Summary of observations of pyroclastic
density currents on Montserrat
| Both these flow types (I am
ignoring surges and secondary flows here) are concentrated granular flows -
but yet they exhibit contrasting mobility characteristics (defined by rows
1-4). This must be explicable by (i) processes governing transport (mode of
formation) or (ii) properties of material involved. |
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| Factor; Mode of formation |
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| Dome Collapse -
avalanche begins as coherent lava and disintegrates to form loose packed
granular avalanche. |
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| Pumice flows - begin
as dilute suspensions and over a period of time condense to form loose packed
granular avalanche which might have a lower frictional resistance. Also
momentum is borrowed from earlier stage (column or surge) |
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| Factor; physical
composition & Material properties |
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| Dome Collapse - dense
particles range in size by 5 orders of magnitude. Coarse blocks often being a
larger magnitude than flow thickness. |
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| Pumice flows - Density
significantly lower, particles better sorted range over 4 orders of magnitude
and are always a fraction of flow thickness. Depth of flow/ particle diameter
ratio different. |
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