Environmental Engineering Reference
In-Depth Information
duration of volcanic eruptions is about 7 weeks; 10% last
no longer than 1 day, most less than 100 days; a few go
on for more than a decade (Simkin 1993). Moreover, as
is the case in Hawaii, eruptions may consist of slow up-
wellings of lavas without any explosions and with energy
releases dispersed along fracture lines or over molten lava
pools.
About half a billion people live within a 100-km radius
of a volcano that has been active during the historical era,
but the number of fatalities and the extent of material
damage caused by volcanic eruptions is highly variable
(fig. 2.11). Hot lava usually spreads only over several
km
2
, ballistic projectiles fall on an area of up to 10 km
2
,
substantial tephra deposits affect areas of 10
2
-10
6
km
2
,
tsunamis generated by large eruptions can cross oceans,
and volcanic dust is transported worldwide. Conse-
quently, it is impossible to calculate any typical impact
power densities. Ejection power densities can be enor-
mous: with tephra clouds rising up to 80 km and with
initial ejection velocities up to 500 m/s (Kittleman
1979), they can be on the order of 10
8
-10
9
W/m
2
, un-
equaled by any other natural terrestrial fluxes.
There is no clear correlation between spectacular ash
plumes and the total energy released by a volcanic erup-
tion, but there is a well-established link between the mass
of dust ejected all the way into the lower stratosphere
and hemispheric or even global temperature declines
during the following months or years (Angell and Korsh-
over 1985; Robock and Oppenhemier 2003). Lamb
(1970) formulated the dust veil index (DVI) in order to
classify these climatic impacts: among the nineteenth-
and twentieth-century eruptions Nicaraguan Coseguina
(1835) had DVI 4000, Tambora (1815) 3000, Krakatau
(1883) 1000, El Chichon (1982) 800, and Mount Pina-
tubo (1991) about 2400. As for the frequency of erup-
tions, it rose from fewer than 20/a before 1800 to more
than 60/a by the late twentieth century, largely because
of improved reporting. Ammann and Naveau (2003)
analyzed sulfate spikes in polar ice and discovered a
strong 76-year cycle of tropical explosive volcanism dur-
ing the last six centuries.
Loss of life and property depends on the prevailing
form of energy release. Kilauea eruptions, with their
slow-flowing, glowing lavas, give plenty of time to evacu-
ate houses. Pyroclastic flows that swept down Vesuvius in
79
C
.
E
. and buried Pompei and Herculaneum, and those
from Mount Pel´e in 1902, which killed all but two of
28,000 people in St. Pierre on Martinique, are the two
most famous examples of instant mass burials (Sigurds-
son et al. 1985; Heilprin 1903). Because of larger popu-
lations the frequency of eruptions that proved fatal rose
from fewer than 40 per century before 1700 to more
than 200 during the twentieth century (Simkin 1993;
Simkin, Siebert, and Blong 2001). Nearly 30% of the
roughly 275,000 fatalities between 1500 and 2000 were
due to pyroclastic flows, 20% to tsunamis. The four
greatest disasters (fatalities in parentheses) were Tambora
(92,000), Krakatau (36,000), Mt. Pel´e (28,000), and
Colombian Nevado de la Ruiz in 1985 (23,000).
The creation, collision, and subduction of plates are
accompanied by frequent earthquakes: about 95% of all
tremors occur along the plate boundaries, 90% of all de-
structive seismic energy is released in subduction zones in
the Circum-Pacific Belt, and all the great earthquakes
recorded since 1900 have been caused by underthrust
subduction in South American, Alaska-Aleutian, and
Kamchatka-Kuriles-Japan zones, whereas the numerous
subduction zones in the Southwestern Pacific have expe-
rienced no large interplate earthquakes (Kanamori and
Boschi 1983; Ruff 1996). On December 26, 2004, the
