Williams, Howel, The Volcano Letter, July-September 1952, number 517, page 7

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VOLCANO NOTES AND NEWS

RECENT ERUPTION ON SAN BENEDICTO ISLAND, REVILLA GIGEDO GROUP, MEXICO

By HOWEL WILLIAMS

The Revilla Gigedo Islands lie 350 to 600 miles west of thc Mexican mainland, approximately on latitude 19 degrees N, in line with the principal chain of volcanoes that includes Orizaba, Popocatepetl, Nevado de Toluca, Parícutin, and Colima. All the islands, with the possible exception of one which may bc composed of granite, consist of volcanic rocks. The two easternmost islands, Socorro and San Benedicto, include several volcanoes that have been active during recent geologic time. The last eruption on Socorro took place in 1848, and solfataric activity still continues there. San Benedicto Island, site of the latest eruptions, is approximately 3 miles long in a north-south direction and 3/4 mile in maximum width. The north end consists of the remnants of a group of coalescing cones of scoria, now deeply eroded and craterless. After these cones had become extinct activity migrated southward to build a cone of white pumice known as the Ash Heap, to a height of 975 feet. Shortly thereafter a new vent opened in the saddle between the Ash Heap and the old cone cluster. First a pumice cone was built then a domical mass of columnar lava rose into the crater. Finally, owing to withdrawal of magma in the conduit, the top of the dome subsided to form a conical depression known as Herrera Crater. The vcnt of the new volcano lies between those of the Ash Heap and Herrera Crater, on a north-south fissure that continues southward to the vents on Socorro Island. Just when activity began is uncertain but probably during July, 1952. The first eruptions to be reported were on August 12, by which time a large cone had already been formed. News of thc activity rcached thc mainland on August 27. On Septcmbcr 13 Robert S. Dictz and Ruy H. Finch flcw to the volcano. It was then about 1,000 feet high and almost a mile across at the base. The intensity of the eruptions had dimhlished greatly. Evcry 20 minutes or so, cauliflower clouds of steam and ash rose from the crater, and the odor of sulphuretted hydrogen was strong. On September 20 the volcano was rc-visited by Dr. Dictz, this time accompanied by the writer. By that time the cone was 1,5000 feet high. Discharge of ash had come to an end; instead, lazy puffs of steam, mingled with hydrogen sulphide, rose only a few hundred feet above thc crater rim at intervals of a few minutes, and on the floor of thc cratcr was a small biscuit-shaped mass of blocky lava a miniature plug. Clearly, there had bcen no further explosive discharge of debris since the plug rose to the surface. Apparently the volcano was approaching extinction; not only was the throat sealed by thc plug, but the vapor pressurc was reduced to a fcw atmospheres. By this time, thc volcano is probably dead.

The San Bnedicto eruption is especially significant as being the first historic pumice eruption in the eastern part of the Pacific Ocean. Propcr idcntification of the naturc of the pumice must await detailed microscopic study, but the white color of the ejecta and the refractive index of the glass suggest that the material is either rhyolite or dacite. Almost surely most of thc new cone was built during the first few weeks, before it was seen. The first explosions seem to have been of vulcanian type, the ejecta being hurled to great heights by strong gas pressure. Subsequently, as the vapor pressure diminished, the vulcanian explosions grcw weaker and were accompanied by explosions that lifted thc ejecta only a short distancc above the crater rim so that they then swept down the outer flanks of the cone as glowing avalanches. Finally, as the vapor pressure continucd to dccrcase, a mass of stiff, viscous lava slowly rose from the conduit onto thc cratcr floor. Few volcanoes have grown as rapidly, few have had such a short life.

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Last Revision June 17, 1995