Volcanoes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 217

carefully done and where the analytical procedures are optimal. Lava formations
from the Taupo Volcanic Zone in New Zealand have been isotopically dated (Gam-
ble et al ., 2003). Dating of select crystalline groundmass separates provided the
most accurate dates. Less precise dates came from whole rock samples, and pure
plagioclase separates. Gamble et al .(2003)foundthat most of the K in the crys-
talline samples was held in glass or groundmass -scale sanidine crystals formed

during late-stage groundmass crystallisation. Groundmass samples could then be
classified into four categories: glassy, moderately glassy, moderately crystalline
and crystalline. The results and precision of the dating varied greatly between
samples and the success of dating events younger than 20 000 years BP was
limited (Gamble et al ., 2003).
Radiocarbon dating cannot date volcanic materials directly as it relies upon
the decay of 14 Cinrelation to the amount of 12 Cinasample of a formerly living
organism. However, radiocarbon can be used to date organisms that have met
their death due to the volcanic eruption and hence mark the actual time of the
volcanic event. Charcoal is useful in this context, for it can form when vegetation
is buried under advancing lava flows. Lava flows are of course very hot and they
will burn vegetation to charcoal relatively quickly as the lava front overrides that
vegetation. Charcoal will burn to ash when temperatures exceed 500 ◦ C. This is
the case whether oxygen is present or not. However, where oxygen is present, the
charcoal will burn rapidly once this temperature is exceeded. Although much
of the charcoal is burned to ash immediately, some charcoal can be preserved
when the availability of oxygen is restricted and the temperatures are below
500 ◦ C. While lava flows will restrict oxygen flow from the atmosphere to char-
coaled organic material, oxygen derived from highly porous, thick soils covered
by lavas can restrict the preservation of charcoal. Entire forest areas in Hawaii
have been covered by lava flows with no evidence of underlying charcoal because
of oxygen rich soils. However, in the absence of these soils charcoal can be pre-
served and used for radiocarbon dating of a relatively young or recent lava flow
(Lockwood and Lipman, 1980).
While some difficulties are inherent in the recovery of charcoal from beneath
lava flows, charcoal has a higher chance of survival when lahars bury vegetation.
Forests in the path of these flows can be destroyed and buried and not be sub-
ject to the searing temperatures associated with lava flows. Cameron and Pringle
(1991) dated forests buried by lahars generated during the eruptions of Mount
Hood in Oregon to determine the recent eruption history of this volcano. Lahars
and pyroclastic flows travelled many kilometres down rivers leading from this
volcano to bury coniferous forests growing on the valley floors and lower slopes.
Some trees were snapped off by high-velocity pyroclastic flows which may have
attained speeds of 140 km h − 1 and the trunks were transported and buried