Volcano Watch: Potential long-term outcomes of recent intrusions in Kīlauea East Rift Zone

“Volcano Watch” is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.

The intense seismicity and ground deformation during the past couple of months along the East Rift Zone of Kīlauea volcano on the Big Island is interpreted to indicate intrusion of magma.

Some of this magma can leak to the surface in eruptions, but there are less likely long-term consequences that have historical analogues.

The Sept. 15-20 eruption in and near Nāpau Crater is an example of a brief eruption in this area.

If a longer and larger eruption were to occur, such as at Maunaulu (1969-1974), most lava flows would head south from their vents because of the topography.

Some, however, could be trapped within craters, such as at Nāpau, partly or completely filling them.

This part of the rift zone is popularly known as the Chain of Craters.

Geologically, all the craters are pit craters, which form solely by collapse of the ground surface without accompanying eruption at the site. Probably all the pit craters are younger than the 15th century.

Sixteen pit craters dimpled the surface before about 1900 from Luamanu to Nāpau. Some coalesced, so there are more craters than crater names; Hiʻiaka had two craters, Pauahi three; ʻAlae two; and Makaopuhi two.

Another crater, Devil’s Throat, formed sometime about 1900.

Beginning in the 1960s and continuing to the early 1980s, this area erupted numerous times. Seven craters were partly filled (Luamanu, Hiʻiaka, Pauahi and Nāpau) or completely filled (Aloʻi, ʻAlae and the deepest pit in Makaopuhi).

Today, the upper East Rift Zone is defined by some to end at Maunaulu, a shield that grew from 1969 to 1974, between now-filled Aloʻi and ʻAlae Craters.

The Chain of Craters is the only section of the East Rift Zone and its submarine extension, the Puna Ridge, that contains pit craters. This probably relates to the proximity of the Chain of Craters to the summit of Kīlauea, where magma rises from the mantle.

It stands to reason that such proximity leads to more intrusions than elsewhere along the rift zone. The infrequent lateral draining of such intrusions is the likely explanation of how pit craters form.

The recent intrusions along the Chain of Craters could have three very different outcomes.

Repeated intrusions might spawn eruptions that could bypass the craters completely, or partly or completely fill one or more craters.

Another possibility is that underground draining of one of the intrusions could lead to formation of a new pit crater.

One or more eruptions seem far more likely (such as the recent Nāpau eruption), but collapse to form a new pit crater happened at least 17 times in the past 550 years, so is not a rare occurrence.

History suggests another potential effect that could accompany continued intrusions is seismicity beneath the south flank of Kīlauea.

The volcano is always in a constant state of balance between extension of the south flank because of pressure beneath the summit and intrusion of magma along its rift zones.

At times, the flank becomes loaded like a spring and even the relatively small force of an intrusion can trigger an earthquake.

The largest recorded earthquake beneath Kīlauea’s south flank happened on Nov. 29, 1975.

A deadly magnitude-7.7 earthquake, the largest on the island since 1868, struck the south flank, unleashing a tsunami that killed two campers and many horses at Halapē and moving the volcano’s south flank several meters southward and downward.

We don’t know what directly led to the earthquake, but about 15 years of repeated intrusions along the Chain of Craters followed and a relationship between the two is a possible interpretation.

Current conditions provide an excellent opportunity to better understand the generation of south flank earthquakes.

All of these extreme scenarios — a new pit crater or seaward movement of the south flank — are less likely than one or more eruptions.

Continued close monitoring of the Chain of Craters area could provide clues down the road.

A holistic, historically based approach to studying Kīlauea helps us appreciate what can happen in the long run.

Evaluation of the past activity of the volcano, no matter how imperfectly known, can influence our thoughts about what could happen.

Even with this approach though, we must remind ourselves that there are no facts about the future.

Volcano Activity Updates

Kīlauea is not erupting. Its USGS Volcano Alert level is at Advisory.

During the past week, about 30 earthquakes were located in the Kīlauea summit region and about 100 in the upper and middle East Rift Zone.

Ground deformation rates have greatly decreased since the Sept. 15-20 middle East Rift Zone eruption, but data show magma is continuing to move at a low rate from the summit to that area of the rift. Future intrusive episodes and eruptions could occur with that continued magma supply.

The most recent measurement of sulfur dioxide emission rate at the summit Sept. 17 was 60 tonnes per day. Sulfur dioxide emissions were not detected Sept. 23 at or downwind of the eruption site.

Mauna Loa is not erupting. Its USGS Volcano Alert Level is at Normal.

No earthquakes were reported felt in the Hawaiian Islands during the past week.

The Hawaiian Volcano Observatory continues to closely monitor Kīlauea and Mauna Loa.

Visit the observatory’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.