Catastrophism and the Geomagnetic Excursion

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by DAVID BROWN | CLEARNFO.com | December 28, 2021

Introduction: 

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Terms from Wikipedia:

  • Catastrophism
  • Geomagnetic excursion
  • Pole shift
  • Magnetic reversal
  • “The Laschamps Excursion.”

Catastrophism

In geology, catastrophism theorizes that the Earth has largely been shaped by sudden, short-lived, violent events, possibly worldwide in scope.[1] This contrasts with uniformitarianism (sometimes called gradualism), according to which slow incremental changes, such as erosion, brought about all the Earth’s geological features. The proponents of uniformitarianism held that the present was “the key to the past”, and that all geological processes (such as erosion) throughout the past resembled those that can be observed today. Since the 19th-century disputes between catastrophists and uniformitarians, a more inclusive and integrated view of geologic events has developed, in which the scientific consensus accepts that some catastrophic events occurred in the geologic past, but regards these as explicable as extreme examples of natural processes which can occur.

Geomagnetic excursion

A geomagnetic excursion, like a geomagnetic reversal, is a significant change in the Earth’s magnetic field. Unlike reversals, an excursion is not a “permanent” re-orientation of the large-scale field, but rather represents a dramatic, typically a (geologically) short-lived change in field intensity, with a variation in pole orientation of up to 45° from the previous position.[1]

Excursion events typically only last a few thousand to a few tens of thousands of years, and often involve declines in field strength to between 0 and 20% of normal. Unlike full reversals, excursions are generally not recorded around the entire globe. This is certainly due in part to them not registering well in the sedimentary record, but it also seems likely that excursions may not typically extend through the entire global geomagnetic field.[1] There are significant exceptions, however.[a]

Pole shift: Cataclysmic pole shift hypothesis

The cataclysmic pole shift hypothesis suggests that there have been geologically rapid shifts in the relative positions of the modern-day geographic locations of the poles and the axis of rotation of Earth, causing calamities such as floods and tectonic events.[1]

There is evidence of precession and changes in axial tilt, but this change is on much longer time-scales and does not involve relative motion of the spin axis with respect to the planet. However, in what is known as true polar wander, the solid Earth can rotate with respect to a fixed spin axis. Research shows that during the last 200 million years a total true polar wander of some 30° has occurred, but that no super-rapid shifts in Earth’s pole were found during this period.[2] A characteristic rate of true polar wander is 1° or less per million years.[3] Between approximately 790 and 810 million years ago, when the supercontinent Rodinia existed, two geologically-rapid phases of true polar wander may have occurred. In each of these, the magnetic poles of Earth shifted by approximately 55° – from a large shift in the crust.[4]

Magnetic reversal

A geomagnetic reversal is a change in a planet’s magnetic field such that the positions of magnetic north and magnetic south are interchanged (not to be confused with geographic north and geographic south). The Earth’s field has alternated between periods of normal polarity, in which the predominant direction of the field was the same as the present direction, and reverse polarity, in which it was the opposite. These periods are called chrons.

Reversal occurrences are statistically random. There have been 183 reversals over the last 83 million years (on average once every ~450,000 years). The latest, the Brunhes–Matuyama reversal, occurred 780,000 years ago,[1] with widely varying estimates of how quickly it happened. Other sources estimate that the time that it takes for a reversal to complete is on average around 7,000 years for the four most recent reversals.[2] Clement (2004) suggests that this duration is dependent on latitude, with shorter durations at low latitudes, and longer durations at mid and high latitudes.[2] Although variable, the duration of a full reversal is typically between 2,000 and 12,000 years.[3]

Although there have been periods in which the field reversed globally (such as the Laschamp excursion) for several hundred years,[4] these events are classified as excursions rather than full geomagnetic reversals. Stable polarity chrons often show large, rapid directional excursions, which occur more often than reversals, and could be seen as failed reversals. During such an excursion, the field reverses in the liquid outer core, but not in the solid inner core. Diffusion in the liquid outer core is on timescales of 500 years or less, while that of the solid inner core is longer, around 3,000 years.[5]

Laschamp event

The Laschamp or Laschamps event[note 1] was a geomagnetic excursion (a short reversal of the Earth’s magnetic field). It occurred between 42,200 and 41,500 years ago, during the end of the Last Glacial Period. It was discovered from geomagnetic anomalies found in the Laschamps lava flows in Clermont-Ferrand, France in the 1960s.[1]

The Laschamp event was the first known geomagnetic excursion and remains the most thoroughly studied among the known geomagnetic excursions.[2]

Background and effects:  Since its discovery, the magnetic excursion has been demonstrated in geological archives from many parts of the world.[2] The transition from the normal field to the reversed field lasted approximately 250 years, while the magnetic field remained reversed for approximately 440 years. During the transition, Earth’s magnetic field reached a minimum of 5% of its current strength, and was at about 25% of its current strength when fully reversed. This reduction in geomagnetic field strength resulted in more cosmic rays reaching the Earth, causing greater production of the cosmogenic isotopes beryllium-10 and carbon-14, a decrease in atmospheric ozone, and changes in atmospheric circulation.[3][4]

This loss of the geomagnetic shield is claimed to have contributed to the extinction of Australian megafauna, the extinction of the Neanderthals and the appearance of cave art.[5][6][7] However, the lack of corroborating evidence of a causal link between the Laschamp event and population bottlenecks of many megafauna species, and the relatively moderate radio-isotopic changes during the event, have cast significant doubt on the real impact of the Laschamp event on global environmental changes.[8]

Because it occurred approximately 42,000 years ago, the period has been termed the Adams Event or Adams Transitional Geomagnetic Event, a tribute to science fiction writer Douglas Adams, who wrote in The Hitchhiker’s Guide to the Galaxy that “42” was the answer to life, the universe and everything.[9][10]

NOAA’s Historical Magnetic Declination

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