Ofu-Olosega Volcano | John Seach



14.17 S, 169.61 E
summit elevation 639 m
shield volcanoes

The islands of Ofu and Olosega are located in in eastern Samoa. They are separated by a narrow strait, and are composed of two eroded, coalescing basaltic shield volcanoes.

1866 Eruption
A submarine eruption occurred in 1866 at the opposite end of the two islands, 3 km SE of Olosega, along the ridge connecting Olosega with Ta'u Island. On 7th September 1866 residents of the islands of Ta'u and Olosega felt 3-4 earthquakes in an hour. On 9th September 39 earthquakes were felt.

On 12th September an undersea eruption was observed between Ta'u and Olosega and appeared like surf breaking on a sunken rock. Initially observers thought it was a whale, or a shoal of fish, however the activity lasted all day. By daybreak on the 13th September eruptions were occurring every hour, and increased to 50 per hour on 15th September. An eruption column rose 2000 ft high. The sea was agitated in a circular area of half a mile in diameter. Sulphur was visible on the ocean surface 10 miles away.

Dead fish were washed up on the islands of Olosega with Ta'u, including large species up to 12 feet long, which the locals had never seen before. Heat, ash, and gas emissions made residents evacuate to the south of Olosega Island. Activity decreased after 3 days, and by 11th November there were 3 eruptions per day.

Further reading
Hart, S.R. and Jackson, M.G., 2014. Ta'u and Ofu/Olosega volcanoes: The “Twin Sisters” of Samoa, their P, T, X melting regime, and global implications. Geochemistry, Geophysics, Geosystems15(6), pp.2301-2318.

Ofu-Olosega Volcano Eruptions