Geoscience Reference
In-Depth Information
99
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5
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0.01
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100
Eruptions per century
Probability diagram of eruptions of individual volcanoes per century (Blong, ©1984, with permission Harcourt Brace Jovanovich Group,
Australia).
Fig. 9.9
20 per cent of volcanoes erupt less than once every
100 years, and 2 per cent less than once every
10 000 years. Both the El Chichon eruption in Mexico
in March 1982, and the Pinatubo eruption in the Philip-
pines in June 1991, exemplify this latter aspect. Neither
had any historic record of eruption and both appeared
extinct. The 25 most violent volcanic eruptions occurred
with a median frequency of 865 years. Of the more
than 5500 eruptions by 1340 volcanoes since the last
glaciation, only 40 per cent are known to have erupted
in the historic past. Thus, most of the world's extinct
volcanoes could become active in the future. On
average, one extinct volcano erupts every five years.
While the above analysis shows the randomness of
eruptions, the historical record of volcanic activity
based on dust that volcanoes have thrown into the
atmosphere illustrates that eruptions are clustered
over time. The amount of dust can be evaluated accu-
rately by examining the attenuation of solar radiation
by volcanic dust and measuring the widths of tree-rings
(Figure 9.10). Professor Lamb in the United Kingdom
compiled indices of volcanic activity based on solar
radiation attenuation going back to the sixteenth
century. His index is referenced to the amount of dust
produced by the Krakatau eruption of 1883 (base value
of 1000) and is known as the
Dust Veil Index
. The
Santorini eruption in the Aegean in 1470 BC produced
an index value of 3000-10 000, while Mt Vesuvius in
79 AD generated a value of 1000-2000. Solar radiation
attenuation, lower temperatures, and increased frosts
reduce tree-ring growth. These effects are enhanced
following major volcanic eruptions, so that tree-rings
should have increased density and diminished width. A
record of the 20 lowest tree-ring density measure-
ments for the last 400 years, from Europe and North
America, is also plotted in Figure 9.10. These records
show that the eruption of Huaynaputina, in Peru in
1601, had the greatest impact on tree-rings, while the
eruption of Tambora in 1815 produced the greatest
amount of dust in the atmosphere.
There have been some truly cataclysmic eruptions
over the past 300 years. The Tambora eruption on the
island of Sumbawa in Indonesia, during its main
eruption in 1815, and the Cosequina eruption in
Nicaragua in 1835, both released four times as much
dust as Krakatau. These eruptions occurred simultane-
ously with many smaller ones, producing a period of
sustained volcanic activity beginning in 1750 and
terminating with the eruptions of Mt Pelée in
Martinique, Soufrière on St Vincent Island, and Santa
Maria in Guatemala in the Caribbean Sea area in 1902.
The tree-ring record indicates that the period between
1600 and 1710 was also one of persistent volcanism.
After the volcanic eruption of Katmai, Alaska, in 1912,
