Pyroclastic Surges

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Nigel Whittington

Posted: Tue Nov 11, 2003 3:30 pm

Guest

Hi folks.

Putting together a talk on my perambulations in Italy I came across
stuff about pyroclastic surges.

I can understand a surge resulting from column collapse, but a skim
through a few books revealed thatr there are several types of surges.

Can anyone attempt to explain the mechanisms behind basal, and other
types of surge to me? And how, exactly is a surge differentiated from
other pyroclastic flows?

Nigel, overawed at the way surges can crate 'current bedding' just
like my beloved fluvial deposits.

Jo Schaper

Posted: Tue Nov 11, 2003 10:48 pm

Guest

Nigel Whittington wrote:

Quote:

Hi folks.

Hi Nigel,

Quote:

Can anyone attempt to explain the mechanisms behind basal, and other
types of surge to me? And how, exactly is a surge differentiated from
other pyroclastic flows?

Flipping through a Google search, it looks like the major
differentiating factors are the violence and temperature of the
explosion, and the speed of the nuee ardente, over dry land, and whether
or not the pyroclastics are cool enough to be carried by water. A lot of
babbly about hot pyroclastics and "cold" pyroclastics. It might be
instructive to look up caldera explosions and rhyolitic ashfalls and
ashflows. I would not want to be the person either clocking the flow, or
holding the thermometer. Yellowstone explosions might yield some info,
however I didn't see many rocks which were obviously flows as opposed to
pumice-like falls.

Quote:

Nigel, overawed at the way surges can crate 'current bedding' just
like my beloved fluvial deposits.

This actually isn't so strange to me. Many of the rhyolite porphyries in
the Ozarks are ashfall and ashflow tuffs. Some rhyolites have ashpits
where cooled pumice hit the flowing pyroclastic rhyolite. Streaky
lighter phenocryst areas are fairly common, as are "ripple rhyolite" and
breccias where the rhyolite picked up loose stone, welding it
together. One geologist here is of the opinion that some of the
rhyolites are reworked: that is they fell and hardened, then were buried
again, reheated to molten, and flowed downhill sometime later. I've also
seen something called a "water-laid tuff"--now that rock is truly
strange. Looks all the world like a fine to medium grain buff to reddish
sandstone, but it is rather made entirely of rhyolitic quartz and
feldspars. Location is between two ash flow tuff layers. Some of these
ashfall and ashflow tuffs are very glassy, others are nearly
devitrified. Of course, they're all 1.5 to 1.1 Ga old, so no one was
there to take pictures, clock them, or take their temperature upon eruption.

Usual disclaimer when I open my mouth about anything igneous...I've got
a really limited sample size here. Less than a month ago, I found/saw a
1 x 3-4 meter basalt xenolith in a rhyolite flow...another strange thing
I've never seen. Not a dike. Say what???

best,
Jo

--
Geo Communications Services -- www.geocommunications.net
Jo Schaper's Missouri World -- http://www.missouriworld.net

Carsten Troelsgaard

Posted: Wed Nov 12, 2003 3:35 am

Guest

Quote:

Usual disclaimer when I open my mouth about anything igneous...I've got
a really limited sample size here. Less than a month ago, I found/saw a
1 x 3-4 meter basalt xenolith in a rhyolite flow...another strange thing
I've never seen. Not a dike. Say what???

lol - you made my day
Carsten

Alastair McDonald

Posted: Wed Nov 12, 2003 6:21 am

Guest

"Nigel Whittington" <coastwatch@my-dejanews.com> wrote in message
news:2449d998.0311111230.7ca1c90c@posting.google.com...

Quote:

To quote "A Dictionary of Geology" by D.G.A. Whitten with J.R.V.
Brooks, first published by Penguin Books in 1972 (not the current
edition) it says;
"Turbidite, Turbidity current. It is a matter of common observation,
and an easily demonstrated fact, that slurries of sediment and
water behave as a discrete fluid phase when poured into fresh
or sea water."

It seems to me that mixtures of dust and air also behave
as if they are a single fluids, with physical characteristis which
depend on the physics of the mixture. Therefore a pyroclastic
flows and lahars are basically the same phenomena, let's call
them slurries, even though the primary fluid is different, air
and water respectively. A snow avalance is another example
of this where the primary fluid is air and the dust is ice crystals.
An example everyone has seen is the dust cloud which
swept down the street after the collapse of the Twin Towers.
A more esoteric case is the combination of mud and sea water
which allow manganese nodules to lie on the sea floor by
floating on the mud.

So long as the temperature of a plinian column is great enough
such that the denisity of the slurry is greater than the atmosphere
then the column will rise. Presumably, if there is molten ash in the
column, when it cools, freezes, and releases latent heat it will
sustain the upward convection in the same way as a thunder
cloud is powered by the condensation of water vapour. Of
course there is also water vapour involved in a plinian columns.

If there is not enough power to shoot the dust and gas high
into the atmophere, then the slurry flows down the side of
the volcano like a liquid with a density equal to the average
of the ash and air. Here, you are possibly talking about a
density similar to that of water, if the dust is thick enough.

The point to realise is that a pyroclastic flow is not like a
rain cloud. It is like a snow avalance. This is because in a
rain cloud the liquid is continually evaporating and condensing,
hence the densty remains close to that of the surrounding air.
In the case of a pyroclastic flow or a turbidite, the density of
the slurry is higher than the host fluid and so it responds to
gravity.

AFAIK, this is the first time a pyroclastic flow has been
compared to a turbidite, or called a slurry.

HTH,

Cheers, Alastair.

Peter Halls

Posted: Wed Nov 12, 2003 9:42 am

Guest

On 11 Nov 2003, Nigel Whittington wrote:

Quote:

As another has already explained well, its about gas-bourne materials
behaving as a fluid.

Quote:

Nigel, overawed at the way surges can crate 'current bedding' just
like my beloved fluvial deposits.

Have you led a sheltered East Yorkshire life, Nigel? The Borrowdale
Volcanics (Ordovician, ~Llandeilo) display a super set of bedded deposits,
ranging from current-bedded tuffs through bedded-tuffs that were almost
certainly pyroclastic flows to andesitic lavas, etc. Sub aqueous,
including pillow-lavas, and sub-aerial deposits are all to be found. The
'marbling' of Lake District 'greenslate', which makes the rock so
attractive, is the result of such bedding. [And its not slate senso
stricta, but cleaved bedded tuffs ...]

Peter - marvelling that a British geologist could have missed his beloved
Lakeland volcanics!

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Jo Schaper

Posted: Wed Nov 12, 2003 1:39 pm

Guest

Carsten Troelsgaard wrote:

Quote:

Good day, Carsten.
Glad I could make you smile *| Wink

However, I am still curious about the
basalt xenolith. What was odd about it was the edges were melted, and
flowed into the rhyolite, not like a trapped cold stone. If I remember
Bowens reaction series correctly, the basalt should have crystallized
before the rhyolite. Nearby we have a magma mixing chamber scenario,
where basaltic and granitic lavas solidified at the same time--a
scenario I describe as the basalt being squirted like the hot chocolate
one puts on ice cream to make a chocolate shell dip cone, and which also
melts the ice cream. Perhaps a similar comagmatic thing happened with
the xenolith. Or perhaps it is just an anomalous bit of stuff.

The relationship of basalt/diabase to the felsic granite and rhyolite is
still somewhat imperfectly understood here. Some is obviously much
younger cross cutting dikes, but some (like this) seems to be
contemporaneous with the ashflows.

--
Geo Communications Services -- www.geocommunications.net
Jo Schaper's Missouri World -- http://www.missouriworld.net

Carsten Troelsgaard

Posted: Thu Nov 13, 2003 5:03 pm

Guest

snip

Quote:

If I remember
Bowens reaction series correctly, the basalt should have crystallized
before the rhyolite. Nearby we have a magma mixing chamber scenario,
where basaltic and granitic lavas solidified at the same time--a
scenario I describe as the basalt being squirted like the hot chocolate
one puts on ice cream to make a chocolate shell dip cone, and which also
melts the ice cream. Perhaps a similar comagmatic thing happened with
the xenolith. Or perhaps it is just an anomalous bit of stuff.

well Jo
it's a question for the hard-core people.
with the same disclaimer: the mixing of chemistry may create a new eutectic
low meltingpoint

Carsten

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