Absolute DatingAbsolute Dating

Absolute DatingAbsolute Dating

• A.D. is used when geologists want to A.D. is used when geologists want to know the know the actual numerical actual numerical age of a rock age of a rock layer.layer.

• A.D. uses the principle of A.D. uses the principle of radioactive radioactive decay decay to find the age of rocks.to find the age of rocks.

Radioactive DecayRadioactive Decay

• R.D. occurs when unstable isotopes R.D. occurs when unstable isotopes decay into stable products.decay into stable products.

Radioactive DecayRadioactive Decay

• R.D. occurs when unstable isotopes R.D. occurs when unstable isotopes decay into stable products.decay into stable products.

TRANSLATIONTRANSLATION

Radioactive DecayRadioactive Decay

• R.D. occurs when unstable isotopes R.D. occurs when unstable isotopes decay into stable products.decay into stable products.

TRANSLATIONTRANSLATIONParent material (unstable)Parent material (unstable)

↓↓Turns into↓Turns into↓Daughter material (stable)Daughter material (stable)

Radioactive DecayRadioactive Decay

• Isotopes occur Isotopes occur naturallynaturally in rocks. in rocks.• If we know the If we know the raterate at which the decay at which the decay

occurs we can figure out how old the occurs we can figure out how old the rock is.rock is.

• The rate of decay for an isotope depends The rate of decay for an isotope depends on its on its half-lifehalf-life..

Half-LifeHalf-Life

• Half-life is the unit used to describe how Half-life is the unit used to describe how long it takes the unstable parent material long it takes the unstable parent material to decay to to decay to halfhalf its original amountits original amount..

Half-LifeHalf-Life

• Half-life is the unit used to describe how Half-life is the unit used to describe how long it takes the unstable parent material long it takes the unstable parent material to decay to to decay to halfhalf its original amountits original amount..

But, why But, why halfhalf? Why not just say how long ? Why not just say how long it will take until it is it will take until it is allall gone? gone?

Half-LifeHalf-Life

Answer: Because radioactive isotopes don’t decay in a straight line.

Half-LifeHalf-Life

Half-LifeHalf-Life

Half-LifeHalf-Life

Half-LifeHalf-Life

Half-LifeHalf-Life

But the path from Parent to Daughter But the path from Parent to Daughter isn’t always the most direct oneisn’t always the most direct one

Example; the natural decay chain of uranium-238

• decays, through alpha-emission, with a half-life of 4.5 billion years to thorium-234 • which decays, through beta-emission, with a half-life of 24 days to protactinium-234 • which decays, through beta-emission, with a half-life of 1.2 minutes to uranium-234 • which decays, through alpha-emission, with a half-life of 240 thousand years to thorium-230 • which decays, through alpha-emission, with a half-life of 77 thousand years to radium-226 • which decays, through alpha-emission, with a half-life of 1.6 thousand years to radon-222 • which decays, through alpha-emission, with a half-life of 3.8 days to polonium-218 • which decays, through alpha-emission, with a half-life of 3.1 minutes to lead-214 • which decays, through beta-emission, with a half-life of 27 minutes to bismuth-214 • which decays, through beta-emission, with a half-life of 20 minutes to polonium-214 • which decays, through alpha-emission, with a half-life of 160 microseconds to lead-210 • which decays, through beta-emission, with a half-life of 22 years to bismuth-210 • which decays, through beta-emission, with a half-life of 5 days to polonium-210 • which decays, through alpha-emission, with a half-life of 140 days to lead-206, which is a stable

nuclide.

Half-LifeHalf-Life

To use radioactive age dating on a rock To use radioactive age dating on a rock sample you must know sample you must know two thingstwo things::

1.1.The amount of stable daughter isotope The amount of stable daughter isotope in the samplein the sample

2.2.The decay rate (half-life) for that isotopeThe decay rate (half-life) for that isotope

Half-LifeHalf-Life

• Carbon – 14 Carbon – 14 is used to find the age of is used to find the age of organic material.organic material.

• Its half-life is Its half-life is 5730 years 5730 years so it is only so it is only useful dating objects useful dating objects less than less than 70 000 70 000 yearsyears.

Half-LifeHalf-Life

• Uranium – 238 Uranium – 238 is often used to date is often used to date rocks because it is the most common rocks because it is the most common type of uranium found in nature.type of uranium found in nature.

• Its half-life is Its half-life is 4.46 billion years4.46 billion years

Watch thisWatch this

The end