It has been said that the science of “volcanology” originated with the
accurate descriptions of the eruption of Vesuvius in A.D. 79 contained
in two letters from Pliny the Younger to the Roman historian Tacitus.
Pliny’s letters also described the death of his uncle, Pliny the Elder,
who was killed in the eruption. Actually, however, it was not until the
19th century that serious scientific inquiry into volcanic phenomena
flourished as part of the general revolution in the physical and life
sciences, including the new science of “geology.” In 1847, an
observatory was established on the flanks of Vesuvius, upslope from the
site of Herculaneum, for the more or less continuous recording of the
activity of the volcano that destroyed the city in A.D. 79. Still,
through the first decade of the 20th century, the study of volcanoes by
and large continued to be of an expeditionary nature, generally
undertaken after the eruption had begun or the activity had ceased.
[Illustration: The U.S. Geological Survey’s Hawaiian Volcano
Observatory, on the crater rim of Kilauea Volcano.]
Perhaps “modern” volcanology began in 1912, when Thomas A. Jaggar, Head
of the Geology Department of the Massachusetts Institute of Technology,
founded the Hawaiian Volcano Observatory (HVO), located on the rim of
Kilauea’s caldera. Initially supported by an association of Honolulu
businessmen, HVO began to conduct systematic and continuous monitoring
of seismic activity preceding, accompanying, and following eruptions, as
well as a wide variety of other geological, geophysical, and geochemical
observations and investigations. Between 1919 and 1948, HVO was
administered by various Federal agencies (National Weather Service, U.S.
Geological Survey, and National Park Service), and since 1948 it has
been operated continuously by the Geological Survey as part of its
Volcano Hazards Program. The more than 75 years of comprehensive
investigations by HVO and other scientists in Hawaii have added
substantially to our understanding of the eruptive mechanisms of Kilauea
and Mauna Loa, two of the world’s most active volcanoes. Moreover, the
Hawaiian Volcano Observatory pioneered and refined most of the commonly
used volcano-monitoring techniques presently employed by other
observatories monitoring active volcanoes elsewhere, principally in
Indonesia, Italy, Japan, Latin America, New Zealand, Lesser Antilles
(Caribbean), Philippines, and Kamchatka (U.S.S.R.).
What does “volcano monitoring” actually involve? Basically, it is the
keeping of a detailed “diary” of the changes—visible and invisible—in a
volcano and its surroundings. Between eruptions, visible changes of
importance to the scientists would include marked increase or decrease
of steaming from known vents; emergence of new steaming areas;
development of new ground cracks or widening of old ones; unusual or
inexplicable withering of plant life; changes in the color of mineral
deposits encrusting fumaroles; and any other directly observable, and
often measurable, feature that might reflect a change in the state of
the volcano. Of course, the “diary” keeping during eruptive activity
presents additional tasks. Wherever and whenever they can do so safely,
scientists document, in words and on film, the course of the eruption in
detail; make temperature measurements of lava and gas; collect the
eruptive products and gases for subsequent laboratory analysis; measure
the heights of lava fountains or ash plumes; gage the flow rate of ash
ejection or lava flows; and carry out other necessary observations and
measurements to fully document and characterize the eruption. For each
eruption, such documentation and data collection and analysis provide
another building block in constructing a model of the characteristic
behavior of a given volcano or type of eruption.
Volcano monitoring also involves the recording and analysis of volcanic
phenomena not visible to the human eye, but measurable by precise and
sophisticated instruments. These phenomena include ground movements,
earthquakes (particularly those too small to be felt by people),
variations in gas compositions, and deviations in local electrical and
magnetic fields that respond to pressure and stresses caused by the
subterranean magma movements.
Some common methods used to study invisible, volcano-related phenomena
are based on:
1. Measurement of changes in the shape of the volcano—volcanoes
gradually swell or “inflate” in building up to an eruption because of
the influx of magma into the volcano’s reservoir or “plumbing system”;
with the onset of eruption, pressure is immediately relieved and the
volcano rapidly shrinks or “deflates.” A wide variety of instruments,
including precise spirit-levels, electronic “tiltmeters,” and
electronic-laser beam instruments, can measure changes in the slope or
“tilt” of the volcano or in vertical and horizontal distances with a
precision of only a few parts in a million.
2. Precise determination of the location and magnitude of earthquakes by
a well-designed seismic network—as the volcano inflates by the rise of
magma, the enclosing rocks are deformed to the breaking point to
accommodate magma movement. When the rock ultimately fails to permit
continued magma ascent, earthquakes result. By carefully mapping out the
variations with time in the locations and depths of earthquake foci,
scientists in effect can track the subsurface movement of magma,
horizontally and vertically.
[Illustration: Scientist, wearing asbestos gloves and gas mask, samples
volcanic gases from active vent.]
3. Measurement of changes in volcanic-gas composition and in magnetic
field—the rise of magma high into the volcanic edifice may allow some of
the associated gases to escape along fractures, thereby causing the
composition of the gases (measured at the surface) to differ from that
usually measured when the volcano is quiescent and the magma is too deep
to allow gas to escape. Changes in the Earth’s magnetic field have been
noted preceding and accompanying some eruptions, and such changes are
believed to reflect temperature effects and/or the content of magnetic
minerals in the magma.
Recording historic eruptions and modern volcano-monitoring in themselves
are insufficient to fully determine the characteristic behavior of a
volcano, because a time record of such information, though perhaps long
in human terms, is much too short in geologic terms to permit reliable
predictions of possible future behavior. A comprehensive investigation
of any volcano must also include the careful, systematic mapping of the
nature, volume, and distribution of the products of prehistoric
eruptions, as well as the determination of their ages by modern isotopic
and other dating methods. Research on the volcano’s geologic past
extends the data base for refined estimates of the recurrence intervals
of active versus dormant periods in the history of the volcano. With
such information in hand, scientists can construct so-called “volcanic
hazards” maps that delineate the zones of greatest risk around the
volcano and that designate which zones are particularly susceptible to
certain types of volcanic hazards (lava flows, ash fall, toxic gases,
mudflows and associated flooding, etc.).
A strikingly successful example of volcano research and volcanic-hazard
assessment was the 1978 publication (Bulletin 1383-C) by two Geological
Survey scientists, Dwight Crandell and Donal Mullineaux, who concluded
that Mount St. Helens was the Cascade volcano most frequently active in
the past 4,500 years and the one most likely to reawaken to erupt, “...
perhaps before the end of this century.” Their prediction came true when
Mount St. Helens rumbled back to life in March of 1980. Intermittent
explosions of ash and steam and periodic formation of short-lived lava
domes continued throughout the decade. Analysis of the volcano’s past
behavior indicates that this kind of eruptive activity may continue for
years or decades, but another catastrophic eruption like that of May 18,
1980, is unlikely to occur soon.
On 18 May 1982, the U.S. Geological Survey (USGS) formally dedicated the
David A. Johnston Cascades Volcano Observatory (CVO) in Vancouver,
Washington, in memory of the Survey volcanologist killed two years
earlier. This facility—a sister observatory to the Hawaiian Volcano
Observatory—facilitates the increased monitoring and research on not
only Mount St. Helens but also the other volcanoes of the Cascade Range
of the Pacific Northwest. More recently, in cooperation with the State
of Alaska, the USGS established the Alaska Volcano Observatory in March
1988. The work being done at these volcano observatories provides
important comparisons and contrasts between the behavior of the
generally non-explosive Hawaiian shield volcanoes and that of the
generally explosive composite volcanoes of the Cascade and Alaskan
Peninsula-Aleutian chains.