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| Furrows on the flank of Barcena filmed Sept. 12, 1952. |
Very few volcanoes have been observed from birth through growth, with Paricutin, Mexico, being the best documented because it was on land. Barcena was visited only briefly during the 8 months of its growth. It was initially studied by Robert Dietz, who later became famous for his pioneering works on both meteorite impacts and sea floor spreading. Dietz (1914-1995) was searching some Navy records for sounds coming from Barcena. He didn't find them but, instead, found explosions arriving from Myojin Reef volcano which he was able to show had blown up and sunk a ship taking all 31 hands aboard to their death. According to Richards (p. 87) volcanologists appeared unaware of this eruption, and Dietz only became aware of it on August 27 when he read about it in the Los Angeles Examiner. Dietz was on two airplane flights (Sept. 12 and 20) over the volcano, but apparently never wrote up his observations, at least in easily accessible literature. (Richards, p. 89) says that Dietz interviewed the pilot who visited on August 12 (see photo below of density current) and compiled a "mimeographed report". The pilot took two 16 mm Kodachrome motion picture films which Richards used for his documentation. The visit of this clipper (the M/V Intrepid) and its plane is the only source of information on Barcena between August 5 and Sept. 12.
Our knowledge of the events at Barcena stems almost solely from a paper** by Adrian F. Richards of the U.S. Navy Hydrographic Office, although two famous geologists, Dietz and Howel Williams (volcanology) both observed the island from two airplane overflights. Williams made a geologic sketch map of Isle San Benedicto from the plane observations and later wrote a brief statement published in something called "Volcano Letter" which we are trying to get hold of at the moment. The Richards paper is part of his 1957 Ph. D. dissertation at UCLA. In it, he says (p. 77) "The majority of the observations......during the period of cone building and lava extrusion were by the men of the California tuna fishing fleet." Specifically, and importantly for the question of when the furrows in the photo shown were formed, three men from the tuna fleet visited on August 1, 1952 on a clipper, two others on August 12 on a different clipper and its seaplane, and three others, including Dietz, on September 12 on a U.S. Air Force B29, 55th Strategic Reconnaissance Squadron.
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| This photo was taken on August 12 by T. Howell from seaplane. |
The eruption began about 0745 on August 1, 1952 with a brief thin pencil-like column of steam ; After a few minutes, this steam column dissipated and a dark gray-black column of ash and steam "shot skyward." This eruption cloud almost immediately began to spread laterally at the base.
Richards sorted through various eyewitness reports and speculations and concluded, along with Williams, that most of the new cone was built to an altitude of 1000' in the first few weeks (p. 92). Richards (p. 89, Table 2) pointed out that "the most striking feature of the initial eruptions" was the fact that the horizontal spread of the eruption column was much more rapid than its vertical rise, and he concluded that it "resembled the base surge of an atomic explosion." At one point (time is not well documented in Richards paper), the altitude of the plume was about 4500 feet and the "maximum lateral extent" (diameter) at the same time was about 11,000', so that the width/height ratio of the plume was 2.7.
Richards considered four possible processes for creating the grooves. He eliminated rain erosion because the furrows did not show dendritic geomorphology; avalanches of bombs because at Paricutin furrows created by bombs were curved, rather than straight and no bombs were found at the base of Barcena; and ash landslides because the furrows originate too far up on the rim for such a process to have occurred. He settled on tephra avalanches or, here, density currents.
On August 12, a density current was photographed (the image shown here is an "unretouched enlargement from a 16 mm duplicate Kodachrome motion picture frame.") Unfortunately no scale is given for the photo, and the only identifiable feature is the crater rim in the upper middle left of the frame. From the motion pictures, Richards stated the following: the tephra avalanche appeared to have a rolling rather than a sliding motion. Bbut what does that mean? It's not a description commonly used in modern volcanology literature. Like other avalanches that he observed at Barcena, the tephra avalanche appeared to issue from the lowest point in the crater rim, and Richards states that "there appeared to be a gaseous dilation from the center of the avalanche." I take the latter to mean that it did not act like an incompressible gravity current, but expanded both vertically and laterally due to gas pressure. There was, unfortunately, not enough resolution in the films to tell whether this avalanche furrowed the cone. During the time that it was spilling from the crater, a vulcanian column of dark ash and gas rose skyward. Slight changes in slope caused "pronounced differences in the behavior of the avalanches."
What was the role of tephra avalanches in building the cone versus eroding the furrows? In Richards summary of the "envisioned activity" (p. 112), he indirectly addresses this question. He concluded that the cone grew rapidly (i.e., over several weeks in early August) by "tephra fallout from the eruptions and deposition from the tephra avalanches." Then, between mid-August and September 12 when it was not observed, the violent phase of cone formation tapered, and tephra avalanches became intermittent. However strong, vertical vulcanian eruptions continued through this period. By September, he says, they probably "slid and eroded" instead of being "rolling" and "caused the formation of the furrows.
Richards description does not allow us to state whether one tephra avalanche event could account for all the furrows, or if there were multiple events. We know, from observation of furrows created by the lateral blast at Mount St. Helens that a single event with a duration of probably only tens of seconds can erode furrows of comparable or even larger scale. Probably the only conclusion that we can draw from Richards accounts and reasoning is that the furrows were carved by one or more tephra avalanches.
