16 January 2011
This post is a precursor to this month’s Accretionary Wedge, where Mountain Beltway’s Callan Bentley has asked us to communicate geology via our favorite tasty treats in a geologic bake sale! I’m quite excited about this, and I’ve got an idea for a baked good that will be featured in a later post to help illustrate the geology I’ll be discussing here.
In my first quick Google search about flow banding, I came up with Cole Kingsbury’s fairly new blog, Chaotically Flow-Banded. He gives a good basic description of what flow banding is: visually distinguishable “bands” or layers in volcanic rock that differ based on composition, texture, or geochemical characteristics. Cole focuses on flow banding in lava, which is a familiar feature to him because of his work on the Obsidian Dome near Long Valley in California. I’m also working on domes now (although slightly less felsic ones), but my first experience with flow banding came from mapping Miocene ash flow tuffs in Utah.
My undergraduate research was conducted on the Fish Lake Plateau in South Central Utah, where I was investigating the geochemistry and structure of the Osiris Tuff, a 23 Ma welded ash flow tuff formed from the caldera collapse of the Monroe Peak caldera, about 40 km to the west. (An ash flow tuff is a pyroclastic rock composed of more than 50% ash, or >2mm -sized particles.) The eastern edge of the Fish Lake Plateau is one of the last places you see the Osiris before you start seeing older tuffs and the sedimentary rocks that underlie the High Plateaus of Utah, and there are some lovely cliff and streambed exposures of the unit. In those exposures, it’s easy to see some pretty dramatic banding:
The “flow” in these examples doesn’t refer to a lava flow, however. Since the Osiris is an example of a “high-grade” tuff or ignimbrite (one that retained enough heat to weld into rock), it likely experienced something called “rheomorphic flow”. I haven’t examined the bands closely, so I don’t know if they’re primary (layering formed as the ash flows were depositing) or secondary (geochemical or textural segregation), but they are certainly deformed, and in pyroclastic rocks this means rheomorphic flow. Rheomorphism is the ductile deformation of hot, welded pyroclastic material – deposits from pyroclastic or ash flows that is hot enough to begin adhering together into rock during and after deposition.
There are multiple ideas for the origins of this deformation. Some say that it is a primary feature, occurring during deposition when parts of the ash flow cool enough to undergo viscous flow. Others suggest that it occurs after deposition but before the tuff has cooled past the brittle-ductile transition point, when gravity can pull it downhill (much like syrup or other viscous fluids). A third suggests that some flow structures may form when an underlying cooled deposit is ‘pulled’ along by shear from another pyroclastic flow passing over it. (This probably wouldn’t account for all of the rheomorphic structures in a deposit, since it would be confined to the upper portions, but could be responsible for some.) Post-depositional deformation seems to show up more often in the literature I’ve looked through, but there’s no reason that all three processes couldn’t be operating to some extent in a pyroclastic deposit.
Utah isn’t the only place I’ve seen this kind of feature in pyroclastic rocks; there’s also a lovely example at the Burro Mesa “pouroff” in Big Bend National Park. The rock in the photo below is part of the Wasp Spring Member of the Burro Mesa formation; it’s a ~ 30 Ma rhyolitic(?) ignimbrite that’s exposed in the plunge pool of a waterfall at the south end of Burro Mesa in the western part of the park. The trail to get there is strewn with examples of flow banding, but this is one of my favorite examples (and one of my favorite photos overall):
Keep an eye out for the next post about flow banding…in cake!
- Branney MJ., Kokelaar P. (1992) A reappraisal of ignimbrite emplacement: progressive aggradation and changes from particulate to non-particulate flow during emplacement of high-grade ignimbrites. Bulletin of Volcanology. Vol 54, pp 504–520
- Branney M.J., Barry T.L., Godchaux M. (2004) Sheathfolds in rheomorphic ignimbrites. Bulletin of Volcanology. Vol 66, pp 485–491
- Chapin C.E., Lowell G.R. (1979) Primary and secondary flow structures in ash-flow tuffs of the Gribbles Run paleovalley, central Colorado. Geology Society of America Special Paper. Vol 180, pp 137–154
- Fisher, R.V. and Schmincke, H.-U. (1984) Pyroclastic rocks. Springer-Verlag, 472 p.
- Gray, J. E. and Page, W.R. eds. (2008), Geological, Geochemical, and Geophysical Studies by the U.S. Geological Survey in Big Bend National Park, Texas. U.S. Geological Survey Circular 1327, 104 p.
- Ragan D.M., Sheridan M.F. (1972) Compaction of the Bishops Tuff. Geology society of America special bulletin. Vol 83, pp 95–106
- Schmincke, H.-U, Swanson D.A. (1967) Laminar viscous flowage structures in ash-flow tuffs from Gran Canaria, Canary Islands. Geology. Vol 75, pp641–664
- Wolff J.A., Wright J.V. (1981) Rheomorphism of welded tuffs. Journal of Volcanology and Geothermal Research 10: 13–34