The researchers investigated how warm you have to make rocks in the Earth - between 450 and 700 km deep - to melt them. They pressurize small pieces of stone and heat them at the same time. Afterwards they could see whether magma had formed at a certain temperature or not.
They concluded that the melting temperature of the stones depends on the amount of oxygen in the rock: the more oxygen, the easier the stone melts. This effect is significant: if you want to melt a rock with a lot of oxygen at a depth of 600 kilometers underground, you have to heat it to 1800 degrees. But rock with little oxygen must be heated to above 2100 degrees. Van Westrenen: “That is a huge difference. For earth scientists it is a bit like saying that ice melts at -15 degrees instead of at 0 degrees.”
Previous results explained
Experiments into the melting of rocks in the Earth have been conducted for decades, and have led to very different outcomes. But the previous studies did not take the amount of oxygen in the rock into account. This study, which was published in Nature Geosciences, has solved the mystery. Van Westrenen calls that “fun, but not that spectacular in itself.” More important are the implications for understanding Earth's evolution.
Van Westrenen: “A question that we can solve, for example, is that the oldest volcanic rocks on Earth contain quite a lot of oxygen, while models for the evolution of the Earth all state that the interior of the early Earth was low in oxygen. At that time, volcanoes could probably only form in regions where there was slightly more oxygen in the earth. We do not find any oxygen-poor volcanic rock from that time because oxygen-poor rock is much more difficult to melt.”
Poor relation
The research results further suggest that volcanoes form more easily on planets with more oxygen than in terrestrial rocks, such as Mars. Van Westrenen: “The oxygen content in rocks has until now been considered relatively unimportant. I hope that our work will ensure that oxygen is no longer a poor relation in research into the formation of rocks.”