Degassing structures in pyroclastic deposits

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.)

Italy (Part II)

...and the rest of my favorite Italian photos.

The Valle del Bove on Mount Etna, with the current lava flows steaming at the summit (upper left of the photo; click to see it larger!)


Dikes in the Valle del Bove on Mount Etna.


A cinder cone (maybe two?) on Etna's south flank that was active during the 2001 eruption, with a channelized a'a lava flow at its base.

A Norman castle in the town of Aci Castello, Sicily. (This was a coolness overload for me; not only is it an 11th century castle, which makes the archaeologist in me drool, it's built on pillow lavas. How's that for geoarchaeology?)


Hydromagmatic deposits and dune structures on the island of Procida, just outside the Bay of Naples. These were chock-full of accretionary lapilli (which unfortunately disintegrated on the trip home).


The neatest spot on Procida, even if there was a lot of trash around - an angular contact between hydromagmatic deposits and the welded layer underlying fines-depleted breccias. The breccias were amazing - huge chunks of pumice, trachytes, obsidian, scoria, all packed together with almost no matrix, and grading into a "typical" ignimbrite. (Volcanologists don't understand a lot about how these are emplaced, but one of the PhD students in my department is working on it for her dissertation.)


The summit crater at Vesuvius, minus the dozen or so souvenir stands.


The Roman city of Hercolano (Herculanium), with Vesuvius in the background. We were lucky that there was almost no one visiting, unlike Pompeii (which I'll have to see another time). It was beastly hot, though.


A street in Herculaneum. You can actually still see all the charred roof beams and window frames - it's almost a little creepy, when you realize that people not only lived here, they died here.


Part of a map painted by Ignazio Danti in the Vatican Museum, showing the Aeolian islands from a 16th-century point of view. Looks like Vulcano and "Strongoli" were both active at the time!


And one of my favorite discoveries on the whole trip...the Pope's rock hammer. He's a closet geologist!*



*Okay, so it was a commemorative builder's hammer from the construction of some ostentatious overly-decorated edifice somewhere. I bet it would make a pretty decent fossil chipper, too.

Italy (Part I)

I'm finally in one place for a couple of weeks this summer, and that means it's time to start posting photos while I delay writing real blog entries. (There's some cool stuff I saw on both my Italy and Utah trips that I definitely want to discuss, but my brain is still adjusting to the DC-area sauna, so they'll have to wait until I"m feeling more creative.)

Anyway, here are a few of the best photos from my trip to Italy in June. (I've also discovered that, while the heat and humidity are nasty in Italy in June, they're nothing like the heat and humidity we have around DC at the same time. I can't speak for August, though.)

Some of the beautiful marbles used to decorate the Vatican (which is, by the way, big and elaborate and overdecorated enough to make your head explode). A lot of the churches in Rome look like this, but the Vatican takes the cake. (This is the tomb of one of the Pope Alexanders, but I was too distracted by the inlay to notice which.)


A medieval street in Rome near the Vatican. All those cobblestones are basalt!


The first Roman mile marker on the Via Appia (which is not only made of basalt, but built on a lava flow).


Ruts in Via Appia basalt.


The amphitheater at Sutri, which is excavated out of volcanic tuff (and right next to even older Etruscan tombs that were dug out of the same unit).


Probably the most spectacular columnar jointing I've ever seen in my life, in a Vulsini District trachyte. (The bush at the top of the cliff is roughly four meters tall.)
The pines of Rome and part of the Palantine Hill. This explains why Pliny the Younger's comparison of eruption columns to pine trees always confused me - Roman pines don't look like American ones!


The Fossa cone on Vulcano. Vulcano is a lovely island, but not if you can't stand the smell of sulfur vents!


Strombolicchio, a volcanic neck off the coast of Stromboli, where an early calc-alkaline volcanic center existed about 200 ka.


One obligatory cute cat photo from a bookstore on Stromboli.


And an obligatory Strombolian eruption photo. If you look closely through the spatter, you can see the lights of the boats watching from about 850 meters below. (Next time, I'm parking myself on a boat for the light show - much more relaxing than the climb!)