Research press release


Nature Geoscience

Earth's inner core may have formed in the nick of time



John Tardunoたちは今回、現在のカナダ・ケベック州東部において5億6500万年前に形成された斜長石と単斜輝石の単結晶内で見つかった小さな磁性包有物に記録された地球磁場の過去の強度と方向を測定した。その結果、Tardunoたちは、この時期の地球磁場強度が前例にないほど低く、またこの時期には地磁気反転が頻繁に起きていたことを見いだし、こうしたことから地球ダイナモは崩壊寸前だったと推測している。

関連のNews & Viewsでは、Peter Driscollが、「内核形成は、地球ダイナモを回復させて地球磁場による遮蔽を保つのに、ちょうどよい時に起きた可能性がある」と述べている。

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.”

doi: 10.1038/s41561-018-0288-0


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