Volcanoes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 216

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0 000000607080
Depth (cm)
Figure 8.1. Tephra chronologies from Skilake Lake, Alaska over the last 500 years
(from Beget et al. , 1994 ).
other areas, and using other methods, can provide a more complete picture of
prehistoric volcanic eruptions (Beget et al ., 1994).
Stratigraphic sequences can also reveal the nature of volcanic eruptions and
in particular, the relative violence of those eruptions. Siebert et al .(1995)exam-
ined tephra, debris avalanche and pyroclastic layers to reveal a history of sum-
mit collapse at Augustine volcano, Cook Inlet, Alaska. Here, 100--500 m of the
volcano summit collapsed three times over the past 500 years during major vol-
canic eruptions. The presence of debris avalanche and lateral blast deposits at
Mt Augustine also suggests that tsunamis may have been a hazard in the past
and will again in the future. The avalanche and lateral blast deposits on Augus-
tine Island, and also the nearby West Island, indicate high-frequency, cyclic col-
lapse and regrowth of the volcano. The deposits indicate a recurrence interval of
about 150 years and at least 11 collapse episodes have been identified for the past
1800--2000 years (Siebert et al ., 1995). The regularity and severity of these events
over this time suggests that similar events will almost certainly occur in the near
future and could cause considerable impact upon local human populations.
Isotope and radiocarbon dating
Direct dating of lavas and pyroclastic deposits is usually undertaken
using Argon--Argon ( 40 Ar-- 39 Ar) and Potassium--Argon (K--Ar) dating. However,
these techniques are not useful for estimating chronologies for young volcanic
formations (<1 Ma) as these isotopes have relatively long half-lives. This is partic-
ularly the case for arc volcanoes as these usually have low potassium (K) content.
However, dating can be accurate where the sample selection and preparation is