An international team of researchers, led by the University of Oxford and the Massachusetts Institute of Technology (MIT), has managed to recover a record of the Earth’s magnetic field from 3.7 billion years ago, the oldest so far.
The biggest surprise of the find is that it appears remarkably similar to the field surrounding Earth today.
Without its magnetic field, life on Earth would not be possible since it protects us from harmful cosmic radiation and charged particles emitted by the Sun (the ‘solar wind’). But so far there is no reliable date for when the modern magnetic field was first established.
In the new study, Researchers examined an ancient sequence of iron-bearing rocks from Isua, Greenland. The iron particles effectively act as small magnets that can record both the intensity and direction of the magnetic field when the crystallization process fixes them in place. The researchers discovered that Rocks dating back 3.7 billion years captured a magnetic field intensity of at least 15 microteslas, comparable to the modern magnetic field (30 microteslas)..
These results have allowed oldest estimate of Earth’s magnetic field strength from whole rock samples. Previous studies used single crystals, which allow for a much less precise and reliable evaluation.
Lead researcher Professor Claire Nichols (Department of Earth Sciences, University of Oxford) said in a statement: “Extracting reliable records from such ancient rocks is extremely challenging, and it was really exciting to see primary magnetic signals starting to emerge. arise when we analyze these samples in the laboratory. This is a really important step forward as we try to determine the role of the ancient magnetic field when life on Earth was first emerging.“.
The movement of life towards the continents
Although the intensity of the magnetic field appears to have remained relatively constant, the solar wind is known to have been significantly stronger in the past. This suggests that the protection of the Earth’s surface from the solar wind has increased over time, which may have allowed life to move toward the continents and leave the protection of the oceans.
Earth’s magnetic field is generated by mixing molten iron in the fluid outer core, driven by buoyancy forces as the inner core solidifies, creating a dynamo.
During Earth’s early formation, the solid inner core had not yet formed, leaving open questions about how the primitive magnetic field was sustained. But the new results leave no room for doubt and suggest that, whatever it was, the mechanism that drove Earth’s early dynamo was equally efficient as the one that generates the planet’s magnetic field today.
Understand how the intensity of the Earth’s magnetic field has varied over time as well It is key to determining when the Earth’s solid inner core began to form. This will help us understand how quickly heat escapes from the Earth’s deep interior, which is key to understanding processes such as plate tectonics.
Unique geological conditions in Greenland
One of the greatest difficulties that scientists faced in reconstructing the primitive magnetic field is the fact that Anything that heats the rock can alter the magnetic ‘signatures’ preserved within it.. And the rocks that make up the planet’s crust often have long and complex geological histories, capable of erase or alter older magnetic data. Fortunately, the Isua supracrustal belt in Greenland, which sits on a thick continental crust that protects it from tectonic activity, boasts some unique geological conditions, which allowed researchers to reconstruct a clear set of evidence supporting the existence of the magnetic field 3.7 billion years ago.
Study co-author Professor Benjamin Weiss (Massachusetts Institute of Technology) said: “Northern Isua has the oldest well-preserved rocks known on Earth. “Not only have they not warmed significantly since 3.7 billion years ago, but they have also been wiped clean by the Greenland Ice Sheet.”
News reference:
Claire IO Nichols, et.al, Possible Eoarchean Records of the Geomagnetic Field Preserved in the Isua Supracrustal Belt, Southern West Greenland. Journal of Geophysical Research. doi.org/10.1029/2023JB027706
