Half a billion years ago, Earth was on track to become just like Mars. Ancient rocks now show what saved our magnetic field

Amazing stuff! Getting to the core of geomagnetism! 

By the way, a geomagnetic reversal is also due! Plus, the very important geomagnetic field has been weakening for the past 2000 years, but in particular over the past 150 years ""At present, the overall geomagnetic field is becoming weaker; the present strong deterioration corresponds to a 10–15% decline over the last 150 years and has accelerated in the past several years; geomagnetic intensity has declined almost continuously from a maximum 35% above the modern value achieved approximately 2,000 years ago." Climate Change anyone!

"Thank Earth’s magnetic field for all the plentiful riches and diversity of life this planet has to offer. Without it, solar radiation would strip away the atmosphere and oceans, leaving behind a barren landscape. That’s in fact what happened to Mars, once believed to be a “blue” water world covered in a planetary ocean, which eventually turned “red” after it mysteriously lost its magnetic field some four billion years ago. ...
About 565 million years ago, scientists found evidence that the strength of Earth’s magnetic field plummeted to just 10% of today’s strength. If this trend had continued, all life on our planet would have been surely doomed. But right before the so-called Cambrian explosion of multicellular life on Earth, the magnetic field bounced back — and, according to a new study by geologists at the University of Rochester, this is all thanks to some fundamental changes in the inner core. ...

But in just 15 million years — almost a flash in geological time — the magnetosphere bounced back. ... this rapid renewal in magnetic field strength can be explained by the formation of a solid inner core about 550 million years ago, which essentially recharged the molten outer core. Then, some 450 million years ago, the growing inner core’s structure changed yet again, cementing a boundary between the innermost and outermost inner core. ..."

From the abstract:

"Paleomagnetism can elucidate the origin of inner core structure by establishing when crystallization started. The salient signal is an ultralow field strength, associated with waning thermal energy to power the geodynamo from core-mantle heat flux, followed by a sharp intensity increase as new thermal and compositional sources of buoyancy become available once inner core nucleation (ICN) commences. Ultralow fields have been reported from Ediacaran (~565 Ma) rocks, but the transition to stronger strengths has been unclear. Herein, we present single crystal paleointensity results from early Cambrian (~532 Ma) anorthosites of Oklahoma. These yield a time-averaged dipole moment 5 times greater than that of the Ediacaran Period. This rapid renewal of the field, together with data defining ultralow strengths, constrains ICN to ~550 Ma. Thermal modeling using this onset age suggests the inner core had grown to 50% of its current radius, where seismic anisotropy changes, by ~450 Ma. We propose the seismic anisotropy of the outermost inner core reflects development of a global spherical harmonic degree-2 deep mantle structure at this time that has persisted to the present day. The imprint of an older degree-1 pattern is preserved in the innermost inner core."

Half a billion years ago, Earth was on track to become just like Mars. Ancient rocks now show what saved our magnetic field Earth's magnetic field bounced back right in the nick of time around 550 million years ago.

Early Cambrian renewal of the geodynamo and the origin of inner core structure (open access)

Fig. 3: Early Cambrian field renewal and inner core growth.