upper slopes and may achieve 60 kph. Deaths by lava are rare in today’s world but
property damage can be immense if lava enters an urban area.
Earthquakesare associated with eruptions and some of these can be major
causing widespread destruction. These short-periodicity motions of Earth’s crust
can result from the rearrangement of magma through injection or extraction of
magma causing crustal rocks to flex and break. Eruptions are usually heralded by
multiple earthquakes.
Volcanic gases and ash vented during explosions are nothing like the composi-
tion of normal air. Water vapor is the most prevalent volcanic gas, but toxic sulfur
dioxide, fluorine, and hydrogen sulfide are also present. Jet aircraft turbines are
easily fouled by ash and eruptions are scrupulously avoided. A frequent event in
an explosion is a pyroclastic flow, which is a lethal mix of gases, ash, and rock
fragments barreling down mountain slopes. Pyroclastic flows can travel at over
160 kph so that there is no escape. Famously, pyroclastic flows obliterated the
Roman city of Pompeii in AD 79 and killed 20,000 people in Amero, Columbia
in 1985.
Volcanic eruptions sometimes devastate low-lying areas by means of lahars,
volcanic mudflows. In the 1980 eruption of Mt. St. Helens, Washington, the
glaciers on the volcano’s flanks catastrophically melted sending a steaming lahar
through the Toutle River valley to the Columbia River over a hundred kilometers
away.
Knowledge ofplate tectonicsprovides scientists with fair knowledge as to the
geography of volcanism around the planet. Large volcanoes and associated forms
are concentrated along plateboundariesand in hot spots. Although scientists
know much about volcanoes and keenly observe precursors such as mountain
bulging and small earthquakes, there is no method by which to predict the time
or severity of eruptions. Such efforts are ongoing in light of the many millions of
persons living in volcanic hazard zones.360 Vulcanism