Possible causes of geomagnetic excursions

partial quote from:

https://en.wikipedia.org/wiki/Geomagnetic_excursion#Occurrence

Possible causes[edit]

Scientific opinion is divided on what causes geomagnetic excursions. The dominant hypothesis is that they are an inherent instability of the dynamo processes that generates the magnetic field.^[3] Others suggest that excursions occur when the magnetic field is reversed only within the liquid outer core, and complete reversals would occur when the outer and inner core are both affected.^[1]

Disorganized dynamo hypothesis[edit]

The most popular hypothesis is that they are an inherent aspect of the dynamo processes that maintain the Earth's magnetic field. In computer simulations, it is observed that magnetic field lines can sometimes become tangled and disorganized through the chaotic motions of liquid metal in the Earth's core. In such cases, this spontaneous disorganization can cause decreases in the magnetic field as perceived at the Earth's surface.^[b]

This scenario is supported by observed tangling and spontaneous disorganization in the solar magnetic field (the 22 or 11 year solar cycle). However, the equivalent process in the sun invariably leads to a reversal of the solar magnetic field: It has never been observed to recover without a full-scale change in its orientation.

Outer-core inner-core opposition hypothesis[edit]

The work of David Gubbins suggests that excursions occur when the magnetic field is reversed only within the liquid outer core; reversals occur when the inner core is also affected.^[1] This fits well with observations of events within the current chron of reversals taking 3,000–7,000 years to complete, while excursions typically last 500–3,000 years. However, this timescale does not hold true for all events, and the need for separate generation of fields has been contested, since the changes can be spontaneously generated in mathematical models.

External driver hypothesis[edit]

Plate tectonic-driven[edit]

A minority opinion, held by such figures as Richard A. Muller, is that geomagnetic excursions are not spontaneous processes but rather triggered by external events which directly disrupt the flow in the Earth's core. Such processes may include the arrival of continental slabs carried down into the mantle by the action of plate tectonics at subduction zones, the initiation of new mantle plumes from the core–mantle boundary, and possibly mantle-core shear forces and displacements resulting from very large impact events. Supporters of this theory hold that any of these events lead to a large scale disruption of the dynamo, effectively turning off the geomagnetic field for a period of time necessary for it to recover.^{[citation needed]}

Substantial cosmic impact[edit]

Richard A. Muller and Donald E. Morris suggest geomagnetic reversal due to very large impact event and following rapid climate change. The impact triggered a little ice age and change of water redistribution more to poles alters the rotation rate of crust and mantle. If the sea-level change is sufficiently large (>10 meters) and rapid (in a few hundred years), then the velocity shear in the liquid core disrupts the convective cells that drive the Earth's dynamo.^[6]