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Four billion years ago, the Earth’s mantle was hotter than that of today, a new study finds. The temperatures were so high that the crust simply melted back into the mantle.

Researchers believed that the dense primary crust went to the mantle in drip form in a vertical pattern. These findings are the opposite of the mechanism through which the tectonic plates move today.

The team of researchers, led by Professor Richard White of the Institute of Geosciences at Johannes Gutenberg University Mainz (JGU) developed an ancient Earth crust and mantle model to show the conditions of the planet during the Archean era.

The simulations proved that the Earth’s magnesium-rich crust temperature remained between 1,500 to 1,550 degrees Celsius.

“The findings add to our understanding of how cratons and plate tectonics, and thus also Earth’s current continents, came into being,” said Tim Johnson, co-author of the study in a press release.

Researchers believed that since the Earth’s crust was hotter back then, the ancient crust might have been composed mainly of magnesium. However, the modern crust is mostly composed of several types of just rocks.

They also found that magnesium that once occupied the crust might have been absorbed by the Earth’s mantle gradually leaving a small amount behind. The evidence of this can still be found in some areas like Northwest Scotland and Greenland.

The study is published in the online journal Nature Geoscience.