U3L4: Absolute Dating

The process used to find the specific numerical age of an object or event.

Atoms of the same element that have the same number of protons but different numbers of neutrons. In geology, certain unstable isotopes are critical for determining the age of rocks.

This occurs when an unstable “parent” isotope breaks down into a new, stable “daughter” isotope. This process occurs at a constant, predictable rate.

As a rock or mineral ages, the amount of the parent isotope decreases while the amount of hte daughter isotope increases.

Uranium-238 (the parent) decays into Lead-206 (the daugher); this process releases particles and energy in the process.

The specific amount of time it takes for exactly 50% (half) of a sample of a radioactive isotope to decay.

Regardless of how much of the isotope you start with, the time it takes for half of it to decay remains the same.

• Carbon-14 is often used for dating organic remains. • It has a half life of 5,730 years.

• Potassium-40 is often used for dating minerals in volcanic rock • It has a half life of 1.3 billion years.

To determine the age of a mineral, scientists measure the ratio of parent isotopes to daughter isotopes

By indentifying what percentage of the original parent is left (e.g. 25%, 12.5%), and knowing the half life of that isotope, the total age can be determined by counting how many half-lives have passed.

• Index fossils are used to help date rock layers that are otherwise difficult to analyze. • The following make a good index fossil: 1. Existed for a relatively short period of geologic time 2. Been geographically widespread (found in many places)’ 3. Been abundant and easy to identify