That volcano is so dangerous because it is a ryolite caldera type volcano the worst of the worst.
"Rhyolite caldera complexes are the most explosive of Earth's volcanoes but often don't even look like volcanoes. They are usually so explosive when they erupt that they end up collapsing in on themselves rather than building any tall structure (George Walker has termed such structures "inverse volcanoes"). The collapsed depressions are large calderas, and they indicate that the magma chambers associated with the eruptions are huge. In fact, layers of ash (either ash falls or ash flows) often extend over thousands of square kilometers in all directions from these calderas. Fortunately we haven't had to live through one of these since 83 AD when Taupo erupted. Many rhyolite caldera complexes, however, are the scenes of small-scale eruptions during the long reposes between big explosive events. The vents for these smaller eruptions sometimes follow the ring faults of the main caldera but most often they don't. The origin of these rhyolite complexes is still not well-understood. Many folks think that Yellowstone, for example, is associated with a hotspot. However, a hotspot origin for most other rhyolite calderas doesn't work; they occur in subduction-related arcs. Examples of rhyolite caldera complexes include Yellowstone, La Primavera, Rabaul, Taupo, Toba, and others."
Very large amounts of gas disolves in this type of magma. It stays there because the lava is extremely thick do to high Si-O2 content 79%. Compared to less to around 45% with hawaii. Mt. St. Helens some where in the middle.
When the Yellowstone volcano errupts the gas boils out of the very thick ryolite magma. This is would be extremely violent.
Volume Comparison of Ash
Sort of a scenario of the event
source(
http://www.rense.com/general41/yellowstoneupdate.htm)
the link is dead but that is the source
By Dr. Bruce Cornet
Geologist, Paleobotanist, and Palynologist
September 8th, 2003
Mt. Sheriden has been rumbling (15+ micro-quakes) between 1:00 pm and now (9/7/03). There were three small earthquakes at Yellowstone lake between 10:00 am and 1:00 pm MT (9/7/03), which were felt at Norris Junction. There were some small quakes between Midnight and 6:00 am (9/7/03) at Norris Junction. There was a whole string of micro-quakes (25 or more) at Madison River between 6:00 am and now, which are continuing. There have been sporadic micro-quakes (32+) all day at Mammoth Hot Springs. Micro-quakes started around Noon and have continued to the present at Mirror Lake Plateau. All in all, activity is picking up from a lull for about two weeks, before which a series of small and large quakes (including a 4.4) occurred. That quake prompted the web report.
Steam pressure is apparently building again, and hydrothermal fluids and steam are working their way up through fractures and vents. I do not expect anything unusual or extreme to happen in the immediate future, but if the trend continues, and the number of earthquakes gradually increase with time, more warnings from geologists will ensue.
What you should be alert to is any report that mentions increasing geyser activity, with new fumaroles and steam vents appearing near or on top of the rising dome. The dome has risen about three feet in the past few years, and magma has risen to within 3.7 km of the surface based on quake data. Earthquake loci measured to within 0.5 km under Mt. St. Helens, and people still didn't think it would erupt.
But everything has to be scaled up for Yellowstone, meaning that 3.7 km is not a safe depth. Ground temperatures in the northwestern part of the park are apparently on the rise (up to 200 dg F in some places), killing the vegetation. Large areas of the park are now closed, including areas with geysers, because their water temperature is now scalding and dangerous for visitors.
If more steam vents appear, that means a continuous pathway for pressure release has been established to the magma chamber. If that happens, the pressure in the magma chamber will continue to drop until it reaches a critical stage when the superheated water within the magma explodes. When that happens the super-volcano will blow violently, blowing out a chunk of its cap-rock and sending millions of cubic feet of ash into the atmosphere in a Pompeii-like explosion, but 100,000 times worse.
When you hear those reports, you will have about two days to "get out of Dodge" before the eruption. Unfortunately, as the steam venting subsides, there will be a false sense of security. People will think it was just another cyclical event, and the danger is over. But that will be the farthest from the truth. It will be the quiet before the storm. A major earthquake will suddenly rock their towns for hundreds of kilometers around Yellowstone, and soon thereafter 1,000+ degree pyroclastic flows will descend on them at hundreds of miles per hour, extending out to 600+ km.
That 600 km radius around the caldera will experience total devastation. The next 600 km out may receive as much as 5-10 feet of ash, depending on wind direction. The thickness of ash will decrease away from the super-volcano, but will reach the crop belt in the Midwest (Missouri, Iowa, Kansas, Nebraska, etc.), destroying most of the fertile croplands of the United States. California will be hit hard by falling ash, with its central wine valley severely damaged (the French will love it). Agriculture will have to shift east of the Mississippi for years. The Garden State will once again live up to its name.
In northern Idaho you will have to contend with several feet of ash and isolation. Roads will be closed. Power will be out. Phones will be out. Communication will depend on Ham radios and local stations that have generators. Rescue will take weeks or months. Some areas will never see rescue teams. The survivalists will be best prepared to make it through the difficult months following the eruption. Make new friends. Have plenty of dust masks on hand, because you cannot breath any airborne ash if you want to avoid lung disease. It's what caused mass kills of plains animals 12 million years ago, resulting in extensive bone beds beneath the ash. Drinkable water will sell at the price of gold.
To recap, I don't expect anything to happen in the near future. But with such an unpredictable event, being prepared is your best ticket to survival."
Dr. Bruce Cornet
It probably wouldn't be the end of the world but Wyoming and many of the mountain states would be wiped clean and buried under hundreds of feet of ash. Much of the bread basket would get at least a meter. There would be a problem of what to do with all that ash. Likely fairly severe global cooling and an overall bad time had by all. Might not happen for 100,000 years though, but it is unlikely it would wait longer than that. In 1987 the caldera started to fall by 2.5 cm, or 1 in., a year, and rose a meter since 1900. It began to buldge again in 1995. So it is definetly still active.
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