Home Fossils Age of fossils


The oldest fossils are bacteria that are 3,5 billion years old. In geologic history more and more species developed. Many species also became extinct, like the dinosaurs and ammonitesClick here for more information ! at the end of the CretaceousClick here for more information ! period. This was probably caused by a meteorite impact.

Geologists researched many earth layers (Stratigraphy). If the layers are not disturbed by folding or mountainbuilding, the youngest layer is always on top (relative age). In every layer different characteristic fossils occur. By doing research about the different rock layers, geologists made a description of the different geological periods.


Geological map of Europe.

Some fossils only occur in a limited timespan. Several abundant fossils are characteristic of a certain geologic period. These fossils are knows as Guide fossils, and are used to determine the age of a certain layer elsewhere.

Absolute Age

The exact age of the layers can be calculated using "radioactive isotopes". An atom can occur with different numbers of "atomic particles". The different varieties are known as isotopes. Some isotopes are unstable and will disintegrate and form another atom, releasing radio activity. This disintegration takes place at a constant known rate. Half of the atoms disintegrated in, for example, 10 million years. After 20 million years only a quarter of the original atoms remain. This "half-life time" is different for every isotope type.

The proportions of some radioactive (unstable) isotopes and stable isotopes when the rock was formed are known. By measuring the two different isotopes in a rock sample, you can calculate the age of the rock layer.

Example how to find out the age of a old lava stream:

A known fact is that the lava contains no "Lead-207" when formed, but only the unstable "Uranium-235" isotope.
The "Uranium-235" decays and over time creates the "Lead-207" isotope .
The half-time of "Uranium-235" is 700 million years.

An old lava stream is investigated and the proportion "Lead-207" : "Uranium-235" = 3 : 4.
After 700 million years the proportion should be the same. The age of the lavastream is 3/4 x 700 = 525 million years old!

Be careful: This is a simplified example!


Several isotopes are used for dating rocks. In the table a list is presented with the different methods. Some methods are only suitable for volcanic rocks. By dating volcanic layers in a sequence of mainly sedimentary rocks, it is often possible to make a relative dating.

Isotope

Decays to:

Is used for:

Carbon-14

Nitrogen-14

Fossils / Archeological finds from 1000 to 65.000 years old.

Uranium-235

Lead-207

Volcanic rock with an age of more than 110.000 years old.

Sodium-40

Argon-40

Volcanic rock with an age of more than 100.000 years old.

Uranium-238

Lead-208

Volcanic rock with an age of more than 10 million years old.

Rubidium-87

Strontium-87

Volcanic and sedimentary rocks with an age of more than 10 million years old.

 


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