(a) What proportion of the original ⁴⁰K and ²³²Th currently remains since the formation of the Earth? (Using the data in Table 2.6, determine how many half-lives of ⁴⁰K and ²³²Th have expired since the Earth was formed.)

(b) Based upon the data in Table 2.6, what was the amount of radiogenic heating in the Earth 4.6 Ga ago, and how does this value compare with that of today?

Table 2.6

Transcribed Image Text:

Table 2.6 Half-lives of common isotopes in the Earth's crust and mantle. The continuing decay of these isotopes is an important internal heat source within the Earth today. Isotope 235U 238U 232Th 40K Half-life (10 yr) 0.71 4.50 13.9 1.30 Present rate of heat generation in the Earth (10-2 Wkg-) 0.04 0.96 1.04 2.8 a Values are in kg of average Earth materials, not of the isotope concerned.