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!
TOURING WITH TUAPOU
Today, your tour will take you to the lava field, you will learn about volcanic shields and receive am indepth talk about what happens during eruptions! Listen carefully.
SALEAULA LAVA FIELD AND RUINS, ERUPTIONS
Some 76 sq km of northeastern Savai’i’s farmland and lagoon were overwhelmed by the Matavanu eruption, four villages were completely buried and Saleaula, the most northerly village, was partly buried. To-day, the long, wide coastal strip of black ropey (pahoehoe) basalt makes an awesome tourist attraction.
Once past Saleaula village the next 2km of road follow the western boundary of the lava field. Walk a few hundred metres to one of the raised pressure ridges to overlook this great, desolate expanse of black basalt with the ocean to the east and the wide arch of Savai’i’s skyline to the west. You will understand the term ‘volcanic shield’.
The surface is a combination of smooth pahoehoe lava showing intricate flow patterns and patches of jagged blocks broken by the pressure of either escaping steam or oncoming molten flow. You might also note that in many places a thin surface bed overlies increasingly thick underlying layers. This is common, the intense early eruptive stage diminishing with time.
Lava ceased spreading over land by late September, 1906 but continued by lava tube until late 1911. A local resident reported in August, 1908, “That the lava continued to run strongly into the sea.” Lava dropping into seawater fragments into black sand. Much of this would have been dispersed by waves and currents but sufficient sand was washed back over the cliffs during the 1990 and 1991 cyclones to start a successful black-sand block industry.
A less successful venture was a coconut plantation. A village family built a rough vehicle track across the lava field to the shoreline, planted hundreds of sprouting coconuts in straight lines but today, 18 years after the planting, many are still alive but only a little taller than the day of planting. Black sand is raw mineral not yet weathered into clay - a requirement for most plant life. You are recommended to walk to the shoreline, not to visit a failed coconut project but to see the jumble of enormous boulders lifted over the cliff by the 1990 and 1991 cyclones - some weigh 30 tons or more. Refer to the front page photograph.
Before visiting the Saleaula ruins, a word about eruptions in general.
ERUPTIONS – THE DANGER FACTOR
All magmas consist primarily of oxygen, silicon, aluminium, iron, magnesium and calcium in different proportions but it is silicon content which largely determines if an eruption will be mild or explosive. The basalts of the Pacific Plate and its intra-plate islands (Hawaii, Samoa) have low silicon content, all of which is combined with the metallic elements as minerals called silicates.
There is no free quartz (silicon dioxide). These lavas have temperatures of about 1200oC on emission and consequently are sufficiently fluid to allow dissolved gasses to come out of solution and escape at the volcano’s vent. Gas pressure doesn’t build up in the volcano causing it to explode. Steam is a gas and the main driving force in all eruptions.
Magmas with high silicon content can be very dangerous. Volcanoes over subduction zones (where a plate of oceanic basalt subducts under and melts an overlying continental plate of highly siliceous rocktypes) can explode violently. Temperature of a siliceous magma on emission can range from 800oC to 1000oC. This cooler magma can be too viscous to allow gasses to escape. Gas pressure will mount until the volcano explodes releasing a searing, rapidly expanding cloud of steam and toxic gasses charged with ash.
Some readers would know about the 1902 eruption of Mont Pelee, Martinique which completely destroyed the original capital St Pierre killing 28,000 residents; and many readers would remember the 1980 eruption of Mt St Helens, Washington State, which killed 57 old-timers who refused to evacuate.
These are Pelean eruptions where a pyroclastic flow (or nuee ardente) blasts laterally and, together with Plinian eruptions where the blast is vertical, are the most dangerous. (Pliny the Elder died in the AD 79 eruption of Vesuvius). The Samoan eruptions are at the bottom of the danger list and are called Hawaiian - type or moderate eruptions. The Matavanu eruption ruined a large area of agricultural land, caused
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’.