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 eruptions. 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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:315541 |
| Date | January 1990 |
| Creators | Young, Simon Rowlatt |
| Publisher | Lancaster University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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