Which two useful properties were used by John Joly and Ernest Rutherford to estimate the age of the earth?

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The question asks which two properties were used by John Joly and Ernest Rutherford to estimate the age of the Earth. Here are the options analyzed one by one.

Option 1: 'Zircon crystals naturally form without lead' — This statement captures a key idea used in early radiometric dating: zircon minerals preferentially incorporate uranium when they form and typically exclude lead, so any lead found in Zircon is mainly from radioactive decay of uranium over time. This makes zircons excellent clocks for dating ancient rocks. The general claim that zircons form without lead is aligned with their crystal chemistry and is why they are used for dating ages of Earth’s rocks. This option is consistent with the historical use of U–Pb dating.

Option 2: 'Zircon crystals form only in the presence of lead' — This is incorrect. Zircon crystallization does not require lead; in fact, zircon tends to exclude lead during formation. Lead abundance in freshly formed zircon is minimal, which is precisely why the uranium-to-lead dating method works well for zircon.

Option 3: 'Lead decays to uranium' — This is the reverse of the actual decay process. In radiometric dating, uranium decays to lead, not the other way around. So this statement misrepresents the decay chain and is incorrect.

Option 4: 'Uranium decays to lead' — This is a correct description of the U–Pb dating scheme. Uranium isotopes decay through a series of steps to stable lead isotopes, and by measuring the ratio of uranium to lead in zircon (or other minerals), scientists can estimate the elapsed time since the mineral formed.

In summary, options 1 and 4 describe true aspects of how radiometric dating with zircon and uranium–lead decay can be used to gauge Earth's age, while options 2 and 3 contain fundamental inaccuracies about zircon formation and radioactive decay processes.

My LMS Subjects Quiz Topic 3: Formation of the solar system

Which two useful properties were used by John Joly and Ernest Rutherford to estimate the age of the earth?

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