Abstract
Temperature can be an important characteristic used to distinguish primary pyroclastic density currents or block-and-ash flows from other collapses not primarily related to an eruption, and also governs the type and level of hazard presented by these mass flows. We examined several mass-flow deposits within the AD1000-1800 Maero Formation at Mt. Taranaki, New Zealand, for field characteristics of hot emplacement - such as the presence of charcoal, baking of soils, or gas-elutriation piping - and conducted a paleomagnetic study of their thermoremanent magnetization (TRM) to determine emplacement temperatures. Results show that the majority of the deposits result from block-and-ash flows emplaced over ~500°C. Some of these deposits were indistinguishable in the field from a re-worked or low-temperature emplaced lahar or landslide deposit, indicating that sedimentary features are not a clear determinant of high emplacement temperature. The high emplacement temperatures suggest that the time between dome emplacement and collapse during this period was usually brief (<30 years), with some events consisting of rapid and repeated growth and collapse of lava domes, possibly within the same prolonged lava effusion episode.
Highlights
Block-and-ash flows (BAFs) are among the greatest hazards presented by growing lava domes at active volcanoes
When there is no historical record of eruptions at an active stratovolcano, proper assessment of BAF hazard to the surrounding areas begins with careful observation and analysis of past volcanic deposits
The paleomagnetic emplacement temperature estimates in this study largely agree with the sedimentary indicators of hot-emplacement and features associated with BAFs confirming that the 1000 year record of the Maero Formation is primarily composed of deposits of hot (>500 ◦C) block-and-ash flow deposits
Summary
Block-and-ash flows (BAFs) are among the greatest hazards presented by growing lava domes at active volcanoes. The high speed and temperature of this type of pyroclastic density current (PDC) makes them deadly and destructive over distances commonly 5 to ∼15 km from the source They may occur in individual episodes over a long period of a volcano’s eruptive record or in quick succession over a short span of activity as in the cases of Mt. Sinabung since 2013 [Nakada et al 2017] and of Soufrière Hills volcano from 1996 to 1998 [Calder et al 2002]. When there is no historical record of eruptions at an active stratovolcano, proper assessment of BAF hazard to the surrounding areas begins with careful observation and analysis of past volcanic deposits. A major difficulty with this analysis is that BAF deposits are often interbedded with—and difficult to distinguish from—
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