Volcano Watch — Kīlauea's East Rift Zone activity likely to continue for decades

The recent history of Kīlauea has been pieced together from geologic mapping and dating, Hawaiian oral histories, and written observations made following the arrival of Christian missionaries in the 1820s. The story begins in the 18th century with the collapse of the top of the volcano, triggered by the draining away of a large part of the magma stored beneath Kīlauea's summit. The collapse formed Kīlauea's great summit depression, or caldera, and ended with the explosive eruption of 1790, which killed part of King Keoua's army as they moved through Ka`u to do battle with Kamehameha. In response to the void left beneath Kīlauea, the magma supply from the Earth's mantle was temporarily increased, resulting in the very rapid filling of the caldera from 1820 to 1840 that was witnessed by the early missionaries. The rift zone eruptions of 1823 and 1840 also had rates of eruption much greater than anything we observe today, judging from contemporary accounts and later mapping.

The 1840 eruption was unique in that movement of magma beneath the entire length of Kīlauea's East Rift Zone was revealed by the appearance of eruptive fissures and steaming cracks at many points between Kīlauea's summit and Puna. The culminating outbreak, from vents near Pahoa, erupted lava rich in olivine, derived from the deeper part of Kīlauea's now replenished magma storage reservoir. After 1840, the rate of lava supplied to Kīlauea's surface slowed, but the magma supply rate remained high, the excess going into the newly opened magmatic core of the East Rift Zone. Kīlauea's subsequent history is interpreted as a long transition from the continuous eruption in Kīlauea's caldera during the 19th century to the continuous Pu`u `O`o-Kupaianaha flank eruption that began in 1983. All activity between 1940 and 1983 can be seen in light of this transaction.

During the remainder of the 19th and early 20th centuries, activity in Kīlauea caldera continued to decline while the rift zone was being wedged apart by the addition of magma underground. The stress put on Kīlauea was relieved in a dramatic way in 1924, when magma intruded beneath lower Puna and spread laterally to erupt on the undersea extension of the East Rift Zone. Evidence for this is provided by the sequence of events from February through May 1924, when, first, lava drained from Halema`uma`u, second, an earthquake swarmoccurred beneath Kapoho, accompanied by cracking and subsidence of the ground surface and, finally, Halema`uma`u collapsed and enlarged through a sequence of spectacular explosions. A tiltmeter installed by the newly founded Hawaiian Volcano Observatory recorded the summit collapse; continued downward tilting indicated that magma drained from the summit to feed the undersea eruption until 1933. During the following 20 years, the deeper rift zone may have been gradually resealed as magma pressure again built up beneath Kīlauea's summit.

The return of lava to Halema`uma`u in 1952 begins the current cycle of Kīlauea eruptions, and the frequency of eruptive activity has steadily increased, right up to the present. Our interpretation of events between 1952 and 1993 is based on the principle that as long as any part of Kīlauea's rift system is open, gravitational forces will favor rift eruption over summit eruption. In other words, it takes less force to move magma sideways than upwards. The complexity of the subsurface structure prevents us from being able to predict the sequence of eruptions, but the tendency will be to favor increased rift activity as new magma conduits open up underground. Under the most favorable conditions, one can expect nearly continuous eruption on the rift zone. The Mauna Ulu eruption (1969-1974) provided evidence that this kind of equilibrium might be achieved. The building of Mauna Ulu, however, was interrupted numerous times by eruptions and intrusions occurring elsewhere on the volcano, such as the return to eruption and intrusion at Kīlauea's summit in 1971 during an eight-month-long repose. By contrast, the current eruption has been interrupted by fewer and briefer pauses, only a few intrusions, with no other eruptions, and has lasted far longer than the Mauna Ulu eruption.

The Kalapana earthquake of 1975 (magnitude 7.2), whose anniversary occurred earlier this week, was important to the transition from a combination of summit and rift eruptions to continuous eruption on the rift zone. The earthquake was probably triggered by the building up of underground stresses related to the absence of a continuous outlet for Kīlauea's unceasing magma supply. The earthquake was located several miles below Kīlauea's south flank and caused the southeast part of the volcano to move down and seaward a distance of several meters. Studies made during the past two decades have shown that the earthquake changed virtually all aspects of Kīlauea's activity. For example, before the earthquake, the magma supplying Kīlauea arrived in batches of subtly differing chemical composition, with new batches arriving every few months to a year. Since the earthquake, individual magma batches have been difficult to identify; the current eruption consists of a single magma batch, many times larger than any previously observed. Evidently, the earthquake opened the deep plumbing beneath Kīlauea's summit to provide a more homogenous magma supply than before.

Before the earthquake occurred, magma pressure applied below the surface was driving Kīlauea's south flank steadily upward and seaward; the movement was temporarily accentuated following intrusions in, and adjacent to, Kīlauea's two rift zones. The area of greatest uplift was close to the rift zone; this proximity suggests a shallow source of pressure. Following the earthquake, the area of maximum uplift shifted towards the coast; this suggests a pressure source at greater depth. The amount of horizontal seaward-directed movement has also greatly increased since the earthquake. The earthquake was located near the base of Kīlauea, and the motion was directed seaward along the interface between Kīlauea and the old sea floor. We hypothesize that the earthquake triggered increased movement of the dense, olivine-rich body deep beneath Kīlauea's summit and rift zones, thereby mobilizing the south flank at a greater depth than had been previously observed.

The new understanding of Kīlauea emerging from the combination of historical research and recent advances in measuring ground movements of the volcano contributes to our ability to predict eruptions and large earthquakes and evaluate the longer-term hazards associated with them. Our interpretation of the historical record indicates that Kīlauea is likely to continue activity on the East Rift Zone for several more decades, at least until the south flank motion abates. Even if the current eruption ends, we can expect an eruption elsewhere on the rift, perhaps after a brief return to activity at Kīlauea's summit or southwest rift. Thus, the hazard to persons living in lava flow hazard zones 1 and 2 (the East Rift Zone and adjacent areas downslope) remains high into the foreseeable future. The only way for Kīlauea to return to continuous activity at its summit is for the rift zone to be effectively sealed. The rift conduits could close if, and when, Kīlauea's south flank stops moving seaward. Our modern instruments allow us to monitor this movement and, presumably, will give us some indication of the end of east rift activity.

The occurrence of large, damaging earthquakes are likewise controlled by the evolving magmatic-tectonicsystem. Both ground deformation and seismicity can be used to make forecasts of upcoming events with sufficient lead time for civil authorities to prepare for them. The pressure being applied to the rift by the current eruption, combined with decreased seismicity in certain areas of Kīlauea's south flank, increases the likelihood of another south flank earthquake with a magnitude similar to that of the 1989 Kalapana earthquake (6.1 magnitude). The recent action by the Hawai`i County Council to adopt the 1991 Uniform Building Code is a timely reminder to citizens of the Big Island to prepare for such an eventuality.

With integrated and ongoing study of the volcano's response to a constant magma supply, forecasting of longer term changes may be possible. For example, the rate of seaward movement of Kīlauea's south flank is an important indicator of relative hazard. A constant or increasing motion correlates with magmatic loading of the rift system and increases the possibility of more large-magnitude earthquakes. Decreasing rates, on the other hand, would suggest eventual return to summit eruption with reduced eruptive, and possibly seismic, hazards as well.