Kīlauea’s 2018 flank eruption, on the volcano’s Lower East Rift Zone, produced approximately one cubic kilometer of lava and was the most destructive volcanic event in the past 200 years in Hawaiʻi, with over 700 structures destroyed. The accompanying collapse of the summit caldera was one of the largest at Kīlauea in centuries, triggering small explosions and tens of thousands of earthquakes that damaged nearby infrastructure. Kīlauea has one of the most comprehensive volcano monitoring networks on Earth, and the 2018 eruption provides an excellent opportunity to understand the complex processes that culminate in destructive flank eruptions, the timescales of priming and triggering in open-vent basaltic systems, and how forecasting such extreme events might be improved.
Prior to 2018, the most recent eruptions from the Lower East Rift Zone occurred in 1955, 1960, and 1961, and in 2014–2015 lava from the long-lived Puʻu ʻŌʻō eruption in the Middle East Rift Zone reached the outskirts of Pāhoa. Based on this historical activity and geologic mapping, volcanologists have long known that the Lower East Rift Zone is at high risk for lava inundation. Likewise, several large collapses of the summit caldera floor have occurred over the past 200 years, primarily during the 1800s. Despite the recognition of long-term hazard in these areas, short-term eruption forecasting must address factors of immediate relevance to hazard mitigation and is largely predicated on interpretation of geophysical and geological monitoring data.
In a paper published in the journal Nature Communications on 6 November 2020, Matthew Patrick of the Hawaiian Volcano Observatory, Bruce Houghton of the Department of Earth Sciences at the University of Hawaiʻi at Mānoa, Kyle Anderson of the California Volcano Observatory, Michael Poland and Emily Montgomery-Brown of the Cascades Volcano Observatory, Ingrid Johanson, also of the Hawaiian Volcano Observatory, Weston Thelen, also of the Cascades Volcano Observatory, and Tamar Elias, agian of the Hawaiian Volcano Observatory, characterise the changes at Kīlauea in the weeks to years leading up to the 2018 eruption.
Patrick et al. highlight that the short-term cause was an increase in magma pressure due to a backup of magma in the shallow plumbing system, which ultimately drove magma into the lower flank of the volcano. Several processes, however, likely primed the magmatic system for years prior to the eruption. A cascading series of events caused a relatively small change atKīlauea’s long-term East Rift Zone eruptive vent to lead to historic consequences at the summit and a major destructive effusive eruption on the lower flank. The 2018 activity highlights the challenges in forecasting the form and timing of low-likelihood, large-volume eruptions that result from a cascade of interconnected processes.
Forecasting volcanic eruptions remains fundamentally challenging, despite ongoing improvements in our ability to measure and understand the processes that prime and drive eruptive activity. Once unrest is detected, volcano observatories must provide actionable assessments to emergency managers and the public that include the likelihood and potential timing of an eruption, its location, and ideally its scale and style. Even with robust monitoring networks and a strong understanding of a volcano’s geological and historical activity, these questions usually cannot be answered with confidence. Magmatic systems are highly complex and cannot be directly observed; therefore, volcanologists infer subsurface processes from often sparse monitoring data and idealised models. In addition, while magmatic systems may recharge and prime over extended intervals, the events that ultimately trigger an eruption occur over much shorter timescales, for example, the collapse of the north flank of Mount St Helens in 1980. These limitations create significant uncertainty in forecasting scenarios.
Explosive eruptions may exemplify the common picture of volcanic hazards, but effusive (lava-producing) events, typical at basaltic systems, can also be dangerous and destructive. In human terms, the stakes of forecasting can be particularly high when lava effusion occurs low on a volcano’s flank, where population centers are common. Eruptions at Nyiragongo (Democratic Republic of Congo) in 1977 and 2002, Etna (Italy) in 1669 and 2001–2002, Piton de la Fournaise (Réunion Island) in 1977, Mauna Loa in 1926, 1950, and 1984, and Kīlauea in 1960 and 1983–2018 are part of the long historical record of the risk to society posed by flank effusions and emphasise the critical importance of accurate forecasts during these events.
Kīlauea’s magma originates in the mantle and rises into a reservoir complex beneath the summit caldera at depths of 1–5 km.The summit reservoir system supplies magma to summit vents and laterally to rift zones that radiate to the east and southwest. Eruptions can occur at the volcano’s summit, where lava lakes have been common over the past 200 years, and/or along the rift zones; Kīlauea’s East Rift Zone has been especially active since the 1950s. Magma can also be stored within the rift zones, but the volume and geometry of this storage remains uncertain, especially below about 3 km depth. Further complicating the picture, Kīlauea’s south flank moves seaward at rates of up to about 8 cm a year, imparting extensional stresses on the rift zones and facilitating rift zone magma transport, dike intrusions, and fissure eruptions. Likewise, magma injected into the rift zones can produce stresses that trigger south flank motion and earthquakes. Thus, the tectonics of the volcano’s south flank are tightly coupled with the magmatic system of the East Rift Zone.
Prior to 2018, the most recent eruptions in the Lower East Rift Zone occurred in 1955, 1960, and 1961 with eruptions focused on the Middle East Rift Zone and summit during the 1960s and 1970s. Kīlauea erupted nearly continuously at or near the Puʻu ʻŌʻō eruption site in the Middle East Rift Zone from 1983 until the onset of the 2018 Lower East Rift Zone eruption. For most of that time, slow-moving lava flowed south to the ocean, producing a 144 km² flow field. In 2008, a lava lake formed in Halemaʻumaʻu crater, at the volcano’s summit 20 km uprift from Puʻu ʻŌʻō, and persisted until the start of the 2018 eruption.
Following several weeks of pronounced pressurization of the magmatic system at both the East Rift Zone and summit eruptive vents, a small, brief fissure eruption occurred on the west flank of the Puʻu ʻŌʻō cone on 30 April 2018. Over the next few days, earthquakes migrated eastward into the Lower East Rift Zone and rift-normal displacements were recorded by Global Positioning System instruments, signaling large-scale injection of magma downrift of Puʻu ʻŌʻō. Magma reached the surface in Leilani Estates subdivision on 3 May marking the onset of the Lower East Rift Zone eruption. The next day, a Magnitude 6.9 Earthquake occurred on Kīlauea’s south flank, the largest earthquake in Hawaiʻi in 43 years. The Earthquake involved southward displacement of the mobile flank and is thought to be a consequence of stress induced by the East Rift Zone injection. Throughout May, 24 short-lived fissures developed in the Lower East Rift Zone, but activity focused on fissure 8 by the end of that month. The fissure 8 lava flow reached the ocean at the eastern tip of the island in early June, destroying several subdivisions and establishing a stable lava channel that persisted for two months.
The Lower East Rift Zone eruption drained magma from the summit reservoir, 40 km away, at rates exceeding 100 m³ per second, causing rapid summit deflation. By mid-May, the summit lava lake had drained and the floor of Halemaʻumaʻu crater disintegrated in a piece-meal fashion, accompanied by several small explosive events. Summit collapses eventually involved larger portions of the caldera floor in June–July, with episodic piston-like failures that released energy equivalent to Magnitude 5.2–5.4 Earthquakes at intervals of 20–50 hours. Significant lava effusion on the Lower East Rift Zone ended on 4 August, roughly coincident with the end of summit collapse, although minor activity continued sporadically within the Lower East Rift Zone eruptive vent for the next month. Since September 2018 there has been no eruptive activity at Kīlauea, although ongoing inflationary ground deformation and a subsurface mass increase since late 2018 indicate that magma is refilling the summit and East Rift Zone.
Kīlauea’s summit magma reservoir complex deflated for two decades following the onset of the Puʻu ʻŌʻō eruption in 1983 as magma drained from the summit to supply the eruption, then inflated from roughly 2003–2007 in response to a surge in magma supply. Sustained deflation returned to the summit with the opening of a new vent near Puʻu ʻŌʻō in July 2007. Inflation recommenced in late 2010 and was followed by a brief deflationary episode due to the formation of a new vent near Puʻu ʻŌʻō in March 2011. Inflation continued into 2012 and subsequent years, likely caused in part by an increase in East Rift Zone vent elevation, and was accompanied by a net rise in the Halemaʻumaʻu lava lake (a proxy for magma reservoir pressure). Inflation and lava lake rise rates at the summit increased in 2016 but leveled off in 2017.
Geodetic data provide evidence for episodic magma transport downrift of Puʻu ʻŌʻō and into the Lower East Rift Zone during the years prior to 2018. Leveling and Global Positioning System data suggest a pause in long-term Lower East Rift Zone subsidence at the eastern tip of the island during 2003–2007, perhaps due to the arrival of magma in the Lower East Rift Zone linked to a surge in magma supply to Kīlauea. Beginning in 2013, pauses in subsidence, and in some cases uplift, were observed at campaign and continuous Global Positioning System stations downrift of Puʻu ʻŌʻō (for example, at JOKA). These changes lasted until the 2018 eruption. The early 2013 change in deformation style at Global Positioning System stations downrift of Puʻu ʻŌʻō roughly coincided with changes in the summit and Puʻu ʻŌʻō eruptions. In January 2013, the Halemaʻumaʻu lava lake rose 50 m over 10 days, and the Puʻu ʻŌʻō lava lake rose several tens of meters, to one of the highest levels of lava in the crater in years.
South flank motion continued at a steady rate in the years prior to the 2018 eruption. Displacement rates were greatest (about 8 cm per year) in coastal areas southeast of the summit, with rates diminishing to under 1 cm per year south of the Lower East Rift Zone. Transient displacements occurred at semi-regular intervals during slow slip of the south flank of the volcano.
Ground deformation data indicate that Kīlauea’s shallow magma system, from the summit to Puʻu ʻŌʻō, showed low rates of deformation during 2017 and the first two months of 2018, but high rates of pressurization were recorded starting in mid-March 2018. Lava lakes at both the summit and Puʻu ʻŌʻō rose to unusually high levels during that time period, confirming pressurisation in the magma system. The summit lava lake produced the largest overflows on the Halemaʻumaʻu crater floor observed during the 10 years of the summit eruption. The small lava lake in Puʻu ʻŌʻō also rose to unusually high levels in April 2018, and the adjacent crater floor was lifted up by roughly 15 m. An increase in shallow (less than 5 km depth) Earthquakes at the summit and Upper East Rift Zone also occurred in April; similar Earthquakes have commonly been associated with summit pressurisation.
Field observations and thermal satellite data indicate that the eruption
at Puʻu ʻŌʻō was waning during early 2018, despite the pressurization
in the summit-East Rift Zone magmatic system. In September–October 2017
the flow of lava at the ocean entry weakened, and the ocean entry shut
down in mid November. From this time onwards, lava breakouts showed
diminishing reach from the vent. MODVOLC thermal satellite data showed
reduced radiant heat flux from the lava flow field55 during March and
April, and sulphur dioxide emission rates from Puʻu ʻŌʻō, used as an
indicator of lava eruption rates, were unusually low in April. These
observations are all consistent with a reduction in the eruption rate
from Puʻu ʻŌʻō during this time.
The rate of lower crustal earthquakes beneath the summit (5–15 km depth) increased in November 2017 and remained elevated into early 2018, with several swarms in March 2018. These rates were higher than in the previous 10 years, during which time these earthquake swarms were commonly associated with brief summit deflation episodes.
Visual observations of changes preceding the 2018 eruption. (a) Normal summit lava lake levels vs. (b) unusually high lake levels due to pressurisation in April 2018. The lake is about 300 m long. (c) Normal lava lake elevation in Puʻu ʻŌʻō vs. (d) unusually high lake elevation due to pressurization in April 2018. The lake is roughly 50 m long. (e) Ocean entry activity typical of the preceding year, with numerous lava streams and a moderate steam plume vs. (f) an inactive ocean entry in November 2017 as East Rift Zone surface breakouts retreated upslope. Patrick et al. (2020).Long-term, seaward sliding of Kīlauea’s south flank may have gradually set the stage for the 2018 Lower East Rift Zone eruption. Sliding of the south flank causes a corresponding increase in extensional strain in the shallow rift zone (under 3 km), which consequently reduces the magma overpressure required for downrift propagation of a magma-filled crack. Along the Middle East Rift Zone, where steady south flank motion amounts to several cm per year, 'passive' intrusions have been known to occur as magma from the Middle East Rift Zone conduit rises toward the surface in response to rift zone extension, for example, in 1997 and 1999. South flank slip rates are much lower in the Lower East Rift Zone than in the Middle East Rift Zone, suggesting that it might take decades for enough extension to accumulate along the rift to facilitate intrusions. Consistent with this idea, a roughly 50-year recurrence interval has been estimated for major flank slip and rift opening events over the past 200 years at Kīlauea. By 2018, 57 years had passed since the previous Lower East Rift Zone eruption and 43 years since the last major south flank earthquake, thus, the Lower East Rift Zone may have been poised for failure. This scenario has similarities with the 2004–2005 eruption of Etna Volcano, which was triggered by extension resulting from long-term motion of the eastern flank of the volcano. Likewise, the 2018 eruption of Ambrym (Vanuatu) was facilitated by tectonically induced extensional stresses that prompted magma flow into the rift zone.
Intrusions and eruptions at Kīlauea are frequently preceded by increases in magma pressure, and Kīlauea’s magma system was unusually pressurised before the onset of the 2018 eruption. Inflation at the summit began in 2010 and was sustained through 2016. By 2018, tilt and Global Positioning System data suggested the system was at its highest level of pressurisation in at least 20 years. Puʻu ʻŌʻō was likewise in a prolonged inflated state since 2010. The highly pressurised magma system would have increased the likelihood of an intrusive event and provided a greater head to drive magma into the Lower East Rift Zone.
In addition, data suggest that leakage of magma downrift of Puʻu ʻŌʻō was occurring by early 2013. The unusually high lava column in Puʻu ʻŌʻō in January 2013 may have provided sufficient overpressure at depth to open or expand a pathway downrift of Puʻu ʻŌʻō, which enabled gradual magma migration during 2013–2018. Could the additional magma flow and heat transfer downrift of Puʻu ʻŌʻō after 2013 have facilitated the 2018 injection of magma into the Lower East Rift Zone? Campaign Global Positioning System data collected annually since 1995 in the Middle and Lower East Rift Zone suggest that at least one period of downrift magma transport occurred prior to 2007, with no accompanying eruption. It is possible that periods of slow magma transport downrift of Puʻu ʻŌʻō have occurred regularly in the past, but this remains poorly understood.
Kīlauea’s magma system began to pressurize much more rapidly during March–April 2018, shown by inflation, rising lava lake levels, and increasing shallow summit and upper East Rift Zone Earthquakes. Increased magma pressure at Kīlauea is sometimes associated with higher eruption rates from East Rift Zone vents, while at other times it is associated with a decrease in eruption rates. In the latter instances, as during early 2018, pressurisation may be explained as the result of reduced output at the Puʻu ʻŌʻō vent, causing magma to backup in the system. The cause of the reduced output is not well understood but, like in 2018, previous instances of backups at Puʻu ʻŌʻō occurred after the vent persisted for several years, suggesting that the shallowest conduit feeding the vent may tend to atrophy due to reduced transport efficiency over time. One strong possibility is that a restriction of some form develops in the shallowest conduit connecting the Puʻu ʻŌʻō reservoir to the vent feeding the lava flow field, inhibiting conduit flow. The efficient hydraulic connection between Puʻu ʻŌʻō and the summit results in magma accumulation throughout Kīlauea’s shallow magma system. Historically, rapid inflation from this process has culminated in the formation of formation of new Middle East Rift Zone vents.
Kīlauea’s shallow magma system is also known to pressurise in response to increases in magma supply from the deeper magmatic system, but we see no clear evidence for an increase in deep magma supply in early 2018. Changes in magma supply had been documented in the mid-2000s and had a significant and direct impact on summit inflation and Middle East Rift Zone eruptive activity. During early 2018, however, the Middle East Rift Zone eruption rate decreased, rather than an increase as might be expected from an increase in deeper magma supply. Furthermore, there was no change in the character of the deeper portion of the magma reservoir complex (3–5 km depth), as had occurred during the mid-2000s. Carbon dioxide emission rates, previously used as a proxy for deep magma supply rates, were not available in the years immediately prior to 2018 due to challenging measurement geometry. There was an increase in lower crustal (5–15 km depth) earthquakes in late 2017 to early 2018 relative to the previous decade, but previous work has shown that these lower crustal earthquakes, common in the 1980s and 1990s, are not clearly related to changes in eruptive activity, and their source mechanism remains ambiguous. In the context of activity that has occurred at Kīlauea since 2008, when the geometry of the magmatic system was most similar to early 2018, an increase in magma supply is not needed to explain the inflation and seismicity during that time.
On 30 April 2018, a small intrusion occurred into the west flank of the Puʻu ʻŌʻō cone, similar to the culmination of previous episodes of rising pressure that created new vents on or around Puʻu ʻŌʻō. This event, however, coincided with a larger injection of magma far downrift of Puʻu ʻŌʻō, creating the first large-scale magmatic episode in the Lower East Rift Zone in 57 years.
What changed in the plumbing system to allow large volumes of magma to enter the Lower East Rift Zone? A persistent feature must have existed in the rift zone that prevented significant downrift magma transport past Puʻu ʻŌʻō during the 35 years of magma supply to the vent. Localised barriers to magma transport have been previously hypothesised in the East Rift Zone based on seismic data, and for diking events at other volcanoes. Multiphase mixture models suggest that a section of rift east of Puʻu ʻŌʻō was exceptionally dense, perhaps making it difficult for new cracks to initiate or propagate downrift. Vents have opened slightly east of Puʻu ʻŌʻō several times during the eruption, but were fed by very shallow dikes that probably emanated from the Puʻu ʻŌʻō feeder system, implying that any long-lived barrier to downrift magma transport was rooted deeper, in the main East Rift Zone conduit. The 2013 and onwards deformation downrift of Puʻu ʻŌʻō (station JOKA) suggests that such a barrier may have been leaky, raising questions on how the feature may have evolved, or degraded, over three decades. The barrier must have been sufficiently resilient, however, to shunt the majority of magma to Puʻu ʻŌʻō despite numerous disruptions during the 35-year eruption. In addition, the entire magmatic pathway downrift of Puʻu ʻŌʻō, having been largely abandoned for decades, may have been so poorly developed as to permit nothing more than a trickle of magma prior to 2018.
Whether due to a localized barrier near Puʻu ʻŌʻō or to the intrinsic resistance to flow in the largely abandoned, vestigial pathway east of Puʻu ʻŌʻō, downrift flow might have been impeded prior to 2018 simply because magma overpressure was insufficient to initiate new cracks. The long-term pressurization of the system, coupled with the short-term perturbation of 30 April, may have finally exceeded the threshold needed to overcome this resistance to flow. Flow into the Lower East Rift Zone was probably aided by long-term dilation of the rift zone due to south flank motion. The combination of rift dilation and magma pressurisation may simply have reached a critical threshold by late April 2018.
There remain unanswered questions in this conceptual model that deserve further study, particularly with regard to the exactfailure process that allowed magma to move east of Puʻu ʻŌʻō. Nonetheless, the onset of the 2018 eruption can be adequately explained by intrinsic magmatic and tectonic processes. Recent work has proposed that heavy rainfall triggered the 2018 eruption, based on a purported lack of significant precursory inflation. The high rates of widespread inflation and lake level rise in the weeks prior to the 2018 eruption, however, indicate that increasing magmatic pressure was the dominant driver, and extrinsic triggers such as rainfall are not required to explain the eruption.
Patrick et al.'s conceptual model explains the buildup to the 2018 activity at Kīlauea, but why was the eruption so large? Previous work has demonstrated that eruptions along Kīlauea’s Lower East Rift Zone tend to be infrequent but relatively large (0.1–0.3 km³ in 1790, 1840, 1955, 1960). This is likely due to the lower elevation of Lower East Rift Zone vents, which require lower overpressures to drive flow, and can drain magma storage zones more thoroughly (including magma stored in the East Rift Zone). Indeed, for 18 East Rift Zone eruptions in the 20th century, summit deflation (a proxy for pressure change) scaled inversely with vent elevation. The volume of the 2018 eruption (approximately one cubic kilometer), however, was large even by Lower East Rift Zone standards, so other factors, noted below, must also have contributed.
An open question regards the role of the May 4 Magnitude 6.9 south flank Earthquake in influencing the magnitude of the Lower East Rift Zone eruption. The timing of the Earthquake, days after the onset of magma moving into the Lower East Rift Zone, suggests that the intrusion triggered the earthquake by stressing the south flank, as proposed for previous episodes where rift zone intrusions apparently induced strong south flank Earthquakes. Did the Magnitude 6.9 Earthquake dilate the East Rift Zone and enable higher rates of magma transport? An apparent increase in the rate of summit drainage after the Earthquake supports the notion that the Magnitude 6.9 Earthquake boosted transport rates in the magmatic system. If the Magnitude 6.9 Earthquake did enhance magma transport to the Lower East Rift Zone, this might, in part, explain the comparatively large erupted volume that triggered structural failure at the summit. In comparison, the 1955 and 1960 Lower East Rift Zone eruptions occurred in a similar area of the rift zone but were much smaller (0.1–0.3 km³), were not associated with south flank Earthquakes with Magnitudes greater than 6, and did not produce large-scale collapse at the summit.
The role of the collapsing caldera at the summit of Kīlauea in the evolution of the eruption also requires further study. Episodic failure of the rock above the summit magma reservoir renewed the pressurisation of the reservoir with each collapse and produced transient increases in eruption rate; these events therefore probably played a role in sustaining the eruption. A quasi-exponential decay of pressure in Kīlauea’s deeper summit magma system over the course of the eruption, however, suggests additional summit processes also affected the magnitude of the event.
How did relatively minor events at Puʻu ʻŌʻō progress to the historic scale of the 2018 eruption? We propose that the 2018 eruption of Kīlauea began and evolved as a cascading series of events, which was difficult to anticipate due to the complexity of the system. Cascading sequences are intrinsic to volcanic eruptions and can occur over a wide range of spatial and temporal scales. In an idealized explosive eruption, for example, pressure in the reservoir drives magma towards the surface, and decreasing pressure eventually allows gas exsolution to occur. Bubble growth then enhances ascent rates and leads to fragmentation, producing an eruption. The resulting eruption hazards may also occur in a cascading manner.
In the 2018 eruption of Kīlauea the cascade sequence was a chain of events that was unforeseen at the onset of unrest. The magmatic system may have been primed for years due to (a) gradual dilation of the rift zone due to south flank motion, (b) the prolonged inflated state of the magmatic system, and/or (c) slow magma leakage into the Lower East Rift Zone. The cascade was set in motion in late 2017 and early 2018 as a restriction developed in the vent conduit supplying magma to the Puʻu ʻŌʻō lava flows (step 1), reducing lava outflow (step 2) and causing magma to backup and pressurize the system (step 3), opening a pathway and/or clearing a barrier near Puʻu ʻŌʻō that allowed a larger scale magma migration east into the Lower East Rift Zone (step 4). Magma reached the surface as a Lower East Rift Zone eruption (step 5). The input of magma into the Lower East Rift Zone imparted stress on Kīlauea’s south flank, which triggered the Magnitude 6.9 Earthquake (step 6), relieving confining stress on the rift zone which, in turn, may have enhanced magma transport to the Lower East Rift Zone eruption site (step 7). The Lower East Rift Zone eruption removed magma at a high rate from the summit magma reservoir (step 8), causing collapse of the caldera floor (step 9) that led to small explosions (step 10) and maintained magma reservoir pressure, in part sustaining the eruption. This convoluted sequence links a relatively small change near Puʻu ʻŌʻō to major, destructive lava effusion on the Lower East Rift Zone (20 km downrift) and historic changes at the summit (20 km uprift), all enabled by an efficient hydraulic connection along the East Rift Zone.
It should be possible to learn for the future by examining how Hawaiian Volcano Observatory scientists assessed the activity as it was unfolding. The long-term inflation at the summit and uplift east of Puʻu ʻŌʻō for example, at station JOKA), were recognized by Hawaiian Volcano Observatory scientists as indicating system-wide inflation, but the gradual nature of the trends did not clearly point to short-term hazard.
The short-term precursors in March–April, pressurization at both the summit and Puʻu ʻŌʻō, were identified immediately. During this phase of pressurisation, the consensus among Hawaiian Volcano Observatory staff was that a new vent would most-likely form on or near Puʻu ʻŌʻō, following patterns in 2011, 2014, and 2016. With the inflation persisting into April, Hawaiian Volcano Observatory issued a Volcanic Activity Notice on 17 April that stated 'Observations…during the past month suggest that the magma system beneath Puʻu ʻŌʻō has become increasingly pressurized. If this activity continues, a new vent could form at any time, either on the Puʻu ʻŌʻō cone or along adjacent areas of the East Rift Zone.' At that time, a primary concern was that such a vent might appear on the north side of the Puʻu ʻŌʻō cone, sending lava into a catchment that could eventually reach populated areas, as happened during the 2014–2015 Pāhoa lava flow crisis. A Volcanic Activity Notice issued on 24 April highlighted the increased pressurisation and high level of the summit lava lake, and the possibility of a new vent forming on or near Puʻu ʻŌʻō.
The expected local intrusion occurred at Puʻu ʻŌʻō at approximately 2.20 pm Hawai'i Standard Time on 30 April, creating a brief fissure eruption and small flows on the west flank of the cone. What was not expected, however, was the continuation of Earthquakes and further magma injection downrift of Puʻu ʻŌʻō that commenced within the subsequent hours. The earthquakes reached the area of Highway 130, 18 km east of Puʻu ʻŌʻō, by midday on1 May. This recorded the first major movement of magma into the Lower East Rift Zone since the 1960s and was an unambiguous signal that larger, and potentially more hazardous, changes were underway. On 1 May (4.54 am Hawai'i Standard Time), the Hawaiian Volcano Observatory issued a Volcanic Activity Notice alerting the public of the evolving hazard and stating that an outbreak of lava in a new location was one possible outcome. The focusing of earthquakes beneath Leilani Estates on 2 May, and the opening of small ground cracks on that day, suggested that an eruption could occur in this area. On 2 May (7.23 pm Hawai'i Standard Time) the status report was updated to indicate that an outbreak of lava from the Lower East Rift Zone remained a possible outcome of the continued unrest. The Lower East Rift Zone eruption began 21 hours later, at about 5.00 pm Hawai'i Standard Time on 3 May.
The Volcanic Activity Notices and status reports released during the precursory phase in April did not forecast significant hazards at the summit, based on lack of recent precedent. Previous intrusions in the area of Puʻu ʻŌʻō, such as in 2011, produced significant summit deflation but no large-scale structural changes to the caldera floor, nor summit explosive activity. The 1955 and 1960 Lower East Rift Zone eruptions caused localized sagging and disintegration of the Halemaʻumaʻu crater floor, but not large-scale caldera collapse or explosive activity. Once the 2018 Lower East Rift Zone eruption commenced and the Halemaʻumaʻu lava lake began rapidly draining, however, Hawaiian Volcano Observatory recognized the possibility of explosive events, similar to those in 1924, which also followed lake draining and a Lower East Rift Zone intrusion. On 9 May a Volcanic Activity Notice stated that the dropping lava level 'raised the potential for explosive eruptions in the coming weeks.' Relatively minor explosive activity began in mid-May and continued throughout the month.
The onset of Kīlauea’s 2018 eruption was forecast accurately in the weeks leading up to the event, but its location and size were not. What can volcanologists learn from these events when responding to future activity at Kīlauea and other volcanoes?
First, the 2018 eruption serves as a cautionary tale against overreliance on recent volcanic activity as a guide for future behavior. Kīlauea’s Puʻu ʻŌʻō eruption had persisted for decades despite numerous perturbations of the magmatic system and appeared to be a testament to the stability of eruptive activity in the Middle East Rift Zone. Based on parallels with magma injections in 1991, 2011, 2014, and 2016, inflation in early 2018 suggested only a new Middle East Rift Zone intrusion or formation of a new vent at Puʻu ʻŌʻō, a fundamental change to the eruption was not expected. In retrospect, the March–April inflation and the sequence of events that was anticipated to result from it served as a point of focus and may have distracted from consideration of Kīlauea’s broader geologic record, which includes four Lower East Rift Zone eruptions in the past 200 years, one (1840) of which triggered collapse of the caldera floor. Several additional large collapses of the caldera floor occurred in the 1800s; however, unlike 2018, none of these previous Lower East Rift Zone events or summit collapses occurred in the midst of an ongoing, multiyear Middle East Rift Zone eruption.
Humans may naturally focus on obvious changes and most likely outcomes at the expense of less obvious changes and less likely outcomes. The predilection to see the future as similar to the immediate past can be considered a kind of tunnel vision, which can have detrimental effects on unbiased, comprehensive consideration of information and illustrates a challenge of forecasting volcanic eruptions using short- to intermediate-term pattern recognition. The risks of tunnel vision may be alleviated in part by considering the broader geologic history of a volcano, which can serve as a useful reminder that other (possibly much larger) outcomes are also possible, even if unlikely. These possible outcomes must be considered during each new phase of evolving unrest, even if a previously recognised pattern appears to be repeating itself.
Evident precursors to the 2018 eruption were relatively small and provided a deceptive underestimate of the scale of the impending eruption. Thus, the 2018 eruption highlights the challenge of forecasting complex cascading sequences of events. At Kīlauea, the extensive rift zone magmatic system has previously exhibited complex interactions with the summit reservoir. In 1924, an intrusion in the Lower East Rift Zone withdrew magma from the summit reservoir and caused the floor of Halemaʻumaʻu crater to collapse, followed by a series of summit explosions. At Stromboli Volcano, increasing magma pressure at the volcano’s summit has previously triggered flank eruptions, leading to rapid decompression of the deeper reservoir and dangerous paroxysmal explosions. The 1980 Mount St. Helens eruption began with a Magnitude 5.1 Earthquake and landslide of the unstable, bulging flank, which then exposed a cryptodome of magma and removed overburden from the central conduit, resulting in a lateral blast and Plinian explosion. In each of these examples, major aspects of the eruption could not be anticipated in a straightforward manner from the immediate signs of unrest. Whether in a cascading sequence or not, unforeseen volcanic events are common from a global perspective, reinforcing the need for implementing forecasting frameworks that account for remote outcomes.
It may never be possible to determine if or when a particular event, such as a small magma injection, might trigger a sequence of cascading events and culminate in a large eruption. Volcanic systems are highly nonlinear and behave chaotically and unpredictably; however, small events can only trigger large events if the state of the volcanic system, such as the volume and pressure of eruptible magma and tectonic stress state, permits it. An important focus of future work should thus be to better understand when systems may be primed such that a small trigger can result in a large eruption. These conditions may be characterised using monitoring data together with conceptual and mathematical models, and interpreted in light of geological and historical records, which can be used to make inferences on the types and recurrence rates of future activity.
Quantitative hazard forecasting tools, such as probabilistic event trees and Bayesian belief networks, allow scientists to rigorously integrate information from geologic mapping, monitoring data, models, and even expert opinion, to obtain probabilistic assessments of possible future activity. In some cases, these tools can be used to obtain not only a forecast of future activity, but also quantitative insight into the state of the volcanic system (e.g. whether or not magma is ascending). Also, importantly, the utilisation of these tools requires careful analysis and discussion of possible outcomes and may thereby reduce the tendency towards tunnel vision. The simple act of carefully discussing possible outcomes may bring a greater awareness of the possibility of low-probability high-impact events and help observatory scientists consider a broader range of outcomes. It should also encourage vigilance for the prospect that ostensibly small changes at a volcano could, given the right circumstances, evolve into much larger and more hazardous activity. Patrick et al. also advocate for observatory scientists to become familiar with these tools, and to use them to develop basic long-term forecasts that can be modified as needed, well before the onset of a volcanic crisis.
Finally, Patrick et al. emphasise that volcanologists must remain humble no matter how sophisticated our data and models become. Volcanoes often erupt in unexpected ways. Stromboli, for instance, has been studied for centuries, and the onset of flank effusion is now recognized by Italy’s volcano monitoring agency as a possible precursor to paroxysmal explosions, an outstanding example of eruption forecasting based on monitoring data, the historical record, and an understanding of the volcanic system. Nonetheless, two paroxysmal explosions in 2019 (one fatal) occurred in the absence of flank activity. As with Kīlauea’s 2018 eruption, these events highlight the limits of current understanding even at relatively well-studied volcanic systems.
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