Question

What does Dawkins mean when he says that a radioactive clock must be “zeroed”? How/when is this achieved in igneous rocks?

What is an “unstable” isotope?

What is meant by “half-life” and how does it help us determine the age of a rock?

How can we use fossils to date strata when there are no igneous rocks present?

Answer 1

When Richard Dawkin said that radioactive clock must be zeroed, he meant if the evolutionary clock could be reset and this would help in calculating the elapsed time. This would basically help in finding out the absolute age of objects. By zeroing the radioactive clock, it would help in finding out how long its been since a particular mineral or material was missing or not present. Basically this would help in finding out absolute age of the rock and absolute time since when a particular element stopped making. The radioactive isotope can be considered as clock as it gradually decays into different kind of atom and by resetting it or zeroing it, it helps in understanding how long a particular conversion takes and when did an element stop making. Through zeroing of the radioactive clocks, it helps in finding how long its been since the decay has started and helps in determining the initial stage.

For the igneous rocks, the potassium/argon timing works and it is found that when the crystals are initially formed or laid down, there is only potassium-40 atom within the molten rock and no argon. However, upon zeroing the radioactive clock, it is the stage when argon is not present and gradually the potassium-40 starts to decay and forms argon-40. The quantity that has been accumulated of argon-40 over time helps in estimating the age of the igneous rock. Through resetting or zeroing it would go back to the phase when there was only potassium-40 and no argon as the decay has not started yet.

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The unstable isotope means that it has the ability to decay over a certain phase of time and gradually it will decay and convert into a completely different element or isotope. The most unstable isotope is francium and it has a half life of 22 minutes. Due to its ability to change, the unstable isotope spontaneously leads to nuclear decay.

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The half life of radioactive substance is the time that is taken by the element to decay half of its atoms. Hence it is the time taken to reduce the half of the quantity through decay. The half life is able to determine age of rock through absolute dating method. In these methods, decay of radioactive elements is determined within the fossils embedded on the rocks and hence the age is determined. There are radioactive elements present in the initial formation of rock, estimating the original amount of the isotope material and comparing to the present help in determining half life and hence age of rock.