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Summary - BA Geography (L700) Volcanology_Reading notes
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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...

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  • August 28, 2023
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Definitions

⮚ Exsolution; a process which occurs when pressure decrease below the solubility line of
volatiles coupled to magma, causing such volatiles to decouple and expand in volume as gas
bubbles that start to ascend at depth
⮚ Buoyancy; force exerted by the contrast in density immersed in a fluid

⮚ viscosity; A metric characterising the material property of a fluid linked with the amount of
deformation to a given amount of applied stress or resistance (depends on several factors,
including its chemical composition (especially silica content), the amount and condition of
included gas, the amount of solid load being carried, and its temperature (Lockwood et al.,
2010)
⮚ magma; Mobile molten rock that comprises of liquid, gas, solid, in the form of crystals and
bubbles suspended in a liquid-crystal medium referred to as silicate melt
⮚ volatile; An element or compound, such as water, sulfur, and carbon dioxide, that either
couples with magma to form a mixture consisting of gaseous bubbles under increasing
magmatic pressures and decreasing temperature otherwise crystallising as anhydrous
minerals (Edmonds and woods, 2018)
o Volatiles constitute the largest volume of material erupted by volcanoes despite
representing only a small proportion of total magma mass, and they have a critical
role in determining the eruption type and behaviour (Lockwood and Hazlett, 2010)
[explosive/effusive]
o volatiles from the mantle and crusts are dissolved in magma and exsolve gas through
processes of decompression, cooling and crystallisation as they move towards the
surface, which are finally released during an eruption or through pre-eruptive
degassing
⮚ eruption; an explosive ejection of fragmented new magma or older solidified material or
effusion of liquid lave onto Earth’s surface.
⮚ ash; volcanic particles < 2mm in diameter

⮚ overpressure; a point when the pressure within volatiles coupled to the magma is in excess
to the surrounding lithostatic pressures, leading to a runaway feedback process culminating
into an eruption (Lockwood et al., 2010)
⮚ Magma chamber; a zone of storage that receives and stores magma within the crust for an
extended period of time and, in doing so, influences the nature of the volcanic system
(Cashman et al., 2017)
⮚ Fractional crystallization; the process in which a cooling magma crystallizes out successive
minerals and in doing so progressively evolves in composition.
⮚ ‘Plinian’; a term used for towering volcanic plumes erupted from a single vent (Cassidy)



Sigurdsson, H., Houghton, B., McNutt, S., Rymer, H. and Stix, J. eds., 2015. The Encyclopedia of
Volcanoes. Elsevier.

Origin and transport of magma

,● Formation of magma occurs in certain parts of the mantle through the partial melting of
peridotite.
● Partial melting occurs through two reasons: first, the lowering of pressure in the mantle
(decompression melting), Second through the lowering of the melting temperature of peridotite
(Solidus) by the intrusion of water into the mantle during subduction.
● Melting occurs beneath plate boundaries where plates either move apart (divergence) or below
converging plates at subduction zones
● Chemical volatiles such as SO2 and H20 exsolve at lower temperatures and pressures closer to
earth’s surface as volatiles (forms a separate gas phase)
● Exsolving gas appears as growing bubbles that expand until fissure, ruptures magma in the
conduit and expel it to the surface as a volcanic eruption.
● Temperature at which melting occurs is controlled by: increasing temperature, decreasing
pressure, changing chemical composition

Rates of magma ascent and storage;

● Volcanic eruptions take in any forms, from violent explosive eruptions that produce rapidly
expanding volcanic ash to PDCs, to effusive eruptions forming lava domes and flows flowing down
volcanic flanks
● Significance of density (mass/volume) in governing the rise of magma beneath volcanoes – if
density of magma is lower than its surrounding environment then it will have positive buoyancy
and rise up the conduit and vice versa (Lockwood et al., 2010)
● As magma ascends, decompression forces water to exsolve from melt, thus resulting in
crystallisation
● The amount of volatile component that can dissolve in silicate melt (magma) increases with
temperature (solubility)
● As magma ascends towards earth’s surface, pressure decreases thereby decreasing solubility of
the volatile and causing them to exsolve (expands volume with lowering pressures in the
surrounding environment relative to internal pressures, a state of overpressure)

Volcanic eruption at surface
● Eruption style is largely determined by the rate of magma ascent and geometry of volcanic
conduit connecting with the volcanic vent at the surface of magma reservoir or shallow crust
● “During magma ascent, the majority of all volatiles are exsolved from the melt because of the
lower solubility at lower pressures. Volatile gases can remain within the melt or escape the melt
as gas through permeable surfaces. Magma ascent, no matter the rate, will allow volatiles to
exsolve”
● Explosivity of magmatic eruptions is generated during fragmentation, when the potential energy
of expanding magma is converted into kinetic energy of the gas phase and individual Pyroclasts
enhanced by thermal expansion of gas-particle mixture within volcanic plumes”

,● Fragmentation by rapid acceleration is initiated at depth by either the intrusion of new hotter
magma or volatile overpressure caused by crystallisation = creates initial increase in pressure =
sufficient enough to create a feedback from volatile expansion = further decompression of
magma, triggering more volatile expansion and exsolution
● (Explosive eruption process) large overpressure, decompression of shallow magma, exsolution
and volatile expansion counteracted by high melt viscosity, hence volatiles in the viscous melt
achieves high overpressures, magma fragments brittle under pressure

Lockwood, Hazlett and Hazlett, Richard W (2010) Volcanoes global perspectives. Hoboken, NJ:
Wiley-Blackwell.

Triggers for Volcanic Eruptions – Why Volcanoes Erupt

“triggers” are mostly related to processes that increase the buoyancy and mobility of a magma body,
and can be separated into three categories: processes taking place below magma chambers (typically
caused by transforming conditions, including melt temperature, volatile content, pressure, shape, and
size), processes within them (rise of new material migrating from storage areas or magma generation
sites in the mantle below), and processes taking place above them (a general weakening of roof crust
because of hydrothermal and fumarolic activity, and short-term non-volcanic phenomena that can
ever-so-slightly reduce confining pressure on the chambers or induce fracturing in their roofs.)

Upon initial fracturing of the chamber walls due to volatile overpressure, loss of pressure causes
additional volatiles to come out of solution (exsolve) which, because of the expansion of gases into
the fractures, forces further fracture development and propagation, reduces pressure in the
remaining melt, and causes volatile exsolution in a sustained, possibly even accelerating process.



Edmonds M, Cashman KV, Holness M, Jackson M. 2019 Architecture and dynamics of magma
reservoirs.Phil. Trans. R. Soc. A 377: 20180298. http://dx.doi.org/10.1098/rsta.2018.0298

What is a magma reservoir
“magma reservoir may be defined as a region of partially or wholly molten rock with varying
proportions of melt, crystals and exsolved volatiles”
“Seismology is an important tool to study the storage and transport of magma” – for example
“Seismic tomography studies of central volcanoes in Iceland have discovered magma storage areas at
depths of 3–6 km and deeper”
“. Fluid saturation resulting in exsolution may occur during decompression (first boiling) or during
isobaric crystallization (second boiling).”

Radiomaetic dating of crystals help tell uss about the “longevity of the reservoir and the timescales of
crystallizatio”

, Cashman, K.V., Sparks, R.S.J., Blundy, J.D., 2017. Vertically extensive and unstable magmatic systems: A
unified view of igneous processes. Science 355, eaag3055.
https://doi.org/10.1126/science.aag3055

I. relate physical processes of crystal-melt-fluid segregation to
magmatic differentiation,
II. anticipate inter-actions between melts and fluids generated and
stored at different depths,
III. determine the physical mechanisms by which magma and
associated fluids move through the crust, accumulate in shallow
chambers, and then erupt.

Focus on magma chamber and vertical connectivity of such systems

+SiO2 = viscosity, greater pressure at depth?. decreasing temp = more
solubility of volatiles = explosive?

Magma chambers heterogeneously distributed and vertically stacked
(such as Toba caldera, Indonesia; Mid-ocean ridges, Iceland) – wide
diversity in crystal phases

Temporal evolution of melts and mushes

Eruptive activity principally driven by the influx of mantle-derived basaltic melt

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