When Science Goes Wrong (11 page)
Galeras presents a real danger, in the sense that it has erupted frequently over recorded history. Significant eruptions occurred in or around 1580, 1616, 1797, 1830, 1865-1869, 1891, and 1936. With a record like this, one always has to keep the possibility of an eruption in mind, especially with a fair-size city so close by.
Galeras is also dangerous on account of the type of magma (molten volcanic rock) that it produces. Known as ‘andesite’, Galeras’s magma is rich in silica and consequently is thick and pasty, especially after it is exuded onto the surface and has a chance to cool. Unlike the more liquid magmas found in Hawaiian volcanoes, which run smoothly down the volcanoes’ outer slopes as incandescent lava flows, the magma at Galeras tends to pile up where it erupts, forming solid domes of lava that eventually seal off the vents through which the magma reached the surface. Thus, if pressure continues to build as more magma is forced upward from below, the result may be a sudden and difficult-to-predict explosion.
Frequent, explosive eruptions are dangerous, but there are also factors that tend to lessen the hazards posed by Galeras. For one thing, the historical eruptions have been fairly small – most of them have been confined to the volcano’s caldera, the mile-wide sunken amphitheatre that was created when the volcano’s summit collapsed during some prehistoric eruption. Also, the occasional lava flows and pyroclastic flows – lethal surges of ash and pumice buoyed by hot gases – have generally exited the caldera toward the west, because the caldera’s walls have been breached on that side. The city of Pasto, on the eastern side of the volcano, is partially protected by the caldera’s 500ft rampart.
One more feature of Galeras limits the hazard it poses to the local population. Because of its moderate altitude, combined with its location barely 80 miles north of the equator, its summit is free of snow or ice. Snow banks or glaciers may enhance a volcano’s beauty, but they also spell danger because an eruption can rapidly melt the ice. The resulting meltwater, mixed with soil, rock, and ash, is likely to rush downhill in the form of all-consuming mudflows. In 1985, an eruption of the 17,400ft Nevado del Ruiz volcano, 300 miles northeast of Galeras, melted its glacial cap: the resulting mudflow travelled more than 20 miles to the town of Armero, which was nearly totally destroyed at a cost of more than 23,000 lives. Such an event could not happen at Galeras.
Whatever its danger level, Galeras was largely ignored by the world’s volcanologists until 1988. In that year, after half a century of inactivity, the volcano showed renewed signs of life. A series of small earthquakes struck the area. In addition, steam began to vent from the volcano. The Colombian government, hypersensitive to volcanic hazards after the Nevado del Ruiz tragedy, sent several volcanologists to investigate and monitor the situation.
Climbing Galeras is a simple matter: one gets into a jeep and drives up. The access road zigzags its way up to the south-eastern rim of the caldera. There, in 1988, was located a small police post and several communications towers. A few policemen were always stationed at the post to guard the towers against sabotage by the leftist guerrillas who were active in Nariño province. From the rim, one could look down on the interior of the caldera: its main feature was a central volcanic cone. About half a mile wide at its base, the cone rose 450ft from the floor of the caldera but did not rise as high as the caldera rim. At the top of the cone was the actual crater of the volcano – a 100ft-deep cavity. Getting from the caldera rim to the edge of the crater was an arduous journey: it involved edging down the very steep eastern rampart of the caldera with the aid of a fixed rope, crossing the floor of the caldera (the ‘moat’), and then climbing the cone. From there, it was another tricky descent into the crater itself.
When the Colombian volcanologists visited the caldera in 1988, they did not descend into the central crater, because it would have meant death to do so: the crater floor was incandescent with heat. Obviously, something very serious was going on inside Galeras. They returned to Pasto and reported their findings to the director of the Colombian National Institute of Geology and Mines, or INGEOMINAS. He called in turn for help from the United States. A few weeks later, David Harlow, of the US Geological Survey (USGS) in Menlo Park, California, brought a small team of scientists to Galeras. They installed several more seismographs around the volcano.
In the spring of 1989, a larger group of Colombian and foreign volcanologists met in Pasto. Two eruptions took place that spring, and though they had both been very small, they got the attention of the people of Pasto. The local government began issuing colour-coded warnings, and as so often happens, these induced more confusion than comprehension. Furthermore, the city experienced serious economic problems, as banks stopped issuing loans to local businesses and tourism dried up.
Soon after the 1989 meeting, Galeras quieted down. Then, in the autumn of 1991, eruptions began again. A lava dome rose slowly from the floor of the crater, eventually reaching a height of 150ft. In response to this alarming development, the governor of Nariño called for yet another meeting of scientists, which took place later that month. Among the attendees was Marta Calvache, a young Colombian volcanologist who grew up in the shadow of Galeras and who had been at Nevado del Ruiz during the deadly 1985 eruption. In the aftermath of that event she had met Stanley Williams, an expert in volcanic gases at Louisiana State University. Calvache later went to LSU and did a master’s thesis with Williams. (Williams moved to Arizona State University in 1991.) The relationship between Calvache and Williams was an obvious and immediate benefit to Calvache, furthering her expertise and her career. But, as it turned out much later, it was an even greater benefit to Williams, for it was Calvache who saved his life while his colleagues died.
Another attendee at the 1991 meeting was Bernard Chouet, a Swiss-born geophysicist with the USGS in Menlo Park. Chouet’s speciality was the interpretation of the seismic signals emitted by active volcanoes. Over the course of a few years before the meeting, Chouet had come to believe that he had discovered a hitherto unknown method for using these signals to predict eruptions.
The seismic signals generated by volcanoes are of two basic kinds. The more common kind are basically little earthquakes: they are produced by the fracturing of rock as magma creates passageways for its ascent to the surface. On seismograms, these events look quite like the common, nonvolcanic earthquakes that are generated by the motion of geological faults: brief, jittery signals that, if sped up and played through a loudspeaker, sound like bangs, pops, rips, crunches, roars or other unattractive noises. They are assigned magnitudes just like regular earthquakes and, though most are tiny, a few range up to magnitude 5 or so, and are thus easily felt by people living in the vicinity of the volcano. They are called volcano-tectonic earthquakes.
The other, less common seismic signals are quite different. They are low-pitched (infrasonic) vibrations that may continue for half a minute or longer. When sped up and converted into sound, they have an eerily musical quality – they may be reminiscent of whale song, a dirge played on trombones, or Tuvan throat-singing.* Unlike volcano-tectonic earthquakes, these events confine most of their energy to a single, very low frequency – a deep-pitched tone – along with some higher-pitched harmonics that add to the musical quality of the sound. They are called long-period events – ‘long-period’ in this context means the same thing as ‘low-pitched’.
Prior to Chouet’s work, much more attention had been paid to the volcano-tectonic earthquakes than to the long-period events as predictors of volcanic eruptions. It is certainly true that most volcanic eruptions are preceded by volcano-tectonic earthquakes. They may start months or years before an eruption, as magma begins to rise from deep magma chambers and collect nearer the surface. These earthquakes are commonly the long-term warning signs that tell volcanologists – and the local populations – that they need to pay attention to the volcano.
Sometimes an intense swarm of volcano-tectonic earthquakes may immediately precede an actual eruption. This happened at the Rabaul caldera in Papua New Guinea in 1994, for example. The port city of Rabaul was evacuated within hours. Next day, the twin volcanoes that guard the harbour entrance both erupted, destroying much of the city. Thanks to the warning provided by the earthquake swarm, only five people died – one of them from a lightning strike.
Yet, just as often, volcano-tectonic earthquakes don’t give timely warning of an eruption. An earthquake swarm may occur without a subsequent eruption, or an eruption may occur without a preceding swarm. The fallibility of volcano-tectonic earthquakes as short-term predictors of eruptions is the main reason why local populations are sometimes kept in a prolonged state of needless anxiety or, conversely, given false reassurance in the face of a looming catastrophe.
During the 1980s, Bernard Chouet turned his attention to the long-period events as possible predictors of eruptions. Chouet had an insight about the nature of these events. He realised that their musicality – the pure tones with their added harmonics – must result from a resonance, that is to say, the reverberation of acoustic waves within a limited space. When Chouet analysed long-period events mathematically with this idea in mind, he was able to define the characteristics of the system that created them. Long-period events, he concluded, occur when a slug of magma jerks forward within a sheet-like crack in the rock. The motion of the magma is quickly arrested by the confining rock, triggering an acoustic wave. This is analogous to the ‘water-hammer’ that may be generated by household water pipes when a tap is turned off too quickly. ‘It’s as if you’re pinging it with a hammer blow,’ Chouet told me during a 2006 interview. ‘The acoustic pulse will travel through the fluid and hit a boundary and reflect, and keep going back and forth.’ Nevertheless, some of the acoustic energy escapes into the solid rock and radiates to the surface, where it can be detected by seismic instruments as a long-period event.
According to this model, long-period events will occur when the path of the advancing magma is at least partially obstructed, rather than being open to the surface, just as water-hammer occurs in closed pipes. Obstructed magma is put under increasing pressure as more magma is forced up from the deep. Thus a series of long-period events may signal an increase in pressure that will end only when the pressure within the magma exceeds the weight of the overlying rock. At this point, an explosive eruption will occur.
During the mid-1980s, Chouet looked through the seismographic records of past eruptions at a number of volcanoes. He found long-period events in the run-up to several eruptions, including the disastrous 1985 eruption of Nevado del Ruiz. Then, on January 2, 1991, he successfully predicted an eruption of the Redoubt volcano, which lies 100 miles southwest of Anchorage, Alaska, and 25 miles upriver from an oil-storage terminal. He made the prediction when there was a sudden increase in the frequency of long-period events – they began occurring every minute or so. In response to his prediction, the terminal was closed down and evacuated by 5pm that same day. Just two hours later the volcano erupted, and the resulting mudflow left parts of the terminal standing in three feet of muddy water.
Thus, when he came down to Pasto for the meeting later that year, Chouet was highly focused on looking for long-period events in the seismographic signals from Galeras. At the time, the volcano was venting excess gases through a long fissure in the lava dome which opened up every few minutes to release a cloud of gas. Because of the action of this ‘safety valve’ the dome’s internal pressure was not building in any dangerous way. Chouet predicted, however, that the crack would eventually seal off, and he advised Calvache and the other Colombian volcanologists to keep a lookout for long-period events once that happened. ‘When it seals,’ says Chouet, ‘the source itself doesn’t know it’s sealed; it’s going to keep on pumping, but now it’s pumping into fractures that are embedded in a solid, so instead of pumping into the atmosphere you are pressurising the whole system.’
During the early part of 1992 the lava dome did in fact seal itself off, as minerals deposited by the venting gases clogged the fissure through which they had been emerging. To all external appearances, the volcano was going back into dormancy. But one day in early July, the local volcanologists noticed an odd-looking signal on their seismographs. It was a long-period event, but a particular kind of long-period event in which the vibration quickly built to a peak amplitude and then gradually faded to silence over a period of minutes.
On the seismographic recording, the event looked like a screw viewed from the side, and the scientists later named it a
tornillo
– the Spanish for ‘screw’. Eight more
tornillos
occurred over the following five days, and then, on July 16, the lava dome exploded, sending a shower of rocks across the caldera that destroyed some of the communications towers on the caldera rim. It was a small eruption – the city of Pasto was never threatened – but it added strength to Chouet’s hypothesis that long-period events could be used to predict eruptions.
Because of Galeras’s continued activity, yet another scientific meeting was planned for January of 1993. This time the organiser was Marta Calvache’s graduate advisor, Stanley Williams. Williams was a very different kind of volcanologist from Bernard Chouet. For one thing, his specialty was not studying volcanoes’ seismic signals, but collecting and analysing the gases they emit – gases such as water vapour, carbon dioxide, sulphur dioxide, and the ‘rotten egg’ gas, hydrogen sulphide. These gases are dissolved in the magma, but as the magma approaches the surface they come out of solution and, if there are open conduits to the surface, they enter the atmosphere at vents called fumaroles. Williams believed that changes in the amount or kinds of gases discharged at fumaroles were potential indicators of impending eruptions. The track record for prediction on this basis was spotty, however.
