Geological notes on Western Samoa
Environmental Monitoring and Governance, Island and Ocean Ecosystems
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.