Absolute Dating

• Absolute age - the actual age, or date, in years when a geological event occurred or a rock was formed.

Isotopes & Radioactive Decay• Elements can exist in several varieties called

isotopes.– Different isotopes of an element have a different

atomic mass.– EX: Carbon-12 and Carbon-14

Isotopes & Radioactive Decay• The nucleus of the atoms of many isotopes are

unstable (radioactive). – Unstable nucleus can spontaneously release particles

and electromagnetic energy during radioactive decay.

• Cosmic rays from outer space, bombard the earth’s upper atmosphere. These cosmic rays collide with atoms in the atmosphere and can cause them to come apart. Neutrons that come from these fragmented atoms collide with 14N atoms (the atmosphere is made mostly of nitrogen and oxygen) and convert them into 14C atoms (a proton changes into a neutron).

• Once 14C is produced, it combines with oxygen in the atmosphere to form carbon dioxide (CO2). Because CO2 gets incorporated into plants (which means the food we eat contains 14C and 12C), all living things should have the same ratio of 14C and 12C in them as in the air we breathe.

Half-Life• The half-life of a particular isotope is the time it

takes for half of the atoms in a given mass to decay.

• At the end of ONE half-life, half of the original atoms will have decayed into other isotopes, and the other half will remain unchanged.

• The halving of unchanged atoms continues indefinitely.

RADIOACTIVE DECAY MODEL

1/2

1/2

1:1

1/4

3/4

1/8

7/8

1/16

15/16

1:3 1:7 1:15

Radioactive element

Half-life(Years)

Decay element

Potassium-40 1.3 x 109 Argon-40

Rubidium-87 4.9 x 1010 Strontium

Uranium-238 4.5 x 109 Lead-206

Carbon-14 5.7 x 103 Nitrogen-14

Half-lives of radioactive elements are unique for each element

Radioactive Dating• The half-life of a radioactive isotope, and the ratio

between the amount of original isotope and decay-product, can be used to estimate the absolute age of a rock sample.

Potassium-Argon dating is useful because Potassium is a very common mineral and is found in sedimentary, metamorphic, and igneous rock. Also, the half-life of potassium-40 is only 1.3 billion years, so it can be used to date rocks as young as 50,000 years old.

The half-life of Rb-87 is so long (49 billion years) that it is normally only used to date rocks that are older than about 100 million years. Minerals such as mica and feldspar (in almost ALL igneous rocks) contain the element Rubidium, and can be dated with this method.

The long half-life of uranium-238 makes it possible to date only the oldest rocks. This method is not reliable for measuring the age of rocks less than 10 million years old because so little of the uranium will have

decayed within that period of time. This method is also very limited because uranium is not found in every old rock. It is rarely found in sedimentary or metamorphic rocks, and is not found in all igneous rocks.

5. Carbon 14 has a short half-life and is therefore used to date newer rocks, or the remains of living things such as wood, bones, and leather.

A Giant Sun FlareThe sun sets off a giant flare. Researchers studying tree rings, and the carbon-14 data left in them by cosmic rays, have created a record of sun activity going back 11,400 years.