There are two faces of the earth: study

Earth appears to have two chemically distinct hemispheric ‘faces’.

Earth’s mantle is currently classified into two main domains, African and Pacific. However, little is known about their formation and history, and they are commonly assumed to be chemically the same.

In a new study by Curtin University, scientists studied chemical and isotopic “make-up” of rocks sourced from thousands of kilometers below the surface. They wanted to understand how the Earth’s mantle responds to plate movements that occur near its surface.

Their study has uncovered how rocks sourced from the Earth’s mantle are linked to the formation and breakup of supercontinents and super oceans over the past 700 million years, suggesting that the Earth is made up of two distinct “faces.”

Lead author Dr. Luc-Serge Doucet, from the Earth Dynamics Research Group in Curtin’s School of Earth and Planetary Sciences, said, “Our team used trace metals such as lead, strontium, and neodymium, from hotspot volcanic islands including the Hawaiian islands in the Pacific Ocean and the La Reunion island in the Indian Ocean, to examine whether these two domains have the same chemical ‘make-up.”

“We found that the African domain was ‘enriched’ by subducted continental materials, which was linked to the assembly and breakup of the supercontinent Pangaea, whereas no such feature was found in the Pacific domain.”

Scientists found that the contents of the two mantle domains are not the same as previously thought. Instead, it appears as the Earth has two chemically distinct hemispheric “faces,” with the Pacific ring of fire being the surface expression of the boundary between the two.

Co-author John Curtin Distinguished Professor Zheng Xiang Li, head of the Earth Dynamics Research Group, said the two chemically distinct hemispheres discovered by the team could best be explained by the different evolutionary histories of the two mantle domains during the Rodinia to Pangaea supercontinent cycles.

“We found that the African mantle domain contains continental materials, which were brought down by the subduction system for at least the past 600 million years. However, the Pacific mantle domain has been protected from the infiltration of such materials.”

“Our research findings are significant as they showcase a dynamic relationship between plate tectonic processes that operate near the surface and the formation and evolution of Earth’s deep mantle structures. The work helps us to understand what drives plate tectonics and the formation and preservation of global geotectonic features such as the pacific ring of fire. The dynamic and interactive nature of the entire Earth system has important implications on the formation of Earth’s resources, the evolution of the Earth’s environment, and even the evolution of life.”

Journal Reference:
  1. Doucet et al., Distinct formation history for deep-mantle domains reflected in geochemical differences. Nature Geoscience, (2020). DOI: 10.1038/s41561-020-0599-9

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