Today you will be led through the Falealupo Canopy Walkway before moving on to look at the Mauga Afi eruption and the Aopo lava field.
Falealupo Canopy Walkway
The magnificent rainforest of 30 years ago has largely disappeared from the area surrounding the canopy walkway so climb it for a geological experience.
There are four young Puapua cinder cones nearby, the small one across the road from the walkway being of interest.
It has been quarried for road construction material leaving an open-cut face showing cinders top and bottom and an irregularly shaped lens of flow lava in between.
Now a word about eruptions in general. Magma, i.e lava while still in the ground, contains an enormous volume of water and dissolved gases under pressure.
On nearing surface gases come out of solution, steam being the driving force of every eruption - either blowing or flowing. Eruptions will often start with magma being ejected by gas, particle size varying from fine ash to bombs, then later by flow. The cone across the road had a fluctuating steam drive.
Mauga Afi (Fire Mountain)
Eruption & Aopo Lava Field
The Mauga Afi eruption was recorded by the French navigator Anton Bougainville when sailing by Savai’i’s north coast in 1768.
It has been ranked as one of the world’s largest lava outflows but, as explained later, this needs re-defining.
The outbreak was from two cones spaced 3 km apart on Savai’i’s high central plateau. Lava from these coalesced and flowed down the island’s steep northern flank forming a wide lava fan over the coastal plain. There is no record of the start or end of this eruption but it covered a very large area to an unknown thickness.
An old legend tells of 100 buried villages - probably an exaggeration but certainly indicating a high pre-eruptive population. Samoans, then as now, lived in villages by lagoons. The basalt cliff along the still smooth coastline probably built up over a coral reef, events that were replicated 150 years later during the Matavanu eruption.
The inland road from Asau to Aopo crosses the 2.5 km wide lava flow about 170 m above sea level. Vegetation changes abruptly to dense scrub.
Rocktype also changes to loose, angular basalt blocks called a’a lava - a Hawaiian term to distinguish it from the smooth ropey pahoehoe lava you will see later at the Saleaula lava field. Several openings in the vegetation along the road allow views over the lava fan - an endless uninhabited wasteland.
You might puzzle over the irregular outline of the Aopo lava field (coloured yellow in figures 6 and 7).
Molten lava flows downhill, can spread laterally over flat surfaces but cannot flow uphill. You are now on the flow path between the two Mauga Afi cones and the lava fan but the areas to the east coloured yellow cannot be part of the same eruption.
Their scrubby vegetation indicates a very young age, probably late Puapua.
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 the fourth part of a series and is taken from ‘A Visitor’s Field Guide to Savai’i’.