April 22 is Earth Day, an annual celebration dating back to 1970. As geoscientists, we celebrate Earth every day by the very nature of our work.
Some of the little-known and fascinating stories of Earth are related to the childhood and teenage years of our planet—the Hadean eon (4.5 to 4.0 billion years ago) and the Archean eon (4 to 2.5 billion years ago).
Let’s look at some of Earth’s deepest-time events: How old is plate tectonics? How old is Earth’s magnetic field? What are the oldest continental crusts?
Enjoy reading, and Happy Earth Day!
Rasoul Sorkhabi
Editor, Core Elements
Earth’s Earliest Recorded Crustal Movements
Vadim Sadovski/Shutterstock.com
As far as we know, Earth is the only planet in the solar system with plate tectonics. When did plate tectonics on Earth start? This depends on how we characterize plate tectonics.
A recent paper in Science by Alec Brenner and colleagues reports on paleomagnetic detection of crustal motion 3.5 billion years ago.
Paleomagnetism:
Earth’s ancient magnetism is preserved in iron minerals in rocks because these minerals record the direction and intensity of Earth’s magnetic field at the time of rock formation.
By knowing the age of rocks, paleomagnetic signals can be used to detect the movement of crustal blocks through various latitudes.
The new study:
Researchers collected 931 samples from 105 sites in the North Pole Dome, East Pilbara Craton in Western Australia.
The samples were mafic volcanic rocks from three formations: North Star Basalt, the Dresser Formation, and Mount Ada Basalt.
These volcanic rocks range in age from 3,481 to 3,451 Ma as determined by uranium-lead dating of zircon.
About cratons:
Cratons are the oldest parts of Earth’s crust. They form the cores of all continents.
They provide vast amounts of minerals necessary for civilization.
Cratons also contain records of Earth’s earliest history.
Studies of cratons are critical to our understanding of how and when Earth’s plate tectonics, magnetic field, and even earliest life forms emerged.
What they found:
The researchers conducted thermal demagnetization of samples, which revealed a variety of magnetization components.
One of these was a high (H) unblocking temperature component at rocks dated 3.48 billion years old. This indicates northward movement of the East Pilbara Craton.
The H component was a secondary magnetization that replaced the primary magnetization by a major hydrothermal event related to felsic igneous intrusions in the craton.
When compared with the Barberton rocks in South Africa:
The researchers compared paleomagnetic results from the East Pilbara Craton with those previously reported from the rocks of the same age in the Barberton Greenstone Belt in the Kaapvaal Craton in South Africa.
Rocks from the Barberton Craton show a stationary position at Equatorial paleolatitudes from 3,481 to 3,456 Ma.
In contrast, rocks from the East Pilbara Craton show a poleward shift from a paleolatitude of 53 degrees to 77 degrees at a rate of 47 centimeters per year over 25 million years.
Why it matters:
This finding from Western Australia is the earliest record of crustal movements during the Eoarchean period.
The previous oldest record of crustal block motion was from a Neoarchean collision between the Superior and Wyoming cratons in North America, between 2.7 and 2.5 billion years old, as reported in 2024 by Jikai Ding in Nature Communications.
Magnetic field reversal: The study of the East Pilbara Craton in Western Australia revealed another interesting result.
The East Pilbara Craton hosts well-preserved rock layers with little faulting or folding.
The researchers compared the orientations of Earth’s magnetic field recorded in these rocks and found there were reversals of the magnetic poles between 3.48 and 3.45 billion years ago.
Why it matters: This study from Western Australia is the earliest record of magnetic reversal in rocks; however, it is not the earliest record of the geomagnetic field.
Go deeper:
Watch the GeoGirl Youtube on when plate tectonics began on Earth.
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In 2024, Claire Nicols and colleagues reported a possible record of Earth’s magnetic field preserved in rocks in the banded iron formation in the Isua Supracrustal Belt in southwest Greenland in JGR Solid Earth.
The rocks were amphibolite-grade metamorphic rocks dated to 3.7 billion years old.
The recorded paleomagnetism was chemical remnant magnetism (CRM) acquired through the metamorphism.
These researchers also estimated the strength of the ancient geomagnetic field recorded by the Isua rocks to be more than 15 micro-Tesla, somewhat weaker than the range of 25–65 micro-Tesla today.
Why it matters:
Earth’s magnetic field is generated as liquid iron circulates in the Earth’s outer core—a phenomenon called geodynamo.
The magnetic field protects life on Earth from harmful solar wind and cosmic radiation.
It seems that Earth’s magnetosphere was well established by the beginning of the Archeon Eon.
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