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Bogoslof reported activity

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EVENT SPECIFIC INFORMATION

Event Name : Bogoslof 1796/5

Start:May 18, 1796 ± 10 DaysObserved
Stop: 1804 ± 6 MonthsObserved

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"Fire", "Glowing", or incandescence: BibCard
Eruption Type:Explosive
MaxVEI: 3 BibCard
Duration: 8 years BibCard
Eruption Product: andesite BibCard
ChemYes
ModalYes
Othermafic

Description: From Miller and others (1998): "A small rocky prominence, called Ship Island or Ship Rock, was observed in 1768 by Russian explorers at the approximate site of Bogoslof Island; it disappeared through marine erosion by the end of the 19th century and little is known concerning its composition and origin (Byers, 1959). An explosive eruption of debris in 1796, followed by extrusion of viscous lava, built a second island by 1804, about 0.6 km south of Ship Rock. The second island was known successively as Ioann Bogoslof, Old Bogoslof, and Castle Rock. The vent agglomerate and hornblende andesite of Castle Rock, exposed on the southwest end of modern Bogoslof Island, represent early and late products, respectively, of this eruption (Byers, 1959)."

There is some discrepancy concerning the start date of this eruption. Grewingk (1850, translated 2003 by Fritz Jaensch) and Dall (1870, 1884) report Baranov writing that this eruption began on the first of May, 1796. Grewingk (1850, translated 2003 by Fritz Jaensch) writes that Kotzebue reported the eruption on the 7th of May (18th new style [Julian vs. Gregorian calendars]). Regardless of the specific date, it seems clear that the eruption did begin in early May,1796.

Grewingk (1850, translated 2003 by Fritz Jaensch) summarizes Baranov's report as follows: "According to Baranov's report there suddenly arose a storm from the north on the first day of May 1796; and the sky grew dark, which lasted the whole day. During the following night the storm increased. On this and the following day a muffled din could be heard, and a far-off crashing sound, which resembled thunder. At the break of the third day the storm abated and the sky cleared. And now a flame was observed between Unalaska and Umnak, and north of the last-named island. It arose from the sea. And soon there was smoke, which lasted ten consecutive days. After that time something white, of rounded shape, was observed rising above the surface of the sea; and it grew very rapidly in size." Grewingk (1850, translated 2003 by Fritz Jaensch) quotes Kotezebue's account of the eruption as follows: "On the 7th (18th New Style) of May 1796, an agent of the Russian American Company, Mr. Kriukov, was on the northernmost point of Umnak. Storm from NW had obscured the view toward the sea. On the 8th, the sky cleared up; and now a column of smoke could be observed rising from the sea a few miles off shore. Toward evening there was something black, which arose beneath the column of smoke just a little above the surface of the water. During the night, fire spouted upward from that location. At times this was so strong, and the amount so large, that on this 10-mile-distant island (25 versts, according to Veniaminov, I., p. 156) all objects could easily be discerned. Now an earthquake shook the island, and a horrible din echoed back from the mountains in the south. The emerging island threw boulders all the way to Umnak. With sunrise the earthquakes abated, the fire diminished. And now the newly emerged island could be seen in the shape of a pointed hat. One month later Mr. Kriukov found the island significantly taller. It had in the meantime continued to expel fire. Since then it has continued to grow in circumference and elevation; but the flames have abated, and only steam and smoke remain to be seen continuously."

Grewingk (1850, translated 2003 by Fritz Jaensch) further summarizes eruption details: "Four years later no more smoke was seen; and eight years later (1804) hunters visited the island. They found that the water was warm; and the ground was still so hot that in many places it could not be walked on. Even a long time thereafter the island continued to grow in circumference and size. A Russian of very healthy judgment reported that the circumference amounted to 2.5 miles; the elevation to 350 feet. In the surrounding 3 miles the sea is covered with rocks. From the middle of the island to the point, he found that it was warm. And the steam, which arose from the crater, had a pleasant odor, probably from the mountain oil." "The elevation of St. J. Bogoslof is probably judged too low, according to Buch. At such a circumference the elevation should easily have been several thousand feet. Langsdorff's expression points up the same thing, when from his own perspective he calls the elevation a medium one. When he got his first view of the island on August 18, 1806, there were four conical mountains visible on the NW side. They rose by steps to a medium and highest elevation. The latter seemed to rise on all sides vertically like a column." Grewingk (1850, translated 2003 by Fritz Jaensch) also writes that Langsdorff report the island to be 10-15 versts in circumference.

Reports of this eruption's impact on human or animal populations is generally not available, although Ransom (1948) writes that "* * * natives huddled in abject terror before their sod houses while fear crawled icily along their spines" and "Whole villages were wiped out as volcanic ash filled the sky and floated like wind-spume on the ocean. There it mixed with millions of bellied-up fish, cooked to a seething pulp by the boiling sea. Then came deluges of drenching rain greater than anything ever before known in those proverbially rainy, stormy regions." Ransom's descriptions are not found in any other historical reference and seem to owe more to imagination than to historical fact.

Newhall and Melson (1983) estimate the size of 1796 lava dome to be about 10x10^6 cubic meters. Based on Sapper's (1927) classification scheme, the Smithsonian Institution's online database of global volcanism estimates a tephra volume of 5.5 +/- 5 x 10^8 cubic meters.

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