Scientists have been trying to figure out exactly what the future entails for the Yellowstone caldera. Unlike other more speculative theories, this most recent study tries to unlock the future by examining the past.
The scientists published their findings in the journal GSA. The Yellowstone caldera has had some large eruptions in the past, but most of these are actually anomalous compared to the vast number of smaller eruptions that have occurred over the last few million years.
The caldera system has had plenty of smaller eruptions, so these are the most likely to occur in the near future. Guillaume Girard and John Stix suggest this and state that small, dome-forming eruptions that are phreato-magmatic (water influenced) will follow pre-existing faults in the caldera, especially along the western ring.
The latest very large eruption happened 640,000 years ago, and since then there have been many smaller explosions that occurred between 174,000 and 70,000 years ago. At least 600 km3 of rhyolite lavas have erupted in the Central Plateau. The reservoir under Yellowstone, based upon the most recent measures, is 10 to 16 km below the surface and is only 5% to 15% molten. Some of the other extensions of the reservoir reach as shallow as 6 km but are less than 30% molten.
The lowest percentage of molten magma for an eruption to occur from a crystal mush is estimated to be between 40% and 45%, implying that none of the magma is ready for an eruption.
There are three areas of the caldera that could yield potential eruptions. These could present potential hazards, however, the scenarios are unlikely to occur.
Reference: “Future volcanism at Yellowstone caldera: Insights from geochemistry of young volcanic units and monitoring of volcanic unrest” by Guillaume Girard and John Stix, September 2012, GSA Today.