Reprinted with permission from Volcanoes: Encounters through the Ages by David M. Pyle, published by the Bodleian Library, University of Oxford, and distributed by the University of Chicago Press. © David M. Pyle 2017. All rights reserved.
The past 150 years have seen a dramatic change in our ability to measure activity at volcanoes, whether they are dormant, or in eruption. New instruments have been designed that can measure earthquakes, sound, gases, and heat at volcanoes, either from the ground or from space. Many of the world’s most active volcanoes are bristling with sensors, beaming back their measurements by cable, Wi-Fi, or mobile phone to the nearest observatory. Fleets of satellites form a global network, like a remote observatory, with on-board sensors detecting changes in the shape of the volcano, or in the amounts of heat and quantities of different gases escaping out of the volcano. In recent years, new global networks of instruments have all been used to spot eruptions that otherwise may have gone unnoticed: the global lightning network; the global infrasound arrays, used to detect very low frequency sounds that bounce around the globe after nuclear explosions; and the ocean buoy network used to measure sounds in the deep channels of the world’s oceans.
Our capacity to record volcanic activity has also been transformed. Before the arrival of the camera, visual records of eruptions could only be snapshots, drawn as fast as the sketcher was able; or composite images that captured some of the features of the eruption. Today, streaming video and webcams mean that we can watch events unfurl from our desktop computers and mobile devices; and then we can go back and watch them frame by frame. Weather satellites capture the spread of ash clouds and their tracks around the globe. While this flood of digital information may mean that we can see more quickly what is happening, it poses new problems for storage and archiving. No one has yet designed a system for quickly reducing these data flows to the “key performance indicators” that might best help describe what is happening now, and diagnose what might happen in the future.
The rapid and ready availability of digital imagery, and the ease of sharing this information widely, through social media, for example, is not always a good thing. Misinformation can spread just as fast as any other sorts of information; and in the hurly-burly of a rapidly unfolding event, recycled images of past events can quickly be adopted as “true likenesses” of the new. As with imagery from past eruptions, it can be hard to detect embellishment or exaggeration in the heat of the moment.
From satellites, the arcuate form of the main islands of Santorini is easy to see. Look more closely, and you’ll see the inconspicuous dark shadows of the two youngest volcanic islands—the “burnt” or Kameni islands—that lie within the great flooded caldera. These islands form the literal “tip” of the present-day volcano, which has grown up within Santorini’s caldera since the last major explosive eruption, the Minoan eruption of ca. 1600 BCE. Historical records and accounts from as far back as the Greek geographer Strabo suggest that there have been at least ten eruptions in and around the Kameni islands since 197 BCE. It is quite likely that there have been more that either weren’t noticed (because they were underwater), or that have been forgotten with the passage of time. Indeed, it has been suggested that the ancient city of Cyrene, in present day Libya, may have been founded in 630 BCE by Greeks who were escaping an earlier eruption of Thera.
In January 2011, Santorini began to show subtle signs of stirring after many decades of quiet—or at least many decades without detectable activity. Tiny earthquakes began to be detected beneath the center of the volcano. These were too small to be felt, but were large enough to be picked up by the local Greek network of seismometers. For the first time, there were more earthquakes occurring beneath Santorini than at its neighbor; an underwater volcano called Kolombos bank, 15 kilometers away. Shortly afterward, we began to see signs of ground movement from radar instruments on satellites and from sensitive ground-based GPS instruments, showing that the whole volcano was beginning to swell. The northern end of Nea Kameni was rising out of the sea at a rate of a few centimeters per year, and all the circling islands of Santorini moving outward, away from the center of the caldera. Apart from a small number of sharp earthquakes that were felt locally, there was nothing significant that would have been noticed without monitoring technology.
The way that a volcano swells tells us about where the pressure source lies. Imagine blowing up a balloon that is buried within a sandpit. If the balloon is very deep, the disturbance at the surface will be quite small, and spread out. If it is shallow, the sand will bulge dramatically over the top of the expanding balloon. Computer simulations of this swelling suggested that the cause of the unrest was the arrival of a small blob of molten rock at about 4 kilometers beneath the volcano, somewhere under the northern part of Santorini’s sea-filled caldera.
New batches of magma must be arriving repeatedl in the magma storage systems beneath slumbering volcanoes. It seems likely that this is how volcanoes slowly prepare for a future eruption; growing drip-by-drip until eventually they are ready to erupt. But we have no way of working out how often these events might have happened in the past, nor of what they might tell us about the future, simply because they are too subtle and too transient to leave any trace in either the historical or geological records of volcanoes. The geological and historical records of past eruptions have strong parallels: both have detection thresholds, below which little or nothing will be recorded or preserved. In both, records are also likely to be biased toward the extraordinary.
Every year, new volcanoes return to life after a long period of repose. Sometimes, there may be an extended warning; but at other times, there may be very little. The reawakening of a long-dormant volcano poses one of the biggest challenges to volcanologists, simply because these volcanoes will not usually have a well-established track record of behavior. In the past fifty years, several of the most troublesome eruptions have been at volcanoes that have erupted for the first time in recorded history. The two most significant explosive eruptions of the past forty years both occurred at volcanoes that were barely known before a fanfare of earthquakes announced their arrival: El Chichón, Mexico, in March 1982; and Pinatubo, Philippines, in June 1991. At Pinatubo, only three months elapsed between the first felt earthquakes and the main paroxysmal phase of the eruption: just long enough to identify the potential hazards posed by the volcano, and to start to prepare the millions of people who lived within reach of the ash clouds and mudflows that were expected to accompany an eruption.
One of the most dangerous volcanoes currently in eruption is Mount Sinabung, a large stratovolcano surrounded by farming lands, on the Indonesian island of Sumatra. In August 2010, the first historical eruption of Sinabung ended at least 400 years of dormancy. Since then, activity at Sinabung has waxed and waned, with repeated episodes of violent eruption, triggering large-scale evacuations of several tens of thousands of people from communities at the foot of the volcano. The most damaging activity a Sinabung is caused by the break-off and collapse of the hot, viscous front of a slowly extruding lava flow. This hot rock avalanche quickly transforms into rapidly moving pyroclastic flows that sweep off the flanks of the volcano. Sinabung is closely monitored by the Indonesian national volcano monitoring agency, but the variable intensity of the eruption, its long duration, and its accessibility mean that people often enter the formal exclusion zone, whether to tend to their fields and livestock, to recover belongings, or for tourism and other reasons. There have been several times when rapid changes in activity have led to multiple fatalities, as people are engulfed in searing clouds of hot ash. Eruptions of this style, like that at Unzen, Japan, in 1991 and at Montserrat in 1995, are inherently unpredictable, even when closely monitored.
In April 2015, Calbuco volcano in Chile burst back to life with less than two hours of warning, after over 40 years of silence. In the era of global satellite monitoring and with proliferating networks of instruments on the ground, why can we still not predict volcanic eruptions? Our capacity to watch, record, and comment once an eruption has started is not yet matched by our ability to anticipate what might happen next at a restless but dormant volcano.
Explosive eruptions typically throw out large quantities of ejecta—the frozen and disrupted remnants of the eviscerated magma reservoir. This often includes pumice: a light and frothy rock that is a network of tiny glassy tubes, sheets and strands, with a void space now filled with air, and would have contained volcanic gas just before eruption. Other components include crystals of different minerals that grew at depth, as the magma cooled and started to solidify, perhaps for decades or centuries.
These bubbles are thought to be the main agent that causes explosive eruptions. At depth, when fresh magma first arrives beneath the volcano, it usually contains quantities of dissolved gases, such as water and carbon dioxide. As the magma freezes, the gases remain dissolved in a smaller and smaller amount of melt, until eventually the melt becomes saturated, and bubbles of gas start to form. From this point, the pressure inside the volcano will begin to build, and eventually, rocks around the magma chamber will crack; bubbly magma will rise through the crack to the surface and start an eruption.
But how can we find out the point at which the magma starts to grow bubbles? This is where forensic volcanology comes in. As magmas freeze, the crystals formed at different times will capture snapshots of the state of the magma reservoir beneath the volcano. With some good fortune, it is sometimes possible to find these crystals after an eruption, and piece together the sequence of events. Some minerals that are common in volcanic rock have turned out to be particularly good time capsules. The green olivine crystals that are often found in basalts can be used to tell us about the temperatures at depth, where they first formed. Often, olivine crystals may trap tiny blobs of liquid magma, deep in the bowels of the volcano. With luck, these tiny blobs will freeze to glass droplets when the olivine crystal is eventually ejected from the volcano; they can then be used to find out more about the magmas at depth.
While colossal explosive volcanic events with a global reach are a real phenomenon, with a well preserved geological record of physical traces of buried volcanic ash, they are also of a scale that has never been seen during the historical era. So, what might the next eruption of a supervolcano look like? Although there have been no supervolcanic eruptions for about 26,000 years (since the Oruanui eruption of Taupo volcano in New Zealand), we have a fair idea of what the immediate consequences of one might be, from field studies and computer simulations.The immediate physical consequences are predictable: an area of up to several thousand kilometers from the volcano will be buried under volcanic ash, disrupting lives and livelihoods. Transportation, communications, and life’s essentials—fresh water, food, warmth, shelter, energy—will be put under considerable stress, if not removed entirely. The resulting consequences for the global economy of a future supervolcanic eruption might be hard to predict—but it is difficult to imagine that the short-term consequences would by anything other than catastrophic. Worrying about the next supervolcano eruption, however, might be a little academic when we haven’t even learned how to live with volcanoes and eruptions of much smaller scale.--DMP