Research by Ben-Gurion University Geologist Dr. Roi Granot helps dispel the assumption that the Earth’s magnetic poles were stable and quiet for ~38 million years between 121 and 83 million years ago.
An article describing Granot’s research study was just published online on Sunday night in Nature Geoscience. Granot undertook the study during his post-doctoral research at Institute de Physique du Globe de Paris (IPGP). He and his two colleagues, Dr. Jérôme Dyment and Dr. Yves Gallet of CNRS, trailed magnetometers across the north Atlantic measuring the magnetization of the rocks that form the ocean’s floor.
“For the last 83 million years, the northern and southern magnetic poles have been flipping back and forth up to 5 times every million years. But, there were ~38 million years without any reversals between 121 million years ago and 83 million years ago. We know that using the magnetization of various geological records from that period,” Granot says.
The obvious question is – why did the geomagnetic field remain stable for so long?
Based on sophisticated numerical simulations, the initial assumption was that the geomagnetic field was very strong and stable during this period of time. However, no firm evidence was available to confirm this prediction since studying rocks on-land means studying an incomplete record.
“Another way to approach this problem is to look at the oceans because the oceans are like giant tape recorders. You can tow a magnetometer behind a ship and measure magnetic anomalies. These anomalies reflect the magnetization of the rocks underneath the ocean and therefore provide a continuous record of the evolution of the geomagnetic field” he says.
The caveat in using this method is that these magnetic anomalies also reflect other geological processes and you may get ambiguous results. One way to compensate for that ambiguity is to compare different oceans.
“If you see the same pattern of magnetic anomalies at different locations around the globe, then you know it’s a geomagnetic field signature. We compared the North Atlantic findings to different magnetic data from widespread oceans - the South Atlantic and the Indian Oceans. We saw that the signal was convincingly similar,” Granot explains.
What Granot and his colleagues discovered was that what has been known until now as the Quiet Zone –actually contained a surprising story.
“The signal was relatively stable in the beginning, then very noisy in the middle, then back to a relatively stable period towards the end,” Granot says.
Tracking the geomagnetic field indicates the evolution of the Earth’s outer core which generates the field.
“What it means is that the conditions in the outer core evolved in a way that we didn’t know or expect. We can use it to better model the convection processes in the core and understand why the poles flip over and why they don’t,” he added.
What’s more, “Traditionally magnetic anomalies are used to date the oceanic basins. That’s how we date the ocean’s crust. These 38 million years, seemingly without reversals of the magnetic poles, couldn’t be dated. Now we hope we can start dating the crust with these new anomalies. This area represents 25% of the Earth’s oceans.”
