18 June 2011
Well, ash-flow tuff got taken pretty quickly, but I’m fairly certain no one will come up with my favorite geology term (or the particular meaning I’m going to talk about). That word is autobrecciation. I’m not talking about the autobrecciation that happens when the surface of a lava flow breaks up and gets incorporated into a lava flow, but the meaning used in several volcanology papers about rockfalls and lava dome collapses, and about which I wrote a term paper a couple of years ago.
To illustrate it visually, watch the first ten seconds or so of this video from the 1991 eruption of Mount Unzen, Japan:
See those blocks disintegrating as they collapse? That’s what I’m talking about when I refer to autobrecciation: volatile-rich, pressurized lava dome rocks fragmenting explosively in response to rapid decompression, which occurs at a critical pressure difference between the overpressurized rock and the surrounding environment (i.e., the point when the pressure overcomes the tensile strength of the rock). As you can see in the video, the rocks basically disintegrate into a lot of fine material (and probably some leftover rock chunks), which is the perfect recipe for a pyroclastic flow.
A more detailed progression of the autobrecciation process goes like this: (1) Crack propagation in dome lava or the fronts of lava flows occurs due to cooling or flow of the rock. (2) Small gas plumes are discharged from the cracks that become the point of detachment. (3) The rock mass becomes unstable and decreases in tensile strength, passing the brittle/ductile transition point and behaving brittlely rather than continuing to flow. (4) The block detaches from dome lobe or lava flow front, preceded by local explosions around the crack “head”. (5) The block fragments progressively from base to top. In many events, fragmentation is instantaneous, which would seem to preclude a contribution to the triggering mechanism by impact with slope. (6) Blocks continue to fragment as the pyroclastic flow moves downslope, providing much of the ash and gases that make up the overriding ash cloud.
The triggers for this process aren’t clear. Some people think that a simple rockfall – where a chunk of dome breaks off because of gravity pulling it downslope – initiates the fragmentation. Another possible trigger could be a small earthquake which causes the pressure situation inside the rock to become too unstable. The process itself has actually been pretty well modeled in the lab; it turns out that the fragmentation occurs in a wave, one “layer” of disrupted rock affecting the next, and so on inward until the whole rock is fragmented. (This also happens in a volcanic vent during an explosive eruption, where it’s usually referred to as explosive fragmentation.) During this process, ash is expelled from the cracks that form in the rock, which in autobrecciation produces all the fine material that forms a cloud around the fragmenting blocks.
Some of the earliest evidence for autobrecciation comes from observations of rockfalls, nuées ardentes and pyroclastic flows generated during the 1974 eruption of Fuego and 1991 eruption of Colima, where researchers were speculating on the mechanisms that could have produced particular pyroclastic deposits. Direct field observations of autobrecciation been made at Mount St. Helens in the US, Mount Unzen in Japan, and Soufriere Hills in the British West Indies. Researchers at Mount St. Helens first noted that relatively small rockfalls from an extruding dome lobe (filmed in April 1982) showed a large amount of fine debris forming as part of the lobe began to fail. Mellors et al. (1988) noted that the phenomenon occurred as the rocks were separating from the lobe, and the production rate of ash increased as the debris avalanche moved downslope. During field visits to the active dome, they also discovered that hot blocks from a young dome lobe would shatter when hit with a hammer, more thoroughly than the impact of the hammer could account for. (This sounds like an really fun and really dangerous thing to be doing to recent pyroclastic deposits, but we’ll give them the benefit of the doubt and assume that they were wearing appropriate safety gear.)
Autobrecciation is a pretty cool process – pretty much the disintegration of solid rock in the course of a few seconds. It’s also potentially a major player in turning rockfalls into pyroclastic flows, a serious hazard on volcanoes (especially ones with lava domes). Although we understand what happens to the rocks involved, we’re still not sure what triggers an autobrecciation event…which could be a thesis topic for another enterprising grad student, yes?
References and additional reading:
Alidibirov, M. and Panov, V., 1998. Magma fragmentation dynamics: experiments with analogue porous low-strength material. Bulletin of Volcanology, 59(7): 481-489. doi:10.1007/s004450050205
Calder, E.S., Luckett, R., Sparks, R.S.J. and Voight, B., 2002. Mechanisms of lava dome instability and generation of rockfalls and pyroclastic flows at Soufriere Hills Volcano, Montserrat. Geological Society, London, Memoirs, 21(1): 173-190. doi:10.1144/GSL.MEM.2002.021.01.08
Davies, D.K., Quearry, M.W. and Bonis, S.B., 1978. Glowing avalanches from 1974 eruption of Volcano Fuego, Guatemala. Geological Society of America Bulletin, 89(3): 369-384. doi:10.1130/0016-7606(1978)89<369:GAFTEO>2.0.CO;2
Martel, C., Dingwell, D.B., Spieler, O., Pichavant, M. and Wilke, M., 2001. Experimental fragmentation of crystal- and vesicle-bearing silicic melts. Bulletin of Volcanology, 63(6): 398-405. DOI: 10.1007/s004450100157
Mellors, R.A., Waitt, R.B. and Swanson, D.A., 1988. Generation of pyroclastic flows and surges by hot-rock avalanches from the dome of Mount St. Helens volcano, USA. Bulletin of Volcanology, 50(1): 14-25. doi:10.1007/BF01047505
Sato, H., Fujii, T. and Nakada, S., 1992. Crumbling of dacite dome lava and generation of pyroclastic flows at Unzen volcano. Nature, 360(6405): 664-666. doi:10.1038/360664a0