The technique known as potassium-argon dating is used to date old lava flows. Th

ID: 774967 • Letter: T

Question

The technique known as potassium-argon dating is used to date old lava

flows. The potassium isotope 40K has a 1.28 billion year half-life and is naturally

present at very low levels. 40K decays by beta emission into 40Ar. Argon is a

gas, and there is no argon in flowing lava because the gas escapes. Once the

lava solidifies, any argon produced in the decay of 40K is trapped inside and

cannot escape. A geologist brings you a piece of solidified lava in which you find

the 40Ar/40K ratio to be 0.12. What is the age of the rock?

half life = ?

Know/K = 2-t/?

so, Ar/k = 1-2-t/?

.12 = 1 - 2-t/?

-t/? = log2.9 =-.23

t = .23 billion y

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