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WHAT makes a volcano blow its top? Earth scientists have identified many
factors that combine to cause violent eruptions, but last week a new culprit
joined the list: the sea. More than 90 per cent of all active volcanoes are
within 250 kilometres of the sea, or surrounded by sea, and my colleagues and I
have uncovered intriguing evidence, published in last week’s Nature(vol
389, p 473), that sea-level changes could significantly affect the activity of
these volcanoes.

Global sea level is gradually rising, probably because of recent warming, but
in the past, changes have been much more dramatic. Four times in the past few
million years, ice sheets have swept from the poles and covered much of the
continents. As the ice waxed and waned, locking up water and then releasing it,
the Earth’s crust and uppermost mantle sank and rebounded in response to the
enormous shifts in the distribution of weight overhead. Through a combination of
these effects, global sea level changed dramatically at the beginning and end of
these ice ages. Around 20 000 years ago, at the height of the last ice age, the
sea level was 120 metres lower than it is today.

But what does sea level have to do with volcanic eruptions? We already know
that individual ice-covered volcanoes—such as those of the
Dyngjufjöll area in Iceland—tend to erupt if the heavy ice cover
melts, releasing the confining pressures that were holding the magma in. So in
1992, my colleagues and I decided to investigate whether changes in ice cover
could also affect volcanoes far away from ice sheets, by altering the sea
level.

The study, called Project Seavolc, was a collaboration between University
College London, the Open University and Brunel University in Britain, the
universities of Milan and Calabria in Italy and the International Institute of
Volcanology in Catania, Sicily. It was funded by the European Union. We decided
to focus on explosive eruptions in the Mediterranean, particularly the volcanoes
of the Bay of Naples, Mount Etna in Sicily and the constantly erupting Stromboli
off the west coast of mainland Italy, and compare their eruption rates with
changing sea levels over the past 100 000 years.

First, we needed estimates of past eruption rates. Reliable records on land
are hard to come by—the dust and ash from explosive eruptions tend to be
washed away by heavy rains or blown away in the wind. But in the sea, falling
ash settles on the seafloor to form a distinct layer which is then protected by
subsequent layers of sediment. We looked at papers describing deep-sea cores
collected by many expeditions in the 1970s and 1980s to the Adriatic, Aegean,
Ionian and Tyrrhenian seas, and the southeast Mediterranean—where most of
the active and recently active volcanoes in the region are found. These cores
contain over 80 volcanic ash layers, all of which had been dated by Martine
Paterne of the Centre for Low-Level Radioactivity at Gif-sur-Yvette in France
and researchers from several other European institutions in the 1980s and early
1990s.

Once we knew when the eruptions had happened we could attempt to correlate
them with past sea-level changes. Fortunately, an accurate record of sea level
already exists, stretching back more than a million years. Global changes in sea
level are reflected in the oxygen isotope content of marine microorganisms that
build their skeletons from elements in seawater. During glacial periods, when
oceans are lower and colder, seawater is rich in oxygen-18. During
interglacials, when oceans rise and warm up, the proportion of oxygen-18 to
oxygen-16 falls. So measuring the ratio of oxygen-18 to oxygen-16 in skeletons
of microorganisms of known age provides an accurate measure of sea-level change.
A number of different sea-level curves have been produced using this
technique.

Armed with this information, in 1995 and 1996 two researchers in the Seavolc
team, Richard Howarth of University College London and Andy Solow of the Woods
Hole Oceanographic Institution in Massachusetts, made a statistical comparison
between the timing of the eruptions and the timing and rate of sea-level change.
They immediately noticed a clear link—greater numbers of ash layers were
deposited when the sea was rising or falling most rapidly. In particular, there
was intense explosive volcanic activity between 15 000 and 8000 years ago, when
sea levels were rising very rapidly, perhaps by as much as 12 metres in less
than 200 years. By contrast, there were many fewer explosive eruptions in less
dynamic periods, when sea levels were relatively stable—for example,
during the height of the last ice age around 18 000 to 25 000 years ago.

Some aspects of the correlation were surprising. In particular, it didn’t
seem to matter whether sea level was rising or falling—as long as it was
changing we still saw an effect. Puzzled, we began to ask more questions. What
causes the correlation? Is it a global phenomenon? What are the implications for
global climate?

We have come up with several possible answers to the first question. Nearly
60 per cent of active volcanoes form islands or occupy coastal sites, and nearly
all of the rest lie within 250 kilometres of a coastline. So changing sea levels
could directly affect the stresses inside nearly all volcanoes, helping to expel
magma explosively. To test this idea, another team member, Andy Pullen of
Imperial College, London, decided to simulate the effects of sea-level change
using a technique known as finite-element modelling. Taking the coastal volcano
Mount Etna as an example, Pullen showed that increasing the sea level by 100
metres next to the volcano weighed down and bent the adjacent crust. This
reduced stresses in the upper part of the volcano, which would allow any stored
magma to burst to the surface.

Under pressure

Geological studies at Etna between 1991 and 1994 by Mauro Coltelli of the
International Institute of Volcanology and colleagues from the University of
Milan support this model. As sea levels rose rapidly at the end of the last ice
age, more extensive and more frequent ash deposits were laid down, indicating
unusually high numbers of explosive eruptions. These layers were thick enough to
survive erosion forces and be preserved.

Next, Pullen tried modelling an island volcano, taking Stromboli as a
template. To his surprise, he discovered that rising sea level has the opposite
effect to that seen for coastal volcanoes. Because the volcano is completely
surrounded by water, raising sea levels by 100 metres actually increases the
confining pressures, reducing the ability of magma to reach the surface.
Reducing sea level by the same amount, on the other hand, dramatically reduces
the pressures acting on stored magma, making an eruption much more likely. As
there are similar numbers of coastal and island volcanoes, one might think that
an increase in the number of eruptions of one type may be offset by a decrease
in eruptions of the other, keeping the total constant.

Sudden collapse

However, these two effects are not the only ones at work. There are other
ways that changing sea levels can trigger eruptions
(see Diagram). If a volcano
is in direct contact with the ocean, for example, the sea can help to buttress
the volcano’s sides. A big drop in sea level could suddenly remove this
buttressing effect, causing a sudden landslide. This would release pressure on
the magma, allowing the volcano to blow. A large rise in sea level could have
the same effect. The sea could induce collapse of a volcano’s flanks due to
rapid marine erosion, or trigger sliding on pre-existing faults.

This may well be one of the causes of the gigantic landslides—some
involving over 1000 cubic kilometres of rock—which are now being
identified in ever larger numbers around oceanic islands such as Hawaii and the
Canaries. Indeed, as part of the Seavolc study in the 1990s, Alessandro Tibaldi
and his colleagues at the University of Milan discovered that a number of
collapses at Stromboli coincided with times of rapid sea-level change. As the
1980 eruption of Mount St Helens graphically demonstrated, removing a
substantial part of a volcano in a giant landslide is a very effective way of
triggering an explosion, by suddenly releasing the enormous pressures acting on
the magma stored inside.

Even if a volcano is not in direct contact with the ocean, it can still
experience the effects of large sea-level changes. Increasing and decreasing
water pressure causes the ocean floor to fall and rise, and this can translate
into slower-acting stress changes in the crust. Such stress changes may promote
the rise of fresh batches of magma up into the volcanoes, or trigger the
explosive expulsion of stored magma due to increased earthquake activity.

If these mechanisms are right, it seems likely that the effect is not
confined to the Mediterranean. In fact, research over the past few years by Greg
Zielinski of the Climate Change Research Center at the University of New
Hampshire, Durham, and his colleagues has revealed a similar correlation between
sea-level change and the numbers of volcanic ash layers in ice cores from
Greenland over the past 110 000 years. Putting their results with ours suggests
that sea-level changes influence volcanic activity across the world.

If so, volcanoes could in turn have a significant effect on the Earth’s
climate. It has been known since the 1950s that explosive volcanic eruptions can
influence the climate by injecting enormous quantities of sulphur aerosol
particles into the stratosphere. These tiny particles are particularly effective
at reflecting solar radiation back into space, cooling the atmosphere in the
process. Following the eruption of Pinatubo in the Philippines in 1991, global
temperatures fell by almost 0.5 °C for nearly two years, temporarily
counteracting the overall warming trend of the past few decades. So rising sea
levels, by increasing volcanic activity, could help to counteract warming. And
for the same reason, falling sea levels could also exacerbate cooling.

Trigger happy

With sea levels again on the rise, do we face a fiery 21st century and
beyond? Certainly, global sea-level rises of the scale and rapidity recorded
over the past million years are not currently on the cards, and even the most
pessimistic estimates predict rises of less than a metre over the next hundred
years or so. The problem is that we have yet to determine how big or how rapid a
change in sea level is required to trigger the eruption of a particular
volcano.

There is increasing evidence that, with at least some volcanoes, only tiny
stress changes are enough to make them erupt. Vesuvius, brooding ominously above
Naples now for over fifty years since its last eruption, seems to have a
tendency to erupt when the Moon is full, suggesting that the small stresses
caused by certain alignments of the Sun and Moon are enough to allow magma to
burst forth. This could be due to the opposing gravitational forces pulling at
the Earth’s crust. There are also tantalising hints that eruptions of Pavlof
volcano on the Alaskan Peninsula appear to be related to short-term, local
sea-level rises of only about 20 centimetres, caused by the passage of
low-pressure weather systems through the Bering Strait in winter.

With such sensitive volcanoes around, if human activity were to cause the
catastrophic melting of the icecaps, this may well be followed or accompanied by
a burst of elevated explosive volcanic activity. Nature’s way of cooling the
planet may serve as an explosive warning about the consequences if we continue
to tinker with the enormously complicated global system that is the Earth. The
message? Just cool it!