Volcano Watch — Kīlauea's eruptive history

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The current activity along Kīlauea's east rift zone is the longest-lived eruption of Kīlauea in historic times. However, the historic period on Kīlauea dates only to 1790, and the first well-documented eruption took place in 1823. This historic period is, geologically speaking, so short that there are eruptive styles that we have probably not yet observed. What is clear from this short record is that eruptive activity at Kīlauea varies greatly in both space and time.

Eruptions have lasted from less than one day to thirteen years, the lenght of the current eruption. Most eruptions last less than a week. Eruptions lasting more than one month are relatively rare, and those lasting longer than one year occurred only at Mauna Ulu (from 1969 to 1974), in addition to the current eruption. However, in prehistoric times, there is evidence for other long-lived eruptions along the east rift zone, such as those that formed large lava shields at Heiheiahulu (erupted around A.D. 1750) and Kane Nui o Hamo (probably 500-750 years old). Other long-lived eruptions occurred at the summit of Kīlauea, including the extensive Ai-laau flows (about 350-400 years old) that flowed towards the east from the Thurston shield, and the "Observatory flows" to the southwest of Kīlauea caldera (perhaps 500-550 years old).

In historic times, there have been only five eruptions along the southwest rift: in 1823, 1868, 1919, 1971, and 1974. In contrast, the east rift zone has seen 24 eruptions, with all but those in 1840, 1955, and 1960 along the middle and upper parts of the rift. The vast majority of east rift eruptions occurred along the upper rift (above the 2,500-foot elevation), although the current, and most voluminous, eruption is in the middle east rift.

More than half of all Kīlauea eruptions are limited to the summit region. During most of the 19th century, activity occurred in the summit almost continuously, although from 1894 to 1907 lava was occasionally visible only during brief periods. From 1907 until 1924, activity was once again nearly continuous at the summit. Following a spectacular phreatic (steam explosive) eruption in Halemaumau Crater in 1924, eruptive activity was restricted to the crater until 1934, when all eruptive activity at Kīlauea ceased for 18 years. Beginning in 1952, Kīlauea has had 35 separate eruptions, mostly along the east rift zone.

The eruption rate at Kīlauea has not been constant through time. A good way to look at time-averaged eruption rates is to plot cumulative volume of erupted lava against time. The slope on such a plot is the eruption rate. The earliest period for which eruption rates can be estimated begins in 1823, when the first detailed descriptions and sketches of Kīlauea's summit were recorded. At that time, the summit caldera was much deeper than it is today, but it was rapidly filling during nearly continuous summit activity, and, by 1840, about 3,000 million cubic meters (3 cubic kilometers) of lava had been erupted in the summit caldera. Between 1840 and 1920, approximately 1,400 million additional cubic meters of lava were added. From 1920 until about 1950, eruption rates were very low as seen in the small increase in cumulative volume and in the shallow slope. Since 1950, eruption rates have increased dramatically, with those of the 1960s greater than those of the 1950s, those for the 1970s greater than those for the 1960s, and those for the 1980s and 1990s greater than those for the 1970s. The present eruption rates are comparable to those for the early 1800s, after the present-day caldera formed in 1790.

Because of the changing character of eruptive activity through time, accurate assessment of lava flow hazards requires knowledge of all the phases of the cycle of activity at Kīlauea, where activity can (1) occur at the summit for more than 100 years, as in the 19th century, (2) be concentrated along the east rift zone, as has occurred since 1955, or (3) be characterized by little or no activity, for example, from 1894 to 1907 and again from 1934 to 1952. With additional detailed mapping and determination of the ages of prehistoric flows, we have learned more and more about the eruptive history of Kīlauea. This new information allows us to refine the boundaries between lava flow hazard zones and to develop statistical models of lava flow coverage.