How many intervals of reversed polarity

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Some of these debates were among the most interesting ones in Earth Science over this period. Nevertheless, the topic has received less attention in recent years because of widespread difficulties and controversies concerning data reliability. A recent article published in Reviews of Geophysics by Jean-Pierre Valet and Alexandre Fournier of Institut de Physique du Globe de Paris provides a mature reflection on the challenges faced in such research, with a critical review of the main reversal features derived from paleomagnetic records and analyses of some of these features in light of numerical simulations.

As well as providing a critical review of past work, this contribution is sure to provide valuable direction for future research on the subject. AGU asked the authors of the article to highlight the important results that have emerged from their research and some of the important questions that remain. Why is this topic timely and important? What recent advances in particular are leading to a new understanding or synthesis?

How often do reversals occur and how long do they last? What is the field morphology when it reverses? Does the field weaken or collapse and then recovers with the opposite polarity? What are possible consequences of reversals for the biosphere?

When should the next reversal happen? Reversals are relatively rare events if we compare their duration with the length of the polarity intervals.

Most basalt magmas contain abundant molten iron. As magma starts to harden into rock, iron-rich minerals solidify first. Once the basalt cools completely into solid rock, the alignment of the iron minerals is fixed. When geologists studied the polarity of ancient rocks, they were stunned to discover that in many of them, iron minerals were aligned toward the south magnetic pole, not the north.

All rocks of the same age have the same polarity. The patterns reflect the creation and spreading of oceanic crust along the mid-oceanic ridges. Basalt forming at the ridge crest picks up the existing magnetic polarity.

Divergence then moves the swath of fresh crust away from the ridge. Such magnetic patterns led to recognition of the occurrence of sea-floor spreading, and they remain some of the strongest evidence for the theory of plate tectonics. Operations Management. Chemical Engineering. Civil Engineering. Computer Engineering. Computer Science. Electrical Engineering. Mechanical Engineering. Advanced Math. Advanced Physics. Anatomy and Physiology. Earth Science.

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