Joseph Herbert on tue 12 jan 99
Veena Raghavan asks about volcanic ash as glaze
ingredient. This is from a previous discussion of ash.
This is material I posted in 1996 in reference to a
thread about Mt. St., Helen ash, with some additions
about specific rock compositions. I apologize to
veteran readers, as necessary.
Using 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 volcanoes that produce
other kinds of rock 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
continental volcanoes like Mt. St. Helens and the
volcanos in the Andes mountains.
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. This is
exactly the process that happens when a bottle of warm
soda is opened. 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.
Here is an additional story about the composition of
volcanic emissions. There was a supposed geologist who
observed the eruption (small scale) of lava at an
African volcano and rushed to collect a sample of the
just solidified lava. The sample was sent off to
whatever American university home was and waited there
for a while. Some time later, the geologist decided to
analyze his rock sample, starting with a good soak and
wash. He never got to the wash because the entire rock
dissolved during the soak. The rock that had been
erupted as a liquid and collected as a solidified
specimen was Sodium Carbonate, common washing soda.
While I cannot document the story, this supposedly took
place in the part of Africa where some deep lakes
contain so much dissolved carbon dioxide that when they
"turn over" people and animals are killed by the
smothering cloud of carbon dioxide that flows away from
the lakes. This indicates a large amount of carbon
dioxide in the water and, by extension, in the
subsurface in general. If the amount of carbon dioxide
dissolved in the melted rock below ground, there really
isn t a reason that sodium carbonate might not form as
a melted material.
What this means to the a glaze discussion is that
erupted material can have about any rock composition.
Some are much rarer than others, but a wide range
exists. The material that is available to any of us at
the local feed, glaze, and video store depends on
factors like transportation and marketing rather than
composition. As we have all noted, shipping pottery
raw materials, which are all heavy and mostly used in
large quantities, is expensive. In days of old, before
the petroleum genie sprang from the bottle, potters
went where the materials were or made do with the
materials at hand.
To speak more specifically about actual rock, and
potential volcanic ash, compositions I have prepared a
group of unity formulae for various rocks. Except for
the granite, any of these could be an extrusive, that
is volcanic, rock. If the table doesn t come out well,
silica content increases by nearly a factor of three
while the alumina content only increases 50 percent.
The total iron is about equal to the fluxes in the
basalt while it is 0.2 to 0.4 in the granite type
rocks. Magnesia, which I did not include in the flux
unity goes from half of unity to 0.03 from basalt to
rhyolite. Potassium increases about 5 times, sodium
about doubles, and calcium decreases by a factor of
three.
Column 1 Olivine Basalt Avg. Unity
Column 2 Tholeiite Basalt Avg. Unity
column 3 Andesite Unity
Column 4 Trachyte, New South Wales Unity.
column 5 Dacite, Mt. Hood, Unity
column 6 Granite, Skye, Inner Hebrides, Unity
column 7 Avg. Rhyolite, Taupo volcano, Unity
1 2 3 4 5 6 7
SiO2 3.13 3.45 3.98 4.61 5.79 8.02 8.24
Al2O3 1.01 1.19 1.32 1.18 1.37 1.46 1.47
Fe2O3 0.28 0.14 0.28 0.10 0.22 0.15 0.10
FeO 0.63 0.48 0.27 0.23 0.21 0.18 0.10
MgO 0.54 0.50 0.24 0.04 0.23 0.06 0.03
CaO 0.72 0.80 0.59 0.14 0.57 0.13 0.18
Na2O 0.21 0.19 0.27 0.45 0.35 0.32 0.47
K2O 0.07 0.01 0.14 0.41 0.08 0.55 0.35
What this all means is that a volcanic rock, which
includes volcanic ash, can have almost any composition
- within a very wide range. The rocks included in my
table run across the range of commonly found volcanic
rocks. So, if you are making a glaze recipe using
volcanic ash, be sure of your source. You must know
that the volcanic material for your next batch is from
the same layer as was your last. The composition of
material erupted from a single volcano can change over
time. Since most of the mined products cut
vertically across deposits that were made at successive
times, the mining process mixes material of different
composition in ways you may not know about. If you are
spending time adjusting an ash recipe, you might want
to collect the ash yourself and note the location very
carefully - maps, photos, markers. Memory fades and
vegetation can fool you.
Joseph Herbert
Joseph.Herbert@att.net
--
Instructional Writer
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