1993 Plinian eruption of Lascar volcano, Chile
Nine percent of the world's population lives within
one hundred kilometres of an active volcano. For these
people, the most important questions are "when
will the volcano erupt?" and "what will
happen when it does?". My research addresses
this second question: the question of volcanic eruptive
Volcanoes show a broad spectrum of eruptive behaviour,
from violent, explosive eruptions, (e.g. Vesuvius,
79AD; Krakatoa, 1883; Mt St Helens, 1980) to more
gentle, effusive eruptions such as those characteristic
of Hawaiian volcanoes and Etna in Italy.
A pyroclastic flow, head-on!
Explosive eruptions are amongst the most hazardous
of natural phenomena. They throw large volumes of
ash and pumice upwards in an eruption column, which
may punch tens of kilometres into the atmosphere,
spreading ash over a wide area.
Explosive eruptions pose a very significant hazard
to the surrounding population. Pyroclastic flows -
avalanches of hot rock, ash and pumice - may travel
many kilometres from the volcano. These flows can
scour sturdy buildings to their foundations and have
been responsible for some of the worst natural disasters
of the last century.
A lava fountain in Hawai`i
At the other end of the scale, effusive volcanic
eruptions are much gentler, and tend to produce lava
fountains and lava flows. Whilst these may be very
spectacular, they pose much less of a risk to human
life. The economic impact of an effusive eruption
may still be significant, however, as lava flows can
destroy buildings and infrastructure, and may leave
agricultural land sterile for centuries.
A lava dome at Mt. St. Helens
Another type of low-energy eruption is the extrusive
eruption. This type of eruption produces a lava dome
- a mound of almost-solid lava that builds up over
the top of the volcanic vent. This can fall away unpredictably,
breaking up to form a pyroclastic flow similar to
those created by explosive eruptions.
The distinction between "effusive / extrusive"
and "explosive" eruption is not as straightforward
as it might seem. Not only is there a full spectrum
of types of eruption between these two cases, many
volcanoes will produce both effusive and explosive
eruptions during their lifetime, sometimes during
a single eruptive episode. Some volcanoes, such as
the Soufriere Hills volcano on the island of Montserrat
in the West Indes, switch unpredictably between the
extrusive formation of a lava dome, to explosive eruption,
over very short timescales.
Figure 1: A schematic
model of an erupting volcano
The degree and nature of the hazard presented by an
eruption is initimately related to the style of the
eruption. Effective hazard management must be built
upon a good understanding of the physical origins
of the hazard itself. I work towards explaining why
some eruptions are explosive, some are effusive, and
some switch between the two. In particular, I aim
to show what physical properties of the magma, and
what physical processes within the volcano, are responsible
for determining the style of the eruption. In order
to do this, we first need to build a general physical
model for a volcanic eruption.
Volcanic eruptions are driven by buoyancy forces arising
from the growth of bubbles of magmatic gas. Figure
1 shows a schematic cross section through a volcano
during an explosive eruption. Deep in the earth, pressures
are high enough to dissolve gases such as water (which
is a gas at these high temperatures) and carbon dioxide
into the molten magma. As magma rises, the pressure
drops and the gas becomes less soluble. When the pressure
drops low enough, some of the gas will come out of
solution, forming bubbles in a process called nucleation.
The bubbles cause a dramatic decrease in the density
of the magma, causing it to rise buoyantly. As the
magma rises further, the pressure drops and more gas
comes out solution, making the magma more bubbly.
The pressure drop also causes the bubbles to grow
decompressively. These effects combine to make the
magma less dense, causing it to rise more rapidly.
This begins a positive feedback of buoyant rise, decompression,
bubble growth and decreasing density, which drives
the magma to the surface, where it erupts.
The upward acceleration and expansion of the magma
gives rise to immense stresses within the magma. If
the stresses are large enough, they can overcome the
internal strength of the magma, causing it to fragment
into ash and pumice. This fragmentation of the magma
is the defining characteristic of an explosive eruption.
The simple physical model of volcanic eruption outlined
above is likely to be broadly applicable to most eruptions.
The question then arises: what features of the model
control the style of eruption? It is becoming clear
that much of the eruptive behaviour of a volcano is
controlled by properties and processes that operate
at the meso-scale: the scale of bubbles and crystals
within the melt. To-date, this area has been somewhat
neglected, as volcanologists have focussed their attention
on two extreme length scales: at the volcano scale
(field volcanologists, conduit flow and eruption modellers,
hazard managers) or at the micro-scale (petrologists,
My volcanology research comprises several parallel
and complementary investigations of fundamental magmatic
properties and processes at this mesoscopic scale.
These investigations combine laboratory experiments
and analytical and computer modelling. My aim is to
bridge the gap between measured properties of the
magma and the eruptive behaviour at the volcano scale.
My current research encompasses the following themes
- links take you to a more detailed (and technical!)
description of each topic:
This describes the way a material flows in response
to a stress. Magma and lava are usually a "multi-phase"
mixture of liquid (molten rock), crystals and
bubbles. This gives them a complex rheology which
influences the way they erupt and flow. I use
a combination of laboratory experiments and computer
simulations to investigate this rheology and assess
the implications for volcanic eruptions.
permeability: During an eruption,
magma may become permeable, allowing gas to escape
from the bubbles within the magma. The rate of
gas escape is an important factor in determining
how violent an eruption will be. I have developed
numerical models to explore how permeability develops
during an eruption.
- Stromboli and other persistently active basaltic volcanoes : Many basaltic volcanoes erupt persistently for decades or centuries. The behaviour of these volcanoes is dominated by the rise of gas bubbles through the relatively low-viscosity magma. I have used laboratory experiments to develop physical models that quantify aspects of this behaviour, including explosivity of strombolian eruptions, and the stability of lava lakes.
- Magma mingling: Eruptions are often triggered when a fresh batch of magma enters a magma chamber. This work uses a variety of experimental and analytical approaches to understand the timescales over which mingling of magmas causes eruptions.
- Fault slip: The numerical models I have developed to investigate magma permeability have application in other geoscience problems. Fault slip in thermally unstable rocks is an example.