Kansas Geological Survey, Bulletin 202, pt. 1, originally published in 1971
Originally published in 1971 as part of Kansas Geological Survey Bulletin 201, pt. 1, p. 9. This is, in general, the original text as published. The information has not been updated. An Acrobat PDF version of the complete bulletin (7 MB) is also available.
Expansion in a volcanic ash shard starts wherever the temperature-viscosity conditions are optimum for expansion.
A fully expanded volcanic ash shard is a mass of sealed bubbles. When viewing such material one cannot answer the questions: Where does the expansion start in the shard?, Does it start at the edge and work inward or does it start at random locations?
During our experimental studies of expanding volcanic ash we found that the fully expanded ash could be separated from the nonexpanded ash by mixing with water and stirring. The fully expanded ash would float to the top; the remainder would sink to the bottom. In the sink portion were found a number of shards in which there was incipient expansion, but the shards had not expanded enough to lower the bulk density to less than that of water.
Shards that had just begun to develop an expanded structure could be separated under a microscope. Photographs of these shards are shown in Figure 1. The assumption that the expansion starts from the edge and moves inward is supported by bubble formations observed in many shards, but in some shards the expansion started in the middle at random locations. One can interpret this to mean that the shard contains an internal pore structure sealed by a thin cap of glass that reaches its softening point before that of the rest of the shard. The expanding gas inside the pore starts to push against the surface, causing the bubble to form. Even along the edge, where there is discrete bubble formation, there is evidence that a sealed capillary structure exists inside the shard.
Figure 1--Volcanic ash shards showing various stages of incipient expansion. 400X
Expansion begins at any location on the shard where the temperature is favorable to formation of viscous deformation accompanied by expansion of confined gas. When temperature conditions are uniform across the piece, expansion begins where the glass is thinnest.
Kansas Geological Survey, Incipient Expansion in Kansas Volcanic Ash
Placed on web May 7, 2009; originally published in May 1971.
Comments to webadmin@kgs.ku.edu
The URL for this page is http://www.kgs.ku.edu/Publications/Bulletins/202_1C/index.html