Geological notes on Western Samoa

Abstract

Four of the six main volcanic formations forming the bulk of Western Samoa comprise interbedded pahoehoe and aa-like scoriaceous rubble in a ratio of between 1 : 2 and 1 : 4. All presumably have very similar bulk densities. The Lefaga Volcanics have substantially the highest void ratio, but are of limited distribution. The Fagaloa Volcanics, the oldest of which have reverse polarity implying an age greater than 1,000,000 years, form much of Upolu and Savai'i at depth, particularly in the east. They have significantly lower void ratios and higher densities than most other rocks, due to compaction before erosion, to vesicle filling, and to deep weathering. There is thus a marked tendency for bulk density to increase with depth, made the more marked because relatively dense vitric tuffs, formed like the young Vini Tuff, by eruptions directly into the sea, are presumably more common in the lower part of the volcanic pile. Faulting features include the major 110°-aligned fractures of the Pacific sea floor and minor gravity-induced faults, particularly on Savai'i, caused by collapse of the flanks of the volcanic pile. All are the sites for subsequent volcanic eruptions and for the congealing of lava into closely-packed dikes. Major unconformities, possibly corresponding to abrupt physical changes, occur within and above the Fagaloa Volcanics.