Branching degassing structures in a pyroclastic deposit in the Colli Albani (Alban Hills) volcanic district. (The ruler is divided into ten-cm sections.)
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).
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.
(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.
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 (These are no longer active, but still visible on the new valley floor.)
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.)
No comments:
Post a Comment