Geoscience Reference
In-Depth Information
the flow then a process called fluidization may allow
sediment particles in dense slurries to move as a fluid
and remain unsorted. Different processes probably
operate at different levels in pyroclastic flows.
Turbulence lifts finer particles into the current higher
up, while at ground level it enhances fluidization.
Similarly, the falling-out of particles from slurries near
the ground entraps smaller particles into a deposit
while expelling water. The latter process also enhances
fluidization. From time to time, turbulent vortices
penetrate to the bed allowing the deposition of alter-
nating layers of fine and coarse material. The resulting
product is a disorganized deposit containing a wide
range of particle sizes with evidence of layering.
Not only can the blast from the flow be destructive,
but the flow can also be extremely hot. Pyroclastic
flows are also called 'glowing avalanches' because of
the presence of heated debris within the flow. The
Mt St Helens flow produced temperatures around
3500°C near the source and 50-200°C near the
margins of the flow. These temperatures were
sustained for less than two minutes. Temperatures in
the 1902 Mt Pelée flow were as high as 1075°C, as
shown by the partial melting of gold coins and bottles.
Temperatures can vary as much as velocities do. For
example, the Mt Vesuvius flow of 79 AD was hot
enough to carbonize wood in places, while food in
other locations was left uncooked. The deposits from
pyroclastic flows can remain hot for considerable
periods of time. Temperatures in excess of 400°C were
found within one deposit from the Augustine, Alaska,
eruption of 1975, several days after the event.
Pyroclastic flows can also travel at high speeds
because they behave like a density flow under the
effects of gravity. Velocity measurements of small ash
flows range between 10 and 30 m s
-1
, while larger flows
can obtain speeds of 200 m s
-1
. At these velocities,
the flows can override high topographic barriers. The
Mt Pelée flow of 1902 refracted around topographic
obstacles, and appeared to increase in velocity seaward.
Both basal surges and pyroclastic flows are exceedingly
deadly because of their speed, and the fact that they are
mixed with large amounts of hot, toxic gases.
Historically, pyroclastic flows have covered substan-
tial areas. The Katmai eruption in Alaska in 1912 - one
of the biggest flows measured - covered an area of
126 km
2
, while the Mt St Helens pyroclastic flows
extended over 600
Beginning of a pyroclastic flow or nuée ardente on
Mt Ngauruhoe, New Zealand, in January 1974. Tephra has
been blasted vertically only to collapse under its own
weight. The pyroclastic flow consists of ash suspended in
hot carbon dioxide, and can be seen as the rapidly moving
cloud closely hugging the ground to the left in the
photograph (photograph courtesy of the United States
Geological Survey, Catalogue of Disasters #0401-09j).
Fig. 11.3
thus vary over short distances. The Mt Vesuvius pyro-
clastic flow of 79 AD knocked over walls and statues in
some places, while short distances away furniture
inside buildings was left undisturbed. Where the cloud
meets the ground, conditions are different. Sediment
concentrations increase and sediment particles ranging
in size from silts to boulders undergo billions of
collisions. The momentum of the flow is equalized
between the particles and the flowing current of air. In
some cases, the grains may flow independent of any
fluid under a phenomenon known as granular flow.
Granular flow tends to expel coarse particles to the
surface; however, if fluid moves upwards through
km
2
. The Rabaul eruption,
1400
years ago, produced a flow 2
m deep over
