Video analysis of the January 8 th 2010 pyroclastic flow from Soufrière Hills volcano (Montserrat): Flow dynamics and modeled evolution
Molle, Alex Nicolas Aime
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On January 8 th 2010, Soufrière Hills volcano (Montserrat) experienced a collapse of the northwestern part of its andesitic lava dome. The ensuing explosive event lead to a 8.3-km-high eruption column and generated a Pyroclastic Density Current (PDC) that propagated down the northern flank of the volcano, moving down the Belham valley. This PDC was recorded from the Montserrat Volcano Observatory using both normal High-Definition and forward Looking Infrared video cameras. The flow front velocity, plus the vertical motion of buoyant thermals ascending above the PDC, were measured by combining the video data with a deposit map made after the event and a high spatial-resolution DEM. A theoretical treatment was then applied to extract key parameters that describe the buoyant motion of thermals ascending above the current (including density, particle mass / volume fraction, and temperature). Changes in the flow dynamics, as well as the properties of the "co-ignimbrite plume" were observed and linked to topographic interactions. These interactions included an increase of the ascent rate of buoyant thermals when the PDC encountered topographic barriers, and deceleration of the flow front velocity due of high degrees of valley sinuosity. A major change in flow dynamics was visible in our data 2–3 km from the dome. Several possible origins can be proposed to explain this change, and these are discussed herein. However, they seem to be attributed to a combination changes in internal factors (changes in the degree of fluidization, sedimentation rate, ash generation from block fragmentation...etc.) and/or external factors (influence of topographic barriers, changes in slope, sinuosity, valley-holding capacity...etc.). Complementary laboratory as well as further in-situ observation using appropriately designed thermal-camera-array deployment should allow better constraint of such topographically-induced changes in the emplacement dynamics of a PDC and their causes.