Coastal Geology of Upolu, Western Samoa

Abstract

Upolu is a volcanic island surrounded by fringing and barrier reefs. Maximum altitude of the rugged interior is about 1150 m. The volcanic rocks range in age from late Pliocene to Holocene and include basaltic lavas, scoria cones, pyroclastics, and tuffs. The coast of Upolu can be subdivided into the following three basic types based on reef characteristics, depositional features, and hinterland geology: wide fringing reef transitional to a shallow barrier reef; cliffed coast with little or no reef development; and fringing reef and narrow coastal strip consisting mostly of beaches, barrier spits, and coastal swamps associated with streams. Engineering structures such as seawalls and groins are common along many areas of the coastline, indicating a widespread coastal erosion problem. Causes for the erosion appear to be (a) possible relative rise of sea level enhanced by island subsidence, (b) reclaimed areas and engineering structures extending beyond the natural shoreline in areas prone to wave attack interrupting littoral transport of sediment, and (c) beach mining of sands and gravels for construction purposes. The well-developed fluvial drainage systems and the wide fringing and barrier reefs are conducive to the formation of numerous sites for potential nearshore construction materials. The potential for economically viable placer mineral resources appears to be limited because good source rocks are not present and the nearshore depositional systems are of relatively small size.