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The two triangle-shaped islands of Ofu and Olosega in eastern Samoa, with a combined length of 6 km, are separated by a narrow strait. The islands are formed by two eroded, coalescing basaltic shield volcanoes whose slopes dip to the east and west. Steep cliffs up to 600-m high truncate the northern and southern sides of the islands. The narrow, steep-sided ridge forming the eastern tip of Ofu Island consists of a dike complex. The shield volcano on Ofu is cut on the north by the A'ofa caldera; bathymetry suggests that a caldera may also exist on the Sili shield volcano of Olosega. The Nu'utele tuff cone, forming a small crescent-shaped island immediately off the west end of Ofu Island, is Holocene in age. A submarine eruption took place in 1866 at the opposite end of the two islands, 3 km SE of Olosega, along the ridge connecting Olosega with Ta'u Island.
Summary of Holocene eruption dates and Volcanic Explosivity Indices (VEI).
|Start Date||Stop Date||Eruption Certainty||VEI||Evidence||Activity Area or Unit|
|1866 Sep 12||1866 Nov 15 ± 5 days||Confirmed||2||Historical Observations||Submarine vent 3 km SE of Olosega|
This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.
|Feature Name||Feature Type||Elevation||Latitude||Longitude|
|A'ofa||Shield volcano||494 m||14° 10' 30" S||169° 39' 47" W|
|Fatuaga Point||Cone||14° 10' 30" S||169° 38' 13" W|
|Maga Point||Cone||14° 11' 53" S||169° 37' 0" W|
|Nu'u||Tuff cone||14° 10' 0" S||169° 40' 59" W|
|Tauga Point||Tuff cone||14° 10' 5" S||169° 40' 44" W|
|Clouds drape the tops of the two triangle-shaped islands of Ofu (left) and Olosega (right) in eastern Samoa in this Space Shuttle image (with north to the upper left). The islands, 6 km in combined length, are separated by a narrow strait. They are formed by two eroded, coalescing basaltic shield volcanoes truncated by mostly submarine calderas. A submarine eruption took place in 1866 about 3 km SE of Olosega, along the ridge connecting Olosega with Ta'u Island.
NASA Space Shuttle image ISS002-E-6878, 2001 (http://eol.jsc.nasa.gov/).
|A narrow strait (just left of center) separates the two triangle-shaped islands of Ofu (left) and Olosega (right) in eastern Samoa. The islands are formed by two eroded, coalescing basaltic shield volcanoes. The narrow, steep-sided ridge forming the western side of Ofu Island is cut by volcanic dikes, and an intrusive plug forms the sharp spire at the left on Ofu Island. A submarine eruption took place in 1866 at the far end of the two islands, 3 km SE of Olosega, along the ridge connecting Olosega with Ta'u Island.
Photo by Peter Craig, 1995 (U.S. National Park Service).
The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title.
IAVCEI, 1973-80. Post-Miocene Volcanoes of the World. IAVCEI Data Sheets, Rome: Internatl Assoc Volc Chemistry Earth's Interior..
Richard J J, 1962. Kermadec, Tonga and Samoa. Catalog of Active Volcanoes of the World and Solfatara Fields, Rome: IAVCEI, 13: 1-38.
Stearns H T, 1944. Geology of the Samoan Islands. Geol Soc Amer Bull, 55: 1279-1332.
Stice G D, 1968. Petrology of the Manu'a Islands, Samoa. Contr Mineral Petr, 19: 343-357.
Stice G D, McCoy F W, 1968. The geology of the Manu'a Islands, Samoa. Pacific Sci, 22: 427-457.
Walker G P L, Eyre P R, 1995. Dike complexes in American Samoa. J Volc Geotherm Res, 69: 241-254.