The Earth’s continental crust, known as the oldest surface layer forming continents, is estimated to be around 4 billion years old. It primarily consists of basalts, volcanic rocks that are 25-50 kilometers thick. Initially, scientists believed that a single lithospheric crust covered the entire planet, with the formation of individual tectonic plates occurring much later, about 1 billion years after this initial crust formation. However, this long-standing hypothesis is currently facing challenges and scrutiny.
The process behind the formation of this continental crust remains somewhat mysterious, and researchers are now proposing that it might have been driven by the phenomenon of plate tectonics. Plate tectonics involves the movement of Earth’s major surface plates across the globe over billions of years, ultimately shaping the landmasses and topographic features we observe today. One theory focuses on the convergence of these plates, often leading to one plate subducting beneath another. This subduction process can result in partial melting, altering the composition of magma. Another line of research investigates mechanisms within the crust itself, occurring at depths of less than 50 kilometers, which are independent of plate boundaries but also contribute to partial melting.
Recent findings published in Nature Geoscience shed light on this topic. The research involved experimental work on analogs for oceanic plateaus, which are large, flat elevations with steep edges. These plateaus resemble the early basaltic crust that formed during the Eoarchean period, around 3.6-4 billion years ago. Dr. Alan Hastie and his colleagues, based at the University of Edinburgh, conducted high-pressure and high-temperature melting experiments on primitive oceanic plateau basalts from the Ontong Java Plateau in the southwestern Pacific.
The results of these experiments indicated that continental crust could not have formed at pressures less than 1.4 GigaPascals (GPa) at depths of up to 50 kilometers. This suggests that such magmas formed during convergent subduction zones. Consequently, this research suggests that even if in a primitive form, plate tectonics existed as far back as 4 billion years ago. This insight is crucial because plate tectonics play a pivotal role in various geological processes on Earth, including erosion, deposition, mountain formation, and volcanic activity. Moreover, it is suggested that gases released from volcanism, particularly carbon monoxide and methane, may have played a role in initiating life on Earth by providing prebiotic molecules that led to the emergence of the first microbial organisms.
This understanding of the early Earth’s continental crust has broader implications. Similar silica-rich continental crust has been identified in smaller quantities on Mars and Venus, offering insights into the role of plate tectonics in the broader solar system.
Dr. Hastie and his team also examined the stability of various minerals at different pressures, ranging from 1.2 to 1.4 GPa, equivalent to depths of approximately 40 to 50 kilometers. The temperatures in the mantle were estimated to reach 1,500 to 1,650°C during these experiments. Key minerals studied included garnet, plagioclase feldspar, rutile, and amphibole. The experiments revealed that garnet and rutile were not stable at pressures below 1.4 GPa, which was higher than previously thought. The team attributed this to the higher magnesium content in their starting oceanic crust, aligning more with the expected composition of the early Eoarchean mafic crust.
In a reverse experiment, the researchers grew garnet crystals at a higher pressure of 2 GPa and then subjected them to the lower pressure of 1.4 GPa, leading to the breakdown of the garnet crystals. This indicated that garnet remained stable at pressures of around 1.6 GPa or greater, corresponding to depths exceeding 50-55 kilometers. This finding challenges the previous belief that garnet was stable only up to 1 GPa and supports the idea that subduction played a significant role in the formation of continental crust.
Modeling efforts also suggest that early magmas underwent fractional crystallization as they ascended through the crust. This process involved crystals separating from the liquid magma, resulting in changes in magma composition as new crystals formed. The research team identified amphibole crystallization as a major driver of partial melting. Amphibole is a hydrous mineral that may have been incorporated into the crust through processes like overturning and burial. This finding aligns with the characteristics of known Eoarchean volcanic rocks, such as tonalites and trondhjemites.
The Isua Greenstone Belt in Greenland and the Archaean Slave Craton in Canada are believed to be remnants of convergent plate margins above ancient subduction zones. In these areas, metabasic magmas, which are derived from metamorphosed basaltic and related rocks, would have mixed with fluids from the melting subducting crust. This process produced new silica-rich magmas, marking the beginnings of a cycle of continental destruction and rebirth that has shaped the Earth’s current geological landscape.