Environmental Engineering Reference
In-Depth Information
13.2 Volcanism on Earth
The size of a volcanic eruption can be expressed in terms of the volume (km
3
)
or mass (kg) of magma produced. The Volcanic Explosivity Index (VEI)
uses volcanological data such as eruption column height and ejecta volume to
measure the relative magnitude of an eruption
s explosivity using an open-ended
logarithmic scale (except for VEI between 0 and 2) (Newhall and Self,
1982
).
The fragmentation of volatile-rich magma during explosive volcanic eruptions
produces large quantities of volcanic tephra, and generally the likelihood that
ejecta reach stratospheric altitudes increases with increasing VEI. There are,
however, examples of VEI 5 eruptions, such as the 1980 Mount St Helens
eruption, which injected little sulfur into the stratosphere, and thus had no impact
on climate (Robock,
1981
).
One of the largest eruptions in the twentieth century, the 1991 Mount Pinatubo
eruption, had a VEI of 6 and produced a bulk magma volume of about 10 km
3
.
For context, the 1815 Tambora eruption (VEI 7) had a bulk volume
>
100 km
3
.
The 1783
'
1784 Laki eruption in Iceland greatly exceeded twentieth-century lava
volumes, yielding about 15 km
3
of extrusives in eight months (Thordarson and
Self,
2003
).
Past episodes of continental
-
flood basalt (CFB) volcanism produced huge
lava volumes on the order of tens of millions of cubic kilometres. Examples
include the emplacement of the Deccan Traps, 65 Ma (Late Cretaceous;
>
10
6
km
3
); or the Columbia River Basalt Group (Western United States),
17
-
10 Ma (Middle Miocene;
210,000 km
3
),
~
including the Roza
ow,
1,300 km
3
). As also discussed in
Chapters 5
and
11
, CFB volcanism
14.7 Ma (
~
is typi
ed by numerous, recurring large-volume eruptive phases, each lasting
decades or longer. On geological timescales, the climactic pulses that build
up an entire CFB province are short-lived
typically about 1 Ma or less. For
the assessment of the environmental perturbations it is, however, important
to recognize the pulsed nature of CFB volcanism, with the duration of non-
eruptive phases (usually centuries to millennia) outlasting the duration of
individual eruptive phases (years to decades). An individual decade-long
eruptive phase would have yielded between 10
3
-
and 10
4
km
3
of extrusives
building up p
ā
hoehoe-dominated lava
fields (see
Chapter 11
for details;
Self
et al
.,
2006
;
2008
). Despite emplacing huge magma volumes, individual
eruptions would rarely have exceeded a VEI of 4 and, typically, volcanic gases
would have been injected into the upper troposphere and lower stratosphere
(see
Chapter 11
). There is, however, evidence that some CFB provinces were
associated with more violent silicic eruptions (e.g. Bryan and Ferrari,
2013
;see
also
Chapter 1
].
ow
