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OCEANIA (INCLUDING FIJI, PNG AND SOLOMONS) 121 the Lau basin, an area of attenuated crust and high heat flow that is interpreted as an extensional back-arc basin Samoa The Samoan archipelago lies between latitudes 13 and 15 S and longitudes 169 and 173 W and extends for 500 km on azimuth 288 , from Rose Island in the east–south-east to Savaii Island in the west–north-west The archipelago comprises a chain of hotspot-related intraplate volcanoes made up of the alkaline and tholeiitic basalts that are typical of Pacific island chains The two largest islands are Savaii (1700 km2) and Upolu (1115 km2) These, together with the small islands of Manono and Apolima, comprise Western Samoa Savaii is the summit of a large basaltic volcano Many small parasitic cones occur about its flanks and the lavas of the historical eruptions of 1760 and 1905–11 are widely distributed across the northern part of the island On Upolo Island, young cones and lava flows overlie older, deeply eroded volcanic sequences that form spectacular topography on the eastern end of the island Similar geology is seen on Tutuila, the third largest island, where posterosional lavas overlie an older group of volcanics Further east, the volcano chain is reduced to subaerial remnants in the Manu’a islands and the easternmost Rose Atoll Pacific island chains typically increase in age westward Dating of Samoan lavas and dredged tholeiitic samples from seamounts west of Savaii confirm that this is the case in the older volcanic rocks of the Samoan chain For example, shield volcanics on Upolu range in age from 2.8 Ma to 1.7 Ma and on Tutuila, to the east, range from 1.54 to 1.28 Ma The post-erosional volcanic activity on Savaii, Upolu, and Tutuila, from 700 000 years ago to the present, does not fit the ‘older westward’ pattern Possibly this is a side effect of the sharp change in the orientation of the Tonga–Kermadec Trench which, at a point about 100 km south of the Samoan islands, swings from a near north–south azimuth to near east–west (Figure 1) Flexural bending of the Pacific plate at this point presumably could cause rifting in the Samoan archipelago and thus provide a conduit for renewed volcanic activity See Also Gondwanaland and Gondwana Plate Tectonics Tectonics: Convergent Plate Boundaries and Accretionary Wedges; Hydrothermal Vents At Mid-Ocean Ridges Further Reading Coleman PJ (ed.) (1973) The Western Pacific: Island Arcs, Marginal Basins and Geochemistry Perth: University of Western Australia Press Hillis RR and Muller RD (2003) Evolution and Dynamics of the Australian Plate Geological Society of Australia Special Publication 22 Sydney: Geological Society of Australia Kroenke LW (1984) Cenozoic Development of the South west Pacific SOPAC Technical Bulletin Suva: South Pacific Applied Geoscience Commission Veevers JJ (ed.) (2000) Billion Year Earth History of Australia and Neighbours in Gondwanaland Sydney: GEMOC Press, Macquarie University General The following Earth Science Series, publications on The Geology and Offshore Resources of Pacific Island Arcs, are published by the Circum Pacific Council for Energy and Mineral Resources (Menlo Park, CA) and are avail able from the AAPG Bookshop, PO Box 979, Tulsa, OK, USA Series 1: Howell DG (ed.) (1985) Tectonostratigraphic Terranes of the Circum Pacific Region Series 2: Scholl DW and Vallier TL (eds.) (1985) Tonga Region Series 3: Brocher TM (ed.) (1985) Geological Investigations of the Northern Melanesian Borderland Series 4: Vedder JG, Pound KS, and Boundy SQ (eds.) (1986) Central and Western Solomon Islands Series 7: Taylor B and Exon NF (eds.) (1987) Marine Geol ogy, Geophysics and Geochemistry of the Woodlark Basin Solomon Islands Series 8: Greene HG and Wong FL (eds.) (1988) Vanuatu Region Series 9: Marlow MS, Dadisman SV, and Exon NF (eds.) (1988) New Ireland and Manus Region, Papua New Guinea Series 12: Vedder JG and Bruns TR (eds.) (1989) Solomon Islands and Bougainville Fiji Colley H and Flint DJ (1995) Metallic Mineral Deposits of Fiji Fiji Mineral Resources Department Memoir Suva: Fiji Mineral Resources Department Hathway B and Colley H (1994) Eocene to Miocene geol ogy of southwest Viti Levu, Fiji In: Stevenson AJ, Herzer RH, and Ballance PF (eds.) Geology and Submarine Re sources of the Tonga Lau Fiji Region SOPAC Technical Bulletin 8, pp 153 169 Suva: South Pacific Applied Geoscience Commission Rodda P (1994) Geology of Fiji In: Stevenson AJ, Herzer RH, and Ballance PF (eds.) Geology and Submarine Re sources of the Tonga Lau Fiji Region SOPAC Technical
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