Monitoring lava lake fluctuations and crater refilling with continuous laser rangefinders

Abstract

The U.S. Geological Survey’s Hawaiian Volcano Observatory (HVO) has developed a new method to continuously monitor lava lake elevations. Since 2018, HVO has stationed a laser rangefinder on Kīlauea’s caldera rim. The instrument automatically measures lava lake elevation each second, with centimeter accuracy. A stream of elevation data flows to HVO’s database and public website, contributing a valuable channel to HVO’s volcano monitoring network. The data display is intuitive for users, providing essential information with a new level of clarity. HVO has used this method to track Kīlauea’s changing lava lake elevations over a series of eruptions, and the time series data show several volcanic processes: crater refilling, gas pistoning, lava lake surface behavior, and endogenous crater floor uplift. This technique is versatile, nimble, and easy to use. Continuous laser rangefinders may also prove useful for tracking lava lakes elsewhere, and for monitoring other hazards such as growing lava domes and debris flows.

Introduction

Lava lake surfaces change constantly on large and small scales. At the U.S. Geological Survey’s Hawaiian Volcano Observatory (HVO), we measure lava lake surface elevations to gauge the pressure of magmatic systems beneath (Anderson et al. 2019, 2024). Underlying magmatic reservoir pressure and variations in shallow outgassing drive fluctuations in surface elevation (Patrick et al. 2019c). Tracking lava lake elevation is a vital part of volcano monitoring. Accurate and timely information on the behavior of the lava lake can improve warnings to those at risk about impending hazards, such as flank eruptions (Patrick et al. 2015; Poland and Anderson 2020).

Until recently, monitoring of lava lake elevation has lacked automated, real-time techniques. Scientists have long used theodolites and handheld laser rangefinders in the field (Patrick et al. 2019c). These measurements are sporadic, because conventional survey methods are labor intensive. Furthermore, field measurements expose staff to hazards such as volcanic gas emissions, acid rain, blowing tephra, ballistic ejecta, and ground collapse. These environments are also hard on scientific equipment.

The U.S. Geological Survey’s Hawaiian Volcano Observatory (HVO) has developed a method to continuously monitor lava lake elevation at Kīlauea volcano, Island of Hawaiʻi. Since 2018, HVO has stationed a ruggedized laser rangefinder on Kīlauea’s caldera rim. The instrument uses an invisible laser to automatically measure the distance to the lava lake below. It reliably measures lava lake elevation at 1 Hz, with centimeter accuracy (Patrick et al. 2019a, 2022a; Younger et al. 2024).

A continuous stream of elevation data flows to HVO’s database and public website (Hawaiian Volcano Observatory 2025), contributing a valuable channel to HVO’s volcano monitoring network. The real-time data provide lava elevation information with a new level of clarity. The time-series data are intuitive for users, and easily incorporated into operational monitoring and public outreach.

This methodology paper discusses HVO’s use of continuous laser rangefinders since 2018 to track lava lake elevation and crater refilling at the summit of Kīlauea volcano (Fig. 1). We share our development work and describe the system’s integration with HVO’s volcano monitoring network. We illustrate several volcanic processes shown by analysis of continuous laser rangefinder data: long-term crater refilling, gas pistoning, lava-lake surface behavior, and endogenous crater floor uplift. The corresponding datasets for the methodology described here are published in Patrick et al. 2019a, 2022a; Younger et al. (2024).

Fig. 1

Laser rangefinder deployment locations at Kīlauea. Map extent encompasses Kīlauea’s summit caldera (Kaluapele), showing the topography before (A) and after (B) the 2018 caldera collapse. (A) In early 2018, the laser rangefinder was on the rim of Halemaʻumaʻu crater and tracked the lava level at the lava lake in the southeast portion of the crater. (B) After the 2018 collapses, the floor of Halemaʻumaʻu dropped over 500 m, and the surrounding area (termed “down-dropped block”) subsided by a lesser amount. This digital elevation model (DEM) shows the 2019 crater geometry before infilling. Since December 2020, the deepened Halemaʻumaʻu crater has been refilled with lava from six separate eruptions at the time of this writing (March 2025)

This technique is versatile, nimble, and easy to use. The instrumentation is unobtrusive and good for field installations with limited power and bandwidth. Continuous laser rangefinders may also prove useful for tracking lava elevations at similar open-vent basaltic volcanoes, and for monitoring other volcanic hazards, such as growing lava domes and debris flows (Iezzi et al. 2024).

Background

Previous techniques to track lava lake level changes

In Hawaiʻi, routine measurements of the elevation of a long-term lava lake at the summit of Kīlauea volcano began in 1912 with the establishment of the Hawaiian Volcano Observatory (Jaggar 1947; Wright and Klein 2014; Tilling et al. 2014). These measurements used conventional survey techniques based on benchmarks, triangulation and theodolites. This labor-intensive field work was generally done weekly, until the termination and draining of the lava lake in 1924. Theodolites continued to be the primary tool for measuring lava lake elevations for decades. During the 1969–1974 eruption of Maunaulu on Kīlauea’s East Rift Zone, geologists used an optical rangefinder to measure the elevation of lava ponds (Swanson et al. 1979).

The 1983–2018 Puʻuʻōʻō eruption formed various lava ponds within the Puʻuʻōʻō cone (Heliker and Mattox 2003). Lava levels were measured sporadically using theodolites for much of the eruption (Heliker et al. 2003). From the early 2000s to the termination of the Puʻuʻōʻō eruption in 2018, a handheld laser rangefinder was used. These measurements referenced heights to benchmarks whose elevation was measured with high-precision GPS (Orr et al. 2015; Patrick et al. 2019b).

The 2008–2018 summit eruption in Halemaʻumaʻu formed a continuously active lava lake (Patrick et al. 2021a). Field measurements of lava level became more frequent, partly aided by the accessibility of the eruption site (Patrick et al. 2022a). In the early years of the eruption, sporadic measurements of the lava lake level used handheld laser rangefinders and a terrestrial laser scanner (LeWinter et al. 2021). Measurement was sometimes difficult due to the lake’s position deep in the fume-filled crater. As the eruption continued, the lava level rose and became easier to measure with handheld laser rangefinders, and measurements were made daily.

HVO also began using the images from a continuously operating thermal camera to provide hourly measurements of lava level (Patrick et al. 2014, 2016a). These image-processed measurements were calibrated daily using the laser rangefinder measurements. These measurements were more frequent, but they relied on manual calibration for accuracy. During subsequent summit eruptive activity in 2020–2022, a terrestrial scanning lidar was used in periodic campaigns. It measured broad areas of the Halemaʻumaʻu crater floor and active lava lake elevations (Carr et al. 2023), much like the campaigns of LeWinter et al. (2021).

Another volcano with long-term tracking of lava lake elevation is Mount Nyiragongo in the Democratic Republic of Congo (Durieux 2002; Barrière et al. 2022). A lava lake was often present in the summit crater during the 20th century, and its gradual rise culminated in a deadly flank eruption in 1977 (Durieux 2002). The eruption’s onset was associated with abrupt lava lake drainage. The crater refilled over the next two decades, in large part due to eruptions in the early 1980s and mid-1990s, until another large flank eruption and crater floor collapse occurred in 2002 (Barrière et al. 2022). Crater refilling then proceeded until a flank eruption in 2021 (Barrière et al. 2022; Smittarello et al. 2022). This eruptive cycle necessitated measurements of the level of lava in the crater, to track the progress of crater refilling and the growing potential for hazardous flank eruptions. Estimates of lake level began in 1928 and were approximations relative to several persistent ledges within the crater. These estimates were sporadic, separated by months or years, and complicated by the difficulties in accessing the summit. More frequent measurements have been possible with satellite data in recent years (Barrière et al. 2022), following a radar-based approach used for Ethiopia’s Erta Ale lava lake in Moore et al. (2019). Modern measurements with unoccupied aerial systems (Barrière et al. 2022), stereo cameras (Smets et al. 2017), and handheld laser rangefinders (Durieux 2002; Burgi et al. 2014) have provided a new level of precision to lava lake measurements.

Elsewhere, the lava lake at Erebus volcano, Antarctica, has been another site for testing lava lake measuring techniques. Terrestrial scanning lidar was used to measure fluctuations in lava level over relatively short periods of several hours during field campaigns in 2008–2010 (Jones et al. 2015). Later, a new radar measurement technique was developed for measuring the lava lake at Erebus, operating during 2016 in extremely challenging, cold conditions (Peters et al. 2018). This radar distance measuring device had the capability of penetrating optically thick volcanic plumes. The instrument was able to record subtle (sub-meter) fluctuations in the lava level at high measurement rates (0.25 Hz). One difference with the laser rangefinder measurements is that the radar instrument averages elevation over a broader area than the small footprint of the laser units.

Recent activity at Kīlauea’s summit

At Kīlauea’s summit, lava lake activity has been well documented within Halemaʻumaʻu—a nested crater inside the summit caldera—over the past two hundred years (Wright and Klein 2014). Lava lake activity was nearly continuous for a century following the first written account in 1823 (Wright and Klein 2014). In 1924 the lava lake in Halemaʻumaʻu drained abruptly, followed by a series of explosive eruptions (Jaggar 1947; Wright and Klein 2014). Sporadic effusive activity occurred within the caldera throughout the rest of the 20th century. The longest of these periods was 9 months of lava lake activity within Halemaʻumaʻu in 1967-68 (Wright and Klein 2014; Mulliken et al. 2024).

Sustained Halemaʻumaʻu lava lake activity resumed with the onset of the 2008–2018 summit eruption (Patrick et al. 2021a; Mulliken et al. 2024) (Fig. 1A). By 2010 lake activity was continuous, with lava rising within a newly formed smaller crater inside Halemaʻumaʻu. Lava levels fluctuated, driven by pressure changes in the summit magma reservoir and shallow gas-pistoning cycles (Nadeau et al. 2015; Patrick et al. 2016b, 2019c). By 2016, the lava lake had risen to an elevation of 997 m above sea level (asl) and was often close to the floor of Halemaʻumaʻu crater, at 1023 m asl (Patrick et al. 2018). High lava levels were maintained through 2017 and early 2018.

In March 2018, an abrupt rise in the lava level was associated with inflation of the summit magma reservoir, signaling that pressure was building (Neal et al. 2019; Patrick et al. 2020). In late April, HVO installed a continuous laser rangefinder to track the ongoing changes in lava level with greater detail (Patrick et al. 2019a). Just two weeks after the installation, the lava level began dropping rapidly. This was caused by drainage of the summit magma reservoir, driven by the onset of a new eruption on the lower East Rift Zone (Neal et al. 2019; Anderson et al. 2019; Patrick et al. 2019a). The lake drained out of view on May 10, with the gradual widening of Halemaʻumaʻu by collapse proceeding for the next few weeks (Anderson et al. 2019). The laser rangefinder that had been installed on the rim was destroyed.

Crater rim collapses near the vent escalated into more widespread collapses of the summit caldera’s floor in early June 2018 (Anderson et al. 2019), and the terrain of the western caldera was changed dramatically as a result. When the collapse events halted in August 2018, the floor of Halemaʻumaʻu crater had dropped more than 500 m, with the narrowest and deepest portion of the conical pit lying at an elevation of 517 m asl (Mosbrucker et al. 2020). (Fig. 1B)

In July 2019, the rebounding groundwater table emerged through cooling rubble at the base of the pit, ponding in the very bottom of Halemaʻumaʻu crater (Nadeau et al. 2024). The water lake reached a maximum depth of 50 m before new effusive lava abruptly boiled off the entirety of the lake on December 20, 2020 (Patrick et al. 2021b).

This marked the onset of the December 2020-May 2021 summit eruption (Carr et al. 2023; Mulliken et al. 2024) (Fig. 2). Within the first five days, lava had filled 180 m of Halemaʻumaʻu crater, up to an elevation of 700 m asl. Lava continued filling the crater for the next five months, tracked with the newly installed continuous laser rangefinder, with the crater floor consisting of an exposed lava lake surrounded by solidified lava. This first post-collapse eruption ended in May 2021. It was followed by the onset of a second eruption in September 2021, which lasted over a year, ending in December 2022 (Mulliken et al. 2024). During this time, the area of active lava retreated to a small lake in the western portion of the crater, with the majority of the surrounding crater floor again consisting of solidified lava flows. Timelapse imagery from stationary webcams showed that this solidified lava surface rose in an endogenous manner during the eruption, akin to inflating an air mattress, indicating that lava was being supplied beneath the surface crust and spreading out across the area of the crater, lifting the solidified crater floor.

Fig. 2

Infilling of Halemaʻumaʻu crater, at the summit of Kīlauea, since 2020. (A) The 2019 digital elevation model (DEM) shows topography following the 2018 caldera collapse. Halemaʻumaʻu formed the deepest portion of the caldera. (B) DEM from 2023 showing the infilling of Halemaʻumaʻu with new lava between 2020–2023. The laser rangefinder, installed in December 2020, is on the western caldera rim. (C) An east-west profile showing a transect across the caldera, and the 400 m of infilling in Halemaʻumaʻu between 2020 and 2023

A third post-collapse eruption began in early January 2023 and continued for three months (Mulliken et al. 2024). During this time, lava lake activity was centralized in the eastern portion of the crater. Webcams again showed evidence of the surrounding solidified crater floor lifting in an endogenous manner. The next eruption occurred in June 2023 and lasted only two weeks. Activity was focused in the southwest portion of the crater, with the primary eruptive vent in the southwestern wall of Halemaʻumaʻu. A subsequent eruption occurred in September 2023 and lasted for just 6 days, with fissures opening east of Halemaʻumaʻu on a portion of the caldera floor that had lowered during the 2018 collapse, informally referred to as the “down-dropped block” (Mulliken et al. 2024). Lava from these fissures flowed west into Halemaʻumaʻu and contributed to further rise of the crater floor.

These first five eruptions occurred in the midst of long-term inflation at the summit, indicating pressurization and refilling of the summit magma reservoir system following the dramatic draining of 2018. Inflation continued at the summit in late 2023, but unrest shifted towards the south portion of the caldera, as well as the upper Southwest Rift Zone. Several apparent intrusions occurred during late 2023, culminating in a larger intrusion in January 2024 that created extensive ground cracking near Twin Pit Craters on the upper Southwest Rift Zone. Inflation continued and led to a brief, 8-hour eruption in this area on June 3, 2024. Similar intrusions have since occurred below the upper East Rift Zone in July and August 2024, producing a small eruption in the middle East Rift Zone in September.

More recently, a sixth summit eruption began in December 2024 and continues at the time of final submission (March 2025). This paper shows only results up to the September 2023 eruption.

Methods

2018 Instrumentation

We started exploring a new method for monitoring Kīlauea lava lake elevations in January 2018. We field tested an industrial laser rangefinder originally designed for foundry operations, a Sick AG model DT1000. From Halemaʻumaʻu “Overlook” observation site, we measured the distance to the glowing lava lake below using an invisible laser. The DT1000 used a 905 nm fiber optic laser with a maximum range of 460 m. The instrument transmitted pulsed laser signals that were partially reflected off the lava surface back to the instrument sensor. The elapsed time of flight of the laser was measured, and the slant-range distance to the lava surface was calculated using the speed of light. The laser rangefinder was classified as a statistical time-of-flight system. A series of laser pulses was emitted for each measurement cycle. Range measurements were calculated for up to eight returned echoes per cycle.

We mounted the instrument on a tripod and aimed it downward from the edge of the crater rim. The instrument’s fixed angle was measured with an inclinometer and used to calculate the vertical distance to the lava lake surface. We compared the stream of continuous range measurements with individual data points from a Safran Vectronix AG Vector23 handheld laser rangefinder. Measurements agreed to within a meter, the precision of the Vector23 instrument.

We recorded range data to the approximately 1100 °C surface of the active lava lake 100 m below, in challenging conditions of volcanic emissions and vent spatter. Signal strength was also recorded as a diagnostic parameter. We logged raw timestamped measurements on a laptop. Power consumption of the DT1000 was 10 Watts.

Our lava lake tests were campaign-style data collections lasting several hours at a time over several separate days. During field experiments, we systematically varied target areas, sampling rates, and inclination angles. We made measurements in a range of visibility conditions and different regimes of lava lake spatter activity. Returned signal strength was related to lava texture. Rougher lava surface texture returned a stronger signal than smooth, shiny lava. At a slant range of about 150 m during testing, the laser’s invisible spot on the lava surface was about 10 cm x 50 cm. Lower measurement cycle times were more successful than higher ones, which were tested but consumed more power and produced noisier data. A measurement rate of 1 Hz was our compromise between temporal resolution and reliability. Data noise was also related to instrument instability. Efforts were made to minimize shaking of the tripod instrument, including using more sturdy tripods and weighing them down with rocks. These improvements produced cleaner range data.

A steeper angle of inclination provided more reliable signal returns than a shallower angle. The location of our observation site in relation to the lava lake surface below determined the steepest angle possible, about 55.4° below horizontal. The surveyed elevation of the observation location and fixed angle of the instrument were used to calculate the elevation of the lava lake in meters above sea level in NAVD88 datum.

To optimize range finding precision, the returned laser signal was processed using internal statistical methods. These onboard algorithms functioned to pick the best return amplitude peaks from the complex echo train received by the instrument. Up to eight return echoes were analyzed for range measurement for each measurement cycle. Our inputs to the onboard processing included setting range gates. These greatly helped to screen out noisy data from aerosol emissions and airborne tephra in the crater. We played with other internal statistical filters to test their potential, but we eventually disabled them in favor of logging raw data.

The initial field data were promising. Range precision of +/- 25 mm was better than the handheld Vector23 by an order of magnitude. The high sampling rate provided fine-scale temporal resolution of lava surface oscillations. Our tests verified instrument performance, telemetry requirements, and power consumption. The next step was integration with HVO’s volcano monitoring network.

Back in the HVO workshop, we built out an instrument package capable of continuous deployment in the volcanic environment. Tephra and corrosive fumes called for special considerations to protect the valuable laser. We made a field-serviceable, sealed enclosure by modifying a Seahorse case. We added a 6 mm-thick glass port window, bezels, bulkhead connectors, and a scope tube. We designed and 3D printed parts in order to rapidly prototype and operationalize this first continuous laser rangefinder.

We deployed the continuous laser rangefinder at Halemaʻumaʻu Overlook on March 12, 2018 (Figs. 1A and 3A). Monitoring network integration provided real-time data of the lava lake elevation for the first time. We connected the instrument to HVO’s instrument telemetry network through a serial device server that converted the RS422 data to TCP/IP data. We transmitted continuous 1 Hz data via digital ethernet radio to HVO’s database. HVO’s existing Halemaʻumaʻu Overlook camera station provided this telemetry, as well as solar power.

Perched on the edge of the crater rim, the instrument enclosure was fixed to an angle bracket on a fiberglass survey tripod (Fig. 3A). We piled the legs with rocks for ballast. As a precaution, we tied the instrument off to a heavy ballistic block several meters back from the edge of the rim. On April 6, 2018, a vent explosion from the lava lake damaged the instrument enclosure, as well as the radio, antenna, and solar panels of the power station. Hot ejecta melted the scope and burned through cable runs. The laser instrument itself survived the blast inside the enclosure. We repaired the telemetry and power station and built a replacement enclosure with a hardened scope, then redeployed the continuous laser rangefinder on April 25, 2018.

Fig. 3

Photos of the field installations. (A) The 2018 laser rangefinder setup, which was installed shortly before the onset of lava lake draining. This photo shows the laser rangefinder, in a grey enclosure, situated on a yellow surveyor’s tripod and weighed down with rocks. At the time of this photo on May 6, 2018, the lake had dropped about 200 m over the previous four days. This unit was destroyed just days later when the crater rim collapsed. USGS photo by Kyle Anderson. (B) The 2021-present laser rangefinder setup. The unit is pointed at the active lava lake in the western portion of Halemaʻumaʻu crater; this lake is surrounded by solidified lava comprising the crater floor. Photo taken September 25, 2022. USGS photo by Matt Patrick

The continuous laser rangefinder delivered an opportune data stream over the next two weeks, which proved to be one of the most dynamic periods in Kīlauea’s observed history. Record high lava levels were followed by a precipitous drop after April 30. Vent collapses and explosions made the Overlook area dangerous for field work, and the lava lake drained out of view on May 10. Caldera collapses became more widespread, and the growing crater claimed the Overlook observation site. All the station equipment, including the first continuous laser rangefinder, was destroyed.

2020-present instrumentation

We started work on an improved continuous laser rangefinder in July 2020. At the time, a groundwater-fed caldera lake was rising in Halemaʻumaʻu (Nadeau et al. 2024). Monitoring the unprecedented (in modern times) water lake was an HVO priority, but the depth and steepness of the crater made the water lake shore inaccessible overland. Consequently, measurements of the water elevation were being collected daily using a handheld laser rangefinder from the western rim of Halemaʻumaʻu. This vantage point was 585 m above the water lake, at a measured slant-range distance of 1200 m. Despite swirling steam from the 80 °C lake, handheld laser rangefinder measurements proved reliable, and showed good comparison with digital elevation models (DEMs) and trending lake growth.

We decided to test a continuous laser rangefinder (Safran Vectronix AG model LRF7047) based on our field experience with the handheld (Vectronix Vector23) laser rangefinder. The continuous laser rangefinder was a bare laser rangefinder module with similar specifications to the handheld Vector23 unit. A 1550 nm infrared fiber laser emitted a series of pulsed measurement signals for each measurement cycle. The receiver photodiode sampled the returned signal echoes from the series. The return signal amplitudes and time of flight were statistically processed to calculate the distance for up to three echoes for each measurement cycle. We designed and built a housing for the laser module, making aluminum and 3D-printed plastic components. The computer-aided design files for this housing are included in Younger et al. (2024). The housing featured dual optical path isolation for the transmit and receive channels, inert gas shielding, and gasketed glass lenses.

In July and August 2020, we field tested the new laser rangefinder from the KWcam site on Kīlauea’s western caldera rim in campaign deployments. We collected slant-range measurements of up to 1220 m to the water lake surface. We mounted the instrument to a portable photography tripod and logged timestamped 1 Hz range data on a laptop. We powered the instrument with a portable DC supply, and measured electrical consumption of 4 W. We aimed the laser by sighting it to the water lake surface, and manually measured the instrument angle using an inclinometer. The laser was pointed 30° below the horizon. Visibility conditions changed over several different days of observation, from clear skies with high surface reflection, to swirling steam and overcast skies. The lake had high turbidity with many different colored patches of water (Nadeau et al. 2024). Surface waves were of variable size and direction. In September 2020 we field tested the laser from another location, on the down-dropped block at the east side of the crater. This observation site sat at a lower elevation in the crater, closer to the water lake. A helicopter pilot flew us in for testing and site reconnaissance. The angle of inclination to the lake surface was steeper, about 35° below the horizon. We collected slant-range measurements of about 620 m, through thick steam and heavy rain.

These campaign field tests built on our 2018 work, with the added variables of a water surface target at a longer range. We compared continuous range measurements with handheld Vector23 data points. The continuous laser range data were collected from the water surface and from the rocks on the shore of the lake. Our elevation measurements were consistent with a new terrestrial scanning lidar DEM of the crater (Mosbrucker et al. 2020). Again, we used range-gate settings to screen out erroneous returns from steam in the crater. Wave action partly contributed to noise in water surface returns. The raw data were +/- 25 cm. We also noted a spurious data artifact that appeared as blank bands of missing data points within the good timeseries data. Multiple levels of narrow +/- 5 cm blank bands were recorded. These erroneous data artifacts were likely caused by internal power mode switching of the LRF7047 as it adjusted signal strength. Ultimately, our tests showed that the laser penetrated much of the steam and returned a repeatable range measurement from the water surface. Our instrument tests verified performance, telemetry needs, and power consumption. A subsequent eruption, the first since 2018, accelerated the installation of the permanent instrument on the rim but shifted the focus of measurement from water to lava.

On December 20, 2020, Kīlauea erupted within Halemaʻumaʻu crater (Mulliken et al. 2024). The ensuing HVO response included finalizing the installation for permanent, continuous use, and included building, deploying and networking the continuous laser rangefinder for lava lake monitoring duty. We built a sealed enclosure around the field campaign housing to protect the instrument for continuous deployment. To check laser alignment and angle, we integrated a dual axis inclinometer. We mounted the continuous laser rangefinder on the crater rim using a sturdy aluminum tripod with an adjustable pan-tilt head (Fig. 3B). We heaped rocks on the tripod legs and winched it down to a heavy ballistic block for ballast. The new continuous laser rangefinder was connected to the existing power and telemetry of the KWcam station, and lava level data went online January 8, 2021.

Over the past several years, only infrequent intervention has been needed to maintain the instrumentation and data stream. Minor maintenance in the field has included swapping desiccant packs in the enclosure and wiping accumulated debris from the lenses. We have periodically re-aimed the laser rangefinder to measure active parts of the lava lake surface. Continuous laser rangefinder measurements have been checked by comparison with handheld Vector23 measurements collected during field crew operations. Other elevation measurement verification has included comparison with digital elevation models of the crater derived from both structure-from-motion photogrammetry and terrestrial scanning lidar surveys (Carr et al. 2023).

Data acquisition

The first version of the data acquisition process, in which a manual distance measurement was taken on occasion by a handheld laser rangefinder, also utilized a more manual approach to ingesting the data into the database. Whoever took the measurement in the field would add the data to a spreadsheet, and then a script would run once per day to search for a new row in the table; if one existed, it would be inserted into HVO’s digital, searchable monitoring database. Once we moved to continuous instrumentation, we also transitioned to a process in which the data is added to the database the moment it reaches the HVO server infrastructure.

All communication with both the DT1000 and LRF 7047 has gone through a serial device server. Both instruments are manufactured with internal computers that support serial interface. The DT1000 uses RS-422 and the LRF 7047 uses RS-232. We wired them to a serial device server and then to an ethernet digital radio for communications to HVO’s data network. The two devices have slightly different requirements for getting data flowing. The DT1000 was configured as a pure streaming device such that once a connection was made, data would flow continuously until the connection was closed. The LRF 7047 operates instead in a command/response mode. The primary command that we use returns a constant stream of 1 Hz data, but the interface exposes much more functionality over the serial interface than the DT1000 did. A small python library was written to take advantage of this interface and has been used for tasks such as testing data output, running hardware tests, and setting distance gates to filter out bad data. This python library is included in Younger et al. (2024).

Nearly all HVO data are deposited in a digital, searchable database via a general data import program that has individual processes for different devices, datasets, etc. With the LRF 7047 a request has to be made for a continuous data stream, but after that the process for both it and the DT1000 are the same. When a message is received, it is first checked for completeness -- length of the line and starting/ending characters as defined for each device. If the message is deemed complete, it is next checked for any error codes. Provided there are none, the message is parsed, and the data inserted into the database, with the process repeating for the next line of data.

Post-processing to reduce noise

The raw 1 Hz data include some false returns from mist or rain in the atmosphere, which appear as unusually high elevation values, well above the actual level of the lava lake or crater floor. We have found that these outlier values are sparse, and can be easily eliminated with a simple median filter. We note that a mean filter would be undesirable because it would average in these outliers, whereas a median filter eliminates the outliers effectively.

Results

Gas pistoning– April 2018

Gas pistoning is a common process observed in lava lakes at Kīlauea. It involves a cyclic rise and fall of the lake surface, with the fall normally associated with increased outgassing and spattering (Swanson et al. 1979; Tilling 1987). This process has been explained as the accumulation of stored gas beneath the surface of the lava during lake rise, and release of this gas is associated with a rapid drop in lava level (Swanson et al. 1979; Orr and Rea 2012; Nadeau et al. 2015; Patrick et al. 2016b).

Gas pistoning was frequently observed in the 2008–2018 lava lake at Halemaʻumaʻu, and provided some of the best opportunities to study the process in detail given the accessibility of the lake (Nadeau et al. 2015; Patrick et al. 2016b). For the two weeks the continuous laser rangefinder operated in late April 2018, its data offered the most detailed quantification of the process to date (Fig. 4; Patrick et al. 2019a). The data show quasi-periodic rise and fall cycles with an amplitude of approximately 5 m and a duration of roughly 4–6 h (Fig. 4C), consistent with results of Patrick et al. (2016b). Seismic tremor, shown by real-time seismic amplitude measurement (RSAM, Endo and Murray 1991) (Fig. 4D), has an inverse correlation with lava level. Higher lava level is associated with decreased seismic tremor, due to reduced spattering in the lava lake as gas accumulates beneath the lake surface, whereas drops in lava level are associated with an increase in spattering and seismic tremor as accumulated gas is released (Fig. 4A, B). Such precise measurements of lava level (Fig. 4C) can help refine gas budgeting in the lake that may provide more insight into outgassing processes (Nadeau et al. 2024; Patrick et al. 2019c).

Fig. 4

Gas pistoning cycles in the Halemaʻumaʻu lava lake during April 2018. A, B) Thermal images of the lava lake showing low and high stands of the lake. Low stands were associated with increased spattering and gas release. C) Lava lake elevation showing cycles of gas pistoning with an amplitude of 3–5 m (Patrick et al. 2022a). D) Real-time seismic amplitude measurement (RSAM) from station NPT showing the inverse relationship between seismic tremor and lava lake elevation, consistent with earlier studies (Patrick et al. 2016b) but shown in more clarity with the continuous laser rangefinder data

Lava lake draining– May 2018

The draining of lava lakes offers important information on the broader magmatic system, providing insight into pressure dynamics as well as the nature of any concurrent flank eruptions. In May 2018, the lava lake at the summit of Kīlauea drained over the course of a week as an eruption on the lower East Rift Zone drew magma from the summit magma reservoir (Anderson et al. 2019; Neal et al. 2019).

The continuous laser rangefinder installed two weeks before the event showed the draining in unprecedented detail (Fig. 5; Anderson et al. 2019; Patrick et al. 2019a). The onset of lake draining appeared to start late on May 1 (Fig. 5D), roughly one day after the start of a large intrusion and magma migration on the East Rift Zone. The lake dropped roughly 200 m over the next five days, at an increasing rate that may have been impacted by a M6.9 earthquake on May 4 (Anderson et al. 2019). Gas pistoning appeared to be present in the initial 1–2 days of draining but disappeared thereafter. The termination of measurements late on May 5 was due to a range gate setting that was fixed on May 6, but only limited data appeared after this, presumably due to the poor visibility of the lake surface through a very dusty plume. The dropping lake surface led to increased rockfalls from the crater walls, producing dust and haze in the crater. After the last data points on May 6, the lake continued to drop, with the thermal camera used for lava level estimates. The lake was last seen on May 9, draining away beneath the rubble at the bottom of the vent by the morning of May 10. The laser rangefinder data were crucial for modeling of the magmatic system during the 2018 draining, and a full description of this event is in Anderson et al. (2019).

Fig. 5

Lava lake draining during May 2018. A-C) Thermal camera images of the 2008–2018 Halemaʻumaʻu lava lake draining following the onset of new eruptive activity on the lower East Rift Zone of Kīlauea. D) Lava lake elevation measured with the continuous laser rangefinder, showing a drop of about 200 m between May 1 and 6. The lake continued dropping and vanished beneath the rubble on May 10, followed by continued crater collapse and lowering of the caldera floor (Anderson et al. 2019)

Lava lake behavior– 2020–2023

The 2008–2018 lava lake at Kīlauea summit often exhibited a cyclic rise and fall that correlated with deflation-inflation (DI) cycles in the summit magma reservoir (Patrick et al. 2015). These DI cycles have been observed for years at the summit, being tracked by local tiltmeters, but they remain poorly understood (Cervelli and Miklius 2003). The close correlation between the summit lava level and ground tilt suggested that the lake behaved like a piezometer, a liquid pressure gauge of the underlying summit magmatic system (Patrick et al. 2015).

During 2022, a small lava lake in the western portion of the crater showed fluctuations that correlated with DI cycles (Fig. 6). Quasi-periodic cycles of rise and fall in the lake had an amplitude of approximately 5–7 m with a period of roughly 15 h. These fluctuations had a close correlation with ground tilt, with increases in ground tilt (representing increases in pressurization of the magmatic system) corresponding with rises in the lava level, and vice versa. Although the geometry of the 2022 lava lake was different than that of the 2008–2018 lake, the variations in lake elevation were also driven by pressure variations in the summit magma reservoir, potentially related to variations in lava supply rate at the vent.

Fig. 6

Lava level fluctuations with deflation-inflation cycles during 2022. A, B) Thermal images from the F1cam webcam (Patrick et al. 2022b) showing high and low stands of the lava level in Halemaʻumaʻu crater. C) Lava lake elevation tracked with the laser rangefinder, showing periodic fluctuations with an amplitude of 5–6 m. D) Ground tilt (radial component) from tiltmeter UWE at Kīlauea summit, showing deflation-inflation cycles in the summit magma reservoir, corresponding to lava level fluctuations (Ellis and Johanson 2024). This correlation was also observed in the 2008–2018 lava lake (Patrick et al. 2015)

Rainfall-triggered lava outgassing– 2021–2022

On several occasions during summit eruptions in 2021–2022, the lava lake was observed to show an abrupt drop during particularly heavy downpours of rain (Fig. 7). The magnitude of the drop was typically about one meter (Fig. 7D). Review of webcam and timelapse camera imagery showed that the drop in lava level was real, and not some artifact of the heavy rains or low visibility. Instead, the images showed that the lava became abruptly and unusually active with spattering and bubbling on the surface during downpours (Fig. 7E), coinciding with the drop in lava level (Fig. 7B). Seismic tremor as shown by RSAM (Fig. 7F) appeared to increase during the event, presumably due to the increase in spattering. It should be noted that this process was only observed during unusually intense rainfall periods lasting several minutes, which only occur a few times a year at the summit of Kīlauea.

The processes responsible for this phenomenon are not fully understood, but it appears that the rainfall somehow triggered enhanced spattering and outgassing of the lava, which then led to the drop in lava level. This bears some similarity with the fall phase of gas pistoning cycles (driven by outgassing) noted above. Several similar episodes of rainfall-induced changes were observed in the 2008–2018 lava lake, but the lava level changes were not measured as precisely at that time to show a definitive correlation. The example here is the first documented instance of rainfall-triggered changes in a lava lake that we are aware of.

Fig. 7

Rainfall-triggered outgassing and lava level change. A-C) Timelapse camera images showing the lava lake in Halemaʻumaʻu before, during and after a brief intense rainstorm (cloudburst). Note an increase in spattering during the rainfall, as well as heavy steaming. D) Lava lake elevation from the continuous laser rangefinder, showing a one-meter drop during the cloudburst. E) Rain gauge data from nearby station UWE (about 2 km away), showing a sharp increase in rain around 13:00. F) Real-time seismic amplitude measurement (RSAM) from station WRM (about 1 km away) shows a possible increase around the time of the cloudburst that may represent increased tremor, presumably associated with the increased spattering. Other RSAM spikes could be earthquakes or brief spattering events

Endogenous crater floor uplift– 2021–2022

Reports of lava lake activity at Halemaʻumaʻu in the late 1800s and early 1900s often spoke of the solidified lava rising around a molten lake, suggesting that lava was being supplied beneath the surface, endogenously (Jaggar 1947). This process was also observed in the 1967-68 summit eruption and was then referred to as “piston uplift” (Kinoshita et al. 1969). Piston-like uplift was also observed during pit crater filling at Nyamulagira in 2019–2020 (Burgi et al. 2021). Until recently, however, the endogenous process has not been clearly illustrated or captured with high-precision instrumentation.

During 2022, a stationary webcam (B1cam) on the eastern rim of Halemaʻumaʻu removed any doubt that the solidified lava forming the majority of the crater floor was being uplifted endogenously (Fig. 8). The continuous laser rangefinder for much of this time was aimed at the western portion of the crater that had a small active lava lake, and webcam imagery shows that this lake rose at a similar rate as the surrounding solidified crater floor, thus making the laser rangefinder data a reasonable proxy of the endogenous growth. Between April and October, the lake and surrounding crater floor rose by approximately 50 m. Short-term (daily) variations in lava level were related to fluctuations in lava supply rate due to deflation-inflation cycles of the summit magma reservoir (Fig. 6). The vast majority of this rise was due to lava supplied beneath the surface crust, lifting the crater floor in a manner akin to inflating an air mattress. Minor extrusions of small lava flows around the perimeter of the crater floor occurred sporadically but did not contribute significantly to the rise. The timelapse sequence from the B1cam webcam combined with the laser rangefinder data provide the clearest evidence yet of the prominent role of endogenous growth in crater refilling periods at Kīlauea.

Fig. 8

Endogenous crater floor uplift during 2022. A, B) Images from the B1cam webcam spanning April to October, showing the intact rise of the crater floor by approximately 50 m. The majority of this rise was from lava emplaced beneath the surface, akin to inflating an air mattress, with only a minor amount of lava emplaced on top of the crater floor. C) The surface of the lava lake tracked by the continuous laser rangefinder, which closely tracked the height of the solidified crater floor surrounding the lake. Timelapse camera images show that the lake rose with the crater floor

Long term crater refilling– 2020–2023

Crater collapse and refilling is a cyclic pattern observed at numerous volcanoes, such as Kīlauea, Nyiragongo, Ol Doinyo Lengai, Stromboli, and others (Jaggar 1947; Wright and Klein 2014; Burgi et al. 2014; Calvari et al. 2005, 2009). Following the 2018 caldera floor collapse and subsidence at the summit of Kīlauea, the subsequent eruptions provided a rare opportunity to observe and study long-term refilling of a crater (Carr et al. 2023). Detailed tracking of the lava level in the crater has been an important component of documenting that process (Younger et al. 2024).

The continuous laser rangefinder data have been the primary data source for tracking the draining and refilling of lava in Halemaʻumaʻu (Fig. 9). Continuous laser rangefinder data captured the lake draining in May 2018, and then came back online in January 2021 to capture the majority of the crater refilling. The water lake phase (2019–2020) and first two weeks of the December 2020 eruption were measured with a handheld laser rangefinder during field visits. The continuous laser rangefinder showed the rapid rise of lava in the initial weeks of the first post-collapse eruption, driven by high eruption rates and the narrow funnel shape of the bottom of the crater. Subsequent eruptions also showed high initial filling rates due to the initially high lava effusion rates, with decreased effusion rates leading to a decline in filling rates later in each eruption. The data show that, as of the end of the September 2023 eruption, the crater floor was at an elevation of approximately 928 m asl. With the bottom of the crater at approximately 517 m asl immediately after the 2018 collapse, this represents a rise of the crater floor of 411 m. Only about 100 m remain to reach the pre-collapse draining elevation. However, given the widening of the crater at higher elevations (Carr et al. 2023), there remains a substantial volume to be filled in future eruptions. Nevertheless, the data show that a significant portion of crater refilling had occurred over just three years.

At the time of final submission (March 2025), a sixth summit eruption that began in December 2024 had added another 30 m of elevation to the crater floor, but those results are not shown here.

Fig. 9

Elevation of lava lakes and the water lake at the summit of Kīlauea, 2018–2023. With the draining of the lava lake in 2018, the floor of Halemaʻumaʻu dropped to an elevation of roughly 517 m asl. In 2019, a water lake appeared and filled to a depth of approximately 50 m. In December 2020, an eruption filled the crater with lava, boiling off the lake. Four subsequent eruptions further filled Halemaʻumaʻu with lava, reaching an elevation of approximately 928 m asl in September 2023. The most recent eruption (onset in December 2024) is not shown here. Most of the data in this plot were collected with continuous laser rangefinders (Patrick et al. 2022a; Younger et al. 2024). Handheld laser rangefinders were used to measure data (a) prior to April 2018, (b) during the water lake era, and (c) during the first few weeks of the eruption that began in December 2020

Discussion

Advantages of laser rangefinders

One of the primary advantages of the continuous laser rangefinder for tracking lava level is the high precision of the measurement. The instrument specifications for the 2018 and 2021-present models claim an accuracy of 1 m, which refers to the absolute accuracy of the distance measurement to a standardized target at maximum range. However, the relative precision appears to be much better than 1 m in our application. By looking at the noise level of the data, the precision looks to be roughly 10 cm. This high precision allows us to detect changes that would otherwise go unnoticed with other measurement techniques, such as the image-based approach or sporadic handheld laser rangefinder measurements. For instance, the minor change in lake level associated with intense rainfall would likely go undetected with other tools.

Another advantage is the small measurement area. The instrument’s measurement footprint on the lake surface is roughly 1–2 m². This increases sensitivity compared to other methods where the distance measurement might be averaged over a larger area, complicating interpretations.

The instrument is suitable for remote monitoring with constrained power and telemetry bandwidth. The continuous laser rangefinder is powered by a portable solar power station, drawing 4 Watts. Due to the fact that this instrument is telemetered over radio in real-time, bandwidth is a vital consideration. Fortunately, the instrument is inherently low bandwidth owing to the singular nature of the measurement, and relatively low sampling intervals. Each reading is about 40 bytes transmitted over the network. In our deployment, the instrument collects at 1 Hz, but this rate can be adjusted to user preference. We emphasize, however, that utilizing a higher rate than needed can be useful when filtering out spurious returns due to poor weather.

The laser rangefinders we use sample three to eight return signals for each 1 Hz measurement cycle. This intensive sampling of the return signal is advantageous in optically noisy environments like the lava lake, where mid-range scatterers of laser energy abound. Undesired return signals from scatterers like aerosols and raindrops are sampled, but they are automatically filtered out in favor of the strongest signal of the series that is returned. This highest amplitude signal is reliably from the lava surface. The single strongest return signal of the series is analyzed for the 1 Hz range measurement value. This is a more numerous sampling of the laser return signal compared to single pulse laser systems, or to constant phase correlation laser systems that are used in other laser rangefinder applications.

Equipment durability has proved good in a volcanic environment. The optical device is well protected by the enclosure design and has required infrequent field maintenance. We have wiped the external lens of accumulated grit and swapped desiccant pouches in the case annually.

A major benefit of the data is the simplicity–a time series consisting of measurement time and elevation. The processing to calculate the slant range value is internal to the instrument, requiring no intensive post-processing to produce a usable elevation value. Furthermore, the data that are output are intuitive and straightforward to interpret–simply the height of the ground surface at the location of the laser footprint. This simplicity makes it easy for public dissemination, which has included posting the real-time data on the HVO public website (Hawaiian Volcano Observatory 2025).

Limitations

One moderate limitation with the instrument is erroneous values that are returned during periods of heavy rain or thick fog. In these cases, the instrument will get returns from raindrops well above the level of the lava surface. We have found that this tends to happen only during moderate to heavy rain, and can be mitigated relatively easily using two approaches. First, setting a “range gate” on the instrument itself will instruct it to only accept returns within a range of distances (e.g. the approximate elevation of the crater floor), which minimizes false returns from raindrops. Second, simple post-processing of the data using a median filter (not a mean filter) is very effective in most cases at filtering out spurious returns given that, even in moderate rain, the majority of the returns are still from the lava surface. Overall, we have observed an impressive capability of the instrument to operate through thick fog. At times when the KWcam optical webcam (co-located with the laser rangefinder) has shown a view completely obscured by thick fog, the laser rangefinder has continued producing clean returns from the lava surface, presumably due to the near-infrared wavelength of the laser being more capable of penetrating the fog compared to the visible-wavelength KWcam.

Another source of measurement error may be contributed by the emission of photon energy from the lava surface itself. The operating wavelengths of the DT1000 (905 nm) and the LRF 7047 (1550 nm) are close to the emission spectra from erupted Kīlauea lava at higher temperature ranges (1100 °C). The DT1000 is manufacturer specified for a maximum target temperature of 1400 °C, with an optional filter available for targets over 1200 °C, which was not used in this study. The LRF 7047 is not target temperature specified by the manufacturer. Unfiltered optical glass was used in both system installations. The lava target temperature varied between freshly erupted lava (1100 °C) to cooled lava surfaces (20 °C). The measurement error resulting from the target emission signal noise source was negligible when compared with other point checks of elevation. On two occasions immediately following new eruptive activity, the LRF 7047 reported an error message, which was fixed by resetting the instrument. The cause of this malfunction is suspected to be related to high levels of emission energy from the lava.

An inherent limitation of this instrument is the small, singular footprint of the measurement on the lava surface. This limits the measurement area to a tiny sample of the lava lake or crater floor surface. For lava lakes this limitation is minor, given the relatively level surface, though recent work has shown subtle but meaningful changes in elevation across the lava lake’s surface at Halemaʻumaʻu in 2008–2018 (LeWinter et al. 2021). For solidified crater floors, however, this limitation is more pronounced, as complex processes such as uplift and subsidence may work in different portions of the crater floor, creating a highly complex surface that varies by meters or even tens of meters. Terrestrial scanning lidar solves this problem by creating a complete picture of crater geometry, but this technique has several drawbacks for continuous monitoring deployments. Hardware concerns include instrument power consumption (Riegl USA Inc. 2025) and fragility. Network concerns include the increased burden of large and computationally expensive terrestrial scanning lidar datasets. Data management concerns include time-consuming collection, processing, and storage. To ameliorate these issues, we have combined periodic surveys with a handheld laser rangefinder and terrestrial scanning lidar to map out surface elevations across the crater floor, as a way to complement the simpler continuous laser rangefinder that measures elevation in a small area.

Future advancements

A step forward is to integrate the continuous laser rangefinder with an electronic pan-tilt stage. This enables programming the instrument to scan across the lava lake and crater floor surface to collect a more comprehensive sampling of elevation points. This method is like a simplified terrestrial scanning lidar with a very sparse scan. While sparse, the advantages of this setup are significant. The continuous laser rangefinder (Vectronix LRF 6019) and the pan-tilt stage (FLIR PTU 5) are far more affordable (thousands USD) than a terrestrial scanning lidar (tens of thousands USD). This setup also requires less station power and telemetry bandwidth, making it compatible with HVO’s existing monitoring network infrastructure, and is better hardened for continuous deployment in a volcanic environment.

Continuous laser rangefinders likely have broader utility in volcano monitoring, beyond simply tracking lava lakes and crater floor changes. Recent work at the U.S. Geological Survey Cascades Volcano Observatory has shown that they have been effective in monitoring debris flows on Mount Rainier in Washington state, and have the potential to be a critical instrument in the lahar detection network on that volcano (Iezzi et al. 2024). Detection of lahars or debris flows with this technique is, theoretically, straightforward when triggering on anomalous height changes of water levels in stream valleys.

Continuous laser rangefinders may be effective in tracking lava dome growth and associated hazards. The 20 + km range of the model we use at Kīlauea would allow it to be placed at a safe distance downslope of an active lava dome, potentially at a site with grid power and internet. Aimed accurately, it could potentially track meter to sub-meter changes in the growth of a dome surface in real-time. Identifying high rates of dome growth in this manner could provide additional insight into the likelihood of dome collapse and pyroclastic flow initiation (Harnett et al. 2019).

Conclusions

Continuous tracking of fluctuating lava lake surfaces and rising crater floors is a vital part of volcano monitoring, but it has been challenging until recently. A new continuous laser rangefinder method has been highly effective for monitoring such activity in recent years at Kīlauea volcano. This method has provided more timely and accurate data than was previously possible using campaign methods. The method has been integrated into the toolkit of operational eruption monitoring at HVO. The lava elevation data are simple and easy to use and have been displayed directly on HVO’s public website. The versatility of this method has been demonstrated in a range of conditions, with durability over several years of field deployment. Volcanic processes have been shown in unprecedented detail, including crater refilling, gas pistoning, lava lake surface behavior, and endogenous crater floor uplift. Continuous laser rangefinders likely have broader utility in volcano monitoring, such as in detecting lahars and tracking lava dome growth.