The problem: By the mid 19th century it was obvious that Earth was much older than 6000 years, but how old? This problem attracted the attention of capable scholars but ultimately depended on serendipitous discoveries.

Early attempts: Initially, three lines of evidence were pursued:

The discovery of radioactivity: Ironically, radioactive decay, which frustrated Kelvin's purpose, ended up providing the true key to the absolute dating of rocks.

Radiometric dating:

Thus, sedimentary and metamorphic rocks can't be radiometrically dated.

Although only igneous rocks can be radiometrically dated, ages of other rock types can be constrained by the ages of igneous rocks with which they are interbedded.


The Earth generates a magnetic field that encompasses the entire planet. In the last fifty years, a new dating method has emerged that exploits two aspects of rocks' interactions with the Earth's magnetic field. It is, in essence a form of relative dating.

The utility of paleomagnetism:

Currently, the paleomagnetic record has been worked out through the Triassic.

Key concepts and vocabulary: