The Earth's mantle was once much hotter than it is today.
Researcher's suggested the crust that formed during the Archean eon four billion years ago was incredibly dense; this composition caused pieces to be "recycled" into the mantle, a Johannes Gutenberg University Mainz (JGU) news release reported.
The team suggested the crust would have "dripped" in a vertical formation; today tectonic plates have a lateral movement. The finding helped researchers gain insight into how the Earth's continents and current tectonics grew up.
The ancient crust can be found in select areas such as Northwest Scotland and Greenland, it is composed of "tonalite-trondhjemite-granodiorite complexes and these are likely to have originated from a hydrated, low-magnesium basalt source," the news release reported.
The composition of the ancient crust suggests it could not have been a direct product of the magnesium-rich source.
The team used thermodynamic calculations and a computer model that simulated Earth's early days to come to their conclusion that "mineral assemblages that formed at the base of a 45-kilometer-thick magnesium-rich crust were denser than the underlying mantle layer," the news release reported.
The computer model showed that a super-thick and magnesium-rich crust would have become unstable in heat above 1,500 to 1,550 degrees Celsius. This would have caused the crust to sink in a phenomenon dubbed 'delamination'.
"The dense crust would have dripped down into the mantle, triggering a return flow of mantle material from the asthenosphere that would have melted to form new primary crust," the news release reported.
As the crust continued melting and got mixed in with primary magmas it most likely formed the hydrated magnesium-poor basalts required in the formation of onalite-trondhjemite-granodiorite complexes.
The process would have left high levels of mafic minerals in the Earth's mantle.
