9 August 2009
One of my favorite features of the pyroclastic deposits that I saw in Italy were degassing structures. A good field description of these features would be “fines-depleted pipes”, since it doesn’t make any assumptions about their origins (something to be avoided in the description section of your field notes!)
These pipes are formed when gases trapped in freshly-deposited pyroclastic material rise to the surface of the deposit as overlying material settles and compacts. The gases usually take fines (ash and small lapilli) with them, leaving behind tubes where clast size is larger than the surrounding deposit, and forming fumaroles on the surface of the pyroclastic deposit. The pipes can branch and join, and the ones I’ve seen range in size from a centimeter or two across to almost half a meter.
If you look closely at a degassing pipe, you will often see that the clast size gets larger and the abundance of clasts (rather than ash and other fine material) increases as you move toward the edge of the pipe. This is an important clue to the processes going on in the structure; the interpretation here is that gases get concentrated around the edge of a pipe, and blow out more of the fine material there than in the middle.
It kind of brings to mind a push-up popsicle: the melted sherbert oozes up around the edges of the cardboard tube while the stuff in the middle more or less stays put.
One of the neatest things about these features is that you can tell something about the emplacement of the deposit. If the degassing pipe cuts through the entire deposit, it’s a good bet that the deposit was emplaced all at once, whether as a single unit or through an episode of progressive aggradation. If there are multiple pipes that terminate on different levels of a layered deposit, the layers must represent different episodes in the eruption.
The ash filled the valley to a depth of 200 meters, and both gases trapped in the ash from the eruption and water vapor from buried streams formed thousands of fumaroles on the deposit’s surface. (These are no longer active, but still visible on the new valley floor.)