Volcano Watch: Kīlauea volcano’s most recent Southwest Rift Zone eruption was on New Year’s Eve 1974

“Volcano Watch” is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates. This week’s article was written by HVO geologist Drew Downs.

Kīlauea volcano on the Big Island is one of the most active volcanoes on Earth, with a majority of its eruptions occurring at its summit or in one of its two rift zones, the East Rift Zone and Southwest Rift Zone.

The Southwest Rift Zone hasn’t erupted since New Year’s Eve 1974, almost exactly 49 years ago.

Recent activity at Kīlauea has been concentrated at the summit (2008–2018 and 2020–2023 eruptions), in the middle East Rift Zone at Puʻuʻōʻō (1983–2018) and in the lower East Rift Zone (2018 eruption). These eruptions have all demonstrated Kīlauea’s awe-inspiring behavior, with sometimes tragic impacts.

Now is an apt time to revisit the 1974 Southwest Rift Zone eruption in the wake of recent elevated seismicity in similar areas. Elevated earthquake activity, sometimes of more than 100 earthquakes a day, has happened since early October in the volcano’s south caldera and Southwest Rift Zone regions.

Kīlauea erupted three times in 1974; twice at the summit in July and September and once in the Southwest Rift Zone in December. Immediately following the September 1974 summit eruption, the summit region started undergoing very high rates of inflation (approximately 30 microradians of ground tilt from September to December).

Seismicity increased in the summit and upper East Rift Zone regions for the month leading up to the December eruption. By the evening of Dec. 30, the summit and upper Southwest Rift Zone were experiencing earthquakes at a rate of 2–4 per minute.

Just after midnight, at 12:10 a.m. Dec. 31, the Hawaiian Volcano Observatory seismic alarm sounded, tiltmeters recorded sharp deflation and increased tremor indicated magma was on the move out of the summit region.

HVO staff did not have long to wait, with the first lava fountains observed at 2:56 a.m. New Year’s Eve in the Kaʻū Desert region of Hawaiʻi Volcanoes National Park. These lava fountains reached average heights of 115–130 feet, but on rare occasions threw lava bombs up to 330 feet high.

More than 10 fissures eventually opened during the first hour of the eruption, to the east-northeast and west-southwest, forming a near-continuous fountain of lava that stretched more than 2.5 miles long.

Lava fountain heights started to rapidly decline throughout the morning and the eruption was finished by 8:50 a.m.

Despite the short duration of this eruption — only about six hours — lava managed to flow approximately 7.5 miles from source fountains, emplacing an uncommonly thin (typically less than 3 feet thick) lava flow across a large area of the upper Southwest Rift Zone.

HVO continues to monitor the unrest in the summit and Southwest Rift Zone regions of Kīlauea, with increased rates of earthquakes and ground deformation indicating magma is moving in this area.

You can learn about this activity in the daily Kīlauea updates posted online.

Whether this increase in activity during the past several months is a prelude to a new eruption in the Southwest Rift Zone in the near future or if another summit eruption will occur is not possible to forecast.

However, the Southwest Rift Zone remains an active part of Kīlauea that will experience lava flows in the future.

Volcano Activity Updates

Kīlauea is not erupting. Its Volcano Alert level is at advisory.

The unrest associated with the intrusion that began in early October southwest of Kīlauea’s summit continues.

Earthquake activity in Kīlauea’s summit region during the past week remained relatively low until an earthquake swarm throughout an about 6-hour period Friday afternoon/evening.

The Uēkahuna tiltmeter, located northwest of the caldera, showed several microradians of net tilt during the past week as did the Sand Hill tiltmeter, located southwest of the caldera.

Unrest could continue to wax and wane with changes to the input of magma into the area. Eruptive activity could happen in the near future with little or no warning.

The most recent sulfur dioxide (SO2) emission rate for the summit of approximately 70 tonnes per day was measured Dec. 5.

Mauna Loa is not erupting. Its Volcano Alert Level is at normal.

Webcams show no signs of activity on Mauna Loa. Summit seismicity has remained at low levels during the past month.

Ground deformation indicates continuing slow inflation as magma replenishes the reservoir system following the 2022 eruption. SO2 emission rates are at background levels.

Three earthquakes were reported felt during the past week in the Hawaiian Islands: a magnitude-3.4 earthquake at 4:42 p.m. Dec. 23 located 8 miles southwest of Volcano at at depth of 16 miles, a magnitude-4 earthquake at 4:27 p.m. Dec. 23 located 8 miles southwest of Volcano at a depth of 17 miles and a magnitude-2.9 earthquake at 12:51 p.m. Dec. 23 located 9 miles south of Volcano at a depth of about 0.6 miles.

HVO continues to closely monitor Kīlauea and Mauna Loa volcanoes.

Visit HVO’s website for past Volcano Watch articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information and more. Email questions to askHVO@usgs.gov.