Supervulkane: sponge instead of ember soup

Magmareseervoir under volcano giants resembles a spongy crystal porridge

Supervolcanoes can cover entire landscapes with lava and ash, but what it looks like inside is still largely unclear. © Julian Grondin / thinkstock
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Surprisingly different: The magma chamber under super volcanoes contradicts common theories. Because it is filled neither with molten melt nor with completely cooled magma. Instead, their content is more like a sponge, as rock analyzes now suggest. Part of the magma crystallizes and forms a network of pores in which liquid magma is stored. This hybrid nature could explain the long life and rare outbreaks of such volcanoes.

Supervolcanos are hidden giants: they usually are hardly like the normal mountains of fire, but hide their fiery nature under an often rather inconspicuous landscape. Often, only geysers, gas leaks and subterranean heat zones bear witness to their activity, such as at Yellowstone or Campi Flegrei in Italy.

Solid or liquid?

However, what happens inside the Super Volcano and why they break out after a long break has only been partially clarified so far. So seismic studies indicate that these volcanoes have at least two magma chambers: one in the upper crust and an even larger reservoir in deeper crustal areas.

However, the shape and consistency of the upper magma chamber is still contentious. Some geologists believe that this reservoir is filled with glowing magma. The volcano would thus be ready to escape at any time. Others, however, assume a completely cooled and solidified upper chamber. Only by a massive heat flow from the earth's mantle, the magma is liquefied again and an eruption is possible.

How exactly is the Magmenkammer under the Yellowstone-Super Volcano beskahffen, you do not know. USGS

Zircon crystals as contemporary witnesses

But which theory is right? To solve this, a team led by Olivier Bachmann from the ETH Z rich examined samples of the magma rock of the primeval super-volcano "Kneeling Nun" in New Mexico. In a violent eruption about 34 million years ago, the volcano spewed enormous amounts of ash and lava, which solidified into sometimes bizarre tufa formations. display

This rock contains zirconium and titanite crystals which, in their structure and composition, store information on both their formation temperature and the time of their crystallization. By analyzing and comparing these crystals, the researchers were able to reconstruct the state of the magma chamber under this super volcano over half a million years.

As well ... as

The surprising result: neither of the two common theories is true. The magma chamber under the "Kneeling Nun" is neither a boiling ember soup that can overflow at any time, nor a completely cooled rock block. Instead, the magma chamber of this super volcano is a mixture of crystalline magma and liquid rock melt - a sort of crystal pulp, as the researchers report.

According to the analyzes, more than 40 to 50 percent of the reservoir is in crystalline form. These solid parts of magma probably form a sponge-like, porous structure, in whose pores the liquid rock melt sits. "The magma is mainly preserved as a crystalline sponge structure. And it must be reactivated by heat before it can break out, "says Bachmann.

These zirconium crystals provided the geologists with crucial information on the state of the magma chamber under the "Kneeling Nun" volcano. Dawid Szymanowski / ETH Zurich

Slow "charging" until the eruption

Thus, the truth is probably somewhere between the two established theories: The already magmatic fraction explains how superculanes can become active again without enormous amounts of heat having to be pumped from below into the crust SEN. The crystalline fraction, on the other hand, ensures that it can take hundreds of thousands of years for such a volcano to be "charged up" again and to erupt again.

The impetus for another eruption is probably the second magma reservoir deeper in the earth's crust. From it, hot rock melt flows upwards, thus helping to melt the half-starved upper magma chamber. A similar mixture of solid and liquid parts can also be found under the super volcano Toba in Indonesia: For him, the Magmareservoir of pancake-like stacked layers, as geologists found out a few years ago.

These new insights provide more insight into the inner processes of a super-volcano - and they explain why these fire giants so rarely erupt. "The eruption of a super-volcano is - fortunately for us - in any case, a very rare event, " says Bachmann. When such a disaster is imminent, however, is still difficult to predict. (Nature Geoscience, 2017; doi: 10.1038 / ngeo3020)

(Swiss Federal Institute of Technology Zurich (ETH Zurich), 06.10.2017 - NPO)