Magnetostratigraphy

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Magnetostratigraphy is a field within stratigraphy that studies the magnetic characteristics of rock bodies.

If the magnetic properties of rocks have measurable differences stratigraphically, that is, from one strata to the next, those differences can be used to identify their relationships and identify varying stratigraphic units. Stratigraphic units are known collectively as magnetostratigraphic units (magnetozones).

The most useful magnetic property for magnetostratigraphy results from a change in the direction of the magnetization of the rocks. Crystals in rocks are magnetically aligned with the Earth’s magnetic field. The Earth’s magnetic field has changed over the eons and those magnetic alignments are ‘recorded’ in the crystals of rocks because the rocks become magnetized in the direction of the Earth's magnetic field at the time of their formation. The change in the earth’s magnetic fields is caused by reversals in the polarity of the Earth's magnetic field, the Earth’s magnetic poles literally change locations. These reversals of Earth’s polarity have taken place many times during geologic history. [1]

Definitions

Magnetostratigraphic classification is based on the organization of rock bodies into units based on their differences in magnetic character.

A magnetostratigraphic unit (magnetozone) is a body of rocks grouped by similar magnetic characteristics which allow the rock body to be distinguished from adjacent rock bodies.

Magnetostratigraphic polarity classification employs the organization of rock bodies into different units that show similar changes in their polarity. That polarity shows their magnetization which is related to changes or reversals in the polarity of the Earth's magnetic field.

The direction of the remnant magnetic polarity recorded in a stratigraphic sequence is used as the basis for a division of each sequence into units with the same magnetic polarity. In this way, rocks characterized by their similar magnetic polarity (magentozones) are recognized as a magnetostratigraphic polarity unit that is distinctively different from adjacent rock bodies.

Transitional strata

The transitional layer between magnetozones are either magnetostratigraphic polarity-reversal horizons or polarity-transition zones. Rocks deposited during a polarity change will show a distinct difference to the magnetic zones below them (older strata) and above them (younger strata)

Magnetostratigraphic polarity-reversal horizons are strata deposited during a relatively short polarity transition duration and are correspondingly thin transition intervals, that is, thin layers of strata. Younger strata above then show magnetic polarity reverses, that means the strata above and below show rock crystals with different magnetic polarities.

Where a polarity change takes place through a thicker layer of strata of approximately one meter in thickness (in other words, the magnetic transition duration is longer), the term magnetostratigraphic polarity transition-zone is be applied. Magnetostratigraphic polarity-reversal horizons and polarity-transition zones show the boundaries between magnetostratigraphic polarity units.[1]

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References and notes

  1. 1.0 1.1 Magnetostratigraphic polarity units International Commission on Stratigraphy

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