9 August 2009

Degassing structures in pyroclastic deposits

Posted by Jessica Ball

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!)

Branching degassing structures in a pyroclastic deposit in the Colli Albani (Alban Hills) volcanic district. (The ruler is divided into ten-cm sections.)

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.

Fines-depleted structures galore! The side of the same cliff in the Alban Hills, with geologist headgear for scale.

An annotated version of the last photo, with the outlines of some of the degassing structures and the top of the pyroclastic deposit (overlain by some paleosols and modern soils).

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.

Detail of the first photo; you can see a strip of scoria and other clasts along the right side of the degassing pipe, and more fine material toward the center.

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.

(There should really be some oozing in this one to illustrate my point better, but apparently Fred Flintstone only gets excited about non-drippy desserts.)

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.

A degassing structure in pyroclastic deposits on Procida Island, near the Bay of Naples. Notice how the bottom of the pipe (which curves around the large central clast) seems to cut off at a layer of cobble sized rocks about a meter and a half above the cliff base. Contemplative volcanologist for scale.

Fines-depleted pipes are an easy way to identify a pyroclastic deposit, and can be a good distinguishing feature if you’re trying to tell apart tuffs and lavas (providing there hasn’t been so much welding and compaction that the degassing structures are obliterated). Another great example of this is the Valley of Ten Thousand Smokes, an area in Katmai National Park and Preserve (Alaska) that was filled by ash flows from the 1912 eruption of Novarupta.

Southeast up the Valley of Ten Thousand Smokes, with the rim of Katmai Caldera on the left skyline. Photo by R. McGimsey, June 10, 1991; from the USGS Photo Library.

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.)