A paper in the latest issue of GSA Today indicates that when it comes to warning signs of an eruption at Yellowstone, we may be looking for the wrong things.
The generally rounded morphology of the quartz crystals and their abundant glass embayments suggest decompression, potentially preceded, or accompanied, by rapid heating. The lack of high-Ti overgrowths also suggests rapid ascent with no storage at shallow levels.That may sound more than a bit obscure, but what it means is that the magma involved in previous eruptions at Yellowstone didn't spend a prolonged period at relatively shallow depths of a kilometer or less. Instead, the mineral composition and crystal growth suggests that the magma shot up from a greater depth of around 9 km.
Why is that important? Because we've always assumed we'd see movement of magma to more shallow pools in advance of eruption, and that this would provide advance warning. That 9km depth? That's where the magma is now.
Before you launch a "she's gonna blow!" Tweet storm, please note this passage from the paper.
A large crustal magma reservoir is interpreted to be present beneath Yellowstone caldera at between 16 and 6 km depth, but there is no indication that this reservoir, or parts of it, are currently in an eruptible state. However, rapid reheating and magma extraction events appear to have occurred episodically during times of Central Plateau magmatism.The Yellowstone super-volcano is the commonly used term for the very large (55km x 35km) caldera located under the geothermal features at Yellowstone National Park. Over the last 18 million years, this site has produced numerous large eruptions, including a dozen "super eruptions" with the three most recent being 2.1 million years before present, 1.4 mybp and 640,000 ybp. The last large eruption at the site occurred 70,000 years ago. The large eruptions can be accompanied by steam explosions and lava flows. The super eruptions by ash and explosive energy on the order of 2,000 Mt. St. Helens. A swarm of small earthquakes in 2009 and 2010 led some scientists to worry that an eruption was on its way, but this earthquake swarm seems to have passed.
Previous efforts to predict activity at Yellowstone have assumed that we would see signs of magma moving to shallow pools in advance of eruption,but the new work suggests that is not the case. Geologists will need to revise the signs they use to predict an eruption or large quake at Yellowstone, and these predictions may turn out to be much more difficult than previously assumed.
Nothing in the paper suggests that the likelihood of eruption is any greater than previously assumed.