HELP on CRUSTAL ABUNDANCE ∞g/kg (microgram per kilogram) (plotted on a logarithmic scale)
This is a plot of an estimate of the average elemental composition of the continental crust. The units are in parts per billion, or ∞g/kg, the same as that plot for terrestrial abundances. The principal constituents of the crust, which is 17km thick, are rocks consisting of aluminium silicates and other complex oxides. Thus oxygen, silicon and aluminium have the highest abundance, with notable amounts of magnesium, calcium and iron. Some heavier elements are radioactive with no long-lived isotopes, and are present in only exceedingly small quantities like protactinium, polonium to actinium, all the elements beyond uranium, and the two lighter elements, technetium and promethium. Other elements, whilst stable, have crustal abundances less than one part per billion, like the noble gases helium to xenon, tellurium, and the platinum group metals such as ruthenium, rhodium, palladium, osmium, iridium and platinum itself, which may instead be concentrated in the Earths core.
