Volcanoes of American Samoa: History, Hazards and Monitoring

American Samoa is the eastern end of a chain of volcanic islands in the South Pacific. Each island has its own eruptive history and could erupt again. The U.S. Geological Survey (USGS) works with the Pago Pago office of the National Weather Service to tell residents and travelers about the hazards, and published this guide in both English and Samoan.

Islands that are volcanoes

American Samoa has four populated islands: Tutuila, Aunuʻu, Ofu-Olosega and Taʻū. Each is only the top of a much larger volcano that continues below sea level. Taʻū volcano, for example, rises more than 12,000 feet (3,600 meters) from the seafloor, but only its top 3,179 feet (969 meters) stand above the sea as Taʻū Island. The most active part of the chain is Vailuluʻu, a submarine seamount east of Taʻū that has not yet reached the surface.

The tuff cone in the northwest of Taʻū Island, formed when hot magma met shallow groundwater or seawater. Photo: D. Downs, U.S. Geological Survey.

The volcanoes formed one after another as the Pacific plate moved from east to west over a volcanic hotspot, much as the Hawaiian Islands did. The islands' eruptive history spans millions of years, and every one of them has erupted within the last 10,000 years. Eruptions are rare, and generations can pass between them, but the geologic record shows that future eruptions are likely at any of these volcanoes.

The 1866 eruption at Ofu-Olosega

On September 7, 1866, three to four earthquakes an hour shook the Manuʻa Islands, which include Ofu-Olosega and Taʻū. Five days later, a column of ash and steam burst from the ocean about 2 miles (3 kilometers) east of Ofu-Olosega. By the third day of the eruption, so much ash filled the air that people on Taʻū could not see Ofu-Olosega, less than 7 miles (11 kilometers) away. Explosions threw ash up to 2,000 feet (600 meters) above sea level, earthquakes shook the islands, patches of ocean turned sulfur-yellow, and many dead fish washed ashore. The eruption lasted about two months.

A newspaper report of the 1866 Ofu-Olosega eruption (Manchester Guardian, April 17, 1867).

The 2022 unrest at Taʻū

From late July to early September 2022, Taʻū volcano went through a period of unrest, and many small earthquakes shook the Manuʻa Islands. At its peak, residents felt as many as 30 earthquakes a day; the largest was around magnitude 4. When it began there was no monitoring equipment in American Samoa, so the USGS Hawaiian Volcano Observatory quickly installed instruments. By early September the earthquakes had fallen back to background levels. The USGS has installed a monitoring network across the islands to measure earthquakes and ground movement at the volcanoes.

The USGS worked with NOAA's National Weather Service, the American Samoa Department of Homeland Security and residents to install monitoring equipment on Taʻū Island in August 2022. Photo: Elinor Lutu-McMoore, NOAA.

What volcanic activity can look like

• Unrest. The most common kind of activity is unrest that never becomes an eruption, as at Taʻū in 2022. A volcano is in unrest when it behaves differently from normal: earthquakes, visible or measurable ground movement, and more gas release, driven by magma moving beneath it and lasting days to weeks. People may feel earthquakes or see cracks in the ground, and volcanic gas can kill plants or fish. Even long or vigorous unrest does not always end in an eruption.
• Underwater eruptions. An eruption can be deep enough that lava and debris never reach the surface. People may still feel earthquakes, smell gas, or see dead fish, floating debris or discolored water — or, if it is deep enough, notice nothing at all.
• Eruptions on land. These can produce lava flows, explosions of rock and ash, and volcanic gas. Lava fountains can feed flows that travel several miles downhill, as glowing orange-and-black rivers or shimmering silver masses that creep forward and sprout smaller lobes. Spatter around a vent can build cinder cones. Vents can open anywhere on a volcano, and explosive activity sends ash and gas downwind.
• Eruptions near the shore. Rising hot magma can meet shallow, much colder groundwater or seawater, boiling it instantly into steam. The result is often explosive, shattering bedrock, throwing a huge column of rock and ash into the air, and scattering debris thousands of feet from the vent.

Taʻū's northwest tuff cone in the foreground and beneath the vegetation behind. Photo: D. Downs, U.S. Geological Survey.

Alerts

The USGS issues Volcano Alert Levels for people on the ground and Aviation Color Codes for flight hazards for American Samoa's volcanoes. Notices go out when activity rises and when it falls, with details of what has changed, the possible hazards and the likely outcomes.

Being prepared

Eruptions can be unpredictable and deadly, and the time from the start of unrest to an eruption can be minutes or years. Monitoring often gives early warning, letting officials start emergency plans and measures that save lives and property. People living near active volcanoes should make an emergency plan for a volcanic event or any other crisis, put together an emergency kit, and follow the advice of local authorities, even when a volcano seems quiet.

Sources

• U.S. Geological Survey, "Volcanoes of American Samoa," Fact Sheet 2023–3022 (English and Samoan): https://pubs.usgs.gov/publication/fs20233022 The photographs and newspaper clipping are reproduced from the fact sheet.