Environmental Engineering Reference
In-Depth Information
magnitude decreases. This means that over long periods
of time randomly occurring eruptions have a nearly con-
stant release of thermal and kinetic volcanic energies
for each energy magnitude category; De la Cruz-Reyna
(1991) calculated it to be equal to about 36 PJ a year.
Invariably, heat is the dominant energy form, at least 10
and up to 1,000 times greater than the other releases,
and hence a careful estimate of heat gives a good idea of
the proper order of magnitude of an eruption's total
energy.
Hedervari (1963) expressed the thermal energy (E
TH
)
In contrast, a virtually identical amount of energy was
released by the 1950 Mauna Loa eruption (1.4 PJ), but
there was no high ash column, just massive flows of
hot lavas characteristic of the Hawaiian type of eruptions.
The Bronze Age Minoan eruption in the Aegean Sea,
about 3,650 years ago, was the largest release of volcanic
energy during the historic period. Its 100 EJ created the
great Santorini caldera (surrounded by the islands Thera,
Therasia, and Aspronisi) by ejecting about 70 km
3
(Fried-
rich 2000). Iceland's Laki released 86 EJ in 1783, Tam-
bora 84 EJ in 1815, Nicaragua's Coseguina 48 EJ in
1835, Krakatau 30 EJ, and Alaska's Katmai 20 EJ
in 1912.
Historic eruptions are dwarfed by VEI 8 (mega-
colossal) events, most recently the creation of the giant
Toba caldera (an oval roughly 30 km by 100 km filled
by a lake) in northern Sumatra about 75,000 years ago,
which produced about 2800 km
3
of ejecta (Rose and
Chesner 1990). This catastrophe is perhaps the best
explanation for the late Pleistocene population bottle-
neck, when small and scattered groups of humans were
reduced to fewer than 10,000 individuals and when our
species came very close to ending its evolution (Rampino
and Self 1992; Ambrose 1998). And in terms of total
lava flow, Toba was a small event in contrast with the for-
mation of the massive Indian and Siberian basalts. Dec-
can Traps (formed 65-60 Ma ago) contain more than
500,000 km
3
of basalt, Siberian Traps (formed @250
Ma ago) about 1.6 million km
3
(Renne and Basu
1991). These massive effusions were not formed by vol-
canic eruption but by prolonged floods of basaltic lavas
accumulating in layers.
Assumptions must be made in order to derive the an-
nual rate of energy released by volcanic eruptions. Elder
(1976) estimated the total annual rate of volcanic heat
as
E
TH
¼
Vd
ð
CT
þ
B
Þ
;
where V is the rate of extrusion of rocks, d their mean
density, C the specific heat of lava (1.25 J/g
C), T its
temperature above the ambient level, and B its heat of
fusion (207.8 J/g). Other energies include the change
in the height of the magmatic column (potential energy),
kinetic and thermal energy of lavas and pyroclastic flows
(mixtures of hot rocks and gases whose high tempera-
ture, up to 700
C, and high speed, more than 80 km/
h, are highly destructive), and three more forms of ki-
netic energy releases: seismic waves of the accompanying
earthquakes, power of the associated tsunamis or air
shock waves, and energy fractioning or deforming the
surrounding crust.
The world's best-monitored volcanic eruption, that of
Mount St. Helens, Washington, on May 18, 1980, pro-
vided an excellent illustration of the dominance of ther-
mal energies (Decker and Decker 1981): heat amounted
to 96% (1.73 EJ) of the total energy release (fig. 2.11).
Ash cloud, accounting for nearly half of all thermal en-
ergy, had enough buoyancy to rise to more than 20 km.
