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Crystals laced with lithium suggest that most of a volcano’s magma isn’t liquefied

As we’ve all been taught, when a volcano erupts it spews ash, noxious fumes, and molten lava— lava of course being a term to describe the melted rocks that were forced to the Earth’s surface. However, zircon crystals, which also get pushed to the surface in a good eruptions, are rewriting this a bit. Formed deep underground, variations in these crystals indicate that there’s more of a distinction between lava and magma than its location. Most of the world’s magma isn’t really a liquid at all, a fact that may help us make sense of how, and more importantly, when, volcanoes erupt in the first place.

Digging miles into the Earth to sample magma “in the wild” isn’t terribly practical, which is part of why understanding the exact state of magma under the Earth’s crust has eluded us for so long. Fortunately, zircon crystals essentially record the condition of the magma they sit in allowing us to get an idea of how hot things were before they were brought to the surface. When a crystal is heated over 1,382° Fahrenheit, it softens and absorbs lithium from it’s local environment. Scientists can then look at how much lithium has been absorbed into a crystal to determine how hot, or cool, the surrounding magma was. Based on samples from New Zealand, most magma isn’t as squishy as we’re often led to believe.

Few signs of flowing stone

This batch of crystals was brought to the Earth’s surface in an eruption 700 years ago. Before that, they’d been buried underground for over 50,000 years, giving them plenty of opportunities to be exposed to hotter or cooler magma in the region. As it turns out, very as few as 40 of those years were spent in truly liquefied rock—  most of the crystals’ time seems to have been in magma as soft as a sort of rocky slurry at best. If anything, it seems that most of the time in truly molten magma was just before the actual eruption that carried the crystals to the surface.

Researchers know that there were more eruptions before that time though, which tells them more about the composition of the buried magma. If some parts of the Taupo Volcanic Zone were hot and pressurized enough to erupt without cooking these particular crystals, it seems that the larger supply of magma is fairly compartmentalized. Hot, liquid rock can apparently form before an eruption, but doesn’t mean that the entire region is floating over a sea of molten magma. Alternatively, eruptions may occur when molten material arrives from even deeper underground, melting a path to the surface. In either case, it seems that the lava we see is very likely very freshly made, at least on geologic timescale.

Source: Magma stored under volcanoes is mostly solid by Maria Temming, Science News

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