American Samoa's geography: steep volcanic islands with room only at the shore

Where the islands sit in the South Pacific

American Samoa is the eastern end of the Samoan archipelago. Britannica defines it as the six Samoan islands east of the 171st meridian west, with Samoa, the independent country, holding the nine to the west. It lies about 2,600 kilometres northeast of New Zealand and about 3,500 kilometres southwest of Hawaii, south of the equator at fourteen degrees.

The inhabited volcanic islands are Tutuila, Aunuʻu and the three islands of Manuʻa: Ofu, Olosega and Taʻū. Rose Atoll, uninhabited, lies 140 kilometres beyond Taʻū. Swains Island is geographically separate, a coral atoll in the Tokelau group about 450 kilometres northwest of Tutuila, administered with the territory since 1925. Tokelau also claims it, and the dispute between Tokelau and the United States is unresolved; this catalogue records that it exists and takes no position on it.

The land is small and the sea is not. Including territorial waters the territory covers about 304,000 square kilometres. Maritime boundaries with New Zealand's Pacific dependencies, Tokelau, the Cook Islands and Niue, have been set by treaty. Boundaries with Tonga and Samoa have not been agreed.

How hotspot volcanoes built steep islands

Except for the two atolls, the islands were formed by volcanic activity within the last seven million years, according to Britannica. Each is a set of overlapping shield volcanoes built from basalt flows over a hotspot on the Pacific Plate, and they emerged in sequence from west to east as the plate moved. Tutuila is the oldest, dated to the Pliocene at roughly 1.24 to 1.4 million years; the Manuʻa islands are younger, probably Holocene.

Erosion then went to work. Much of the lava broke into angular breccia, and what stands out on the ridges today are plugs of trachyte and outcrops of tuff left behind when softer rock wore away. The interiors are high and rugged and fall steeply to deep inlets.

Taʻū shows the process at its most abrupt. It is what remains of a shield volcano that collapsed in the Holocene, and the collapse left sea cliffs about 910 metres high. An inland escarpment on the island, the Liu Bench, is a mass-wasting feature that could still slump into the ocean.

The hotspot is not finished. A submarine eruption was detected just east of the territory in 1973. The Vailuluʻu seamount, 45 kilometres east of Taʻū, was discovered in 1975 and has an active cone growing inside its summit crater, named Nafanua after the Samoan goddess of war. It is read as the next Samoan island in the making.

The same plate boundary that feeds the volcanoes also shakes them. The earthquake of 29 September 2009 struck on the Kermadec-Tonga subduction zone, and its tsunami ran up to 1.6 kilometres inland on Tutuila.

Why Tutuila holds almost everyone

Tutuila is about 33 kilometres long and at its widest a little more than 4.8 kilometres from north to south. Its land is about 68% of the territory's total. A rugged range runs its length from west to east. The north coast is steep cliff broken by small bays, and the south has flatter ground.

Pago Pago Harbor, a deep inlet beneath Matafao Peak, divides the island nearly in two. The government centre at Utulei, the legislature at Fagatogo, the port and the cannery at Atuʻu all sit around its shore. Southwest of it the Tafuna urban area runs as a string of villages from ʻIliʻili to Tafuna, and the international airport stands on the Tafuna plain, about 11 kilometres from central Pago Pago. A coastal road of about 50 kilometres runs from Fagamalo in the northwest to Onenoa in the northeast.

The census puts figures on this. In 2020, 97.5% of the population lived on Tutuila, and the Manuʻa islands had been emptying for decades: about 20% of the territory's people lived there in the 1930s and about 6% by the 1980s. The Manuʻa islands' own record gives the reasons as a lack of work and the pull of the way of life on Tutuila.

Aunuʻu, a small island off Tutuila's southeast coast, covers about 1.5 square kilometres and had 476 people at the 2000 census. Its Pala Lake is a freshwater wetland, and the whole island was named a National Natural Landmark in 1972.

How Manuʻa rises straight from the sea

The Manuʻa group lies about 100 kilometres east of Tutuila, and its three islands have a combined land area of 56 square kilometres. Ofu and Olosega are separated only by the Āsaga Strait, 137 metres wide and now bridged. Taʻū covers 44 square kilometres of that and carries Lata Mountain, 963 metres.

The islands are high and mountainous, and rainforest runs from their summits to the coast. All of Manuʻa's land is communally owned by its families, including the land inside the national park, which is leased. In January 1915 a hurricane uprooted an estimated 75% of Manuʻa's coconut trees and washed away the village of Sili, and contact with Tutuila took 22 days to restore.

What the two coral atolls add

Rose Atoll is nearly square, about 2.6 by 2.7 kilometres, a reef ring around a lagoon with an 80-metre channel into the lagoon near its northern point. Only 0.05 square kilometres is dry at high tide, on two islets: Rose Island, 3.5 metres high and vegetated, and bare Sand Island, 1.5 metres high. It has been a national wildlife refuge since 1973 and is part of a marine national monument managed by the United States Fish and Wildlife Service with the territorial government.

Swains Island is different in origin and in history. It is a coral atoll of 2.43 square kilometres around a lagoon, owned privately by the Jennings family since 1856 and worked as a copra plantation until 1967. The 2010 census counted 17 people there.

Why rain falls in metres on Pago Pago

The climate is tropical, humid and even through the year: temperatures average about 26.7 °C and vary by about 8 °C across it. Britannica gives average temperatures from about 21 to 32 °C and average humidity of 80%. Southeast trade winds blow almost daily, and the mountains above Pago Pago Harbor catch the clouds they carry: the town receives about 5,000 millimetres of rain a year, according to Britannica.

Much of that water never reaches the sea on the surface. Streams run fuller in the highlands than near the coast and sink into the porous basalt, so coastal wells supply much of the territory's water.

The seasons are defined differently by different sources, and the difference is worth stating. Britannica places the wet season from November to March. The National Park Service tells visitors to expect frequent showers from October to May and treats June to September as the dry season. Tropical cyclones arrive between November and April. The cool months, such as they are, fall in the southern winter from June to September.

What lives in the forest and on the reef

About 86% of the land was forested in 2020, 17,130 hectares, down from 18,070 hectares in 1990. The National Park Service describes it as the one paleotropical rainforest in the United States national park system, with species that originated in Southeast Asia and more than 475 plant species.

The only native mammals are bats: two fruit bats, the Pacific flying fox and the Samoa flying fox, known locally as peʻa, and a small insect-eating sheath-tailed bat. The government banned hunting fruit bats in 1992. Sixteen of the Samoan islands' 34 bird species are found nowhere else, among them the tooth-billed pigeon; four occur in Manuʻa and not on Tutuila, including the blue-crowned lory, the territory's only parrot. The one amphibian is an introduced one: biologists estimate more than two million cane toads on Tutuila.

Two sea turtles nest in the territory. Hawksbills, listed as endangered, come ashore on Tutuila's beaches, around fifty nesting females a year, while green turtles are most common at Rose Atoll. Pola Island, off Vatia on Tutuila's north coast, and Rose Atoll hold the territory's breeding colonies of red-footed boobies. Freshwater habitat is scarce and small: the main wetlands are the pala lagoons at Nuʻuuli and Leone and Pala Lake on Aunuʻu, home to the Pacific black duck, the Samoan islands' one duck.

The reef is where the numbers grow. The National Park Service counts more than 900 fish species and more than 230 coral species in its reefs, and humpback whales pass through in the southern winter. The National Marine Sanctuary protects old and massive Porites coral heads, along with deep reefs and hydrothermal vent communities. The park also names the crown-of-thorns starfish among the threats to it.

Which hazards the islands live with

The hazards follow from the geology and the latitude. Cyclones come in the wet months, and the record of what one can do is the storm of 9 January 1915, which cut Manuʻa off from the outside world, left no vessel on the islands able to reach Tutuila and killed three people in Fitiʻuta, Ofu and Olosega. It was the first time Congress and the American Red Cross sent disaster aid to the territory.

The larger threat comes from the sea floor. The 2009 tsunami came ashore in four waves and struck the villages along Tutuila's coast, Leone and Pago Pago among them. The account kept with the national park's record gives 34 confirmed deaths in the territory, more than a hundred injuries and about 200 homes and businesses destroyed; the park lost its visitor centre and main office. Geologists also watch the Liu Bench on Taʻū, whose collapse into the ocean could send a tsunami as far as Fiji.

Slower change shows up on the reef. The National Park Service lists coral bleaching, severe storms and invasive species among the pressures on the reefs, and reports that parts of the Ofu lagoon have tolerated rising water temperatures better than reefs elsewhere, which is why scientists study them.

How the land is held and protected

Geography here cannot be separated from ownership. More than 90% of the land is native land, held communally by extended families under their matai, and only about 2% is freehold. The territory's constitution makes any change to land law wait for two successive legislatures.

That shaped conservation directly. Congress established the National Park of American Samoa on 31 October 1988, but the Park Service could not buy communal land. The problem was solved on 10 September 1993 with 50-year leases from the village councils, and the park now covers more than 8,000 acres across Tutuila, Taʻū and Ofu, about 4,000 acres of it under water. Villagers keep their fishing, gathering and farming inside it. Seven National Natural Landmarks were designated on Tutuila in 1972, among them Matafao Peak, Rainmaker Mountain and Aunuʻu Island.