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Oxygen-rich rock melts faster, volcanism models revised

2 November 2021
New experiments by researchers from Vrije Universiteit Amsterdam, including Professor Wim van Westrenen, and from China have shown that oxygen is essential for rock to melt. The more oxygen you add to rock, the more easily you can make magma.

The results of the research have been published in the journal PNAS. These new insights can lead to a new interpretation of the role of oxygen inside planets and in causing volcanic eruptions on Earth.

Enormous differences in oxygen levels
There is an enormous difference between the oxygen levels of the Earth’s atmosphere and its core: the former is rich in oxygen whereas the latter contains very little. The level of oxygen in the rock between the atmosphere and the core varies. According to many geoscientists, these different oxygen levels do not have much significance. Differences are primarily thought to be linked to the effect of oxygen on the oxidation or reduction of iron (which occurs in many types of rock). If rock contains a lot of oxygen, iron is oxidized (as is the case in rust); if there is little oxygen available, iron can be present in the metallic form (as is the case in a steel beam).

However, new experiments carried out by researchers from VU Amsterdam and the Centre for High Pressure Science and Technology Advanced Research (HPSTAR) in China have now shown that high oxygen levels have another effect. Rock that is rich in oxygen melts much more easily, that is, at considerably lower temperatures (as low as 100 degrees Celsius), than rock that contains little oxygen. The experiments were carried out on synthetic rock, which contains no iron atoms at all, so this effect is not caused by the oxidation of iron but by the oxygen atoms themselves. Yanhao Lin (HPSTAR and, of late, Veni-funded researcher at VU Amsterdam), Wim van Westrenen (VU Amsterdam Earth Sciences and HPSTAR) and Dave Mao (HPSTAR) carried out these experiments.

La Palma
The study clearly demonstrates that current models of volcanoes on Earth, like the one we have been seeing in action on La Palma during the last few months, are incomplete. The effects of oxygen levels on the melting properties of ‘earthly rock’ are not yet known. The follow-up research now being carried out by the team of researchers suggests that the rock that makes up the Earth melts much more easily if there is a surplus of oxygen present.

This could mean that not only the temperature of a rock determines whether magma (liquid rock) is produced, but that the oxygen level can also be a very important parameter. Oxygen can thus, literally, ‘wake rock up’.

These results also throw new light on the evolution of exoplanets (planets that orbit stars other than our own sun). The experiments show that exoplanets with a lot of oxygen will have a small metallic core (because little iron is reduced) and can be covered with a deep ocean of magma (because rock melts easily).

Because of degasification of vast quantities of magma, the atmosphere of these planets will comprise large quantities of oxygen-rich gases, such as CO2 and water vapour. On the other hand, exoplanets with little oxygen will have a large metallic core and only melt to a small extent. Limited degasification takes place on such planets and their thin atmospheres will consist mainly of small quantities of low-oxygen gases such as CH4 (methane).

Planned space missions, for instance the launching of the James Webb telescope in December 2021, will discover many new exoplanets and conduct analyses of their atmospheres in the coming years. This data will tell us more about the planets surrounded by these atmospheres.