Volcanic Geoheritage of Landslides and Rockfalls on a Tropical Ocean Island (Western Samoa, SW Pacific)

Abstract

Landslides and rockfalls on volcanic islands in tropical climate are characteristic landscape shaping features. Their common formation poses potential hazard for island communities; hence, understanding their formation and recognition on modern landscapes are vital element of educating local communities and providing mitigation strategies for future events. As landslides and rockfalls are continuously shaping tropical islands’ landscapes, they contribute significantly to the volcanic geoheritage of those islands. Rockfall and landslide hazards are commonly associated with the Fagaloa Formation and Salani Formations of Western Samoa in the SW Pacific. Four case studies (Mauga-o-Fao, Mauga-o-Vaea, Fagaloa Bay, and Leagi’agi Hill) are reported here, based on highly populated areas which are predicted to be vulnerable in generating rockfalls and landslides in the future. Field observations and interpretation of aerial photographs and satellite images were used to identify landslide and rockfall hazards in the region. The lack of records and previous studies related to rockfalls and landslides in the region is the major challenge for this investigation. Commonly, rockfall and landslide scenarios in Western Samoa are associated with the presence of high angle faults, highly weathered, extensively jointed, and fractured rocks. Surface and groundwater could expand the size of the joints, and existing fault scarps trigger the rock face to become more unstable by losing its support. There are two networks of jointing patterns commonly occurring in both old (3 Ma to 800 ka) and young (200 ka to 3 ka) volcanic rock formations: parallel and perpendicular with lava flow axis. These joint networks would increase the instability of tabular lava flows especially nearby to fault scarps and thick columnar jointed lavas. It is suggested that the major faults on the main islands almost running perpendicular to the central volcanic rift (elongated north east to south west) could be other main drivers of the rockfall and landslide hazards in the region.