geochemistry
A number for “the Earth” is not a number at all until it says which Earth. Bulk Earth, bulk silicate Earth, the crust and the upper continental crust are four different volumes with four different compositions.
About: Bulk Earth · Bulk silicate Earth · Continental crust (reservoir) · Upper continental crust · Average crust
Bulk silicate Earth means the planet without its core: mantle plus crust. Because the core holds most of Earth’s iron and its siderophile elements, bulk Earth and bulk silicate Earth are not competing estimates of one composition — they are the compositions of two different volumes.[1]
Crustal composition is a third thing again. The crust is a thin, chemically evolved skin produced by repeated partial melting, enriched in exactly what the mantle would not keep.[1]
And the upper continental crust is a fourth: the part actually available for sampling, weathering and erosion, whose composition differs from that of the crust as a whole.[1]
No sample of bulk Earth exists or can exist. Estimates of it are built from meteorites and from models of accretion and differentiation — model outputs, not measurements, and Lithoen labels them so.[1]
The core is in the same position: nothing from it has ever been held, and its composition is inferred from density, from the behaviour of seismic waves, and from meteorites.[1]
Oceanic crust is sampled easily but is compositionally quite unlike the continents: young, basaltic, thin, and continuously destroyed before it can evolve.[1]
Average crustal abundance figures — including the rare-earth values on this site — are quoted against the crust taken as one averaged body, without distinguishing continental from oceanic. That coarseness is part of what the numbers mean, and transferring them to either crust specifically would be an error.[1]
The same discipline applies at the other end of the scale. Soil concentrations mean nothing without knowing which soils, at what depth, sampled how: the USGS national survey specifies one site per 1,600 square kilometres, samples from 0 to 5 centimetres, a composite of the A horizon, and a deeper sample from the C horizon.[1,2]
Seawater is governed by different rules again: concentrations there reflect solubility and residence time rather than crustal abundance, so an element common in rock can be vanishingly rare in the sea.[1]
Often said
“The composition of the crust is the composition of the Earth.”
What the evidence supports
The crust is a thin differentiated skin, enriched in the elements repeated melting concentrated into it. Bulk Earth includes the core, which holds most of Earth’s iron; bulk silicate Earth excludes it. The three are different volumes with different compositions.
Supporting records: crust-not-whole-earth · bse-excludes-core · bulk-earth-not-measured
Often said
“An abundance figure applies wherever the element is found.”
What the evidence supports
An average crustal abundance is an average over a heterogeneous body. It says nothing about the concentration at any particular place, and nothing about whether the element can be recovered anywhere.
Supporting records: average-crust-definition · lanthanum-crustal-abundance · ree-not-rare
Bracketed references link to the structured entity records whose claims support each paragraph. Last updated 2026-09-06.