Around four billion years ago, when the Earth's mantle was hotter than it is now, the crust was unstable and disintegrated, according to researchers from the Johannes Gutenberg University Mainz in Germany.
The research published this month in Nature Geoscience used model calculations to determine that the high density of the primary crust caused it to subside back into the mantle.
The high temperatures of the mantle during the Archean eon ensured a thick crust rich in magnesium. Researchers suggest little of this crust remains, with most recycled into the Earth's core.
Although Archean crusts have survived in areas like Northwest Scotland and Greenland, they have low magnesium, and do not originate from the primary crust.
“Archean crust exposed today is composed mostly of tonalite-trondhjemite-granodiorite (TTG), indicative of a hydrated, low-magnesium basalt source, suggesting that they were not directly generated from a magnesium-rich primary crust,” the authors wrote.
Computer models suggest the base of the thickened and magnesium-laden crust would have been unstable at mantle temperatures greater than 1,500 to 1,550 °C and this would have caused it to drip down in a process called delamination.
[Johannes Gutenberg University Mainz] [Nature Geoscience]