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A geomagnetic excursion, like a geomagnetic reversal, is a significant change in the Earth's magnetic field. Unlike reversals however, an excursion does not change the large scale orientation of the field, but rather represents a dramatic, typically short-lived decrease in field intensity. These events, which typically last a few thousand to a few tens of thousands of years, often involve declines in field strength to between 0-20% of normal.

Scientific opinion is divided on what caused geomagnetic excursions. The dominant theory 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 magentic field as perceived at the Earth's surface. In truth, under this scenario, the Earth magnetic field intensity does not significantly change in the core itself, but rather energy is transfer from a dipole configuration to higher order multipole moments which decay more rapidly with the distance from the Earth's core, so that the expression of such a magnetic field at the surface of the Earth would be considerably less, even without significant changes in the strength of the deep field. This scenario is supported by observations tangling and spontaneous disorganizations in the solar magnetic field. However, this process in the sun invariabily leads to a reversal of the solar magnetic field (see: solar cycle), and has never been observed such that the field would recover without large scale changes in field orientation.

A minority opinion, held by such figures as Richard A. Muller, is that geomagnetic excursions are not a 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 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.

Except for very recent periods of the geologic past, it is not well known how frequently geomagnetic excursions occur. Unlike geomagnetic reversals, which are easily detected by the change in field direction, the short lived excursions can be easily overlooked in long duration, coarsely resolved records of past geomagnetic field intensity. Present knowledge suggests that they are at least as abundant as geomagnetic reversals and may be considerably more abundant.