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# Earth Science 3.3 : Absolute Dating: A Measure of Time

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• 1. Earth Science 3.3
Absolute Dating : A Measure of Time
• 2. Objectives:
Identify four types of radiometric dating.
Determine the best type of radiometric dating to use to date an object.
Absolute dating
any method of measuring the age of an event or object in years.
To determine the absolute ages of fossils and rocks,
scientists analyze isotopes of radioactive elements.
Isotopes
atoms of the same element that have the same number of protons but different numbers of neutrons.
• 4. Most isotopes are stable, meaning that they stay in their original form.
Other isotopes are unstable.
Radioactive isotopes tend to break down into stable isotopes of the same or other elements.
Scientists can use the relative amounts of stable and unstable isotopes present in an object to determine the object’s age.
Dating Rocks — How Does It Work?
In radioactive decay, an unstable radioactive isotope of one element breaks down into a stable isotope.
The stable isotope may be of the same element or of a different element.
Parent isotope
Daughter isotope
The stable isotope produced by the radioactive decay of the parent isotope.
• 6. The rate of radioactive decay is constant
so scientists can compare the amount of parent material with the amount of daughter material to date rock.
The more daughter material there is
the older the rock is.
Determining the absolute age of a sample, based on the ratio of parent material to daughter material.
If you know the rate of decay for a radioactive element in a rock
you can figure out the absolute age of the rock.
Half-life
the time needed for half of a sample of a radioactive substance to undergo radioactive decay.
After every half-life, the amount of parent material decrease by one-half.
• 8. Types of Radiometric Dating
Scientists use different radiometric-dating methods based on the estimated age of an object.
Potassium-Argon Method
Potassium-40 has a half-life of 1.3 billion years, and it decays leaving a daughter material of argon.
This method is used mainly to date rocks older than 100,000years.