Some definitions:

"Active" ; whether erupting, rumbling or merely producing steam and gas, a volcano is active.

"Dormant" ; not currently active, but known to have been active within a measurable period of time, and in a tectonic setting which means it is liable to become active again.

By this definition, the Yellowstone region is active. St. Helens and Pinatubo are two examples of dormant volcanoes which suddenly came back to life.


Here are two US Geological Service links on The Snake River Plain and Yellowstone

Yellowstone sits on top of a "hot spot", a source of rising hot material in the mantle. The rising material is a basic (i.e. low in silica) magma which forms basalt when it erupts. Oceanic crust is largely basaltic, so a hot spot under oceanic crust (such as Hawaii or Iceland) generally builds itself islands in a comparatively undramatic fashion (though there is the possiblity of very large tsunami triggered by undersea collapse of unstable slopes).

Under continental crust, which is acid (i.e. high in silica), the hot spot will melt some of the crust to form an acidic magma which is known as rhyolite when it erupts. These can be much more violent than basaltic eruptions because as a magma, rhyolite is much more viscous than basalt. If there are lots of volatiles (e.g. steam, carbon dioxide, sulphur dioxide, and other gases present, they all come out of solution in the magma when the eruption begins and the pressure on the magma is released, and cause an explosive or "paroxysmal" eruption.

(To reproduce the effect, purchase a 2-litre plastic bottle of lemonade. Kick it around the floor for a bit. Then remove the top. And go and get a cloth.)

"Supervolcanoes" are distinguished by their magnitude i.e. the sheer volume of rock involved, and their paroxysmal nature. Note that Krakatoa was a mini-supervolcano in this respect; its violent eruption was paroxsymal in nature, but the volume of rock involved as ash, pumice and so on was little more than that produced by, say, Pinatubo. The final cataclysmic explosions were produced by Krakatoa collapsing into its partially voided magma chambers.

I suspect that in the case of Yellowstone exploding, we will have lots of warning signs, which will precede the bang by years or even decades years. The area will bulge several tens of metres or even hundreds of metres; there will be lots of earthquakes and gas will be evolved. ("Old Faithful" and other geysers involve superheated ground water. Water evolved from the hot spot or a rhyolite intrusion will be loaded with sulphides and other minerals). Long-period harmonic seismic waves will indicate that magma is being evolved or is rising within the existing chamber. There may also be small preliminary basaltic eruptions.

Unfortunately, geology isn't an exact science. Most predictions are hedged with uncertainty and qualifications. The periodicity of the preceding blasts isn't necessarily a guide as to when the next will occur.