Environmental Engineering Reference
In-Depth Information
dormant, do not break surface. Those which do often form long volcanic island chains,
best seen in the Hawaiian and other Pacific islands. Slow, intermittent sea-floor effusion
of magma above a 'geostationary' hot spot has studded the main Pacific plate with a line
of volcanoes 6000 km long in its journey north-west during the past 70 Ma. A shift in
plate direction away from the East Pacific Rise, adjusting to neighbouring plates
c
. 40 Ma
ago, realigned islands in the Hawaiian and adjacent chains. Hawaii itself is at the active
point in the chain. Its shield volcanoes of Mauna Loa, Kilauea and three others rise 10 km
from the sea floor at shallow angles, due to low-viscosity
Figure 12.7
A reconstruction of Ordovician fissure eruptions
and caldera collapse, Snowdonia, North Wales.
Source: After Howells et al. (1991).
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 (see Colour Plate 8 between pp. 272 and 273). 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
