Chronometric dating techniques are based on

The time between reversals has ranged between less than 100,000 to tens of millions of years with an average of about 300,000 years. It takes 1,000-8,000 years for one to be completed. It is thought that as a reversal approaches, the earth's magnetic field weakens.

It has been gradually weakening for the last 250 years.

A compass needle would have pointed to the south pole during some periods and to the north pole during others.

Lava and volcanic ash deposits often contain the thermoremnant magnetic records of these reversals.

Fortunately, there are other methods available to researchers.

All trees of the same species in an area usually have roughly the same pattern of growth.

Since weather patterns tend to run in cycles of a number of years, the sequence of tree-rings in a region will also reflect the same cycling, as illustrated by the graph below.

In addition, any argon that existed prior to the last time the rock was molten will have been driven off by the intense heat.decay or the rate of other cumulative changes in atoms resulting from radioactivity. The various isotopes of the same element differ in terms of atomic mass but have the same atomic number..One half-life is the amount of time required for of the original atoms in a sample to decay.Subsequently, a diminishing field is likely to result in a significant rise in the frequency of radiation induced cancers in the future.Despite this limitation, paleoanthropologists have found dating by association with polar reversals to be a helpful additional method of tracking the evolution of our fossil ancestors in regions such as East Africa where there has been frequent volcanic activity leaving clear thermoremnant magnetic evidence.

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