Absolute and Relative

A numerical (or "absolute") age is a specific number of years, like 150 million years ago.

A relative age simply states whether one rock formation is older or younger than another formation.

The Geologic Time Scale was originally laid out using relative dating principles.

Absolute numerical dating takes advantage of the "clocks in rocks" - radioactive isotopes ("parents") that spontaneously decay to form new isotopes ("daughters") while releasing energy.

An unstable atom that will give off energy and decay into another type of atom.

Decay of the parent isotope Rb-87 (Rubidium) produces a stable daughter isotope, Sr-87 (Strontium), while releasing a beta particle (an electron from the nucleus).

("87" is the atomic mass number = protons + neutrons.

Radioactive isotopes decay at constant rates

The rates are different for each type of isotope

A graph of isotope decay over time is called a decay curve

Many minerals contain radioactive isotopes. In theory, the age of any of these minerals can be determined by: 

1) counting the number of daughter isotopes in the mineral, and  

2) using the known decay rate to calculate the length of time required to produce that number of daughters.

The amount of time it takes for 50% of the parent radioactive isotope to decay to its stable daughter isotope

The radiocarbon dating method has evolved into the most powerful method of dating fossils, artifacts and geologic events up to about 50,000 years in age.

N-14 in the atmosphere is converted to C-14 by cosmic radiation

C-14 enters food chain through photosynthesis

C-14 decays back to N-14

In order to date older fossils, scientists must use other radioactive isotopes.

The element potassium 40 (K40) is found in most rock-forming minerals

half-life = 1.25 billion years

allowing measurable quantities of Argon 40 (Ar40) to accumulate in potassium-bearing minerals of almost all ages.

Absolute Dating can only be performed in igneous rocks, not sedimentary rocks

Fossils older than 50,000 years cannot be dated, their age must be estimated using the surrounding rocks

Most fossils are found in sedimentary rock layers

Scientist then age the igneous rock layers above and below the fossil to determine an age range

The rate of decay (half-life), and the branching ratio, of K-40 have not changed.

The material in question lost all its argon at an identifiable time, the reset time.

No argon has been lost since the time the rock was reset, or set to zero.

No argon except atmospheric argon, with today's Ar-40 / Ar-36 ratio, has been gained since the reset time of the rock.

No potassium has been gained or lost since the reset time, except by decay.

The ratio of K-40 to total K is constant. The total K, Ar-40, and Ar-36 in the material

in question can all be measured accurately.