Volcanoes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 202

8
Volcanoes
historical volcanoes
Volcano and eruption characteristics
More than 90% of all volcanoes are associated with Earth's plate tectonic
boundaries. Hot spots account for the remaining 10%. The most explosive volcan-
ism occurs where plates converge, principally at subduction zones, but over 80%
of magma released from the mantle by volcanic eruptions occurs at oceanic
spreading centres (Abbott, 1999). Eruptions at spreading centres and over hot
spots tend to be less violent than at convergent plate margins. This is princi-
pally because the former locations sit over the high-temperature asthenosphere
where the rock has low densities of SiO 2 . The pulling apart action by the oceanic
plates here makes the upward movement of magma through fissures easier. The
basaltic magma that rises at oceanic spreading centres releases gas easily and
produces lava flows rather than explosive eruptions. This magma also has a high
temperature with low viscosity. Because of these characteristics, these types of
volcanic eruptions are the least hazardous to humans. Magma that is released
at subduction zones incorporates subducted crust resulting in a higher water
and SiO 2 content. It also has a higher viscosity (resistance to flow in a liquid)
and explosive potential as gas is trapped in the magma and not easily released.
Hot spots are located where plumes of slowly rising mantle rock occur. This
mantle rock is considerably hotter than the surrounding rock resulting in the
mantle plume having a lower viscosity allowing it to rise. Tectonic plates ride
over hot spots resulting in chains of volcanic islands marking the trace of the
plate movement. The Hawaiian Islands are a classic example of such an occur-
rence and here the western islands of the chain are the oldest and least volcani-
cally active as they have moved away from the hot spot. Hot spots can occur
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