Not long after Yellowstone Park was officially created, a small group of campers were killed by Nez Perce Indians on the run from US troops1. More recently, the last time I was in the area, a ranger was killed by a Grizzly Bear (so was his horse) on the edge of the park. A quick glance at my sister’s newspaper archives (Lightning Fingers Liz a.k.a. Caldera Girl has been running newspapers in the region for nearly forty years) shows a distinctive pattern of danger in the Caldera, mainly in relation to the lack of turning lanes on highways with poor visibility and other traffic related hazards.
So, no, the Yellowstone Caldera is not especially safe, what with cars, humans, and griz everywhere. Oh, and every now and then somebody falls into a geyser. But you are probably here because you are interested in a different question: Is the Yellowstone Caldera, the volcanic feature, not the natural and cultural landscape, dangerous? In other words, is one of the largest volcanoes to exist on the earth ever gonna blow? Like this?
(Photograph from UFO Digest)
Over the last decade the caldera has shown interesting and sustained activity, with a large number of small earthquakes and a steady rising of the crust. In all, the surface of many parts of the caldera have risen as much as a foot vertically, which is considered to be a lot.
Over the last couple of years, however, the rise of the surface of the semi-dormant super volcano has slowed considerably. It is now thought that an episode of increased activity of magma 7 plus kilometers beneath the earth occurred and is mainly over.
Geologists infer the three dimensional structure of the stuff under our feet (rock, magma, water, etc.) in part by observing energy as it moves through the earth’s crust. For example, a geologist might set off a series of explosive charges and measure the timing (and other details) of the shock waves coming from those charges to sensors. This sonar-like approach allows the mapping of underground three dimensional structure. Deep earthquakes resulting from either faults or from the movement of magma are natural experiments that geologists can use to map subterranean structure, so when the numerous extra earthquakes started to occur in the Yellowstone area several years ago, geologists were able to develop a much more detailed three dimensional concept of what the rock and magma looks like underneath the griz and human infested surface of this part of Montana, Idaho and Wyoming.
A recent paper by Chang, Smith, Farrel and Puskas summarizes the last several years of activity:
… measurements of Yellowstone ground deformation from 2006 to June 2010 reveal deceleration of the recent uplift of the Yellowstone caldera following an unprecedented period of uplift that began in 2004. In 2006-2008 uplift rates decreased from 7 to 5 cm/yr and 4 to 2 cm/yr in the northern and southwest caldera, respectively, and in 2009 rates further reduced to 2 cm/yr and 0.5 cm/yr in the same areas. Elastic-dislocation modeling of the deformation data robustly indicates an expanding sill at ∼7-10 km depth near the top of a seismically imaged, crystallizing magma reservoir, with a 60% decrease in the volumetric expansion rate between 2006 and 2009. Reduction of hydrothermal-volcanic recharge from beneath the northeast caldera and seismic moment release of the 2008 and 2010 large earthquake swarms are plausible mechanisms for decelerating the caldera uplift and may have influenced the change in recent caldera motion from uplift to subsidence.2
Don’t ever ask a geologist to write up the end of year holiday letter. What does this mean? Two things. First, look at this graph from the University of Utah:
This plot shows the up-down movement of the LKWY GPS station at the north end of Yellowstone Lake. After moving downward about 50 mm (2 in.) between 1997 and 2004, this area moved 90 mm (3.5 in.) upward from mid-2004 to mid-2006. The most recent data shows that as of October 2007 the total uplift at LKWY had reached 140 mm (5.5 in.). Source
Notice that the crust moves up and down all the time, but during the mid 2000’s mostly went up. Now, look at this graph also from the University of Utah:
The orange shapes in this image represent the magma chamber — a chamber of molten and partly molten rock — beneath the giant volcanic crater known as the Yellowstone caldera, Photo Credit: Wu-Lung Chang. Source
The blob on the top of the box is the outline of the Yellowstone Caldera. The giant orange three dimensional blob down inside the box is the inferred magma chamber beneath the caldera. Now, here we must have a small digression to explain something that people often get wrong. What you see here is a fairly typical arrangement causing volcanic activity (but on a relatively large scale). It is not the case that volcanoes typically come from the earth’s magma through the crust and into the surface. Rather, within the crust there are numerous magma chambers some of which come, in turn, from sub-crust magma, others formed within the crust as ‘potent geological forces’ melt the crust in place. The earth’s crust is tens of kilometers thick, but this magma chamber is less than 10 kilometers in depth.
Anyway, look at the red “wedge” thingie sticking in the orange blob. This is approximately the location of fresh magma that has intruded from some other source into the chamber, expanding it, since mid-year 2004. That is the event that happened in the first decade of the 21st century which got everybody all excited about the Yellowstone Super volcano.
In the future, this magma chamber could cause some real serious volcanic eruptions, but here’s the thing: Volcanic sources usually have a lifespan, and quite often, it is a lifespan linked to large scale movements of the crust over the much deeper mantle. This is much more obvious when looking at certain oceanic island chains, forming a string of volcanic islands on oceanic crust (which is much thinner than the continental crust). A “hot spot” in the magma will sit relatively in one location while the crust moves over it. Magma chambers will form over the hot spot, and spew out one or two volcanoes, then that part of the crust crust will move move off the hot spot and over time cool, with the volcano eventually becoming dormant. The Hawaiian islands are an excellent example of this phenomenon:
The smallest islands off to one side of the arc of volcanoes are dormant and, indeed, eroding back into the ocean. The largest islands are more active.
Something like this can also happen on land. Look at this:
You can see an arc-shaped sequence of highly volcanic areas that may have picked up their own local magma chambers from a hot spot in the mantle. Over time, any area of the crust affected by a mantle plume should move away from that source of heat and even if it remains a hot spot for some time it should eventually cool down. It is quite possible that the Yellowstone Caldera is a dormant caldera, in the sense that it will not explode violently in the future as it did three times in the past. This does not rule out smaller volcanic eruptions, however. And, at the scales we are speaking of here, a “smaller” volcanic eruption could be the size of Mt. Saint Helen’s 1980 blast.
On the other hand, the magma chamber below Yellowstone is large, and recently was re-estimated to be even larger than previously thought by about 20 percent. The information that allowed the new size estimate is a result of the placement of additional senors as part of volcanic research and monitoring …
… as well as the swarm of data that comes with a swarm of earthquakes, as mentioned above.
This recent research has resulted in the following three dimensional model of the magma underneath Yellowstone:
Compare this to the three dimensional model above. They are totally different scales, the first looking only about ten kilometers deep, the second looking tens of kilometers deep. This huge amount of magma along with the fact that the mass of magma is growing a little now and then (as evidenced by the intrusion of new magma during the last decade) suggests that maybe the Yellowstone Calder has not really moved off of any hot spot and still has a good chance of “blowing.” (An interesting discussion of whether or not the caldera is underlain by the kind of “plume” that might feed a major blow up can be found here.)
Also, look at this map showing the relationship between the Yellowstone Caldera (B) and the location of the so-called Yellowstone Volcano Observatory (A) where the geologists watching the caldera live and work:
That’s pretty far away!3
If you ask geologists today whether or not they think there will be an “eruption” in the Yellowstone Caldera, they say that yes, there will be, but the chance of it being a major super-volcanic eruption is very low. But, they say that this chance is small in relation to a given human lifetime, not that it is small in relation to a volcanic system’s lifetime. In other words, geologists seriously entertain the idea that the caldera will “blow” again at some time in the future, in a super-mega-eruption of some kind. The most likely kind of eruption to happen in this area is a steam-based eruption that is caused by water super-heated by the same volcanic heat that causes the hot springs and geysers of the region. Such eruptions can form craters up to a kilometer or so wide and have been fairly common in the area. Less likely are the more typical volcanoes like Mt. St Helens (or much smaller or even a bit bigger) spewing out some combination of lava, gas, rock, and stuff. But the full-blown blowout of a caldera style eruption, very rare on Earth these days and quite catastrophic, is a possibility. The best summary of these kinds of activities is probably this one.
So, to return to the question at hand: “Is the yellowstone caldera safe?” The answer is complex:
1) Wear a seat belt when driving around in the region;
2) Don’t feed the bears and make sure you understand bear safety; and
3) Somebody is going to get blasted by some kind of volcano in the area some day, but even if you live there the chances are it won’t be you.
1Chittenden, Hiram Martin. 1985. Yellowstone National Park,: Historical & descriptive
2Chang, W., Smith, R., Farrell, J., & Puskas, C. (2010). An extraordinary episode of Yellowstone caldera uplift, 2004-2010, from GPS and InSAR observations Geophysical Research Letters, 37 (23) DOI: 10.1029/2010GL045451
3I’m joking, of course. The geologists are not really scared. That’s just where their home institution is. Right?