It is very easy to be distracted by Yellowstone as the only caldera in town, but there are plenty of other caldera systems worldwide -- many of which have been much more active in geologically recent times (last 10,000 years) than Yellowstone. Two of these "restless calderas" include Santorini in the Aegean Sea off of Greece and Long Valley in eastern California. Both have had massive eruptions -- the most famous being the ~1600 B.C. Minoan eruption of Santorini (technically, the island is Santorini and the volcano is Thera) and the Bishop Tuff eruption of Long Valley ~760,000 years ago. They have also both seen eruptions since the "big one" -- Santorini as recently as 1950 and Long Valley in ~1350 (as part of the Mono-Inyo Crater chain). I've gotten a number of e-mails lately about people noticing seismicity under these calderas, so I thought I'd take a quick peak at the action at both. Santorini (Greece)
Geologically speaking, Santorini's big eruption was yesterday -- and even speaking from the human historical perspective, it was quite recent. This means it comes as little surprise that there have been a number of historical eruptions within the Thera caldera since the Minoan eruption -- at least 11 listed on the Global Volcanism Program website. Many of these have been shallow submarine eruptions that have been rebuilding the volcanic edifice after the Minoan collapse, although some of the largest (VEI 4) have been to the northeast of Santorini at the Colombo Bank, including one in 1650 that generated a tsunami. Recently, there has been an increase in seismicity around Santorini, stretching from directly under Thera to the northeast toward the Colombo Bank. VolcanoDiscovery has been posting on the seismic swarm under Santorini as it waxes/wanes and their most recent update suggests that Thera/Santorini isn't seeing much anomalous seismic activity. You can check out the seismometers on/near Santorini yourself as part of the Hellenic Unified Seismic Network -- just zoom in on the island in the middle of the Aegean arc -- and the Santorini Seismic Network also has real-time data. The Institute for the Study and Monitoring of the Santorini Volcano (ISMOSAV) also has listing of recent seismicity along with a webcam pointed into the caldera. If there was another eruption from Santorini/Thera in the near future, the mostly likely event would be another small, dome-building eruption around Nea Kameni inside the Minoan caldera. Most of these eruptions have been VEI 2 eruptions that had phreatic explosions and the extrusion of lava domes/flows. It is very unlikely that we'd seen around Minoan-scale eruption, although Santorini/Thera has seen a number of caldera-forming eruptions (at 180,000, 21,000 and 3,600 years before present). Long Valley (United States)
Unlike Santorini, it has been quite a long time since the caldera-forming eruption at the Long Valley caldera in California. The product of that eruption, the Bishop Tuff, is date at ~760,000 years old and can be found around much of western North America. Since that grand volcanic eruption, there have been smaller eruptions both inside and outside the caldera, with the most recent Long Valley eruption being ~50,000 years ago. Nearby, the most recent volcanism is a chain of domes to the northwest of the caldera, the Mono-Inyo craters. These rhyolite domes and craters run from Mono Lake to the edge of the caldera and are likely not directly related to the Long Valley caldera but rather a discrete magmatic system. (However, I lump them together due to their geographic and volcanologic similarity.) Much of this volcanism is due to Basin-and-Range extension across the Owens Valley. The Long Valley caldera itself has seen its share of restlessness -- there are active hot springs in the middle of the caldera and intermittent fumarolic activity occurs as well. There has been multiple incidents of tree and animal deaths due to carbon dioxide from the caldera rim (especially near Mammoth Mountain) and numerous seismic swarms have occurred over the past 30 years. These swarms have even tricked the experts as the USGS warned of a potential Mammoth Mountain eruption in the 1980s that is thought to have been a dike of magma being emplaced at depth. Every few months, seismicity increases slightly somewhere across the caldera -- mostly at depths between 3 and 12 km. However, these are all well within the "normal" background activity at the caldera, which still sits at alert level Green. It would take a much larger increase is seismicity than a few dozen earthquakes over a week for the alert to be raised at Long Valley. It would likely also need to be closely associated with increased fumarolic and hot spring activity, along with carbon dioxide emissions (sound familiar?) Again, much like Santorini, the most likely event at Long Valley, if it were to erupt again, would be a small dome-building eruption similar to the Mono-Inyo chain. The Long Valley Observatory has some real-time monitoring data but the only webcams in the area will get a lot more view of snow on Mammoth Mountain than anything. So, two restless calderas that remind us that active volcanic systems are always rumbling (to some degree), but keeping a close watch on them allows us to know when the noise becomes more than just the daily routine.
Image 1: Santorini in Greece. Image from ilkerender/Flickr.
Image 2: The Bishop Tuff in Long Valley. Image from Erik Klemetti/Eruptions.