JJHerb@aol.com on mon 30 sep 96
>The word "ash" for the material that is expelled during volcanic eruptions
is misleading and not helpful when thinking about its composition or origin.
I suppose the idea of ash, as in wood ash, is left from the times when
volcanoes were thought of as chimneys from inside the earth. It was obvious
to the ancients that there was definitely smoke there, likely fire there, and
so the material that rained down, being a gray, powdery material, was ash.
(insert appropriate Latin phrase)
>The eruption of oceanic, basaltic volcanoes is relatively simple and benign
- as these things go - and are relatively easy to study. The eruptions
usually go on for a long time, have "predictable" phases, and rarely do
astonishing things. The molten rock material involved in these eruptions is
similar to basalt in composition. It is silica poor and very fluid. While
all erupting volcanoes emit lots of gas, the presence of the gas dissolved in
the liquid rock is not as important in these eruptions as in some others.
>The molten rock material in an oceanic volcano often forms a lake inside a
crater or cauldera and runs back down into the vent from time to time. Lots
of mixing. Sometimes, the wall of the crater is breached and the contents of
the lake flows down the side of the mountain in a spectacular river of liquid
rock that is some 1700 degrees F or more in temperature. It may be that this
material is in the process of crystallization with crystals of the higher
temperature minerals forming while the material flows. There might be some
crystals of olivine and high calcium plagioclase in the liquid rock as it
goes toward the sea. Other times the side of the mountain fails in a fissure
and the contents of the lake runs out through the crack. This kind of
eruption is dangerous because the fissure may be far from the crest of the
mountain and a large volume of liquid rock is released in an unexpected place
very quickly.
>In contrast, the eruption of other kinds of volcanoes takes a different and
more violent course. Specifically, volcanoes that erupt material that is
relatively rich in silica behave very differently. The presence of large
amounts of silica in molten rock makes the material very viscous - it does
not flow out during an eruption. The molten material does move inside the
earth where the temperatures are higher and, more importantly, the amount of
dissolved gas is greater. The dissolved gas, most of which is water is the
key to the behavior of volcanoes like Mt. St. Helens.
>Before ending the story, I would like to try to amplify on the difference in
viscosity of the two kinds of melted rocks. A non-oceanic volcano in South
America erupted (over the course of several days) a spine of molten rock that
extended straight up 1300 feet from the vent where the eruption originated.
This spine glowed red, and was a very viscous liquid - when the exterior of
it solidified, pieces broke off and fell. A spectacular sight.
>When a volcano that erupts a high silica material starts to erupt a couple
of things happen rather suddenly. The gas rich liquid material melts its way
up into the area below the volcano s cone. At some point, the material
encounters a zone of weakness and the state of the material in the ground
begins to change. First gas is released into the zone of weakness. This is
actually a foaming process because the gas is dissolved in the liquid rock
and when the pressure is released, the liquid foams. Now, an interesting
thing happens, the rock that was flowing because of the dissolved gas
becomes stiffer because the gas has left it. So the liquid that flowed well
enough to form a bubble, suddenly stiffens once the bubble is formed. The
gas is still expanding and soon the bubble breaks. The pieces of the broken
bubbles of the foaming liquid rock are carried out of the volcano with the
gas. This is volcanic ash.
>In the case of Mt. St. Helens, the landslide that removed part of the
mountain s summit relieved the restraining pressure on a large mass of liquid
rock and all of it foamed up and blew out at once. The chemical composition
of the contents of the magma chamber depends on the original composition of
the melted rock, the changes in composition as it melted previously erupted
material, and what ever removal of material there might have been by loss of
early forming crystals. Because this kind of rock is not very liquid when
liquid, there could be some variation in composition between the top and
bottom of the rock mass. However, as this mass of material was being broken
up and spewed into the air, there was mixing. I would be surprised to find
large differences in composition of fallen ash from place to place. I have
not, however, done any research that might injure that particular prejudice.
>As a practical matter, erupted volcanic ash from silica rich volcanoes is
composed of glass shards of various sizes. It is bad to breath them and they
do settle rapidly in water. The glass may not be a very good glass and will
probably leach soluble materials rather easily, especially when you consider
the surface area available in fine powders. When left in the ground long
enough, the "ash" turns to Bentonite clay. It is possible that some of the
Bentonite used in yesterdays glaze batch could be traced to a particular
volcanic eruption a few million years ago. Perhaps the famous Mount Mizuma,
now Crater Lake, contributed that to your glaze.
Joseph Herbert
JJHerb@AOL.com
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