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helium clock A method for measuring the passage of geologic time based on the accumulation of helium in the Earth's atmosphere. As one of the rare or ‘noble’ gases, helium is essentially inert chemically. There is very little of it on Earth and this is attributed to the fact that its low atomic weight allows it to escape from the Earth's gravitational field. The mean residence time in the atmosphere is only a few million years. No helium in the atmosphere can therefore be residual since the formation of the Earth, but must be the product of alpha decay in the Earth's crust. Thus a state of equilibrium must exist between the continual loss of helium into outer space and the supply of new radiogenic helium.