Volcanoes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 218

further down slope. Where slopes were shallower and flow velocities lower, the
flows buried the roots and lower trunks of the trees and the remains of these
forests remain in situ .Six buried forests have been found in this region result-
ing from three separate eruption events over the past 2000 years. Trees buried
by these separate eruptions returned radiocarbon ages of 1700 ± 700, 560 ± 150

and 455 120radiocarbon years BP. Two of these erup-

tions clearly occurred over the last 500 years with the last occurring from approx-
imately AD 1760 to1810.

± 135, 260 ± 150 and 185 ±

Desktop studies

Desktop, as opposed to field-based, studies can also be useful in recon-
structing the volcanic history of a region (Crattan et al ., 1999). Maars are volcanic
craters formed from phreatic or phreatomagmatic explosions. Carn (2000)sug-
gests that the morphology of prehistoric maars can be used to determine the
relative age or 'freshness' of the formation based upon their diameter--depth
ratio. Young maars have a diameter--depth ratio of 4--6. This value increases
with age as erosion processes widen and fill the crater. Older maars have a
diameter--depth ratio of about 10. Carn (2000)used this technique to estimate a
relatively young age (14 000--40 000 years BP) for the maars and other volcanic
activity in the Lamogan Volcanic Field (LVF), Indonesia. The crater diameter can
also be used to estimate the ejecta volume from the eruption. Carn (2000)used
the total ejecta volume ( V E )for each maar (seven in all) to constrain the volume
of juvenile magma ( V J )bysubtracting the crater volume ( V M )fromthe dense
rock equivalent (DRE) of ( V E ). Conversions to and from DRE were achieved using
an ejecta density of 1.3 g cm − 3 and a juvenile tephra density of 1.8 g cm − 3 .
The approach assumes that the crater has not been significantly widened since
formation. This is reasonable when considering the relatively small diameter--
depth ratios of maars. ( V M )ratios are subject to errors where the maars contain
lakes; however, this is not an issue where the lake depth is known (Fig. 8.2).
Ejecta calculations can also be applied to cinder and spatter cones with well
preserved craters. Volumes of exposed lava flows ( V L )are calculated and used
in estimations of total cone eruption volume ( V tot ). The cone basal diameter to
height ratio ( W co / H co )andslope angle ( ) can then be used to estimate cone
age (Fig. 8.3). Existing data suggest that W co / H co increases and decreases as
cones are eroded over time. Relatively fresh cones have indices of about 5--6.
Low estimates of W co / H co can in some cases be a result of underestimates of
W co , which is poorly constrained for older cones that may have been buried by
younger deposits. It should be noted that without radiometric dates, the age
estimates are relative to younger and older formations in the same area. Also