This is part of a series of illustrated articles taken from ‘A Visitor’s Field Guide to Savaii’ by geologist, Tuapou Warren Jopling. Tuapou takes tour groups around Savaii and his knowledge is legendary!
The North Savai’i (Ologogo) Fault
Landslides make the news all too frequently. Never a month goes by without some disaster in China, Peru, Nepal, wherever. Most are local when part of a mountain collapses but what you are looking at from this steep hill overlooking Sataua was gigantic. A great slice of Savai’i dropped and slumped northwards towards the ocean. (Fig 9)
Figure 9 is adapted from the original 1956 field mapping of New Zealand’s Geological Survey. It shows a curved fault from Sataua to Safune and two step faults further east - a slump 42 km across and extending 13 km inland.
The broad curvature of the main fault indicates the collapse of a thick stack of poorly consolidated volcanic ejecta and flow lavas, probably destabilized by rapidly changing sea level. The fault displaces Salani and Mulifanua beds and is partly covered by Puapua and Aopo flows.
Consequently it is young, probably relating to rising sea of the last glacial meltdown. Cones of late-Mulifanua age and those of the 1760s and 1902 eruptions are on or close to the surface trace of this main fault.
It has a vertical displacement of 150m south of Aopo and cuts the coastline at Safune in the east and Sataua in the west forming steep scarps into these villages. Visitors stop to photograph the magnificent coastal panoramas from the road above these villages without realizing the geological significance of these slopes.
Cones are concentrated on Savai’i’s high central plateau with many lining the fault on both the upthrown and downthrown sides. Neither this faulting nor that of the south-western coastline has been dated but both are geologically young and possibly concurrent, one triggering the other.
Savai’i’s coastline has changed considerably over the past 20,000 years by rising sea, faulting and volcanism. Coastal Savai’i was submerged by the 120 m post - glacial rise in sea level and faulting has displaced large coastal section into the ocean. Conversely, eruptions may have heightened parts of the interior but have certainly increased land area within the past 5,000 years by lava flows filling lagoons.
Savai’i has been extremely active volcanically and tectonically during the past 20,000 years. Such activity is predicted to continue until the island has moved to the northwest beyond the influence of the Tonga Trench.
Lava tubes of Aopo, Letui and Paia
A lava tube (or tunnel) forms when molten lava is channelled into a topographic depression such as a dry stream bed. The surface cools and consolidates insulating the underlying flowing lava from the air. Flow continues with diminishing volume until the eruption peters out leaving a 4 m to 5 m diameter sloping tunnel with a flat floor.
Savai’i’s 20 known lava tubes have been exposed by surface collapse but there are probably many more. Three tubes of this northern area are open to visitors; one a few kilometres west of Aopo (but not yet seen by this writer) is highly recommended by visitors; a 40 m long tube near Letui is alongside the road, convenient for a quick visit and the Dwarfs Cave 3 km from Paia is for adventure seekers. Enter via a steep slope into a large underground chamber then continue downwards by short steep drops, each separated by a shallow pool.
Lava tubes are exciting but don’t enter without a local guide and adequate lighting. Take your own flashlight as a back-up. Polynesian white- rumped swiftlets nest in them. Dont panic if you’re told they’re bats.
ABOUT THE AUTHOR
Tuapou Warren Jopling is an Australian geologist who retired to Savai’i to grow coffee after a career in oil exploration in Australia, Canada, Brazil and Indonesia. Travels through Central America, the Andes and Iceland followed by 17 years in Indonesia gave him a good understanding of volcanology, a boon to later educational tourism when explaining Savai’i to overseas visitors and student groups. This article is taken from ‘A Visitor’s Field Guide to Savai’i’.