Earth system
Elements and isotopes
An element is defined by one number. An isotope by a second. And the atomic weight of an element is not a single number at all — which turns out to be the most useful fact of the three.
An element is defined by its atomic number: the count of protons in the nucleus. Silicon has 14, oxygen 8, iron 26. Nothing else defines it.[1, 2, 3]
The standard atomic weight is a different quantity — a property of the element as it is actually found, averaged over its isotopes. For elements whose isotopic composition varies in nature it is published as an interval rather than a value.[1, 2]
That variation is precisely what makes isotopes useful. If the ratio of oxygen-18 to oxygen-16 were fixed, it would record nothing; because it shifts with temperature and process, it records both.[1, 2]
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Atoms, elements and isotopes
Three numbers describe an atom, and geologists use all three for different purposes. Confusing them is the commonest error in writing about geochemistry — and one of them is not a single number at all.
Dating the Earth
Radiometric dating is precise, well understood, and does not measure what people usually think it measures. Understanding what it actually records explains why the age of the Earth had to come from meteorites.
Major rock-forming elements
The elements that build the silicate Earth. Between them they account for the overwhelming bulk of crust and mantle.
Chemical element
Oxygen
The most abundant element in the Earth’s crust and mantle by atom count, and the framework of nearly every rock-forming mineral. Silicate structures are built of oxygen; everything else occupies the spaces between.
Chemical element
Silicon
The element that, with oxygen, defines the silicate minerals. Its tetrahedral coordination by four oxygens is the single structural motif from which most of the solid Earth is assembled.
Chemical element
Aluminium
A major crustal element that can substitute for silicon in tetrahedral sites and also occupy larger octahedral ones — a dual role that makes it central to feldspars, micas and clays.
Chemical element
Iron
The dominant element of Earth’s core and a major constituent of mantle and crustal minerals. Its two common oxidation states make it the principal recorder of redox conditions in rocks.
Chemical element
Magnesium
A major element of the mantle, and the lighter partner in the iron–magnesium substitutions that run through olivine, pyroxene and mica.
Chemical element
Calcium
A major element of plagioclase feldspar, pyroxene and carbonate minerals, and the element whose partitioning drives much of the compositional variation in igneous rocks.
Chemical element
Sodium
A major element of alkali and plagioclase feldspars, readily mobilised by fluids and among the first elements lost during chemical weathering.
Chemical element
Potassium
A major element of alkali feldspar and mica whose long-lived radioactive isotope makes it simultaneously a rock-forming element and a heat source.
Chemical element
Titanium
A minor element that concentrates in its own oxide and silicate phases, and a common host for other elements too large to enter the main rock-forming minerals.
Elements of the deep and the surface
Elements whose behaviour illustrates a geochemical principle rather than a rock-forming role.
Chemical element
Carbon
The element of carbonate minerals and of organic matter, and the pivot of exchange between rock, ocean and atmosphere.
Chemical element
Hydrogen
Present in rocks chiefly as hydroxyl and water, whose presence lowers melting temperatures and changes how minerals deform.
Chemical element
Sulfur
The element that makes sulfide minerals possible, and therefore the principal carrier of chalcophile metals.
Chemical element
Phosphorus
A minor element concentrated almost entirely in phosphate minerals, which in turn concentrate many trace elements.
Chemical element
Manganese
A minor element with several oxidation states, which makes its solubility and mineral host strongly redox-dependent.
Chemical element
Chromium
A trace element strongly compatible in mantle minerals, and therefore retained in the residue when the mantle melts.
Chemical element
Nickel
A trace element that partitions strongly into olivine and into metallic phases — behaviour that links it both to mantle rocks and to Earth’s core.
Chemical element
Copper
A trace element with a strong affinity for sulfide, and therefore a standing example of chalcophile behaviour.
Chemical element
Zirconium
A trace element that concentrates in zircon, a mineral robust enough to survive weathering, transport and metamorphism and to carry a datable record through all of them.
The rare earths and the radioactive clocks
Elements whose atomic structure or instability gives them a special evidential role.
Chemical element
Lanthanum
The first of the lanthanides and the lightest rare-earth element.
Chemical element
Cerium
The most abundant rare-earth element, and unusual among them for adopting a tetravalent state as well as the trivalent one.
Chemical element
Neodymium
A light rare-earth element whose isotopic composition is used to trace the evolution of mantle and crustal reservoirs.
Chemical element
Lutetium
The last and smallest of the lanthanides, and the end point of the lanthanide contraction.
Chemical element
Yttrium
Not a lanthanide, but grouped with the rare-earth elements because its charge and ionic radius make it behave like one.
Chemical element
Uranium
A radioactive trace element whose decay both heats the Earth and provides its most precise clocks.
Chemical element
Lead
The daughter of uranium and thorium decay, and therefore the element whose isotopic composition records the age of the material containing it.
Isotopes
Nuclides that share an element’s chemistry and differ in mass — which is what lets them measure time and trace process.
Isotope
Oxygen-16
The dominant isotope of oxygen. Ratios between it and the heavier oxygen isotopes shift with temperature and with phase change, which is what makes oxygen isotopes a thermometer.
Isotope
Oxygen-18
The heavy stable isotope of oxygen whose ratio to oxygen-16 records temperature and process.
Isotope
Carbon-12
The dominant stable isotope of carbon and the reference against which carbon isotope ratios are reported.
Isotope
Carbon-13
The heavier stable isotope of carbon, whose ratio to carbon-12 distinguishes carbon that has passed through biological processes.
Isotope
Silicon-28
The dominant stable isotope of silicon, making up more than nine tenths of the element in normal materials.
Isotope
Hydrogen-2 (deuterium)
The heavy stable isotope of hydrogen, whose abundance in natural waters varies enough to trace their history.
Isotope
Potassium-40
A rare radioactive isotope of potassium that is simultaneously a geological clock and one of the three principal sources of Earth’s radiogenic heat.
Isotope
Uranium-238
The most abundant uranium isotope, a long-lived heat source and the parent of a decay chain ending in lead-206.
Hub last updated 2026-09-06. Every entry above is a structured record; the bracketed references link to the entities whose claims support each statement.