Earth's Unstable Crust 4 billion Years Ago Dripped Down into Mantle

The Earth's mantle was much hotter 4 billion years ago during the Archean eon than it is today, according to a new study that sheds light on how plate tectonics - and thus current continents - came about.

The new study was led by Tim Johnson who is currently studying the evolution of the Earth's crust as part of a research team led by Richard White of the Institute of Geosciences at Johannes Gutenberg University Mainz. Through a series of model calculations, Johnson determined that the Archean crust was so dense that much of it was recycled back into the mantle.

Unlike today's tectonic plates, which largely move laterally, this dense primary crust would have descended vertically drip by drip, and would have been extremely thick and rich in magnesium due to the high temperatures. Little of this original crust was preserved, meaning that most of it was recycled into the Earth's mantle.

In those places where it did survive, including Northwest Scotland and Greenland, the Archean crust is largely comprised of TTG (tonalite-trondhjemite-granodiorite) complexes that would have come from a hydrated, low-magnesium basalt source. These pieces of crust, Johnson concluded, are among the oldest parts of the Earth's crust.

Through a series of calculations, Johnson and his colleagues determined that the minerals that formed at the bottom of a 45-kilometer thick crust packed with magnesium was denser than the mantle layer beneath it. In order to better understand the physics at play during this process, Boris Kaus of Mainz University's Geophysics work group put together new models to simulate the conditions present when the Earth was still young.

The results indicated that the base of a magmatically thick, magnesium-rich crust would have been gravitationally unstable at mantle temperatures more than 1,500-1,550 degrees Celsius, causing it to sink. As the dense crust dripped down into the mantle, it would have triggered a return flow of mantle material that then melted to form a new primary crust. 

"Continued melting of over-thickened and dripping magnesium-rich crust, combined with fractionation of primary magmas, may have produced the hydrated magnesium-poor basalts necessary to provide a source of the [TTG] complexes," Mainz University wrote in a statement.

The residue of all of this likely still resides in the mantle today, the researchers hypothesize.

The study was published in Nature Geoscience.