Before clay is fired and while lava is still in a molten state, the very weak magnetic fields of individual particles are randomly oriented. Later, its thermoremnant magnetism is measured with a magnetometer.
By comparing these data, a researcher can determine the direction of magnetic north at the last time the sample had been exposed to a high temperature..
It is based on the fact that amino acids (the building blocks of all proteins) exist in two mirror image forms, both of which otherwise have the same chemical structures.
The L-amino acid molecule form has an extension to the left, while the D-amino acid form has an extension to the right.
The L- and D-amino acid ratios are determined by gas and liquid chromatography..
Likewise, it can occur in molten rock from a volcano.
They are called chronometric because they allow one to make a very accurate scientific estimate of the date of an object as expressed in years.
The L-amino acids change to D-amino acids more or less steadily following death. As a result, remains of organisms that died long ago will have more D-amino acids than ones that died recently.
Aspartic acid (one of the 20 amino acids) is usually extracted from samples for this dating technique.
That may mean that another reversal is coming, but not for a few thousand years.
The earth's magnetic field helps block out dangerous ionizing radiation from our sun.