Volcano Watch — Magma returns to an old haunt (or, where did all the magma go?)

Episode 54 of the Pu'u 'O'o-Kupaianaha eruption began early in the morning of Thursday, January 30, and manifested itself in a number of different way. Pu'u 'O'o itself became inactive; flows to the sea via lava tubes stopped; about 300,000 cubic yards of lava was erupted from new fissures in, and just uprift and downrift of, Napau Crater and now covers some 65 acres; and strong seismic tremor and earthquakes shook the summit of Kīlauea and the Napau-Pu'u 'O'o area.

One of the most intriguing aspects of the episode, however, was the least visible: the summit deflation. Visitors to Hawai`i Volcanoes National Park could not have known that beneath their feet, more than 7 million cubic yards of magma was leaving the summit storage reservoir and migrating down the east rift zone. This eerie transfer of magma from beneath the summit was detected by geodetic instruments and showed that the summit region was collapsing as if it were a balloon.

By mid-day on Friday, the ground at the Hawaiian Volcano Observatory (HVO) had tilted toward the caldera about 40 microradians (the slope resulting from inserting 40 pennies at the end of a bar one mile long)—a tiny amount in human terms but more than has occurred at Kīlauea since the start of the Pu'u'O'o-Kupaianaha eruption in 1983. Distances measured across the caldera shortened more than 4 inches as the "balloon" lost magma. This movement of magma was ultimately responsible for the seismic tremor and felt earthquakes.

Where did all the magma go? Geodetic measurements at two stations about 1.2 mi uprift of Napau Crater show that the east rift zone widened about 10 inches. Most likely the widening was caused by injection of magma into the area, and the obvious source of the magma was the summit. We have no geodetic stations at Napau, but probably that area widened considerably more than 25 cm (10 inches), because that is where fissures opened and lava erupted.

HVO scientists are not especially surprised by this turn of events. For 30 years or more geodetic and seismic data have shown that magma storage is a common event in the area between Makaopuhi and Napau Craters. And so are eruptions. This is the eighth time since 1961 that lava has erupted in or near Napau Crater, and two other historical eruptions (in 1840 and 1922) have also occurred in this area.

Astute readers will note the great disparity between the large volume of deflation at Kīlauea's summit and the small volume of lava erupted. The rest of the magma—the "savings account" in Kīlauea's magma "budget"—is stored in the Makaopuhi-Napau area. Can that savings be used for later eruptions? You bet. Chemical studies have shown that stored magma is often erupted at some later time. But much of the stored magma may cool so much that it can't erupt; it solidifies underground. What proportion of the stored magma will later erupt is anybody's guess.

Episode 54 has provided not only an exciting chapter to the Pu'u 'O'o-Kupaianaha eruption story but also shows the potential for more future eruptions in the Napau area. Meanwhile, will eruptive activity return to Pu'u 'O'o? Perhaps that question will have been answered by the time this article is read.