Environmental Engineering Reference
In-Depth Information
volcanoes can be fed by a crack caused by elastic bending
of a plate.
One conclusion is clear: these grand geotectonic pro-
cesses had an enormous impact on the evolution of life.
For the past 3 Ga they have made it possible to keep a
significant share of the Earth's surface well above sea
level, allowing for the evolution of complex terrestrial
forms of life. Diffusion and diversification of land biota
have been influenced by changing locations and sizes of
the continents. These changes have also created different
patterns of global oceanic and atmospheric circulation,
the two key determinants of climate. Two massive land
features that affect the climate for nearly half of humanity
(the Himalayas and the Tibetan Plateau) are direct con-
sequences of the continuing breakup of Pangea (An
et al. 2001). So is the warm northward flow of the Gulf
Stream, which moderates the climate in Western Europe.
And plate tectonics generates volcanic eruptions and
earthquakes. The overwhelming concentration of both
active volcanoes and the areas of the most frequent and
most powerful earthquakes along the subduction edges
of tectonic plates leaves no doubt about their genesis.
Volcanic eruptions have been the most important natural
source of CO
2
and hence a key variable in the long-term
balance of biospheric carbon. Intermittently they have
been also by far the largest source of aerosols that can
be injected all the way to the stratosphere and whose
high atmospheric concentrations can produce a hemi-
spheric or global cooling detectable for months or years
after an eruption (Lamb 1970; Briffa et al. 1998; Soden
et al. 2002; Robock and Oppenheimer 2003). During
the twentieth century no other natural disaster has been
responsible for more deaths than earthquakes, whose
devastating effects recur in regions that are now inhab-
ited by roughly one half of humanity.
As spectacular and devastating as they often are, volca-
nic eruptions account for a surprisingly low share of the
total geothermal release. A very liberal assumption of 30
Pm
3
of lavas ejected since the Cambrian period (ended
505 Ma ago) would imply the total cooling and crystali-
zation loss (averaging 1.7 MJ/kg) of some 170 YJ
(about 11 GW), or a mere 0.025% of the total planetary
heat loss during the same period (at least 7
10
29
J even
when assuming the current rate of 44 TW). Obvious
measures of the intensity of volcanic eruptions are the
volume of ejecta and the height of the ash column. New-
hall and Self (1982) modified Tsuya's 1955 scale using
both these criteria to construct a volcanic explosivity
index (VEI). Fedotov's more quantitative scale relates
eruptions to the rate of ejection (kg/s), the logarithm of
thermal power output, and the height of the eruption
column.
Indices less than 4 include eruptions that take place
somewhere on the Earth daily or weekly and that pro-
duce less than 1 km
3
of tephra (airborne fragments rang-
ing from fairly large blocks to very fine dust) with
maximum plume heights below 25 km. Mount St. Hel-
ens (1980) had VEI 5 (paroxysmal eruption, the same
magnitude as Vesuvius in 79
C
.
E
.), producing just 1 km
3
of ejecta (Lipman and Mullineaux 1981). Krakatau's
(1883) VEI was 6 (colossal eruption with 18 km
3
), and
Tambora's (1815) supercolossal eruption that ejected
150 km
3
of solids was the largest historic event. On
Fedotov's scale, eruptions with intensity XI (equivalent
to VEI 6) have columns reaching 28-47 km and releas-
ing thermal energy at the rate of at least 100 TW.
Analysis of a fairly complete set of VEI for continental
eruptions since 1500 shows that the mean number of
events in each category increases in the same propor-
tion as the energy released by each eruption of that
