Geoscience Reference
In-Depth Information
Volcanic hazards
HUMAN IMPACT
Volcanic eruptions pose major human and environmental hazards. Blast, flowing lava, tephra and ash ejected from
explosive volcanoes and frightening pyroclastic flows driven by ash fall-out from the atmosphere have the most direct
impacts, bringing instant death and burial of homes and farmland. Second-phase geophysical effects, interacting with
rivers or glaciers, unstable volcano slopes and the ocean produce lahars, landslides and tsunami in turn. Annual death
rates vary considerably but it is estimated that volcanic hazards have killed 250,000 people in the past 400 years.
More than half these were in Indonesia and fewer than half were by indirect means (famine, disease).
Almost all active volcanoes lie within 200 km of a coastline and much Quaternary volcanism was probably triggered
by coastal stress responding to rapid, climate-driven sea-level changes. They may, in turn, have generated climatic
feedback. Spectacular post-eruptive sunsets hint at gaseous and particulate inputs to the atmosphere, with short-
term influences on radiation and moisture balances. Annual global average yields of 1-3 km 3 of new magma and 20
Mt of related sulphur dioxide can be matched or exceeded in single large eruptions considered capable of a small
but significant role in global climatic change.
Plinian-style stratovolcano eruptions, named after Pliny the Younger's graphic account of the eruption of Vesuvius
(Italy) in AD 79, are the most violent. Blast flattened a large area of forest on Mount St Helens (Washington State,
United States) in 1980 and slope disturbance disrupted hydrological and vegetation systems, generating landslides
and debris avalanches aided by catastrophic ice melt. A cubic kilometre of erupted magma spread ash over 600 km 2 .
Pinatubo (Philippines) erupted ten times this volume of ash and a further 3 km 3 of dacite magma in pyroclastic flows
and ash columns during 1991. Herculaneum and Pompeii were destroyed by pyroclastic flows and ash falls during
the AD 79 eruption of Vesuvius, with over 25,000 casualties, and ash flows travelling at up to 500 km h -1 from Mount
Pelée in the Caribbean killed 26,000 in 1902. Lahars of liquefied ash and other debris, named from Indonesia, where
they are a major hazard, killed over 21,000 people around Nevado del Ruiz (Colombia) in 1985. Area impacts and
other destructive effects from the blast of these and other recent volcanoes often cover 10 3-4 km 2 .
uppermost 4 km above sea level. Other island shield
volcanoes are located in the Galapagos (Pacific Ocean),
Azores, Cape Verde, Madeira and Canary island groups
(Atlantic Ocean). Basalt effusion rates are more easily
assessed on land, where they appear as narrow, linear
fissures or vast flood basalts. The 100 km long Laki fissure,
where the Atlantic mid-ocean ridge goes onshore in
Iceland, discharged over 5,000 m 3 per second for several
weeks in 1793. Its sulphurous exhalations spoiled
Benjamin Franklin's visit to Paris, 2,000 km away. More
impressive still, the Late Cenozoic Roza eruption in
Oregon probably contributed peak flows of 1,500 km 3 per
week to the Columbia basalt plateau.
changes in form commonly involve low-magnitude
compaction by rock or water overburden pressures,
dewatering (dehydration), degassing and even small
thermal effects. They create textural and chemical changes
generally regarded as the final stages of lithification of
previously unconsolidated rock material. Higher tempera-
ture/pressure levels such as those experienced in tectonic
activity, however, trigger more substantial changes.
Magmatization is the ultimate response to temperature/
pressure changes in the lithosphere. Similarly, tremendous
mechanical forces generate wholesale crustal reorganiza-
tion through subduction or uplift. Both interlinked
processes transform original rock character beyond
recognition. Metamorphism alters texture or mineralogy
permanently, without a liquid phase - i.e. short of melting.
Metasomatism is minerochemical change through
infusion by high-temperature fluids and is particularly
important in oceanic crust. Migmatization represents the
extreme range of metamorphism at the boundary with
magmatization. All are further distinguishable from rock
deformation , which is mostly a mechanical effect on the
THE ROCK CYCLE (2)
METAMORPHIC PROCESSES AND
LANDSYSTEMS
Rock material is subjected during its formation to
immediate syngenesis or progressive diagenesis . These
 
 
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