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Volcanic Ash

Section: Materials, Subsection: General


Joseph Herbert overviews the technical and practical aspects of this interesting group of materials

Article Text

Joseph Herbert 

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 volcanoes 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.

Out Bound Links

By Tony Hansen

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