Photos of Mt. St. Helens on the 15th Anniversary of the Eruption (18 May 1995).

A Geological Guidebook to Dante's Peak

For the General public

Our Commentary:

First - go see the movie (we thought it was OK). Dante's Peak is an all-too-rare opportunity for you to enjoy a good adventure/"nautural catastrophe" movie, and at the same time, get a feel for how geologists (and other scientists) work. You can also learn some science along the way. Of course, the writers, producers and directors did exercise quite a bit of artistic license in the movie, as they should; otherwise, our heros and heroines would have been killed off as soon as the action started! So, you will find some "scientifically imprecise" embellishments in the movie, and we find this quite understandable. We can think of no movie/television series that portrays scientific concepts and scientific practice as well as this flick does. As we said, our aim here is to point out the scientifically related plot elements that we think are accurate and realistic, and those wherein the writers stretched things a bit to make the movie exciting.

OK, let's get started:

The Movie is set in the Cascade Range of the Pacific Northwest, U.S.A. Dante's Peak is, of course, not a real Cascade volcano, and although Idaho (where some scenes are shot) is adjacent to states with Cascade Volcanoes (Oregon and Washington), and another state with an active magma chamber (Wyoming-magma exists under Yellowstone National Park), there are no Cascade (or any other active or dormant volcanoes) in Idaho (there is, however, plenty of evidence of past volcanic activity, as there is in most parts of the world including many of the 50 United States). Anyway, as a keen observer has pointed out to us, the Mayor's LandCruiser has a Washington State licence plate, and Washington does have active volcanoes.

Wait! We need to start at the beginning (well, not exactly the beginning...):

What is a volcano?

When Volcanoes in the Cascade Range and Alaska erupt, they frequently do so explosively and produce pyroclastic flows, ash falls and "mud" or debris flows (lahars). According to the USGS, "Lahars destroyed houses, bridges, and logging trucks during the May 1980 eruption of Mount St. Helens, and have inundated other valleys around Cascade volcanoes during prehistoric eruptions. Lahars at Nevado del Ruiz volcano, Colombia, in 1985, killed more than 23,000 people." Near a volcano, the falling volcanic ash is quite heavy (high density), and the newspaper used as a proxy for volcanic ash in the movie looked more like snow (low density) as it fell. For geologists who have seen the movie, the hot, runny lava, seen issuing from the volcano, is the most bothersome issue. Generally, runny, fast-flowing lava (basalt) erupts from Hawaiian or "shield" volcanoes; but we understand that this makes for a more exciting movie. Lava flows at Cascade volcanoes are usually thick, and rarely move far from the vent (for example Mount St. Helens Dome: see the center photo at the top of this page: the dome is a mound of rather thick lava that has partially or wholly solidified) unlike the Hawaiian-type flows and lava fountains shown in Dante's Peak. Stratovolcanoes like the Cascades do not usually produce pyroclastics and lava in the same eruption. Also, lava is VERY hot (over 1500 F), and most flammable material (rubber, wood, people) brought near the lava would burst into flame. The radiative heat alone is sufficient; flammable materials do not even have to touch the lava. We think a car would last only seconds on a lava flow before it would burst into flames, consuming the occupants. It is worth pointing out here that lava itself does not burn (most of the constituents of lava are already in their "most oxidized" state), and what we see burning around lava flows consists mostly of grass, houses. trees, shrubs, animals, etc.

We noticed that the lake near Grandmother's house becomes acidic quite fast in the movie, but hey, you need to pack a lot into a movie these days to draw a crowd. There are very acid lakes around volcanoes and yes, you would not want to swim in them... and yes metal parts can corrode in acid lakes; a thin metal cable might dissolve on a movie timescale. The acids may be sulfuric and hydrochloric acid, plus others, and the acidity (roughly measured by how low the pH is) can be quite high, hundreds to thousands of times as acidic as lemon juice or vinegar, or EVEN Coca Cola! A pH on the order of 0 to 2 would not be unexpected.

The hot springs in any geothermal area (e.g. Yellowstone; Long Valley, CA) can turn on or off, or change temperature abruptly, but these changes are usually related to rapid changes in the "underground plumbing" (the system of cracks and fractures present in underground rock that allows hot or cold underground water to flow from one spot to another brought about by earthquakes that are usually not directly related to the magmatic activity in the area). Since most undergound hot water flows through fractures that may be active as faults, changes in the fracture/fault system during earthquakes can redirect flow, or increase or decrease it. However, hot springs (like the one that parboils a "couple in love" in the movie), do not turn "red-hot" when they heat up(!) as shown in Dante's Peak. Increased steam discharge and bubbles would be likely if the temperaure increased. So, changes in hot spring activity suggest recent strong earthquakes, with or without an impending eruption.

Trees and animals killed by magmatic carbon dioxide? Strange as it seems, yes, carbon dioxide gas released from undergound magma chambers can accumulate in soil and kill plants and trees or suffocate animals. Trees have died by having their root environments flooded with carbon dioxide. Note that sugars, formed by photosynthesis during the day, are USED by the plants as food at night (plant respiration), and plants need oxygen just like we do at those times! Ground dwelling animals (squirrels, ants, etc.) can also be killed as carbon dioxide displaces oxygen gas-bearing air in soil pore spaces and subterranean critter dwellings. About 100 trees have been killed around Mammoth Mountain, CA since 1989. USGS scientists have suggested that the gas is bubbling out of magma that lies a few miles under Mammoth Mountain. Note, however, that the magma is not currently moving toward the surface; so you decide if Pierce Brosnan is correct in predicting an eruption at any moment!

Volcanic tremors are different from normal earthquakes, but you probably can't feel the difference - Geologists can see the difference on a tracing (a seismogram) made by an instrument called a seimograph, that detects the amount of ground shaking produced by an earthquake. Volcanic quakes usually register a 4-5 or less on the Richter Scale; therefore, they usually do not produce the type of destruction seen in the town of Dante's Peak upon the start of the eruption. The destruction in the movie suggests a 6-7 magnitude temblor, although the amount of destruction is a complex function of distance to the quake, duration of the earthquake, the type of rupture, the nature of the soil/rocks in the area, and the type of building constrcution.

Regarding the suddeness of the activity of Dante's Peak, volcanoes can become restless and erupt with only days to weeks warning. According to a USGS website, "The first steam eruption at Mount St. Helens on March 27, 1980, was preceded by only 7 days of intense earthquake activity. The climactic eruption, on May 18, followed seven weeks later. An eruption of Redoubt Volcano in Alaska on December 13, 1989, was preceded by only 24 hours of intense earthquake activity. But other volcanoes have been restless for months or years before an eruption occurred, and sometimes a period of unrest doesn't produce an eruption at all." So, the suddenness of the eruption of Dante's Peak was not was not a dramatic effect.


Other Sites of Interest

Alaska Volcano Observatory

Cascades Volcano Observatory

Hawaiian Volcano Observatory

Michigan Technological University Volcanoes Home Page

Dante's Peak-The Movie

The USGS Perspective

Degassing on Mammoth Mountain?

Mt. Pinatubo

Lahars

Gases at Lake Nyos, Cameroon