Earth's Magnetic Field

It is a fundamental tenet of geophysics that the Earth’s magnetic field can exist in either of two polarity states: a “normal” state, in which north-seeking compass needles point to the geographic north, and a “reverse” state, in which they point to the geographic south. Geological evidence shows that periodically the field’s another; rather, the process involves a transition period that typically spans a few thousand years.

Though this much is known, the underlying causes of the reversal phenomenon are not well understood. It is generally accepted that the magnetic field itself is generated by the motion of free electrons in the outer core, a slowly churning mass of molten metal sandwiched between the Earth’s mantle (the region of disturbs the heat circulation pattern of the outer core fluid, and with it the magnetic field.

Several explanations for this phenomenon have been proposed. One proposal, the “heat-transfer hypothesis,” is that the triggering process is intimately related to the way the outer core vents its heat into the mantle. For example, such heat transfer could create hotter (rising) or cooler (descending) blobs of material from the inner and friction and turbulence near the outer core-mantle boundary and initiating a reversal of the magnetic field.

How well do these hypotheses account for such observations as the long-term increase in the frequency of reversal? In support of the asteroid-impact model, it has been argued that the gradual cooling of the average ocean temperature would enable progressively smaller asteroid impacts (which are known to occur more frequently than larger by means of the thermodynamic state of the outer core and its effect on the mantle.