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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…

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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…

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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. Usually it forms…

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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…


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