Up here in Alaska, this time of year (writing in November), we are well into our first month or two of winter. The days are short and mostly dark. The sun sits low on the horizon. There is not yet a lot of snow, barely enough to ski on. And while it has been cold, the ice on lakes and streams is not yet thick enough for serious ice skating. It is enough to make one start thinking about warmer parts of the world. How about a look at volcanoes in the tropics? This time around, Samoa.
American Samoa sits on the Pacific Plate north of Tonga. This is a complex tectonic region with multiple volcanic island groups and many, many interesting volcanoes. Many of them grabbed our attention over the years. Granyia covered Fiji in 2019. I covered volcanoes in Vanuatu with posts on Ambryn and Kuwae in 2015. I covered Havre Seamount and Niuafo’ou in Tonga in 2018. With that introduction, on to Samoa.
The Samoan Islands have been settled for around 3,500 years. Inhabitants are closely related to those in neighboring Fiji and Tonga. Before first contact with westerners, there were multiple waves of invasion, raids and intra-island warfare. This was sufficient to eliminate long-term spoken tradition among remaining islanders. Contact with Europeans took place in 1722 with a French explorer naming them the Navigator Islands. British missionaries, whalers and traders began arriving in the 1830s. American arrived in 1821. By the 1880s, there were regular German visits. All three nations took an economic interest in the islands. The residents gave as good as they got, being known for headhunting in the 19th Century.
As usual, nobody is able to play well together, and a pair of civil wars broke out among the visitors and inhabitants at the end of the 19th Century. This was settled with a partition that gave the eastern islands to the US, the western islands to Germany. The British traded their interest in Samoa for a German withdrawal from Tonga and other locations. The Germans administered German Samoa until 1914 when New Zealand forces invaded and seized control from Germany. New Zealand ruled German Samoa until 1962 when it ratified Samoan independence. Samoa today is an independent nation.
Samoa is classified as an economically developing nation. Its agriculture produces mainly copra, cocoa beans, and bananas for export. Tourism is a rapidly expanding economic sector, up to 25% of the economy today. It has two main islands, multiple smaller islands, with a total area around 2,800 km2, and a population over 202,000.
American Samoa is five main islands and a pair of coral atolls. Its total dry land area is around 200 km2 with a population just over 55,000. It is the southernmost territory of the US. Tuna is the main export.
Climate in Samoa is tropical with a dry and wet season. It is warm, tropical and humid. Trade winds created precipitation shadows much like they do on the Hawaiian Islands, with wet sides of the mountains and dry sides in the rain shadows. The islands also are in the path of cyclones and do get hit regularly.
The Samoan Islands are all volcanic in nature, thought to originate from the Samoa hotspot. The islands in the chain are geologically young, having formed in the last 5 Ma. It has high volcanic mountains, submerged reefs. Larger islands are to the W, with the smaller ones to the E. Like the Hawaiian Islands chain, the head of the hotspot is thought to be at the eastern end of the chain. Unlike Hawaii, the larger islands are the older ones on the western end of the chain. Worse, there are active volcanoes on both ends of the chain.
The submerged volcanic portions of atolls along this chain may stretch as far as Combe Bank near Fiji, some 790 km SW of Savai’i. Bathymetry of the Samoan chain found multiple seamount ridges 100 – 250 km long. Many are linked end to end as longer curving lineaments broken by short gaps. General concavity of the curving lineaments is oriented toward the trench.
If this is a typical hot spot chain of volcanic islands, why are the larger islands on the western end of the chain? Worse, why is the one on the western end, supposedly farthest away from the hot spot or plume head, still volcanically active? Editorial comment: Typical volcanoes. The more you think you know, the less we actually do.
We will take a look at the volcanic islands generally from west to east, starting with Savai’i, Upolu, Tutuila and on to the Manu’a Group which includes Ofu, Olosega, Ta’u Islands. Farther east, we find the active Vailulu’u Seamount.
Volcanic activity has generally migrated from W to E as the Pacific Plate moves toward the W. The sequence of events on each island has been construction of a massive (or multiple) shield(s) from eruptive centers aligned along a rift. These are mostly effusive eruptions with thin, runny lavas produced. After a couple million years or so of island building, shield building shuts down and the islands begin to erode and subside back into the ocean. After a hiatus of some hundreds of thousands to millions of years, rejuvenated volcanism begins on some of the islands. In the case of Samoa, this rejuvenated activity has been most prevalent on Savai’i, neighboring Upolu, and Tutuila, with decreasing amounts of erupted lavas as you work your way E along the island chain.
There have been historic eruptions on Savai’i and Upolu Islands, and a historic eruption in the Manu’a Islands. Like Hawaii, rejuvenated volcanism has covered the western islands with new volcanic products, 99% of the surface of Savai’i and around 50% of Upolu. This is far larger coverage than Hawaii, where rejuvenated volcanism generally covers less than 6% of the surface of each island (other than Ni’ihau and Kaua’i at 35%). Tutuila has only 15% coverage by rejuvenated lavas.
The Wallis and Futuna Islands are located some 375 W of the main Samoan Island of Savai’i. As such, they are not formally part of Samoa, but part of French Polynesia. Volcano Discovery grouped them into its discussions of Samoa, so we will include them today. Wallis and Futuna Islands include the main island of Uvea and 22 smaller islands, all surrounded by barrier reefs. Uvea measures 13 x 7 km, with an area of 60 km2. It is a broad expanse of basaltic lava flows cut by explosion craters, capped by tuff and cinder cones. The island appears to exhibit much the same life cycle as the Samoan islands: initial shield construction followed by some period of quiet before rejuvenated volcanic activity finishes. Dated samples on the island range from 500 – 80ka, though rocks of recent age are present.
Volcanism in the Wallis Islands is too young to be directly related to passage of the Pacific Plate over the Samoan hot spot. The basaltic magmas erupted here are thought to be generated by a later thermal disturbance that may be related to deformation along the boundary between the Pacific and Indo-Australian Plates.
The highest point on Uvea is Mt Lulu at 145 m near the island center. Lakes and marshes fill 6 craters. The most spectacular of these is Lake Lalolalo, surrounded by 40 m cliffs. Rich brown soils cover most of the island to depths of 2-3 m. There are no streams on the island even though the island gets 250 cm of rain/year.
Earliest inhabitants appear to date to at least 1200 AD. The island was invaded multiple times from Tonga, which set up a fortified settlement dated 1450 AD. Total population of the islands is just over 15,000.
Savai’i is the largest shield volcano in the South Pacific. The island measures 70 x 46 km, around 1,700 km2. It has just over 43,000 residents. The shield tops out at Mount Silsili, 1,858 m. There is what appears to be a collapse caldera that was refilled with subsequent lava flows. The existence of the caldera is inferred by semicircular contours of valleys surrounding the crest of the shield.
Volcanic cones are common on Savai’i. There are more than 400 present in varying stages of erosion. The greatest density of these is along the Tuavisi Ridge. Cones are spatter, agglomerate, tephras, or a combination of all three. Activity forming them has varied from explosive to effusive. There is no apparent pattern to cone location and type of activity that formed them. There are few pyroclastic deposits recognized. They are generally base surges generated by explosive hydromagmatic activity.
There is a WNW – ESE trending rift zone that splits into two rifts on the E side of the island. There may be up to five rift zones on the island. The oldest deposits above sea level are up to 2 Ma. Those on the subaerial flanks date to 5 Ma.
The life cycle of Samoan volcanoes is in many ways similar to that of Hawaiian volcanoes. Starting some 5 Ma, initial shield formation built the main island shield. Once this shield was built, there was an extended period of relative quiet starting some 2.5 Ma while erosion and subsidence worked on returning the island back to the sea. Coral reefs grew around the island during this period. Rejuvenated volcanism began 1.0 – 0.5 Ma and has continued into today. This volcanism has covered 99% of the island with new lavas and scoria. It mostly takes place from monogenetic cones and vents along the multiple rifts on the island. More recent lava flows partly buried the surrounding reef system. There are numerous cinder and lava cones on the crest of the island. There are additional cones on the north central flank. A large number are found on the south-central part of the islands.
There have been three historic eruptions producing large volume lava flows that reached the sea. The lava flow fronts are up to 15 km wide. Mount Matavanu is the most recently active volcanic center on Savai’i Island. It most recently began erupting in Aug 1905. Eruptions from it lasted into 1911 producing a’a and pahoehoe lava flows. Lava flows from the eruption covered over 100 km2 of the island, flowing into the sea, destroying a lagoon and coral reef. Some of the lava flows were up to 100 m thick. Other recent volcanic eruptions took place in 1902 and 1725.
There are multiple offshore volcanic islands mostly located in the strait between Savai’i and Upolu. These are generally simple to complex tuff cones built by the explosive interaction between magma and seawater. There are coral and mollusk fragments in the pyroclastics that indicate initial activity took place in shallow seafloor. A 2009 paper by Nemeth and Cronin investigated oral traditions of local residents and found little in the way of enduring oral traditions among the residents. This is likely due to cultural turbulence, wars and invasions. The only available sample of an offshore eruptive product was dated around 1,900 years ago.
There is a range of eruption styles from Savai’i. These include long-term lava flow eruptions (1905 – 1911 and 1760), short-term, small scale spatter cone eruptions (1902 eruption, Seuseu crater, and other W Savai’i cones), short-term, small scale lava-water littoral explosions (S Savai’I lava coastline Taga-Gatavai), large-scale magma-water interactions and explosive eruptions (Tafua Savai’I, East Savai’i), and submarine flank collapses (S Savai’i).
Upolu island is the second largest island in Samoa. It is a massive basaltic shield some 75 km long, 26 km wide, 1,125 km2 in area. It is the most populated of all Samoan islands with over 143,000 people. The highest point is Mount Fito at 1,113 m. It is immediately to the east of Savai’i, across the 18 km wide Apolima Strait.
Upolu is a deeply eroded mass of Pliocene lavas surrounded by a drowned barrier reef, partly buried by more recent lavas. The volcano was submerged so quickly that only a narrow fringe reef grew on the steep shores of the earlier lavas while a wide barrier reef grew on the gentle slopes of the more recent lavas.
The island was built during two periods of volcanic activity over the last 5 – 1.6 Ma. While it should be newer than neighboring Savai’i, precise dating is so far unavailable, hence the wide range of dates. Like neighboring, Savai’i, the initial shield building stage was followed by an extended period of quiet where erosion and subsidence predominated before volcanism resumed over the last 400 ka or so erupting rejuvenated magmas which have covered some 50% of the aerial surface of the island.
Upolu Island has several volcanic areas dated earlier than 5,000 years ago. It also has several volcanic features that are fresh in appearance. The most prominent cone in western Upolu, Tafua Upolu, may be evidence of recent activity. The word “Tafua” can be traced back to “fire mountain” used in Tonga to describe active volcanic islands. Some more recent activity is dated 1,915 years ago, the maximum age of a tuff cone just offshore Cape Tapaga, E Upolu. Recent volcanism has been reported and described farther eastward along the same structure on the island.
The most extensive recent volcanism took place along a 20 km segment of the central axis (rift) of the island. The most recent lava flows are estimated at a few hundred to thousands of years old. These were erupted from vents near the crest of island. One of these flows reached the north central coast with a flow front at the ocean 1.5 km wide. Several of these flows traveled down river valleys. There are multiple areas along the shoreline likely formed by recent lava flows that have reached the seas.
Neighboring Apolima Island in the channel between Upolu and Savai’i is a Holocene tuff cone too young to be fringed by coral reefs. Like neighboring Savai’I, rejuvenated volcanism on Upolu is considered to be monogenetic, single eruption volcanic centers.
An underwater archeological site was discovered while dredging for a ferry landing at Mulifanua on Upolu. The pottery shards were dated at 2.8 ka and found 2.25 m below current sea level. The calculated shoreline subsidence was calculated at 1.4 mm/yr. Subsidence of Savai’i and Upolu is offset by upward flexure caused by lateral flexure of the Pacific Plate which is thought to have caused the rejuvenated volcanism on both islands, so subsidence on the two islands has been variable over time and in different locations.
Part 2 will follow with the islands of American Samoa, Tectonics and conclusions.