Marcia Selsor on tue 23 may 00
Dear Michael, Lawrence, Paul, Dave, Mel,
My scientist husband explained the source variations this way;
Cascades are the Pacific ocean crust is being pushed under the N.
American Continent'-basalt and ocean floor
The Hawaiian Island ash is from a hot spot melting ocean floor -basalt
Yellowstone is also a hot spot melting earth crust-more silica (there is
also obsidian sites in the park -very siliceous)
Iceland is the spreading of the plates, more basalt but deep mantle stuff.
As for my ash, I think it is pretty pure except for the gravel either
rocks that flew or those from some of the parking lot. This is not a
cobalt blue but black/navy blue
The bags have been wrapped in plastic and untouched for 20 years. I will
have it analyzed and post/send you a copy and we can compare the details.
I still have some of the handout from Michael from Pullman. The glazes
from his recipes fired to ^10 in my oil kiln didn't do what what was
described with the ash from Couer d lane.
Anyway, I think it is curious to consider the flying factor and weight
distribution as changing the chemical composition. It seems so logical.
Another friend mention all the forest fires started by the eruption and
the ash that was created by those.
Glorious weather and time for garden work.-me too.
Thanks for all the input. It is on the back burner for a while. I have
to pack up books and empty files, etc.
Marcia
Michael McDowell wrote:
>
> I've been preoccupied with my gardening of late, so I've not chipped in on
> this thread before now. Marcia, I would be suspicious that your Mt. Saint
> Helens Ash sample has somehow become contaminated if it is coming out with a
> dark blue tinge. In twenty years, things do happen. I do concur with Paul
> Lewing's information on the geographical variation in samples tested around
> the Northwest at the time of the erruption. My sources were somewhat
> different, but they also showed that the material was pretty consistent
> throughout the range where it was deposited. There was just a small dropoff in
> iron content with distance from the volcano as the most significant systematic
> geographic variation.
>
> Jeff Lawrence's work with volcanic deposits from the Jemez Mountains in New
> Mexico, and the analysis he has shared here, shows how different a material
> can come from such a similar (also volcanic) source. We potters should be
> careful then to be specific as to source when offering or requesting
> information regarding volcanic ash.
>
> In this vein, I'd like to ask Frank Martin to revise the glaze formulas he
> just posted for "volcanic ash" glazes by specifying which erruption from which
> volcano, if he knows it. If not, then at least the spot from which it was
> recovered, or the supplier from whom it was purchased would help clarify his
> meaning. To say "Volcanic Ash" in a glaze formula is only slightly more
> informative than saying "Powdered Rock".
>
> Michael McDowell
> Whatcom County, WA USA
> mmpots@memes.com
> http://www2.memes.com/mmpots
>
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--
Marcia Selsor
selsor@imt.net
http://www.imt.net/~mjbmls
http://www.imt.net/~mjbmls/spain99.html
http://www.silverhawk.com/ex99/selsor/welcome.html
Paul Lewing on wed 24 may 00
Marcia Selsor wrote:
> Another friend mention all the forest fires started by the eruption and
> the ash that was created by those.
Marcia, I don't remember there being many forest fires from the Mt. St.
Helens eruption. Trees near the mountain were vaporised, and trees in
the rest of the blast zone were flattened, but there was not that much
heat associated with the blast. At any rate, wood ash would be such a
tiny part of the total that I wouldn't think it would be detectable in a
chemical analysis. Remember that the mountain blew out about a third of
a cubic mile of rock. This, by the way, is not a large eruption. When
Mt. Mazama blew, it threw out about three cubic miles of stuff. That
caldera, which now contains Crater Lake in Oregon is 6 miles across.
Now visualize the Yellowstone caldera, which occupies most of the park-
maybe 20 miles across.
Another note on volcanic ash glazes, in light of Mel's posting his 60/40
ash and GB recipe. This is a really serendipitous combination, as the
ash settles like a rock, and the Gerstley keeps it in suspension. It
will be interesting to see if any of the new GB substitutes coming on
the market work as well for this, or whether suspenders will be needed.
(Or maybe a belt AND suspenders, just to be safe?)
Paul Lewing, Seattle.
Sorry, couldn't resist.
Michael Banks on sat 27 may 00
The Mt St. Helens ash is a good example of a natural frit. It is mostly
composed of glass shards which geologists call vitric ash. Under a
microscope, vitric ash is seen to be scalloped slivers which are fragments
of the walls of the original gas bubbles in the magma. Frothing magma is
torn to shreds when the gas bubbles violently expand, the pieces instantly
freeze solid and are carried up into the atmosphere by the convecting gas
column above the volcano.
Not all volcanic ash is vitric though, and some ash has a high proportion of
crystals or tiny rock fragments called lithics. Crystal or lithic-rich ash
would not be nearly as fusable into a glaze, as vitric ash. The crystal
components of volcanic ash clouds do usually fall out first and the small
proportion of iron-rich crystals in the dacite ash from Mt. St. Helens
dropping out close to the mountain, may be the reason why the iron content
is slightly lower at distant locations. The St. Helens ash was erupted as
predominantly vitric in composition however, so that most of the iron
content is dispersed in the glass fragments and cannot be separated by
magnetic means.
I concur with Paul Lewing that incinerated forests around Mt. St. Helens did
not materially contribute to the widespread ashfall in 1980. In fact,
virtually all of the ash fall was generated by a nine-hour plinian eruption
which occurred after the spectacular lateral blast event that felled the
forests. Plinian eruptions generate high altitude, vertically ascending
columns of gas and ash which produce widespread distributions of ash and are
driven by violently degassing magma deep in the throat of the volcano.
The earlier lateral blast from St. Helens conversely, was driven by the
flashing of superheated groundwater and pressurised gas liberated by a
landslide. The resultant debris flow was not hot enough to burn trees and
any fire would have been snuffed out in any event -being buried under the
dense suffocating pyroclastic flow cloud. The lateral debris flow was
largely composed of fragmented older rocks from the mountain and had less
than 1% fresh magma component, according to one study. The ash collected
and used by potters is from the subsequent 100% fresh magma eruption, which
did not cause the spectacular felling of the forests. But the big ash
generating eruption was triggered by the landslide/lateral blast, because of
the consequential abrupt depressurisation of the gas-rich magma reservoir
lurking deeper down in the mountain.
Michael Banks,
Nelson,
New Zealand
> Marcia Selsor wrote:
>
> > Another friend mention all the forest fires started by the eruption and
> > the ash that was created by those.
> Paul Lewing wrote:
> Marcia, I don't remember there being many forest fires from the Mt. St.
> Helens eruption. Trees near the mountain were vaporised, and trees in
> the rest of the blast zone were flattened, but there was not that much
> heat associated with the blast. At any rate, wood ash would be such a
> tiny part of the total that I wouldn't think it would be detectable in a
> chemical analysis. Remember that the mountain blew out about a third of
> a cubic mile of rock. This, by the way, is not a large eruption. When
> Mt. Mazama blew, it threw out about three cubic miles of stuff. That
> caldera, which now contains Crater Lake in Oregon is 6 miles across.
> Now visualize the Yellowstone caldera, which occupies most of the park-
> maybe 20 miles across.
> Another note on volcanic ash glazes, in light of Mel's posting his 60/40
> ash and GB recipe. This is a really serendipitous combination, as the
> ash settles like a rock, and the Gerstley keeps it in suspension. It
> will be interesting to see if any of the new GB substitutes coming on
> the market work as well for this, or whether suspenders will be needed.
> (Or maybe a belt AND suspenders, just to be safe?)
> , Seattle.
> Sorry, couldn't resist.
>
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