Working as a seasonal ranger at Yellowstone National Park I get asked a lot of questions by a lot of visitors. Or more precisely, I get asked a lot of the same questions over and over again.
|Old Faithful does its thing approximately every 90 minutes|
Where is the restroom?
What time does Old Faithful geyser erupt?
Where can we see bears?
Where can we hike where there aren’t any other people?
What is there to see here?
When is this thing gonna blow, anyway?
At 2.2 million acres Yellowstone encompasses a considerable quantity of northwestern Wyoming real estate (along with small slices of Idaho to the west and Montana to the north). In fact, until Death Valley was made a national park in 1994, Yellowstone was the largest park in the lower 48 states. With over 10,000 thermal features and surface dimensions of 30 miles by 45 miles, the collapsed crater of Yellowstone’s caldera is one of the world’s largest active volcanoes.
|Clepsydra Geyser is in the Fountain Paint Pots area|
|Fountain Paint Pots are acidic cauldrons of boiling, burping mud|
|Surging Spring surges occasionally in West Thumb Geyser Basin|
And that doesn’t even begin to include the massive magma chamber that lurks a mere few miles beneath our feet. That shifting mass of partially molten rock has been imaged and found to be at least 400 miles deep and 150 miles wide.
When this magma rises but does not erupt onto the surface, the floor of the caldera can bulge upward into small hills called resurgent domes. Mallard Lake Dome east of Old Faithful and Sour Creek Dome (identified by astute reader Larry in this previous post) north of Fishing Bridge are both resurgent domes inside the caldera.
Once viscous underground flows of rhyolite, these flows were pushed upward by magma from below but never erupted.
|Mallard Lake resurgent dome is the hill beyond the white of Biscuit Basin thermal area|
Seismic imaging shows that there is a conduit of partially molten rock located beneath both Mallard Lake and Sour Creek Domes, each at a depth of 5–7 miles.
Resurgent doming formed the Sour Creek Dome around 630,000 years ago and the Mallard Lake Dome around 130,000 years ago.
|Part of Sour Creek Dome (the hill on the left with the toupee of trees) is easily viewed from the Mud Volcano trail|
Between 1924 and 1984 the caldera floor was uplifted about 3.3 feet. The bulge of Sour Creek Dome caused the northern shore of Yellowstone Lake to subsequently rise while the southern shore has been submerged. The bulge then began to subside and dropped a total of eight inches after a swarm of small earthquakes occurred between 1985 and 1995. Between 1995 and 2000 the caldera bulged once again, and over the next few years continued to uplift. In July 2004 the rate of uplift was an extraordinary 2.8 inches per year. At Sour Creek Dome a GPS station recorded over 6.7 inches of uplift between 2004 and 2007. It was during this time that the entire country held its breath and wondered if the next Yellowstone supervolcano explosion was about to occur.
The West Thumb of Yellowstone Lake is a caldera within a caldera, having exploded around 175,000 or so years ago. It is very likely that it was once a resurgent dome.
|West Thumb of Yellowstone Lake is a caldera within a caldera|
So...Is the Yellowstone caldera going to blow again, and if so, when?
The presence of hot, molten magma just beneath the surface indicates that future volcanic eruptions are likely to happen, but no one really knows when this might occur. The amount of heat still flowing underground, frequent earthquakes, over 10,000 thermal features including hot springs, mud pots, geysers, and steam vents, and the rising and subsiding of the caldera floor all point to the likelihood of continued volcanic activity.
Cataclysmic events such as supervolcano eruptions are infrequent, though, compared to smaller, less explosive events. Hydrothermal explosions of steam and rock are the most likely to occur, followed by lava flows of either rhyolite or gentler flowing basalt from faults and fractures in the caldera floor.
Visit Yellowstone before it visits you!
Image courtesy http://www.pbs.org/wgbh/nova/volcanocity/dead-08.html
My very helpful resources:
Fritz, W.J. and Thomas, R.C., 2011, Roadside Geology of Yellowstone Country, 2nd edition.
Smith, R.B. and Siegel, L.J., 2000, Windows Into the Earth – the Geologic Story of Yellowstone and Grand Teton National Parks