What lurks beneath: learning from lava ooze outs — USGS Volcano Watch

Halemaʻumaʻu has been progressively filled by six eruptions at the summit of Kīlauea since 2020, and it continues to accumulate new lava with each successive fountaining episode of the ongoing eruption. Let’s dive beneath the solidified crust on the floor of the crater to explore the complicated mix of molten material below. 

Thermal map of Halema‘uma‘u surface on April 2, 2025 (episode 16). Primary, lava fountain fed flows reached to the east (annotations 1-4), next to the region where patchy distal ooze outs were being squeezed from below the crust (annotation 5). During episode 16, HVO field crews sampled both the fountain-fed flows and a nearby ooze-out for analysis in the lab.

Kīlauea has had six summit eruptions inside Kaluapele in the past 4 years: 2020–2021 (five months), 2021–2022 (1.25 years), January–March 2023 (three months), June 2023 (1.5 weeks), September 2023 (one week), and the ongoing 2024–2025 eruption (five months so far). Each of these eruptions has filled the area that collapsed in 2018, and lava now reaches more than 425 meters (1394 feet) deep (for comparison, the Empire State Building is 380 meters or 1250 feet tall). 

During many of these eruptions, USGS Hawaiian Volcano Observatory (HVO) field crews have documented “ooze outs”—where lava squeezes out from underneath the solidified surface crust onto the floor of the crater, usually in places that are far away from where eruptive vents are located. These ooze outs tell us that despite the solid crust at the surface, some portions of the interior have remained molten, even in the periods between eruptions. 

In the earlier eruptions, field crews had limited access to sample the eruptions in Halemaʻumaʻu due to vents being located deep inside the collapsed area of Kaluapele. As the surface of the crater, and thus the elevation of the vents, rose and expanded laterally through time with every eruption since 2020, safe opportunities to sample lava flows became possible.

Photograph of a polished Kīlauea lava sample collected from an ooze out in Halemaʻumaʻu before the onset of episode 12 during episode on March 4, 2025. Each light-colored blotch (example in red circle) on the black background is a cluster of minerals that has an approximate diameter of less than 1 millimeter (0.03 inches). USGS photo. 

With permission and coordination from Hawaiʻi Volcanoes National Park, HVO field crews have recently been able to directly sample the lava ooze outs for the first time since 2020.  As part of HVO’s agreement with Hawai‘i Volcanoes National Park, HVO staff are typically available at the Volcano House and Wahinekapu (Steaming Bluff) overlooks during overflight missions in which conditions are safe for sampling, to answer questions about our work and field operations.

The location of ooze outs are documented with photographs and thermal images. The thermal images are used to create a thermal map, which often show the ooze outs occurring on the eastern side of Halema‘uma‘u, far from active surface flows that are being fed by lava fountains. These different types of lava flows can be seen from public overlooks at many points around Kaluapele in Hawai‘i Volcanoes National Park (sometimes, even during periods between the eruptive episodes). 

Ooze-out lavas are spiny pāhoehoe; they have a rough surface texture compared to fresher pāhoehoe fed by hot lava fountains. These spiny flows are more viscous (they flow more slowly) because they have had time to cool. They also have lower gas contents (are denser) and analysis in the laboratory shows that they are also very crystal rich. 

The most common mineral in Hawaiian eruptions is olivine (which is green), but other minerals are found in lavas that have had more time to cool (such as white plagioclase and dark green/brown pyroxene). The ooze out lavas collected from Halemaʻumaʻu are dominated by clots of plagioclase and pyroxene with minor amounts of small (<0.5 mm or 0.02 inches) olivine crystals.

Ooze outs also contain larger (1-2 mm or 0.04-0.08 inches) olivine crystals that have been recycled from previous eruptions or episodes. Originally these larger crystals were formed in the magma reservoirs underneath Kaluapele. They were erupted in one of the previous lava lakes, and then were re-erupted in a recent ooze out.

We know these crystals had this complex history because of careful laboratory work by two University of Hawai‘i at Hilo undergraduate students examining samples from the January–March 2023 and June 2023 Halemaʻumaʻu eruptions. Their work has documented minerals found in those eruptions, and they have identified lake-recycled minerals through chemical and textural analysis, similar to what we see in the recent ooze-outs.

Additional clues will come from comparing the recent ooze-out samples to rock core drilled from the Kīlauea Iki lava lake in the 1960s and 1970s. Studies on these found that the lava lake interior, fed by 17 episodes during the 1959 Kīlauea Iki eruption, took decades to cool resulting in crystallization of a variety of minerals. 

Each eruptive episode, and successful sampling mission, provides further insights into how Halemaʻumaʻu is evolving and helps us understand eruptive behavior of Kīlauea.

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Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates. Today's article is by HVO geologist Kendra Lynn.