Transcript
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/"natural 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 heroes 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 Land Cruiser has a Washington State license plate, and Washington does have active volcanoes. Wait! We need to start at the beginning (well, not exactly the beginning...): 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. Strato-volcanoes 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 underground 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 temperature 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 underground 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 seismograph, 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 construction. Regarding the suddenness 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. More Movie Fact or Fiction
MOVIE FACT OR FICTION?
Q: Is the eruption depicted in Dante's Peak realistic? A: In many but not all respects, the movie's depiction of eruptive hazards hits close to the mark, especially as regards the enormous power unleashed during an eruption. Stratovolcanoes in the Cascade Range and Alaska erupt explosively and produce pyroclastic flows, clouds of volcanic ash, and debris flows (lahars) that behave much as shown in the movie. Lava flows at these volcanoes, though, are usually thick and slow moving, unlike the fluid flows in the movie. Fastflowing flows of basalt lava are common in Hawaii, though. Real eruptions may be considerably larger or smaller, and affect larger or smaller areas, than those shown in the film. Q: Can eruptions really threaten helicopters, as in the movie, and other aircraft? A: Yes. Encounters between aircraft and clouds of volcanic ash are a serious concern. Jet engines and other aircraft components are vulnerable to damage by fine, abrasive volcanic ash, which can drift in dangerous concentrations hundreds of miles downwind from an erupting volcano. During the past 15 years, at least 80 aircraft have accidentally encountered volcanic ash clouds, and in 6 cases jet engines temporarily lost power. An international consortium of government agencies, including the U.S. Geological Survey, Federal Aviation Administration, and National Weather Service, is now
monitoring ash-producing volcanoes and tracking volcanic ash clouds to reduce the likelihood of future encounters. Q: Can the temperature of hot springs near a restless volcano change quickly enough to injure bathers? A: Temperature changes can and do occur, but usually more slowly than shown in the movie. In fact, the temperature of hot springs may increase, decrease, or stay the same during volcanic unrest. Increases in water temperature, when they do occur, usually take days or weeks to develop, rather than a few seconds as shown in the movie. In rare cases, earthquakes can suddenly disrupt a volcano's hot groundwater system, changing its temperature. And earthquakes have been known to temporarily increase the flow of water from hot springs, sometimes causing geyser-like activity that could threaten bathers. Q: Do earthquakes large enough to collapse buildings and roads accompany volcanic eruptions? A: Not usually. Earthquakes associated with eruptions rarely exceed magnitude 5, and these moderate earthquakes are not big enough to destroy the kinds of buildings, houses, and roads that were demolished in the movie. The largest earthquakes at Mount St. Helens in 1980 were magnitude 5, large enough to sway trees and damage buildings, but not destroy them. During the huge eruption of Mount Pinatubo in the Philippines in 1991, dozens of light to moderate earthquakes (magnitude 3 to 5) were felt by several hundred thousand people. Many houses collapsed, but not primarily because of the shaking. Heavy, wet ash from the eruption and a hurricane accumulated on roofs and crushed them. Stronger earthquakes sometimes DO occur near volcanoes as a result of tectonic faulting. For example, four magnitude 6 earthquakes struck Long Valley caldera, California, in 1980, and a magnitude 7.2 earthquake struck Kilauea Volcano, Hawaii, in 1975. Both volcanoes were quiet at the time. The Hawaii earthquake triggered a small eruption at the summit of Kilauea. No eruption has yet occurred at Long Valley, but the area has been restless since the 1980 earthquakes. Q: Can a town's water supply become contaminated when a volcano is restless? A: Yes, but probably not as quickly as shown in the movie. If a town's water supply originates directly from a volcano's groundwater system or from a stream that has been covered with volcanic ash, the water could become contaminated with foul-smelling gases or fine ash and other sediment. Some volcanic gases such as sulfur dioxide dissolve in groundwater, making the water acidic. Sulfurous odors, however, are caused by hydrogen sulfide gas, which smells like rotten eggs. Q: Do scientists drive across moving lava flows?
A: No. Any attempt to drive across an active lava flow, even one that has partly solidified to form a thin crust, is likely to lead to disaster. With a temperature of 1,700 degrees Fahrenheit or higher, fresh lava will quickly melt rubber tires and ignite gas tanks. And if a vehicle gets stuck in moving lava, well, you know the rest of the story. Q: Can carbon dioxide gas from volcanoes kill trees and wildlife? A: Yes. At several volcanoes around the world, carbon dioxide gas released from magma has accumulated in the soil in sufficient concentrations to kill vegetation or has collected in low areas and suffocated animals. At Mammoth Mountain in California, carbon dioxide has killed about 100 acres of trees since 1989, and visitors to this area have occasionally suffered symptoms of asphyxiation when entering cabins or below- ground excavations. USGS scientists have concluded that the gas is escaping from a magma body beneath Mammoth Mountain. The magma itself is not currently moving toward the surface, but the USGS is monitoring the situation carefully. Q: Can volcanoes suddenly become restless and erupt within one week of the first signs of activity? A: Yes. 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. Q: Are robots used by the USGS to monitor volcanoes? A: No. We rely on observations and measurements made by experienced scientists and on critical data sent by radio or satellite relay from monitoring instruments installed around a volcano. These instruments include seismometers, tiltmeters, Global Positioning System (GPS) receivers, gas sensors, mudflow (lahar or debris flow) sensors, and temperature probes. NASA has tested a robot named Dante at Mount Erebus volcano in Antarctica and Mount Spurr volcano in Alaska. The USGS believes that, on Earth, experienced volcanologists are a better and more cost-effective alternative for monitoring dangerous volcanoes. Q: Can volcanoes produce large explosive eruptions and rivers of fluid lava at the same time? A: Not usually. During a single eruption, a volcano CAN produce both lava flows and ash, sometimes simultaneously. The red, glowing lava fountains and lava flows in Dante's Peak (including the active flow across which Harry Dalton drives) are characteristic of a fluid magma, called basalt. In contrast, explosive gray ash columns and pyroclastic flows shown in other scenes are characteristic
of more viscous magmas, called andesite, dacite, or rhyolite. It's uncommon for a volcano to erupt magmas of widely different composition at the same time. Q: Can lakes near volcanoes become acidic enough to be dangerous to people? A: Yes. Crater lakes atop volcanoes are typically the most acid, with pH values as low as 0.1 (very strong acid). Normal lake waters, in contrast, have relatively neutral pH values near 7.0. The crater lake at El Chichon volcano in Mexico had a pH of 0.5 in 1983 and Mount Pinatubo's crater lake had a pH of 1.9 in 1992. The acid waters of these lakes are capable of causing burns to human skin but are unlikely to dissolve metal quickly. Gases from magma that dissolve in lake water to form such acidic brews include carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen chloride, and hydrogen fluoride. However, the movie's rapidly formed acidic lake capable of dissolving an aluminum boat in a matter of minutes is unrealistic. Back To: Volcanoes Natural Hazards SCIWEB