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Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Christy Till, assistant professor with the School Of Earth and Space Exploration at Arizona State University.

As we learned in an earlier Caldera Chronicles post, volcanoes don't erupt at regular intervals or on a schedule, which is why they can't be "overdue" for an eruption. So what does cause an eruption at volcanoes like Yellowstone?

To answer this question, we need a black box recorder of sorts—one that can tell us the processes leading up to past eruptions and how long it took. Luckily over the last 10-15 years we have developed just such a recorder: crystals!

Crystals?! That may sound surprising to you. How can crystals record anything? And what do they have to do with volcanoes?

When volcanoes erupt, they spit out a mixture of gas, liquid magma, and crystals formed in the magma chamber. The crystals go on to be part of the ash deposited during explosive eruptions, or lava flows during less explosive ones. The crystals in the deposits from both big and small eruptions in Yellowstone's past have concentric zones, like the rings in trees. A previous Caldera Chronicles article detailed how these crystal zones can be used to determine the age of the magma. The information stored in these zones can also allow us to reconstruct the "climate" in the magma chamber as the crystal grew. Specifically, they tell us about changes in the temperature, pressure, and chemistry over time leading up to past eruptions. The youngest zones—those at the outer edge of the crystal—tell us what was happening in the magma chamber immediately before the eruption.

So what have we learned from looking at these crystal recorders?

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There are number of different processes in the magmatic system below volcanoes that can cause an eruption, such as the movement of magma from a deeper storage region to the shallowest one below the volcano, or the mixing of two different kinds of magma. The crystals in deposits from Yellowstone's last big eruption about 631,000 years ago, the Lava Creek Tuff, and also in lava flows erupted about 256,000 years ago both tell us that the movement of magma from a deeper storage region to a shallower one created sufficient pressure to cause an eruption.

Based on how seismic waves travel, we know that Yellowstone's magma system extends from the base of the Earth's crust to the surface. So this movement of magma from a deeper storage area to shallow is not all that surprising. In both cases (the explosion and the lava flow), it took several decades between the time the crystals first recorded the magma movement to the eruption.

This is not the end of the story, but rather just the beginning. More data are needed to confirm these results! For example, these types of studies will be done on many of the eruptions from Yellowstone's past to make sure the results are consistent. But just from these two eruptions, one big and one small, it seems that there is a common process that causes Yellowstone's eruptions.

So if Yellowstone were to erupt again (which it may never do, mind you), it is possible that the eruption could be caused by a similar movement of magma from deeper in the crust to the shallowest magma chamber. This is something that would be easy to detect, as such a movement of magma would cause major earthquake swarms (much different and more vigorous than what we see normally), significant ground deformation (far greater than the few inches per year of uplift or subsidence that are typical), and potentially even changes in gas or thermal emissions. These parameters are well monitored, so there will be ample warning of any potential future eruption.

This work is a good example of how geologic investigations of volcanoes can help us understand what has happened in the past and might happen in the future, and how volcano monitoring systems can be tuned to best detect the lead up to an eruption.