Physical volcanology of holocene airfall deposits from Mt Mazama, Crater Lake, Oregon.
The 6845±50 yrs BP caldera-forming eruption of Mt Mazama (Crater Lake,
Oregon) was preceded within 200 years by two plinian eruptions producing
voluminous airfall deposits followed by lava flows. This study concentrates on these
two airfall deposits as well as the complex airfall deposits from the climactic
eruption, which are distributed over = 1.7 million km2 of northwest America.
Tephro-stratigraphic mapping of airfall units throughout south-central Oregon
has revealed the presence of five lobes of coarse pumice deposits and two widespread
ash units which are important marker horizons. Detailed grain size data have been
generated by sieving and measurement of maximum clast sizes, and these are used to
characterise each deposit and as input data for clast dispersal models of plinian
airfall eruptions. Geochemical variations between each deposit generally support the
models already developed for Mt Mazama, and geochemical techniques have been used
to deduce the source of distal 'Mazama ash'. The role of volatiles in each eruption is
reviewed and, along with the rate of vent and conduit erosion, is found to be vital in
controlling eruptive evolution.
Deduction of column height and mass eruption rate for various stages during each
eruption has been possible using clast dispersal models and, when combined with
eruptive velocity and vent and conduit dimensions, has produced a detailed physical
model of eruptive development. This has then been linked to field characteristics to
provide significant new information about the physical volcanology of plinian airfall
Revised volume estimates for the climactic airfall eruption, including distal fine
ash, give a volume of = 20 km3 (dense rock equivalent), with a maximum column
height of = 55 km occurring immediately prior to column collapse and ignimbrite
generation. This eruption is thus one of the most intense and voluminous ultra
-plinian eruptions yet documented.