Eighty years ago, 12 cubic metres of molten rock burst from the ground in the wilderness of southwest Alaska. The magma forced its way to the surface, erupting from a new volcanic vent, now called Novarupta, 10 kilometres from Mount Katmai. Gas that had been dissolved in the molten rock expanded and literally blew the volcano apart, filling the air with ash. After two-and-a-half days, when the eruption died down, a layer of ash 30 centimetres thick had fallen on the settlement of Kodiak, 170 kilometres downwind. But closer to the volcano, ash mixed with water and sediments flowed along the ground to form the strangest feature of this eruption 鈥� a valley that was, quite literally, steaming. The eruption had filled a broad glacial valley to a depth of 200 metres with hot ash, which covered the damp valley sediments; the moisture gave rise to innumerable smoking vents.
There was almost no one in the Aleutian mountains to observe what is now known to be the biggest eruption of its type for nearly two thousand years. But when news of the event reached the outside world, geologists realised that it had been something out of the ordinary. In 1915, the American geologist Robert F. Griggs led a team of three into the Alaskan wilds to see what had happened. They found the strange new landscape of hot springs, smoking vents and ash everywhere 鈥� and gave the area a new name: the Valley of Ten Thousand Smokes.
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The Novarupta eruption was one of an important class of volcano 鈥� one involving magma with a high proportion of silica (silicon dioxide, SiO2). This type of molten rock is especially thick and sticky, and its intrusion into the upper layers of the Earth鈥檚 crust leads to the most dangerous explosive eruptions. These volcanoes pose a significant threat to human life. They also bring benefits by setting up major systems of water circulation in the crust, transporting and concentrating valuable minerals and even helping to form the continental crust.
The 1915 Griggs expedition and the larger teams he led in later years produced a unique archive of information about the aftermath of this massive eruption. Geologists now want to capitalise on this wealth of data 鈥� unavailable elsewhere in the world 鈥� by taking a closer look at the workings of the volcano. Expeditions have returned to Katmai National Park, to combine mapping with geophysical exploration. But to go further, they need to drill a borehole into the heart of the volcano, to reveal its foundations. Drilling can be a messy business, but the team of volcanologists backed by the US Geological Survey, the National Science Foundation and the US Department of Energy have already shown that they can work with minimum effect in this beautiful landscape. All they need now is the permission and money to go ahead; without it the lessons from this unique volcanological laboratory will go unlearnt.
The early investigators believed that most of the magma that erupted came from a vent on Mount Katmai itself, but more recent studies show that virtually all of it came from Novarupta, a vent some ten kilometres away. Eyewitnesses reported that within hours of the onset of the eruption, two of three summits of Mount Katmai began to collapse. This important observation established the link between the two volcanic centres.
The mountain collapsed to form a large, basin-shaped crater, or caldera, 1 kilometre deep and 3 kilometres across. It is now ringed by fractures and appears to bulge in the centre. The vent at Novarupta is probably a funnel-shaped structure that grew widest at the surface by a combination of attrition during eruptions and slumping afterwards. Most of this funnel was filled by rubble, called tephra, and ash as each stage of the eruption died back. The vent 2 kilometres wide that supplied the ash for the first day鈥檚 eruption, then filled with volcanic debris and ash, contains the vents of the later stages of the eruption. The first phase of the eruption produced ash especially rich in silica, known as rhyolite. On the second and third days, ash with a distinctly lower proportion of silica emerged through a 500-metre wide vent within the rubble of the first eruption, and within that vent is the 400-metre diameter vent that produced the last phase of the eruption. Lava solidified in the vent in the final burst of eruption, leaving a plug of rock called the Novarupta Dome to mark the site.
Griggs and his teams made valuable observations and measurements in the years after the eruption, particularly on transient phenomena, such as the fumaroles. Six years after the eruption they found that some of them were still as hot as 654 掳C. The heat needed to generate and sustain the fumaroles came from the sheets of ash that had flowed across the valley. The water came from the plentiful rain and shallow ground water flowing into and over the ash layer. The resulting steam also hastens the cooling of the ash. Griggs and his team found that the fumaroles were also emitting gases that had been dissolved in the molten rock before it erupted.
Most of the fumaroles had cooled and died out by the mid-1930s except around the Novarupta vent. The few that remain today, with temperatures as high as 90 掳C, are still an attraction for weary hikers.
As geologists accumulated evidence pointing to Novarupta, not Mount Katmai as the focus of the eruption, they needed wider-ranging investigations. If there is a large hole filled with tephra or hot igneous rocks in the sedimentary basement of the Valley of Ten Thousand Smokes, researchers reasoned that geophysical methods should reveal it. Expeditions returned to Katmai in 1989 and 1990 to investigate the physics of the region where the vent is thought to lie on the basis of geology.
The team set up a grid of 150 reference points over six square kilometres spanning the position of the vent inferred from topography and patterns of fractures. They measured gravity and magnetic characteristics at each point in the grid, and left 17 sites marked so that they can return to them in the future. The team also used the grid as a reference for surveys of heat flow, resistivity, and seismic properties. In addition, the team made a low-altitude aeromagnetic survey and continued geological and geochemical investigations.
Environmental care
Any researchers working in the wilderness of Katmai must take more than usual care to minimise their impact on the environment. The 1989 expedition established a camp on the east edge of the vent, in a narrow barren valley. Helicopter use was kept to a minimum and most of the power for the equipment came from solar cells supplemented occasionally by a sound-insulated generator. Meticulous fuel management cut down on exhaust gases and food and waste were sealed in bear-proof containers. Research procedures were carefully thought out to use as few flags and surface markers as possible.
Afterwards, the markers were removed, footprints raked away, and everything, including human waste, was flown out. Park wardens conducted frequent inspections to make sure that the team was complying with environmental laws and the conditions of the permit for the research. In 1990, the team made the most of having people and helicopter support by renovating and tidying the dilapidated shelter on Baked Mountain that they used as a base.
Volcanologists understand a lot about how volcanoes work: models for intrusion, eruption, cooling and alteration are well developed and widely accepted. But hardly any of them have been tested. The best way to do so would be to drill into a volcano that has erupted recently enough to be still cooling and changing chemically. The Katmai volcanic system, with its sequence of scientific observations dating back to the time of the eruption, provides a unique opportunity to do this.
Novarupta is an ideal volcano for such a drilling project. First, it is the right size 鈥� probably the only eruption in historic times to produce a sheet of ash large enough and hot enough to weld together, and the only historic rhyolite eruption in the United States. Although eruptions between 10 and 100 times as large are know to have happened, it falls into the class of eruptions both large enough to have serious consequences and frequent enough to represent a plausible threat.
The age of the eruption is also a bonus. It is recent enough for there to have been eyewitness accounts and erosion and weathering of deposits of rubble and ash has been minimal. Most importantly from the point of view of drilling, high temperatures persist at reasonably accessible levels, perhaps within the first kilometre underground, but the surface has cooled enough not to be dangerous. The researchers hope that through drilling here they will be able to observe directly for the first time how igneous rocks cool underground. The relatively short time since the eruption also means that there will have been little weathering of the fragile minerals formed at high temperatures in the fumaroles, and relatively few volatile gases will have leaked away. This will help researchers to understand how the circulation of water and other vapours to the fumaroles carries other elements, such as the metals important for ores.
Matching patterns
The essential task in interpreting data from any borehole drilled through a volcano would be to assign the rocks and thermal and chemical gradients that the core intersects to an individual eruption. This would be a formidable problem in systems such as Mount Fuji in Japan, where small batches of magma of roughly the same composition repeatedly intrude the central conduit of the volcano. A borehole at Novarupta would be much easier to interpret. The Katmai eruption was a single burst of magma erupting through uniform sedimentary rocks at a site where there had been little previous volcanic activity, and none involving magma of the same composition. Thermal, chemical, lithological and structural observations at depth can all be related unambiguously to the 1912 event.
The combined results of the geological and geophysical investigations at Katmai strengthen the case for drilling there. The geophysical surveys in particular support the hypothesis of a funnel-shaped vent below the Valley of Ten Thousand Smokes. The magnetic properties of the fractured region differ from its surroundings and, with the gravity data, suggest twin intrusions, one coinciding with the inner vent beneath Novarupta Dome and one concealed beneath a hill known as The Turtle. Both are within the outer vent funnel.
Seismic data indicates that the ash-flow sheet is at most about 200 metres deep, quite thick enough to have stayed warm since 1912. Although most of the fumaroles that Griggs saw have vanished, patches of hot ground and weak steam vents persist today. Measurements of the heat flowing from the ground show that most of the shallow portion of the vent is cold. But there is evidence that, in places, the ash sheet retains some heat deep down. Warm ground water emerges from springs by the River Lethe in the upper part of its valley, suggesting that nearby ash is still warm.
Geochemical investigations reinforce the researchers鈥� models. Beforehand, the magma held a quantity of water equivalent to 4 per cent of its own weight. With this amount of water, the molten rock would have risen to within about 500 metres of the surface before the water boiled into steam, shattering the rock into the many tiny fragments that erupt as ash. This estimate is consistent with the surface signs that the vent lies within the upper kilometre of the ground surface 鈥� well within reach of a borehole.
The magma contained other volatile elements before it erupted. Chlorine and fluorine were only partly lost in the eruption, so there must have been plenty of them in the ash sheet after the eruption. As vapour, chlorine and fluorine could have transported metals while the ash sheet was hot, and glass within ash fragments was crystallising. The fumaroles had a large role in chemical changes after the eruption. Analysis of the chemistry of fumaroles in the ash-flow sheet indicate that much of the movement happened very early in the evolution of these systems. A borehole penetrating the ash sheet would provide evidence for how this movement actually happened.
A borehole could also help to determine how fast the Novarupta area has cooled so far, and how fast its cooling continues. Direct measurements in a borehole, combined with estimates from the groups of minerals formed at different stages in the evolution of the system will help the researchers to establish both the rates and mechanisms of cooling. The problem is simplified because the volcano itself is simple and its age is known precisely.
The researchers plan to tackle these questions by drilling twice into the volcano. One hole will slant across the main vent and finish in the wall of the crater, a path that will enable researchers to measure how far and how fast the heat from the 1912 eruption has spread. A second hole will be vertical, and deep enough to penetrate the hot root of the volcano. In addition, there will be a third core into the ash-flow sheet. This will not be as deep as the other two, but it will reveal characteristics of the hidden interior of the ash.
The drilling programme that researchers hope for should lead to a much better understanding of the eruptive dynamics of silicic volcanism and the transport of elements in vapour. Researchers need quantitative understanding of the processes in a young, simple system before we can hope to understand older or more complex volcanic events. The Valley of Ten Thousand Smokes is a unique geological laboratory that should not be allowed to go to waste.
James Papike is director of the Institute of Meteoritics at the University of New Mexico. John Eichelberger is a researcher at the University of Alaska.
Further Reading The August 1991 issue of Geophysical Research Letters includes 15 papers about the Katmai investigation.
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DRILLING IN A PROTECTED WILDERNESS
The core from a borehole into a volcano is tiny compared to the whole volcanic system, but it can provide a continuous sample through the structure. By drilling, geologists can take measurements and sample rocks and gases at various depths, and relate these to the thermal and chemical evolution of the volcano. Moreover, drilling provides a means of sampling materials and measuring conditions at known times after magma movement, as igneous systems move toward thermal and chemical equilibrium.
But Katmai is a protected area, and the drilling proposed there is a major research project that will bring in people and machinery. It can be justified only if the research rewards cannot be gained elsewhere. All stages in the planning process have had to take into account the likely impact on the surroundings as well as the unique qualities of the scientific resource.
The project is part of the US Continental Scientific Drilling Program. It is funded jointly by the Department of Energy, the National Science Foundation and the US Geological Survey. It is guided through their Interagency Coordinating Group (ICG) for Continental Scientific Drilling.
The project began formally in January 1987 at a workshop in Durango, Colorado, attended by about 80 scientists. The ICG accepted the concept of the project in 1988, appointed a Science Experiments Panel to guide further modifications, and completed the existing science plan in 1989.
It is now at a stage between the surface investigations and the proposed drilling. The science plan has been reviewed by the National Academy of Sciences, and formed the basis for the Katmai Operations Plan, which describes in detail the logistical and engineering aspects of the drilling.
The environmental impact of the drilling plans is now being evaluated under the National Environmental Policy Act. The National Park Service, as the manager of Katmai National Park, must judge whether or not drilling may take place on protected public lands.
In 1989, the National Academy of Sciences set up a Panel on Volcanic Studies at Katmai, at the request of the Park Service and with funding from the ICG. The panel gave its support to the science plan鈥檚 statement that the 1912 vent is uniquely suited to a drilling investigation of explosive volcanism. Now the researchers are waiting for approval and funding for the final stage of this innovative project.