earth’s crust that slide across the underlying mantle as distinct units and are bounded by plate margins.
pluton An igneous body formed by a magma slowly cooled at depth.
polymer A large molecule formed by repetition of smaller and linked units, the monomers.
postglacial rebound Isostatic uplift of continents caused by the removal of mass by melting of ice sheets from the last glacial period.
p-process Nucleosynthetic process leading to the buildup of heavy nuclides by addition of a proton.
pressure release melting The process by which a rock crosses its solidus and begins to melt by mantle upwelling so that the pressure on the rock decreases. It occurs because the melting temperature (solidus) is temperature sensitive, and it decreases markedly as pressure declines.
prokaryotes The simplest single-celled organisms, lacking cell nucleus and organelles. Bacteria and Archea are both prokaryotes. Most of the genetic diversity of the planet resides in prokaryotes. To be contrasted with the larger and more complex eukaryotes.
protein A complex organic molecule made up of combinations of amino acids; proteins are essential building blocks of living cells.
proton One of the basic units of the atom; characterized by a positive electrical charge.
protoplanets Inner planets of the the solar system at its early stage. Intermediate in size between planetesimals and planets.
purines Used to refer to the nucleobases adenine and guanine.
pyrimidines Used to refer to the nucleobases cytosine, thymine, and uracil.
pyroxene A group of minerals consisting mainly of the elements iron, magnesium, silicon, and oxygen. Slightly richer in Si than olivine, pyroxenes are primary constituents of mafic igneous rocks, such as basalt, gabbro, and peridotite.
quartz SiO2, a primary constituent of granites and sandstones.
radioactive decay The spontaneous transformation of a radionuclide from one element to another through one of its three forms, or by fission.
radionuclide A radiogenic isotope that will decay.
rare earth elements Usually abbreviated as REE, also known as the lanthanide contraction series; a group of fourteen chemical elements in the Periodic Table that have the very useful geochemical characteristics that all of them have a common outer electron shell structure and similar chemical behavior that varies slightly but regularly from one element to another.
Rayleigh number A dimensionless quantity that can be used to estimate the likelihood and character of convection.
red giant A very large star that burns fast and hot until its fuel is exhausted, at which time it undergoes an explosive death.
reductant The element that donates an electron in a reduction-oxidation reaction.
r-process The buildup of heavy nuclides during an intense neutron bombardment that occurs during supernova explosions.
salt domes Plugs of solid salt that push up through overlying sediments.
sandstone A coarse-grained rock formed when beach, dune, or river sands become lithified; consists largely of the mineral quartz.
schist A common metamorphic rock formed when shales are recrystallized.
sedimentary rock A rock formed when material deposited from water, ice, or air becomes cemented into a coherent unit.
shadow zone For a given earthquake, the region where there is no arrival of shear waves or compressional waves due to the liquid inner core (for shear waves) or the abrupt change in velocity (for p-waves).
shale A fine-grained sedimentary rock that originates as mud at the bottom of a standing body of water; consists largely of clay minerals from soils.
sheet silicate A silicate group of minerals (phylosilicates) characterized by parallel sheets of silicate tetrahedra. Mica, chlorite, and clay minerals are common sheet silicates.
siderophile Metal-loving elements that prefer to exist in the metallic phase rather than silicate phases; nickel, gold, tungsten, silver, copper, and platinum are common siderophiles. Iron is either lithophile or siderophile, depending on how much oxygen is available.
snowball catastrophes Episodes, named by Joseph Kirschvink, during which Earth became totally frozen; they may have occurred during the Neoproterozoic period from 580 to 750 million years ago.
solar nebula The initial cloud of gas and dust around the sun from which the solar system formed.
solidus The line in pressure-temperature-composition space that delineates the boundary between no melt present and the initiation of melting. Contrast with liquidus.
spreading axis The plate margin at an ocean ridge where two plates move directly apart from one another and new ocean crust is formed.
spreading rate A measure of the spreading velocity of the world’s plates.
s-process The construction of heavier nuclides through a slow process of neutron addition. Occurs predominately in stellar interiors.
stable isotope fractionation See mass dependent fractionation.
star The basic unit of matter within galaxies; distinguished from other objects (i.e., planets, moons, etc.) by the nuclear fires that burn in their interiors.
stishovite The high pressure form of quartz considered definitive evidence of an impact origin for numerous circular, nonvolcanic craters on Earth.
stromatolites Carbonate sediment structures created by photosynthetic microbiological communities that live in shallow seas; they are among the most ancient structures created by living organisms but are also generated in some modern locations.
subduction zones Convergent margins where one plate moves beneath another and is returned to the mantle.
sulfides A nonsilicate mineral group with sulfur as the major anion; pyrite (FeS2) is the most common and important sulfide.
sulfur mass independent fractionation (SMIF) The most secure line of evidence of a change in atmospheric oxygen about 2.4 Ga.
supernova The explosive demise of very large stars; an event where heavy elements are formed.
tectonic thermostat The hypothesis that a feedback relates subduction and volcanic outgassing of CO2 to changes in weathering, thereby keeping Earth’s climate in a relatively stable state where liquid water is present.
terminal cataclysm The marked increase in cratering intensity for a short period of time, between 3.9 and 3.8 billion years; also called late heavy bombardment (LHB).
thermal convection The process by which Earth’s mantle and core flow, due to differences in density caused by temperature variations.
T-Tauri Massive ejection of gas by violent winds that occur during a small star’s earliest history.
ultramafic nodules Rock fragments from the mantle that are carried to the surface in mantle-derived rocks, such as kimberlites, that are brought to the surface by explosive volcanic eruptions from great depth.
universal common ancestor The forefather from which all life evolved.
volatility A measure of the tendency of a substance to vaporize. Volatile compounds transform to liquid and gas at lower temperatures than refractory compounds.
volcanoes Points on Earth’s surface where liquid silicate (i.e., magma) created in the Earth’s interior is erupted at the surface.
white dwarf A former star of moderate size that has exhausted its nuclear fuel supply.