The Earth’s magnetic field was at its lowest intensity about 565 million years ago, with its powering dynamo on the verge of collapse, suggests a paper published online this week in Nature Geoscience. As the formation of the Earth’s solid inner core would have strengthened the geomagnetic field, this finding suggests that the inner core had not fully begun solidifying by that time.
Estimates of when the inner core solidified vary widely, ranging between 2.5 billion and 500 million years ago. However, these timings can be refined by analysing rocks that record the past nature of the geomagnetic field. Solidification of liquid iron at a fledgling inner core boundary would have become an important energy source for the geodynamo - the convecting currents of liquid metal in the outer core that power the magnetic field. Simulations have predicted that this energy boost would be preserved in the rock record of geomagnetic field strength.
John Tarduno and colleagues measured the geomagnetic field’s past intensity and direction as recorded in tiny magnetic inclusions found within single crystals of plagioclase and clinopyroxene formed 565 million years ago in what is now Canada’s eastern Quebec. They found unprecedentedly low geomagnetic field intensities, and infer that there was a high frequency of magnetic reversals at that time, suggesting that the geodynamo was on the point of collapsing.
In an accompanying News & Views article, Peter Driscoll writes that “the nucleation of the inner core may have occurred right in the nick of time to recharge the geodynamo and save Earth’s magnetic shield.”