A "groovy instability" (and its role in pyroclastic density currents)

 A small volcano, Volcán Bárcena,  480 km off the west coast of Mexico, displays an unusual set of grooves that were carved during an eruption in 1952. The volcano came to life, grew to 335 m elevation, and ended eruptions all in the short span of 7 months.  There were three phases in this eruption: (1) formation of its base by eruptions starting on August 1; (2) creation of a large crater in the summit; and (3) eruption of lava at the base.  The eruption was only intermittently observed and those observations were documented by Adrian Richards, a scientist who was working in the region in 1952 and undertook to document the "terrestrial and submarine geology" of the area.  His observations were only the second observations of the evolution of a new volcano following the observations of Paricutin, Mexico, which erupted from 1943 to 1952 through a cornfield in Mexico.

      Richards argued that the straightness (lack of sinuousity) and lack of dendritic pattern of the grooves indicated that they were not formed by rain (seev^^^below), but they were formed by erosion by "tephra avalanches." He avoided the use of the term "pyroclastic flow" because he did not see the incandescence implied by the term "pyro."

It is not clear when the grooves were formed, but almost certainly as one of the last stages of the eruption sequence. Fortunately, Adrian Richards  listed seven features of the grooves that must be addressed by any theory for their origin: (1) their origin close to the crater rim; (2) U-shaped profiles in cross section; (3) the straightness of the grooves; (4) the (generally) non-coalescing nature of the grooves; (5) increasing width downstream; (6) "turbulent patterns" on the cone, which are interpreted to be a herring bone pattern; and (7) local deposition at the lower ends.

In a new paper (March 2021) Kieffer et al. (2021)*** propose that the grooves were carved into an erodible substrate (volcanic ash, cinders) during passageof a high-energy pyroclastic density current (PDC) that contained streamwise vortices. In a survey of the volcanic literature, we found several volcanoes that have streamwise grooves, some of which--but not all--might have been carved the the same mechanism that we propose for the grooves at Volcán Bárcena. 

We suggest that the grooves terminated in the downstream direction by passage of the supercritical flow in which they were embedded through a hydraulic jump. The jump caused the flow to decelerate, resulting in the formation of dunes.

 

***Kieffer, S.W., Meiburg, E., Best, J., and Austin, J. The mysterious grooves of Volcán Bárcena: a review of the role of streamwise counter-rotating vortices during erosion by dilute pyroclastic density currents. Bulletin of Volcanology v. 83, article number 26 (2021). 

^^^Footnote regarding the effect of rain on the grooves: The two images below show the effect of rain on the initially straight channels, with one image taken in ~1952 and the other in 2005 or 2006 (Google Earth). 

  (left) Barcena from 1952 (Richards) and (right) 10/27/2006 (Google Earth). 

 

In the early image small sinuous rills are confined inside the larger structure of the grooves, indicating that the grooves were a preexisting structure when the rills formed. By the time of the later image, the sinuosity is more developed, the rills have been deepened and have a coarser transverse structure (wavelength). The effect is particularly pronounced where the sinuous channels have cut through the dunes (lower right in these two images; another image below).