Principles dating sedimentary rocks
The two major discontinuities that are universally recognized are the Wiechert–Gutenberg Discontinuity, which separates the mantle from the core.
The latter discontinuity exists at a depth of 2,900 kilometres (1,800 miles); it is marked by a sudden increase in density, from about 5.7 at the base of the mantle to 9.7 at the top of the core.
The mantle comprises that part of the Earth between the Mohorovičić and the Wiechert–Gutenberg discontinuities.
It makes up 83 percent of the volume of the Earth and 67 percent of its mass and is thus of decisive importance in determining the bulk composition of the planet.
The evidence for the composition of the core is all indirect because no means have yet been devised for directly sampling the deep interior of the Earth.
The moment of inertia of the Earth indicates that there is a concentration of mass around the centre, and seismic data have shown that below the Wiechert–Gutenberg Discontinuity the density of the material is high, ranging upwards from 9.7.
Concurrent with these advances in chemical understanding, from about 1850 onward there was a steadily increasing output of analytical data on the Earth’s rocks, minerals, and waters, mainly from laboratories in Europe and North America.
The output from North America was materially increased following the establishment of the United States Geological Survey in 1879 and the appointment of Frank W. Clarke’s name will always be linked with the study of the geochemical distribution of the elements—indeed, the term clarke was proposed as the unit for the average percentage of an element in the Earth’s crust by Soviet scientists and has been generally adopted.
Sulfur, with atomic number 16, and carbon, 6, are relatively abundant in meteoritic matter, and the presence of minor amounts of these elements in the core would effectively reduce the mean atomic number.
In estimating elemental abundances in the mantle, however, the same difficulty as with the core arises: direct sampling is not feasible.