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How do magnetic reversals preserved in rocks help build the geologic time scale?

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Messinian Stage — Tertiary Period Gypsum cones, which resulted from the evaporation of the Mediterranean Sea during the Messinian Salinity Crisis, in the Sorbas basin, Spain.

Magnetic reversals are essential to the geologic time scale because they provide a global, recordable sequence of events that can be dated and correlated across the globe.12

As igneous and sedimentary rocks form, magnetic minerals within them align with and "lock in" the orientation of Earth's magnetic field at that time. Because the magnetic field periodically reverses polarity—switching between "normal" and "reversed" states—these rocks create a chronological record of these shifts.134

Scientists build a paleomagnetic time scale by:
* Dating: Applying radioisotopic dating (such as potassium-argon dating) to volcanic rocks to determine the absolute age of specific polarity intervals.13
* Mapping: Identifying strips of normal and reversed polarity in oceanic crust and deep-sea cores, which act as a continuous tape recorder of magnetic history as seafloor spreading occurs.324

By integrating this magnetic-polarity succession with biostratigraphy and isotopic ages, geologists create a high-resolution, internally consistent geologic time scale.3