Volcano Watch — Monitoring the rise of a new pu‘u on Kīlauea
Volcano Watch is a weekly update produced by scientists and affiliates of the U.S. Geological Survey’s Hawaiian Volcano Observatory (HVO). This week’s piece is authored by HVO geologist Natalia Deligne.
Tephra refers to rock fragments blasted out during an eruption, and it includes Pele’s hair. Although tephra has been falling on a closed section of Crater Rim Drive since the current eruption began, a new pu‘u began forming in late March to early April. At first, HVO scientists roughly tracked tephra buildup by photographing how a road sign was progressively buried. By the end of episode 21 on May 11, only a small portion of the sign remained visible, signaling that this approach wouldn’t be sustainable much longer.
To improve measurements, just before and after episode 22, geologists set up a line of garden stakes about 5 feet tall at regular intervals along the closed road. The goal was to quantify tephra deposition at each spot after each eruption — similar to how rainfall gauges or water gauges monitor rainfall or water levels at a location. That method also proved insufficient: episode 23 on May 25 buried five of the twelve stakes, and episode 24 on June 4 buried three more.
The rising pu‘u grew too quickly for these low-tech approaches during lava fountain events. Higher-tech methods required conducting foot-based transects with a handheld, high-precision GPS unit. This was trialed at several points after episodes 23 and 24, with complete transects along the then-buried Crater Rim Drive completed after episode 25.
GPS measurements are taken at multiple sites, including the closest stable point near the pu‘u’s peak (the exact peak location shifts with each episode, and the top can become unsafe due to slumping or cracking). At a measurement site, the instrument first needs a strong satellite fix (typically 10–20 satellites) to achieve vertical accuracy of 30 centimeters (about 1 foot) or better. This fix can take from a few seconds to several minutes. Then, a location data point is recorded once per second for 240 seconds (4 minutes).
A full transect takes 2–3 hours to complete. Back in the office, the data are corrected using a nearby permanent GPS monitoring station. The final averaged positions at each site achieve vertical accuracy around 10 centimeters (roughly 4 inches). This data enables tracking of the pu‘u’s growth over time. The original ground surface was derived from high-precision elevation data collected by lidar in 2019.
HVO has also conducted a few surveys of the new pu‘u using its helicopter-mounted airborne lidar system, providing high-resolution three-dimensional scans of the entire feature. This work occurs about once a month, but data processing remains complex and ongoing. Attempts to use photogrammetry to monitor elevation changes across the crater floor have been unsuccessful, because the new pu‘u’s surface is too uniform and lacks distinctive features.
The dataset reveals how rapidly the new pu‘u can accumulate material during lava-fountaining episodes. For example, at the site of the now-buried road sign, tephra deposits reached 3.4 meters (11 feet) thick by May 31, 2025 (episode 23). During episode 24, over roughly 7.5 hours, another 7.8 meters (25.6 feet) of tephra piled up. Episode 36 added about 7.5 meters (24.6 feet) in just 5 hours!
The pu‘u grows most dramatically during high-fountaining episodes that exceed about 300 meters (1,000 feet) in height. When lava fountains remain below the crater rim, tephra has a harder time reaching the new pu‘u from Halemaʻumaʻu. Pele is now building where visitors once drove around Halema‘umaʻu between the former Jaggar Museum site and the Halema‘umaʻu Overlook.
HVO will continue to monitor the evolution of this new feature on Kīlauea, which illustrates how rapidly Hawaiian volcanoes can reshape their landscapes.
Volcano Activity Updates
Kīlauea has been erupting intermittently from the summit caldera since December 23, 2024. The USGS Volcano Alert level remains WATCH.
Episode 37 lava fountaining lasted about 9 hours on November 25. The summit is inflating again, with glow intermittently visible at night from both vents. Another fountaining event is anticipated between December 6 and 9. No unusual activity has been noted along Kīlauea’s East Rift Zone or Southwest Rift Zone.
Mauna Loa is not erupting, with its Volcano Alert Level at NORMAL.
One earthquake was felt in the Hawaiian Islands in the past week: a magnitude 3.5 event located 1 km west of Pāhala at a depth of 32 km, recorded on November 29 at 6:39 p.m. HST.
HVO will continue close monitoring of Kīlauea and Mauna Loa.
For more past Volcano Watch articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information, and more, visit HVO’s website. Questions can be emailed to askHVO@usgs.gov.
