Hi I'm Anna, an Earth Sciences PhD
student at the University of Oxford.
I'm interested in volcanoes and why
sometimes they erupt with a really big explosion
but sometimes with a smaller
and much less dangerous one.
One thing we think is really important
in figuring out how explosive an
eruption will be is the amount of
pressure that builds up beneath the volcano.
If we can understand a bit more
about these levels of pressure, and how
they might change, this can help us to
understand the scale and type of the next eruption.
The island of Montserrat in the Caribbean is a tropical paradise.
But in 1995 the Soufrière Hills volcano on
the island started to erupt.
This eruption destroyed the capital city of
Plymouth and forced two-thirds of the
islands residents to flee. Most of the
destruction was caused by short-lived
volcanic explosions and pyroclastic
flows. These are super-hot mixtures of
volcanic gas and rock that can travel at
speeds up to hundreds of kilometres an
hour. But between these bouts of epic
destruction the volcano goes through
much quieter phases, slowly pushing out
lava into a dome at the surface, or just
pumping out gas through cracks in the
summit.
Deep within the Earth's crust is a liquid we call magma.
This magma is
what we eventually see erupted at the
surface as lava. Within this magma,
there are different amounts of mineral
crystals, like quartz, and also bubbles of
gases, such as carbon dioxide and water.
Let's keep things simple and imagine
that all of this magma is stored within
one big magma chamber, or reservoir.
Sometimes the pressure within this
reservoir increases and so the magma
pushes outwards on the rocks surrounding
it. If the pressure of the magma is too
great, then the crystal rocks around it
can fail and break, and then the magma
rushes to the surface in an explosive
eruption - just like what happened in the
1980 eruption of Mount Saint Helens in
the United States of America.
A volcano like Soufrière Hills on Montserrat,
even after the magma has found a pathway
to the surface, the pressure within that
magma reservoir can still vary depending
on how much new magma arrives from
deeper in the crust.
Sometimes, the pressure in the magma reservoir is
balanced out by the pressure exerted by
the column of magma above it, and then in
these cases the volcano is quiet and
relatively safe. But, with little warning,
these pressures can get out of balance
and the volcano can explode - sometimes
with fatal consequences.
At different depths within the volcanic system, the
magma is under different amounts of pressure.
We can estimate these pressures using the equation:
pressure equals height times density times gravity.
So in our example, the pressure within the
magma reservoir is equal to the height
of the magma column above it multiplied
by the density of the partially liquid
magma multiplied by the force of gravity.
This is just like you working out how
much pressure you experience when you dive to the bottom of the swimming pool.
Magmas, and the crystals and bubbles
within them, behave differently at the
different pressures and temperatures
beneath the volcano. For my PhD project I
used this equipment to do experiments to
work out how the crystals within magma
change as they rise from deep within the
Earth's crust up to the Earth's surface.
Here is some volcanic rock called pumice
which interrupted from the Soufrière Hills
volcano on Montserrat.
I grind up this rock, put it into these
capsules, and load them into these
pressure vessels. Then I turn this valve
to create high pressures, which forces
water through these little pipes and
onto a really small area in contact with
my capsule of rock. As pressure equals
force divided by area, this is a really
efficient way to focus the force of the
water and increase the pressure on my
rock sample. As well as applying high
pressures, these furnaces also heat the
rock up to hundreds of degrees Celsius -
just like real magma.
Once my experiment is finished, I cool
and depressurize the system and take out
my rock. Now it's no longer a fine powder
but a fragment of glass, bubbles and
crystals. So now I can look at the shapes
and numbers of the crystals and bubbles
within this sample to tell me more about
how the magma will move beneath the volcano.
Hopefully one day this will help us to
make better predictions about when a
volcano is next likely to erupt, helping
to save lives.
