A Visitor’s Field Guide to Savai’i – Touring Savai’i with a Geologist A Visitor's Field Guide to Savai’i Touring Savai'i with a Geologist Warren Jopling Page 1 A Visitor’s Field Guide to Savai’i – Touring Savai’i with a Geologist ABOUT THE AUTHOR AND THIS ARTICLE 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. His 2014 report on Samoa's Geological History was published in booklet form by the Samoa Tourism Authority as a Visitor's Guide - a guide summarising the main geological events that built the islands but with little coverage of individual natural attractions. This present article is an abridgement of the 2014 report and focuses on Savai'i. It is in three sections; an explanation of plate movement and hotspot activity for visitors unfamiliar with plate tectonics; a brief summary of Savai'i's geological history then an island tour with some geologic input when describing the main sites. It is for nature lovers who would appreciate some background to sightseeing. Page 1 A Visitor’s Field Guide to Savai’i – Touring Savai’i with a Geologist The Pacific Plate, The Samoan Hotspot, The Samoan Archipelago The Pacific Plate, the largest of the Earth's 16 major plates, is born along the East Pacific Rise. Figure 1. Pacific Ocean Plates It is a 4 to 5 km thick plate of oceanic basalt moving at 7 cm yearly to the west-northwest across the Earth's mantle. In the area encompassing Samoa the plate is some 100 million years old but the islands it carries are much younger. Fig 2 shows the 400 km long Samoan Archipelago with Savai'i at the west and Mount Vailulu'u, an enormous submarine volcano at the east. Figure 2. The Samoan Archipelago. Page 2 A Visitor’s Field Guide to Savai’i – Touring Savai’i with a Geologist Mt Vailulu'u rises 4.5 km from the ocean floor, has a crater 400 m deep and 2 km wide, and is continously venting mineral - charged hot water. This is one of the Pacific Ocean's seven widely scattered hotspots - a hotspot being a stationary area deep within the mantle where heat escapes as a rising plume of molten magma; this burns through the slowly moving overlying plate to form a volcanic island or, if magma supply is deficient, an underwater seamount. These features are slowly carried away as new ones form. Mt Vailulu'u is presently 590 m below sea level but will possibly emerge in another 50,000 years or so to add another island to the eastern end of the archipelago. Savai'i, as shown by its oldest rock outcrops, formed over the hotspot 5 million years ago and Ta'u, the easternmost island of American Samoa, 500,000 years ago. The Samoan archipelago is the youngest and easternmost segment of a long volcanic trail of islands and seamounts that extends to the northwest of Samoa through the atolls of Tuvalu, Kiribati and the Marshall Islands (Fig 3) Figure 3. The Samoan Volcanic Trail. Savai'i's Volcanic History Savai'i built up over the hotspot 5.5 to 4.5 million years ago then was exposed to 4 million years of sub- aerial erosion before a second phase of volcanism commenced 390,000 years ago. Eruptions have continued periodically into historic time but are not related to the now-distant hotspot. Savai'i and Upolu are close to the northernmost extremity of the Tonga Trench where the heavy, westerly moving Pacific Plate subducts (pushes under) the lighter rocks of the Tonga Micro Plate, which is moving eastwards at 19cm yearly (Fig 4). Page 3 A Visitor’s Field Guide to Savai’i – Touring Savai’i with a Geologist Figure 4. Tonga Trench Rollback. The Pacific Plate encompassing Savai’i and Upolu is heavily deformed with molten magma from an underlying magma chamber rising along fractures to cover the old eroded rocks of hotspot origin (locally known as Fagaloa formation). Savai’i was completely covered by post-Fagaloa 4olcanic and Upolu partially covered but later stream erosion re-exposed several small Fagaloa outcrops in Savai’i (Fig 5). Potassium- Argon dating shows an age of 5 million years. Figure 5. Fagaloa Formation. Post- Fagaloa eruptions over Savai’i have built a stack of interbedded basalt lava flows and tephra (volcanic ejecta) to a height of 1863 m above sea level. Fig 6 shows the distribution of Samoa’s rock units (formations). Areas coloured various shades of green are designated as the Salani, Mulifanua and Lefaga Page 4 A Visitor’s Field Guide to Savai’i – Touring Savai’i with a Geologist formations which range in age from 390,000 to 10,000 years ago; red, Puapua formation from 10,000 to 1722 when Samoa was first sighted by a Dutch navigator and yellow, Aopo formation, from 1722 onwards. (Fig 6) (Puapua and Aopo are very beautiful Savai’ian villages). Figure 6. Samoan Rock Formations. Green areas have soils suitable for farming, red areas (Puapua) have very thin topsoil unsuitable for ground crops but sustain tree growth when roots penetrate soil-filled joints between basalt blocks and yellow areas (Aopo) are either barren or covered by scrub. It is immediately obvious from Fig 7 that much of Savai’i’s coastal area is covered by Holocene (Puapua and Aopo) rocks whereas Upolu has several Puapua cones and one small area of outcrop along the central south coast. Aopo volcanism has so far been restricted to Savai’i. This has impact on population distribution . Samoans mostly live by subsistence farming so Upolu, the smaller island, has a population of 145,000 as compared to Savai’i’s 45,000. Page 5 A Visitor’s Field Guide to Savai’i – Touring Savai’i with a Geologist Figure 7. Aopo and Puapua Outcrops. Touring Savai’i Your tour starts at Salelologa, Savai’i’s only commercial village and takes you clockwise visiting and discussing sites of interest. 1 The Tafua- Savai'i volcano. Tafua - Tai 13 The Mauga Afi eruption and Aopo Lava Field, Aopo 2 The Afu A'au waterfall, Vailoa 14 Lava tubes, Aopo, Letui, Paia 3 A freshwater spring, Satupaitea 15 MataLeAlelo Spring, Mangroves, Safune 4 The Mu Pagoa waterfall and emerged shorelines, Puleia 16 The Mauga Mu eruption - Discussion only 5 An abandoned eucalypt forest, Gataivai 17 Mount Matavanu, Paia 6 Sea arches and iron bound coastline, Taga 18 Saleaula Lava Field, Saleaula 7 Alofa'aga Blowholes, Taga 19 Mauga Crater 8 Coral sand and beachrock, Satuiatua 20 Samalaeulu 9 Faulting, Fagafau 21 Mt To'iavea, Mt Tagotala 10 Moso's Footprint, Fagalele Bay 22 Puapua to Salelologa 11 Canopy Walkway, Falealupo 23 Mt Silisili 12 The North Savai'i Fault, Sataua 24 Pulemelei Step Pyramid 25 Tapa Page 6 A Visitor’s Field Guide to Savai’i – Touring Savai’i with a Geologist 1. Tafua - Savai'i Volcano Visitors nearing Savai'i by ferry will see on the port (left) side an irregularly shaped hill rising above the treeline. It is not a particularly impressive sight and most would be surprised to hear that this is the largest of Savai'i's 450 volcanic cones. It was also the source of an enormous eruption during early Polynesian times and is now a wonderful eco-tourism attraction. Tafua-Savai'i started life less than 2000 years ago and is Savai’i’s only volcano located on a shoreline. The cone is elongate in a WNW-ESE direction, and with a 130 m high central ridge separating two craters, east and west. A crater to most visitors means a deep, wide depression surrounded by vertical or sloping walls. The western crater fulfils this description but not the eastern where the entire northern wall has collapsed leaving a flat crater floor planted with village food crops. Before climbing to the western crater a word about Tafua-Savai'i's eruptive history will explain the two very different rocktypes you will see. Molten magma on reaching surface is called lava. This fragments explosively with the seawater contained in the underlying surface rocks to produce tuff, a brown unstratified, granular rocktype containing fragments and blocks of the underlying black basalt. This initial violent phase of hydro-volcanism built high tuff rings around the two vents (eruptive centres). Molten lava pooled over the vents and surging caused spill over their rims to consolidate as thin basalt beds, gassy at top, dense at the base. The western lake drained (by lava tube?) leaving a 150m wide and 50m deep vertically walled crater. Marine tuff can be seen in quarries on either side of Tafua-Tai village and along the plantation road that encircles the volcano. It is compact in outcrop but crumbles into sand under pressure and can be mixed with cement to make concrete flooring. It provides good soil for garden crops. You see thin basalt strata in the crater's wall and you can't help walking on loose slabs that have been dislodged by tree roots. Enough geology! The purpose of your Tafua visit is to see some of Samoa's wildlife in a beautiful natural setting. An easy 10 minute walk up the volcano's north flank takes you to a low point of the western crater.