Volcanoes and Volcanic Erputions AS notes.

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Volcanoes and Tectonics
Volcanoes are associated with two types of plate boundaries:
Constructive or divergent plate boundaries ­ e.g Mid Atlantic Ridge and East Pacific
Destructive or convergent plate boundaries ­ e.g Nazca and South America plates
forming the Andes and the Japanese Volcanic Arc
Volcanoes are also associated with hot spots and can occur in the middle of tectonic plates
(intraplate) e.g. Hawaii and the Pacific plate
Why do we get volcanoes at these boundaries?
Constructive Plate Boundaries
Upwelling and diverging mantle convection currents bring mantle rocks towards the surface. As
these solid rocks rise they start to partially melt because the rock is decompressing through the
reduction in confining pressure. This is much like a chocolate chip cookie on a radiator ­ only the
chocolate chips melt and not the entire cookie.
Mantle rock (peridotite) partially melts to form basalt
The basaltic melt then rises towards the surface due to its lower density first forming a magma
chamber from where it erupts at the surface as basaltic lava.
Destructive Plate Boundaries
Converging convection currents cause two plates to collide. The denser plate of the two (colder,
older more dense oceanic crust) is pulled back into the mantle through a process called
subduction. As the subducting plate moves into the mantle it undergoes compression, which
causes earthquakes to occur. As the plate gets deeper within the mantle the crust starts to heat up
and water within the oceanic sediments and crust is driven off. This causes the melting point of the
oceanic lithosphere (crust) to partially melt. The melt then rises towards the surface again due to its
lower density where it is erupted at the surface as andesitic lava.
Basalt (Oceanic crust (lithosphere)) partially melts to form andesite

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What is the difference between basalt and andesite?
Basalt is a low viscosity lava/magma i.e. it is runny
Andesite is a high viscosity lava/magma i.e. it is sticky
What controls viscosity?
Water content, temperature, amount of gas but most importantly silica (SiO2) content
governs viscosity.
The higher the SiO2 content the higher the viscosity.…read more

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Andesitic volcanoes
Explosive ­ how explosive depending on viscosity and gas content of the magma
The common shape of an andesitic volcano is steep sided and stratocone or composite
Infrequent eruptions ­ long periods of dormancy (inactivity)
Lava if it is erupted doesn't flow very far.…read more

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BGS and island's government produced a hazard map for the island designating areas, which
were considered to be safe all the time, and those, which could only be entered at certain times or
if the conditions were correct.
They also set up a series of responses according to the hazard posed. Responses and hazard
maps changed over time after the eruptions progressively became more dangerous. This is
because the dome forming within the crater was gradually filling and overflowing its rim.…read more


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