Geologists from Massachusetts Institute of Technology (MIT) and Oxford University have discovered 3.7-billion-year-old rocks in Greenland that contain the oldest known evidence of Earth’s magnetic field. The study titled Possible Eoarchean records of the geomagnetic field preserved in the Isua Supracrustal Belt, southern west Greenland has been published in the Journal of Geophysical Research: Solid Earth. The findings extend the known existence of Earth’s magnetic field by 200 million years beyond previous estimates of 3.5 billion years.
Geological Context and Dating
The team analysed banded iron formations, composed of iron and silica-rich layers, likely deposited in ancient oceans before atmospheric oxygen rose significantly. They used uranium to lead ratio methods for confirming the age of the minerals. This helped the scientists better understand the timing and importance of Earth’s magnetic shield in supporting early life.
The northeastern region of Isua Supracrustal Belt underwent two significant metamorphic events—approximately 3.69 billion years ago and 2.85 billion years ago—along with a later hydrothermal alteration around 1.5 billion years ago. During the initial metamorphic event, banded iron formation developed a chemical remanent magnetisation. Notably, this magnetic signature was not completely erased by the later geological processes.
Paleomagnetic data suggest that the Isua Supracrustal Belt has retained a record of the Eoarchean geomagnetic field, offering valuable insight into Earth’s early magnetic history.
Importance of Earth’s Magnetic Field
Life on Earth would not be possible without the planet’s magnetic field. It serves as a vital shield against cosmic radiation and charged particles emitted by the Sun, known as the solar wind. Despite its significance, until now, the exact date in Earth’s history when the modern magnetic field first emerged has remained uncertain.
Earth’s magnetic poles have reversed or ‘flipped’ hundreds of times over the past 160 million years. The most recent major reversal, known as the Matuyama-Brunhes (M-B) geomagnetic reversal, occurred about 780,000 years ago.
Magnetic Signature in Ancient Rocks
Scientists analysed an ancient sequence of iron-rich rocks from Isua, in Greenland. Iron particles within these rocks behave like microscopic magnets capable of recording the strength and direction of Earth’s magnetic field at the time of their formation. These particles become locked into place during crystallisation, preserving magnetic signatures. The researchers discovered that these Greenland rocks, dating back 3.7 billion years, preserved a magnetic field strength of 15 microteslas—comparable to today’s magnetic field, which measures around 30 microteslas. This finding represents the oldest strength of Earth’s magnetic field which was obtained from whole rock samples. These types of samples provide a more robust and reliable dataset, compared to earlier studies that focused on individual crystals.
Significance of the Study
New Clues about Atmospheric Evolution The findings further provide a potential link between Earth’s magnetic field and the evolution of its atmosphere. One enduring mystery is the loss of Xenon, an inert and heavy gas from the atmosphere more than 2.5 billion years ago. Because xenon is not reactive and relatively heavy, it should not have simply escaped into space. A new hypothesis, which is under investigation, suggests that charged xenon atoms may have been carried away by interactions with the magnetic field.
Evolving Shielding Power of Earth’s Magnetic Field Although the overall strength of Earth’s magnetic field appears to have been relatively stable over time, it is known that the intensity of the solar wind was significantly stronger in the past. This suggests that the effectiveness of the magnetic field in protecting Earth’s surface has increased over time. Such enhanced protection may have allowed early life to move onto the continents by venturing out from the oceans.
Origin and Sustenance of Earth’s Dynamo The Earth’s magnetic field is generated by the motion of molten iron in the outer core, which is driven by buoyancy forces caused by the gradual solidification of the inner core, which creates a dynamo. However, during Earth’s early formation, the inner core had not yet solidified. This raises the important questions about how the early magnetic field was sustained. The new findings imply that the mechanism powering Earth’s early dynamo may have been as effective as to the solidification process that generates the Earth’s magnetic field today.
Probing Earth’s Interior Studying changes in the strength of the Earth’s magnetic field over time is also essential for identifying when the Earth’s solid inner core first began to form. Understanding this timing provides valuable insights into the rate at which heat escapes from Earth’s deep interior, a factor that plays a crucial role understanding the processes such as plate tectonics.
Geological Significance of Isua Supracrustal Belt
Many rocks on Earth have undergone complex and prolonged histories that obscure their original magnetic records. However, the Isua Supracrustal Belt in Greenland presents a unique geology. This region rests atop thick continental crust, which has largely shielded it from tectonic activity and deformation. As a result, the researchers were able to construct a solid body of evidence supporting the existence of a magnetic field 3.7 billion years ago.
Professor Benjamin Weiss of the MIT, a co-author of the study, remarked that the northern part of Isua contains the oldest well-preserved rocks on Earth. These rocks have not experienced significant heating since their formation 3.7 billion years ago and have been kept clean of modern contamination by the Greenland ice sheet, making them ideal candidates for studying early Earth conditions.
Future Research Direction
Looking ahead, researchers aim to explore additional ancient rock formations in regions such as Canada, Australia, and South Africa. These investigations seek to further illuminate the nature of Earth’s magnetic field before the rise of oxygen in the atmosphere approximately 2.5 billion years ago. By improving our understanding of the early strength and variability of Earth’s magnetic field, scientists hope to determine whether magnetic fields are critical for supporting life on planetary surfaces and how they influence the long-term atmospheric evolution.
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