It is hard to believe, but it has been thirty years since the eruption of Mount St Helens  . At 8:32, the eruption began; by noon, most of the top of the mountain had disappeared in a plinian eruption  that spread ash across fifteen states. The mountain shrank from 9,677 ft tall before the eruption to 8,363 ft tall after, removing nearly 4 billion cubic yards of rock in the largest landslide in recorded history. The blast cleared the forest for 17 miles from the northern side and spawned a mudflow that blocked the Columbia River shipping channel 70 miles away.
Amazingly, only 57 people were killed in the explosion. This was because scientists had been monitoring the volcano for years, recognizing that it was potentially active, not inactive . They had established a number of seismometers to measure the earthquakes associated with magma movement and has flown over the volcano several times to capture images of the incipient eruption and to put instruments into place to measure the tilting of the ground and its temperature (among other things). This information allowed them to predict when the volcano would erupt  and to get most of the people to safety.
And now for the question that is on everyone’s mind – when will it happen again?  Since the eruption, Mt St Helens has grown a new dome and has had some major magma movements. Looking to the past, it has had at least four major eruptions in the past 500 years , so they are not as uncommon as we might think. Though most of those who study Mt St Helens do not think that it will erupt any time soon, they are prepared to be surprised.
More interesting (to the volcanologists, at least) is the associated question of “When will the other volcanoes in the Cascades erupt?” You see, Mt St Helens isn’t the only volcano in the area. There are over 100 volcanoes in the chain, stretching from California to British Columbia. All of them are caused by the subduction of the Juan de Fuca and Gordo plates under the North American plate; this is also what has created the Cascades mountains and the large number of large earthquakes in the region . Many of these other volcanoes have just as violent a history as Mt St Helens – and the statute of limitations hasn’t run out on any of them. So don’t be surprised if some morning you wake up and hear “The government has ordered the immediate evacuation of Portland due to the imminent eruption of Mt Hood”; we may lose some good beer, but think of all the great geology that we’ll discover .
 The mountain was named for the British diplomat Alleyne Fitzherbert (the Baron St. Helens) by Commander George Vancouver on H.M.S.Discovery during its northern Pacific coast survey (1792 to 1794).
 Plinian eruptions [a] are those with characteristics similar to the ones that Pliny the Elder saw during the eruption of Vesuvius back in 79 CE. In these eruptions, a large column of ash and gas shoots up into the stratosphere, with nearly continual gas explosions that eject large volumes of pumice. Plinian eruptions are most common in volcanoes near subduction zones, where the magma is thick and forms a solid seal against the gas pressure.
 Popular science classifies volcanoes as “active” (erupting), “dormant” (has erupted in the past 500 years), or “extinct” (can’t erupt). Scientists classify volcanoes as “active” (erupting), “potentially active” (has erupted in the past 10,000 years), and “inactive” (hasn’t erupted in the past 10,000 years). Notice that, except for the first category, there is no agreement. That is because scientists know that there is no such thing as an extinct volcano, just ones that have been dormant for a short time – and they know that 10,000 years is a very short time indeed (to a volcano).
 But notice that they were only able to make the prediction for about a month before the event. This situation has not changed much today; we still cannot predict volcanic eruptions more than a month ahead of time. Though this is plenty of time to evacuate a small town, it isn’t nearly enough to deal with the complexities of saving a major city such as Naples or Seattle.
 Notice that the question isn’t “Can it happen again?”, as it most assuredly will. Just as there will be more major earthquakes and hurricanes, there will be more volcanic eruptions. And, as long as we insist on living in places where these things happen, people will die and property will be damaged by them.
 “Aha!” cries the typical arm-chair statistician. “500/4=125; therefore, we don’t have to worry until 2105!” And, as always, the arm-chair statistician is sadly wrong. Major eruptions (like major earthquakes) are quasi-random; they happen at intervals that are influenced by other factors but which have a large portion of “dunno” built into them. Thus, there could be another major eruption in 125 years, or in 250 years – or next month. The only rule of thumb is that there are no rules of thumb [b].
 Seattle is also overdue for a major earthquake (Mb 7+). Just to make life interesting, any earthquake is also likely to create a tsunami that might strike Japan and will definitely strike Vancouver.
 For a geologists, that is about the ultimate in “Buridan’s ass” situations: is it better to save Portland’s beer or to watch the volcano erupt?
[a] Didn’t know that there were different types of eruptions? There are three major classes, with about eight types based on the type of magma and the interaction with the enviroment. Phreatic eruptions may be the most common; rising magma meets a water table and creates a steam explosion such as the one at Krakatoa. Phreatomagmatic eruptions happen when hot magma erupts under water (liquid or solid); there are two types. Subglacial eruptions happen beneath a glacier (duh), creating a flat-topped tuya and massive floods. Submarine explosions happen underwater (again, duh), and are most common along the world’s longest mountain chains, the mid-ocean ridges; most of the Earth’s volcanoes are actually submarine volcanoes that never reach the surface. And there are Surtseyan eruptions, where the water is just deep enough to give the explosion a little oomph! Next come the ones that everyone thinks of when they think ‘eruption!”- magma, rocks, gas, and assorted other debris being hurled into the air like candy from a pinata of death. There are Strombolian eruptions that create fire fountains like those at Stromboli. Hawaiian eruptions are just like Strombolian eruptions, only more so; with the less viscous and dryer basaltic lava of the Hawaiian hotspot, these eruptions form spectacular fountains and long, winding flows; they have also created the Earth’s tallest mountain (Mauna Kea). There are Vulcanian eruptions, after the critter that gave volcanoes their name, Vulcano; these are short, sharp, shock like the blast of a Christmas cracker – only with hot rocks and ash instead of prizes and confetti. And then there are Peléan eruptions, which have hot, glowing clouds of ash and gas flowing downhill at up to 100 mph in a nuée ardente (“Glowing cloud” – who says science is tuff?). The 1902 nuée ardente from Mount Pelée engulfed the town of Saint-Pierre on Martinique and killed 30,000 people in less than an hour; the only two people left alive were a prisoner on a barge in the harbor and a farmer on the edge of the city.
[b] “Don’t walk on hot lava” isn’t a rule of thumb, just a bit of common sense that gets overlooked all too often.