For months or years, probably a decade or more, the signs accumulate. Sensitive equipment detects fifty or 100 earthquakes one month, 500 the next, then less than 100, and so forth. People sitting still might notice a tremor every few months. One rumbles through that knocks knickknacks off of shelves, and maybe someone falls off a chair. "Are they getting worse?" people ask the next day. A GPS device shows that the ground has risen another foot in just a week, but it has done that before, and often subsided.
Finally, one day a series of shocks that can knock a fellow down is followed by an eruption of smoke and a little ash. A new crack becomes a fumarole, then as the ground rises more rapidly, ash and hot rocks pop out.
This is new. This could be the beginning of a major eruption. This isn't Mount Etna or Vesuvius, which are dangerous enough. This is Yellowstone, and the modern park almost encompasses a set of three overlapping calderas (volcanic craters) big enough to hold Vesuvius and Etna both...not their craters, but the entire mountains.
It may be that these new events herald an explosion similar in size to Krakatau. That simply removed an island the size of New York City. Or it could be much bigger. Two million years ago, the oldest Yellowstone caldera was formed in the fourth largest eruption geologists have studied. It has erupted twice since, one just 640,000 years ago being about half the magnitude of the first, and one between the two in time, but about one-tenth the scale. A "smaller" eruption occurred just 70,000 years ago. That one was still ten to fifty times as destructive as Mt. Vesuvius when it destroyed Pompeii.
There are just under fifty known ashfall beds that Geologists classify as "Supervolcano" eruption debris. Each released at least 500 cubic km (120 cubic miles) of ashy lava. The largest, which spread the Fish Canyon Tuff in Colorado 28 million years ago, erupted 5,000 km3. The earlier Yellowstone ashfall was about half that, with the later one about 1,000. The ashfall 70,000 years ago was a "mere" 100 km3.
Within a few years from today, and perhaps in a much more compressed time frame, a new caldera could appear at Yellowstone, and whenever it does, an area the size of Connecticut could be covered in several feet of ash...or hundreds. Human civilization could be completely reworked as a result, or vanish entirely for generations. Then again, it may not occur for centuries. But the chances are deemed to be about five times as great as another asteroid impact like the one that wiped out the dinosaurs.
All this is my own synopsis of Greg Breining's thesis in Super Volcano: The Ticking Time Bomb Beneath Yellowstone National Park. My sketch is just a middle-of-the-road speculation, based on the history of Yellowstone and the Snake River plateau over the past 16-18 million years.
Think Hawaii: A series of islands that arose, one after another, over 20 million years or so. The Big Island is still erupting but a new island is forming to its northeast, and will grow above sea level in a few hundred years. This chain of island volcanoes is symptomatic of a hot spot anchored in Earth's mantle, somewhere at least a few hundred miles down. The Pacific Plate is moving over this stationary hot spot at a rate of about 5 cm yearly. From time to time, this starves off the heat supply to one island, so a new conduit to the surface is melted through, forming a newer island.
The North American Plate is moving over a similar hot spot, at perhaps 2 cm yearly. Our first bits of evidence for it are 350 miles WSW of Yellowstone, along the Nevada-Oregon border. Geologists have pieced together evidence of 142 major eruptions spanning about 17 million years. Many of these had Super status (500 km3 of ash or more). Events the size of the one 70,000 years ago are hard to discern because the larger ones tend to erase the smaller ones. A "70,000" style eruption would be the biggest in recorded history.
Hawaii and other island chains erupt much larger total volumes of lava, and very little ash, because the oceanic crust has a composition that, when melted into magma (when magma is ejected it becomes lava), is runny and flows readily. A hot spot that melts continental crust produces a much thicker magma. It is much hotter than Hawaiian magma, but so sticky it cannot flow at all. It allows pressure to build up until an explosion occurs. A "small" explosion reduces pressure at the top of the magma body, which allows water to expand into steam, leading to a bigger explosion; this process cascades until a large portion of the magma is blasted out. The caldera forms when the steam pressure falls and the overlying, "cooler" rocks collapse into the emptied magma chamber.
This can happen fast! The thick ash left behind by Yellowstone's largest known eruption is a single "welded tuff" that could have fallen in a single day. Near the caldera it may have been more than a mile thick.
When Mt. St. Helens erupted 27 years ago, it loosed a couple of cubic km of ash. I lived in South Dakota at the time. We had an ash fall of a quarter inch or less of fine dust a few days later. Had that been a supervolcano eruption, the thickness would have been three or four meters. Shoveling snow is bad enough! You gotta get ash like that off the roof of your house before the next rain, or it soaks up the water and your roof collapses! And it is likely to rain soon. Such eruptions usually trigger heavy rainfall.
The book's chapters survey the history of Yellowstone and of volcanic disasters through both recent Geologic time and historical time. The timing and scale of the largest eruptions points up the problem we have: There has been no super volcano eruption in the last 10,000 years, so we don't have any idea what events lead up to it, how rapidly it actually occurs, or how long the biosphere (and human culture) take to recover.
Here is the latest update from the Yellowstone Volcano Observatory:
day, December 3, 2007 10:27 MSTWhen might it happen? The timing of past eruptions is quite irregular, so we could be overdue, or it could be thousands of years. Such an eruption takes time to gather its energies, so there will be signs, but like "false labor" that can come and go for a month before a baby is born, nobody can predict which set of gathering precursors will cascade into a huge blast. Knowing human history, when Yellowstone really blows, there will probably be tens of thousands of people there, inside the present caldera, enjoying the extra geyser activity and excitedly peeking at the new, smoking, spitting fumaroles. Not even their DNA will be found afterward.
Volcano Alert Level: NORMAL
Aviation Color Code: GREEN
NOVEMBER 2007 Yellowstone Seismicity Summary
During the month of November 2007, 69 earthquakes were located in the Yellowstone Region. The largest of these shocks was a magnitude 2.9 on November 4, 2007 at 11:43 PM MDT, located about 2 miles south southeast of West Thumb, WY. This earthquake was slightly felt at West Thumb and Old Faithful but produced no damage. There were no earthquake swarms during the month.
Earthquake activity in the Yellowstone region is at relatively low background levels.
Ground Deformation Summary: Through November 2007, continuous GPS data show that most of the Yellowstone caldera continued moving upward at similar to slightly lower rates as the past year. The maximum measured ground uplift over the past 37 months is ~17 cm at the White Lake GPS station. An example can be found at: UNAVO time series
The general uplift of the Yellowstone caldera is scientifically interesting and will continue to be monitored closely by YVO staff.
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