Volcano Watch: Volcanoes of American Samoa

Ta‘u volcano in American Samoa experienced volcanic unrest in the form of an earthquake swarm that was felt throughout the Manu‘a Islands (Ofu-Olosega and Ta‘u Islands) from late July through early September 2022.

At its peak, as many as 30–40 earthquakes were detected per hour. Most earthquakes were too small to be felt, but some days dozens were noted by residents of the Manu‘a Islands, likely magnitude 2–4.

In response, the U.S. Geological Survey’s Hawaiian Volcano Observatory rapidly deployed monitoring equipment and staff, who met with federal and local partners to assess the situation, and help respond to any future hazards (https://www.usgs.gov/volcanoes/ta-u-island/news/volcano-watch-hvo-responds-american-samoa-earthquake-reports).

American Samoa comprises the easternmost islands of the Samoan archipelago in the south Pacific. These include the islands of Tutuila (the population center) and the Manu‘a Islands about 60 miles (100 km) to the east. These islands are the tops of shield volcanoes, which are mostly submerged to 15,000 ft (4,500 m) beneath the ocean surface. Other volcanoes created by the Samoan hotspot are still completely below the ocean, such as the Vailulu‘u seamount located about 25 miles (40 km) east of Ta‘u.

Despite being near the Pacific’s famed ‘Ring of Fire,’ the volcanoes of the Samoan Islands were created by a hotspot in much the same way as the Hawaiian Islands. This type of volcanism tends to produce shield volcanoes known for their distinctive broad shape. The volcanoes in nearby Tonga are of a different type related to subduction at the Tonga Trench, and consequently, an eruption like the one that occurred at Hunga Tonga–Hunga Ha’apai, in January 2022, is extremely unlikely in the Samoan Islands.

Vailulu‘u is the most recently active volcano, within American Samoa, with three eruptions since the 1970s. These eruptions occurred nearly 2,000 ft (600 m) below the ocean surface and produced lava flows. It is difficult to confirm Vailulu‘u eruptions without ship-based ocean floor mapping. Occasional sea-floor mapping, from time-to-time, looks for changes in topography of the volcano.

Most eruptions in American Samoa produce relatively slow moving lava flows that are similar to eruptions in Hawaii. Rarer are small explosions where magma and water interact. There is evidence, in the past, of small explosive eruptions that threw out bombs of molten rock and colder, dense rock a few hundred yards (few hundred meters) away from their volcanic vents. These bombs can reach over a yard (meter) in size and are usually in tuff cones that are mostly made of volcanic glass shards with a texture similar to sand and flour. These types of eruptions occur when magma comes into contact with shallow groundwater or near the coast in the shallow marine environment.

Such an eruption happened about 2 miles (3 km) offshore to the east of Ofu-Olosega in 1866. On September 7, 1866, residents of the Manu‘a Islands began feeling earthquakes. Five days later, an eruption started and continued for at least 2 months. The most dramatic of the volcanic activity occurred about 3 days into the eruption, when there was so much volcanic ash ejected that people on Ta‘u Island could not see Ofu-Olosega. The cone at the site of the eruption remains submerged below the ocean, but at the time of the eruption, volcanic ash reached 2,000 ft (600 m) above sea level. Earthquakes were felt throughout the 2 months of the event, and the surrounding ocean was agitated with an occasional sulfur yellow hue and dead fish washing ashore.

Tutuila is the westernmost, and most populous, island of American Samoa, it too has been impacted by eruptions. Volcanic ash deposited above soils, containing ceramic shards, indicate that people likely witnessed eruptions on Tutuila about 1,400–1,700 years ago. Youthful-looking lava flows on Tutuila, Ofu-Olosega, and Ta‘u suggests there have been other eruptions within the past 10,000 years.

Since September, there has been little earthquake activity beneath Ta‘u Island. The monitoring network that HVO set-up across American Samoa in response to the earthquakes earlier this year will help assess potential future volcanic unrest in American Samoa, and associated hazards.

Volcano activity updates

Kilauea volcano is erupting. Its USGS Volcano Alert level is at WATCH (https://www.usgs.gov/natural-hazards/volcano-hazards/about-alert-levels). Kilauea updates are issued daily.

Over the past week, lava has continued to erupt from the western vent within Halema‘uma‘u crater in Hawaii Volcanoes National Park. Sulfur dioxide emission rates remain elevated and were last measured at approximately 600 tonnes per day (t/d) on November 9. Seismicity is elevated but stable, with few earthquakes and ongoing volcanic tremor. Over the past week, summit tiltmeters recorded two deflation-inflation (DI) events. For more information on the current eruption of Kilauea, see https://www.usgs.gov/volcanoes/kilauea/recent-eruption.

Mauna Loa is not erupting and remains at Volcano Alert Level ADVISORY. This alert level does not mean that an eruption is imminent or that progression to an eruption from the current level of unrest is certain. Mauna Loa updates are issued daily.

This past week, about 300 small-magnitude earthquakes were recorded below the summit and upper elevation flanks of Mauna Loa — the majority of these occurred at shallow depths less than 15 kilometers (9 miles) below sea level. Global Positioning System measurements show continued ground deformation consistent with inflation of a magma chamber beneath the summit. Gas concentrations and fumarole temperatures at both the summit and at Sulphur Cone on the Southwest Rift Zone have remained stable over the past week. Webcams show no changes to the landscape. For more information on current monitoring of Mauna Loa, see: https://www.usgs.gov/volcanoes/mauna-loa/monitoring.

There were 5 events with 3 or more felt reports in the Hawaiian Islands during the past week: a M3.6 earthquake 21 km (13 mi) E of Honaunau-Napoopoo at 2 km (1 mi) depth on Nov. 9 at 6:21 p.m. HST, a M2.7 earthquake 7 km (4 mi) SW of Kahaluu-Keauhou at 2 km (1 mi) depth on Nov. 8 at 5:08 a.m. HST, a M2.8 earthquake 9 km (5 mi) E of Pahala at 32 km (20 mi) depth on Nov. 5 at 3:54 p.m. HST, a M2.9 earthquake 3 km (1 mi) SW of Pahala at 36 km (22 mi) depth on Nov. 4 at 11:46 p.m. HST, and a M3.7 earthquake 21 km (13 mi) ENE of Honaunau-Napoopoo at 3 km (2 mi) depth on Nov. 3 at 9:43 p.m. HST.

HVO continues to closely monitor Kilauea’s ongoing eruption and Mauna Loa for any signs of increased activity.

Email questions to askHVO@usgs.gov.