Environmental Engineering Reference
In-Depth Information
modelling efforts to test whether, and to what degree, CFB eruptions caused or
contributed to environmental changes in the geological past.
11.2 Volcanic plume rise during CFB volcanism
Studies of modern eruptions, such as Pinatubo in 1991, suggest that the short-term
climatic effect of an eruption is greatest if gases such as sulfur dioxide (SO
2
) reach
the stratosphere (see also
Chapters 10
,
12
and
14
for discussion of other gases
released). In general, basaltic lava compositions that characterize CFB volcanism
erupt less explosively than their more silicic counterparts, resulting in lower
eruption columns. It is therefore important to understand whether, and under what
circumstances, basaltic eruption columns from both modern-day and CFB erup-
tions can reach the stratosphere. Early attempts were made by Stothers
et al
.(
1986
)
and Woods (
1993
), suggesting that near-stratospheric heights could be attained by
high-intensity basaltic eruptions. More recent studies in this area have built upon
and improved the formulation of these earlier models, re
ning plume-height
estimates for linear vent sources (Glaze
et al
.,
2011
).
In addition to SO
2
and other volcanogenic volatiles, volcanic eruptions are
capable of redistributing large volumes of water from the lower atmosphere into
the stratosphere (Glaze
et al
.,
1997
). A 25-km-high eruption column rising through
a wet, tropical atmosphere can transport up to 4 Mt of H
2
O per hour up through
the plume (we use the term
'
plume
'
to describe the vertical eruption column
and downwind ash/gas cloud). This water, if injected at the top of the plume at
stratospheric heights, may be important in generating stratospheric aerosols (Textor
et al
.,
2003
). Carbon dioxide (CO
2
) is also released during basaltic volcanism but,
due to its longer atmospheric lifetime, the height to which it is injected is not critical
for its atmospheric residence time and environmental impacts.
11.2.1 Modern observations of buoyant plumes from basaltic volcanism
Observations in the literature have occasionally noted sustained buoyant plumes
associated with historic basaltic
fire-fountain events (
Figure 11.2
). While the
overall scale is much smaller than CFB volcanism, basalt magma should behave
similarly in terms of the individual eruptions. An important and well-documented
example is the Pu`u `
Ō
`
ō
eruption, K
ī
lauea, Hawai`i. Early episodes in 1983 and
1984 generated multiple
fire-fountains, with typical heights of 100
-
200 m, occa-
sionally up to
7-km height above sea level
(ASL). The 1984 eruption of Mauna Loa, Hawai`i, produced somewhat larger
~
400 m, and sustained plumes of 5
-
fire-fountains (up to 500 m high) along a 2-km-long active
ssure that generated