Volcanoes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 215

and the temperature regime that existed during the eruption from which indi-
vidual tephras were ejected and the subsequent chilling of the magma. One
great benefit of using magnetic susceptibility to identify tephra horizons is that
these layers can be identified in cases where field identification is difficult or
even impossible. The magnetic properties of the tephra can give information
about the type, concentration and grain size of the magnetic mineral and thus
temperatures and cooling processes following the eruption (Gonzales et al ., 1999).
Tephra magnetostratigraphy is quick, easy and economical compared to geo-
chemical methods. Along with ice cores, tephra layers can also be identified in
soils and sediments. Soils will also show magnetic characteristics, but individual
tephra layers can be discriminated because the back-field isothermal remnant
magnetizations (IRMs) highlight the presence of different types of magnetic min-
erals for the soils (magnetite) and tephras (paramagnetic minerals). Curie tem-
perature curves also differ between magnetite and paramagnetic minerals and
these curves can be used to further help differentiate between soil and tephra
layers (Gonzales et al ., 1999).
Beget et al .(1994)used magnetostratigraphy along with geophysical and geo-
chemical techniques to identify tephra layers in sediments in Skilake Lake,
Alaska. Sediment cores from the lake floor contained at least nine separate,
thin layers with anomalously high magnetic susceptibility. Petrographic analy-
sis showed these horizons consisted of volcanic ash. These layers were not easily
visible to the naked eye as the sediment cores consisted of several dark and light
layers that were usually between 0.1 and 0.5 cm thick. Volcanic glass shards were
separated from the tephra layers and individually characterised using an elec-
tron micro-probe (EMP). Each grain was analysed individually, which avoided
theproblem of misidentifying detrital contaminates and density fractionation
during transport and emplacement. The source volcanoes for these tephra lay-
ers were identified by comparing the separate tephra layers with reference geo-
chemical data sets from previously known and dated tephras. Four volcanoes
were identified as the source of the nine tephra layers. Historic eruptions that
deposited tephra layers in the lake sediments occurred in AD 1912, 1902 and
1883, and prehistoric eruptions occurred around 250--350, 300--400, 350--450 and
500 years ago (Fig. 8.1). Even older tephras were identified lower in the core
but these were difficult to identify and date. The tephra layers and historical
records show that volcanoes in this region erupted every 10--35 years during the
20th Century and ash falls accumulated in the lake at least once every 50--100
years over the past 500 years. The ash fall deposits in the lake alone probably
underestimate the volcanic history of the area. Eruptions may not have been
recorded if they did not have a significant ash deposit or if the ash was trans-
ported in an opposite direction. Comparison with records and chronologies for