Geodynamo and geomagnetic polarity reversals

7 problems

• What powered the geodynamo before the inner core existed?

  Open

  Rocks show Earth had a magnetic field at least 3.4 billion years ago, long before the solid inner core probably formed. If the core conducts heat as well as recent calculations say, ordinary heat-driven stirring may have been too weak to run the dynamo (the self-sustaining generator that turns flowing liquid iron into a magnetic field).

• Did Earth have a magnetic field 4.2 billion years ago?

  Contested

  Tiny crystals called zircons from the Jack Hills of Australia, some older than 4 billion years, contain magnetic grains that may record an ancient field. Researchers disagree on whether the magnetization formed with the crystals or was added much later.

• Can an impending reversal or excursion be detected from today's field?

  Open

  The main part of the field has weakened over the last two centuries and a weak patch over the South Atlantic is growing. It is unknown whether such signs announce a coming flip.

• What triggers geomagnetic polarity reversals and sets their rate

  Open

  The magnetic poles have flipped hundreds of times at irregular intervals, sometimes several times per million years and sometimes not at all for tens of millions of years. The flow pattern in the liquid core that starts a flip is unknown.

• Do numerical dynamos reach the asymptotic regime of Earth's core?

  Partially resolved

  Computer models of the core are still far too viscous compared to the real thing. It is unclear whether their results scale correctly to Earth.

• Do mantle heat-flux changes cause superchrons and reversal-rate swings?

  Open

  The rate of pole flips has changed slowly over hundreds of millions of years, perhaps because the overlying rocky mantle changes how fast heat leaves the core. Whether that explains long flip-free periods is untested.

• Is the top of Earth's liquid core stably stratified?

  Contested

  Some seismic and magnetic data suggest the outermost few hundred kilometers of the liquid core form a calm layer that does not churn. Other analyses find no such layer, and its presence changes how the field is generated and how fast the core cools.