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A significant discovery has emerged from recent research, offering new insights into the formative stages of Earth’s geological history and prompting a reevaluation of prevailing theories about continental formation and the initiation of plate tectonics.
A study published in Nature on April 2 indicates that Earth’s primordial crust, believed to have formed around 4.5 billion years ago, exhibited chemical characteristics strikingly similar to those of contemporary continental crust.
This finding implies that the unique chemical signature associated with our continents may have originated during the very early phases of Earth’s existence.
Professor Emeritus Simon Turner from the Faculty of Science and Engineering at Macquarie University spearheaded this research, collaborating with colleagues from various institutions in Australia, the UK, and France.
“This finding significantly alters our perspective on Earth’s earliest history,” states Professor Turner.
“Traditionally, scientists have maintained that for tectonic plates to generate the chemical profile evident in continental formations, they would need to subduct beneath one another.”
However, Professor Turner asserts, “Our research indicates that this chemical fingerprint originated in the protocrust, which suggests a need to reassess existing theories.”
Rethinking early Earth formation
For years, scientists have sought to determine when plate tectonics began, a milestone linked to the advent of early life. Distinct rocks formed in subduction zones display a particular chemical fingerprint, notably lower levels of niobium.
Researchers focused their efforts on identifying the age of the earliest low-niobium rocks, believing it would be indicative of the onset of plate tectonics. However, the findings from various groups revealed substantial inconsistencies.
“I began questioning whether we were approaching the right issue,” reflects Professor Turner.
Working alongside collaborators from six universities, he developed mathematical models that simulated conditions in early Earth, characterized by the formation of its core and an ocean of molten rock enveloping the planet.
The results indicated that the protocrust—formed during the Hadean eon (approximately 4.5 to 4 billion years ago)—could naturally acquire the same chemical characteristics observed in modern continents, independent of tectonic activity.
Chemical clues to formation
Initial modeling outcomes suggested that under the reducing environments of early Earth, niobium would behave as a siderophile, gravitating toward metal and ultimately sinking into the Earth’s core from the global magma ocean.
“I began to see a relationship between the initial core formation, patterns of high siderophile elements, and the notorious negative niobium anomaly found in continental crust,” explains Professor Turner.
This unique continental crust signature corresponded to material likely extracted from the mantle following core formation but preceding the bombardment of early Earth by meteorites—shedding light on the ubiquitous presence of this chemical signature across nearly all continental rocks, regardless of their age.
Early Earth’s evolution
“Our findings indicate that the chemical signatures observed in continental crust were formed during Earth’s earliest days—independent of surface activity,” states Professor Turner.
This primordial crust was modified and enriched in silica due to meteor strikes, fragments of crust detaching, and the nascent movements of tectonic plates.
It is thought that the initial crust fragmented into sections, which became thicker in certain regions, laying the groundwork for the formation of continents.
As these fragments shifted horizontally, the molten magma filling the gaps generated crust akin to what is currently observed on ocean floors.
Meteor impacts and plate tectonics
The intense meteor bombardment characteristic of this early period led to significant disruption and recycling of the crust.
Plate tectonics may have operated intermittently during this time, potentially initiated by meteor impacts until about 3.8 billion years ago, when the frequency of meteor collisions diminished, allowing for a more stable orbital environment in the early Solar System.
After this transition, plate tectonics likely established a continuous and self-sustaining process.
“This discovery fundamentally alters our comprehension of Earth’s initial geological mechanisms,” concludes Professor Turner.
Moreover, it provides a fresh perspective on how continents might emerge on other rocky planets throughout the universe.
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