Class notes BA Geography (L700) Paper 10: Volcanology
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Course
BA Geography (L700)
Institution
Cambridge University (CAM)
This course explores volcanic processes, and their climatic, environmental and human consequences. It begins by probing what goes on beneath a volcano – the physical and chemical evolution of magma chambers; why bubbles are so important in triggering and driving eruptions, and how dramatic (and d...
This course explores volcanic processes, and their climatic, environmental and human
consequences to achieve knowledge of how volcanoes work; the nature of their eruptions
and products; the interactions between volcanism, atmosphere, climate, and environment;
and the complexities of volcanic risk management.
● what goes on beneath a volcano – the physical and chemical evolution of magma
chambers; why bubbles are so important in triggering and driving eruptions, and
how dramatic (and deadly) switches in eruptive style can occur.
● examines eruption clouds and ash deposits, and how the latter provide a record that
can be combined with tree-ring, ice core and archaeological evidence to unravel the
environmental and human impacts of eruptions in the distant past.
● broader role of volcanism in the Earth system, as well as on aspects of volcano
monitoring and risk management (including the impact of ash clouds on aviation).
Lecture 1: Introduction and eruption triggers
Explosive
● Mt Pinatubo, Philippines, 1991
● Mt Redoubt, Alaska,
o The eruption plume comes from behind the volcano because ash flow ran
along the ground and then became buoyant behind the volcano where it
lifted off the ground. The Source of plume was point 5-6 km away from
volcano.
● Soufriere Hills, Montserrat, 1994 (features: lava domes)
o Monitoring; Sound waves determine magma depth (evidence and tools
important, tiltmeter (altitude), seismometer) 🡪 detected cyclic behaviour with
a time-lag in frequency of eruptive behaviour
▪ When eruption stopped tilt increases showing magma accumulating in
volcano.
▪ As more and more pressure pushes ground above it out in both ways
so get tilt from volcano, as erupts the ground pushes down again and
so has less tilt.
o Frequency; erupted every 2 years
, ▪ produced a dome that then stopped for about 2 years, then erupted
again for 3.5 years, then stopped again for about 2 years and then
started again.
o Three main eruption episodes with three different magma bodies erupted
▪ Deep chamber which is deep down and it drives magma to a deeper
chamber and then to a shallower reservoir.
▪ As the shallow chamber wanes in pressure, eruption stops and has to
be built up by deep chamber.
o Erupted 0.5 kilometres cubed of magma
● Eyjafjallajokull, iceland, 1991
o Eruption; multiple earthquakes at about 10 km depth on 5th May.
o On 11th May earthquakes occur at 20-18 km depth for a period of 2-3 days.
They stop and then there are small earthquakes 14-15 days after.
o There were 3 eruptions at the surface associated with having 3 magma
chambers.
o Magma initially rose to surface and started erupting. After finishing eruption
pressure decreased. As that chamber decreased pressure the below chamber
started expanding as there is a big pressure gradient
● Mt Saint Helens, Washington, 1980
o main eruption was lava dome that carried on for 4 or 5 years on until 1985
from 1980. Series of discrete events where magma exploded but lava dome
gradually grew with time.
o Intermittent explosions from this volcano. Intermittent growth of magma
from this system.
o Valve of plug from surface and as pressure builds up it pushes the magma
out, as pressure drops, plug would form again and you have to build up
pressure again.
o Provided by deeper reservoir of magma - Deep source and then shallow
magma chamber, Lower chamber is gradually dropping.
o Ash cloud formation; ash spreads distances of 500 km with patterns of
deposit measuring up to 2-inch deep in the near local radius. Further
downwind, contour value decreases but see local maximum.
Basaltic
● Mt Erebus, Antarctica (Lava Lake)
● Stromboli, Italy (low viscosity)
o Forms at a subducting plate, the African plate going beneath the Eurasian as
they meet
o demonstrates a tight eruption rate of discrete explosions of gas and
fragments. Very regular cycling of every 20 mins or so. At moment many
, people try to understand models for intermittency of eruptions, due to
presence of crystals in magma at shallow depths
Submarine
● Havre Volcano, Kermadec Islands South Pacific, 2012
● Kavachi, Soloman Islands, 2022
o erupted and caused stream of pumice. Very intricate pattern on sea surface.
Pumice rafts tend to occur in submarine eruptions as pumice is full of holes,
very bubbly rock, bubbles form as magma rises from deep, when gas is in
solution, tends to float provided not waterlogged. Can get floating pumice
that travel a long way before sinking.
o In submarine environments, erupting magma is Cooled down very quickly by
water and get cooing structures as magma cooled from top.
Hot spot; Hawaiian systems of Kauai, Oahu, Molokai – formed from a mantle plume where
there has been a heat transfer from the core of the earth rises upwards and decompresses,
causing melt formation that accumulates below the crust to cause periodic eruptions of
basaltic volcanos.
- inter-plate where crust is brittle, mantle penetrates – low H20 content – effusive
eruption (lava flows, fire fountains)
Model of Krakatoa eruption
“Tank of salty water add fresh water to it. A plume develops where freshwater rises up into
salty water and reaches height where all the saltwater which reaches density of freshwater
so it spreads out as has a maximum height and spreads up to cause an extrusion.
particle plume rises to maximum height and as spread out has a different shape as particle is
falling out and develops this shape that has a sedimenting zone throughout the plume. This
is a way to model some of the shapes of eruption clouds. Red dye shows where fluid of
source goes to.”
Importance of lab experiments; Lab experiments try to understand dynamics as
experiments are very controlled , know all the conditions in the tank, real eruption don't
always know the eruption rate so trying to find out is part of the exercise, control
experiments allow us to test models
According to the type of plate boundary, different volcanic styles and eruptive behaviour
(viscosity, water content) occurs.
1. Subduction – high pressure, temperature – partial melts – silica rich, greater
viscosity - explosive eruption
2. Divergence – partial melt as plates move away – magma effuses as lava on Earth’s
surface – black/ white smokers – effusive eruption (Lava flows)
a. Difference with hot spot is that magma comes from a different origin
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