| Summary: | Pyroclastic density currents are among the most dangerous hazards during explosive
volcanic eruptions, typically having catastrophic and lasting impacts on society,
infrastructure, and landscape evolution of the area. After the eruption of Unzen
volcano, Japan, in 1991, during which 43 people were killed when pyroclastic surges
unexpectedly separated from the parental flows, the possibility of decoupling in
block‐and‐ash flows and the potential hazard of this was recognized. In the following
years, decoupling has been documented at several composite volcanoes, but still not
enough is known about the mechanics of pyroclastic currents, which allow the
detachment of ash cloud surges. In this thesis, several processes thought to initiate
decoupling in pyroclastic currents, such as entrainment of substrate at the flow base
or of air at the flow front, elutriation of fines into the upper ash cloud surge or
simple gravity segregation, are investigated. These mechanisms lead to increased
non‐uniformity and stratification, which is a prerequisite for the onset of decoupling
in small‐volume block‐and‐ash flows. Other mechanisms such as topographic control
over block‐and‐ash flow dynamics are also considered, with examples confirming the
importance of topographic influence for flow stratification and decoupling in block and‐
ash flows. Detailed field studies at Tarawera Volcano, New Zealand, have
provided comprehensive descriptions of the distribution and sedimentology of
block‐and‐ash flow deposits emplaced during the Kaharoa eruptive episode in AD
1314 ± 12, and these confirm the importance of changes in topography on flow
dynamics. Topographic variations causes channeling, blocking and deceleration of
the basal flow parts at Tarawera Volcano, while the upper flow parts are unconfined
and decoupled, leading to detached ash cloud surge deposits beyond the limits of
the main block‐and‐ash flow deposits. Interaction of the advancing flow with the
substrate resulted in dynamic interaction. Deformation features and erosion gullies
confirm the highly erosive nature of the flows. Laboratory‐scaled simulations of
aqueous glycerol solutions and glass particulate currents are used as quantitative
semi‐guides for pyroclastic flow behaviour, with special regard to decoupling caused
by irregular topographies.
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