Question

Lord Kelvin argued that, based on the temperature of rocks, the earth could not be as old some geologists thought. What turned out to be wrong with his reasoning?

a) He claimed that the temperature of surface rocks should be used in calcuations, but these calculations were shown to be unreliable.
b) He did not realize that the planet's interior was being heated by radioactive decay, which had not been discovered yet.
c) The upper layers of the earth are cooler than Kelvin realized.
d) all of the above

Answer 1

Lord Kelvin's reasoning was found lacking due to his oversight of the planet's interior heating caused by radioactive decay, an aspect undiscovered at his time, leading to an underestimation of the Earth's age.

Step-by-step explanation:

He did not realize that the planet's interior was being heated by radioactive decay, which had not been discovered yet.

Lord Kelvin calculated the Earth's age based on the assumption that the planet was cooling from a molten state and erroneously concluded it could only be tens of millions of years old. However, he did not know that radioactive decay within the Earth's interior was producing heat, which greatly affected the cooling time. Radioactive isotopes like Uranium-238 and Potassium-40 have long half-lives, which means they have been providing heat for billions of years, sustaining the Earth's molten core and altering the expected temperature profile based on just heat transfer concepts. Thus, his calculations did not account for this continuous heat source, leading to an underestimated age of the Earth.