Down Under in Savai’i

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Satuiatua, Coral Sand & Beachrock.

Satuiatua, Coral Sand & Beachrock.

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 fabulous Taga blowholes and then on to the coastal village of Satuiatua. 

 

Taga – Alofa’aga Blowholes

There are many blowholes around Savai’i’s basalt shoreline but those at Taga are one of the Pacific’s best tourist attractions. Local legend states that two drowned Tongan princes were washed ashore where their bodies rotted through a low-lying coastal shelf to from blowholes. A more scientific explanation is that wave-cut caves underlie the shelf, each with an overhead opening, that of the main blowhole being a 1.3m diameter fumarole. Waves surge into the cave, compress with air, and spray shoots skywards with a thunderous roar rising 30 metres (providing the trade wind is not blowing too strongly). A popular ploy is to toss coconut husks into the hole. With strong swell and split-second timing they rise 50 metres. Stand up-wind. The weaker blowhole blows spray seawards at a 45 degrees angle through disarranged surface blocks. 

Pacific guide books state the best time to visit is high tide. This might help marginally if swell is weak but don’t plan your day around a tide timetable. The blowholes are great at any time. And if you are visiting between August and mid-November keep an eye on the ocean. The blowholes attract humpback whales. They come close in to see if it’s the big daddy of all whaledom. 

 

Satuiatua, Coral Sand & Beachrock

Raised sections of compact coral sand outcrop along many sections of coastline protected by coral reef. Coral sand has already been mentioned when discussing hydro-isostatic rebound but this is your first sighting. Satuiatua, like many coastal villages of Savai’i and Upolu, is built on it, the name Tafagamanu sand coming from a village in Upolu. This is also your first sighting of beachrock. 

Storm erosion of Tafagamanu Sand plus the addition of new sand by wave abrasion of the coral reef ( and parrot fish grinding up live coral to extract the algae and protein) have built to-day’s beaches but, quite often, access to the lagoon is obstructed by bands of dense, slippery rock deeply pitted at surface. This, previously called coquina, has been re-named beachrock. It occurs in the intra-tidal zone between a beach and lagoon. 

Coral sand is composed of calcium carbonate and, like limestone, is slowly soluble in cool fresh water. Fresh water of a Ghyben-Herzberg Lens on seeping through Tafagamanu Sand will become charged with calcium carbonate which, on entering the tidal zone, will be precipitated by the chloride ion of the salt water thereby cementing unconsolidated sand grains into beachrock. 

The width of the tidal zone and consequently that of the beachrock outcrop varies with the slope of the beach. When steep, 3 or 4 narrow, closely spaced bands of rock will be exposed with falling tide, each band being progressively higher towards the shoreline. You will see a fine example of this much later when driving by the beach at Faga. But what you are looking at now at Satuiatua is puzzling - two broad bands of beachrock with the seaward band slightly higher than the landward band. Evidence of ongoing uplift? 

The walled-in pool at the western end of the beach contains freshwater from the Ghyben-Herzberg Lens. 

 

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 the fourth part of a series and is taken from ‘A Visitor’s Field Guide to Savai’i’. 

© Samoa Observer 2016

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