# Why do most east-rift eruptions start along the lower Chain of Craters?

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The Father's Day eruption last June occurred in the most frequently active segment of Kīlauea's east rift zone. What is this segment, and why are eruptions so common there?

Diagram of the East Rift Zone

(Public domain.)

The east rift zone extends 52 km (31 miles) east-northeast from the summit to Cape Kumukahi and another 75 km (45 miles) to the eastern tip of the submarine Puna Ridge. It consists of eruptive fissures, ground cracks, stored bodies of solidified or molten magma, and a conduit system that carries magma from the summit reservoir. The recently active part of the east rift zone is at least 2 km (1.2 miles) wide, to judge from eruption locations during the past 40 years.

The Father's Day eruption took place near the eastern end of a 10-km- (6-mile-) long segment of the east rift zone between Hiiaka and Napau, along the lower Chain of Craters. Twenty-one eruptions began in this segment since written records started: in 1840, 1922. 1923, 1962, two each in 1963, 1965, 1968, 1969, in 1970, 1972, two in 1973, in 1979, 1980, 1983, 1997, and 2007. Of these eruptions, 18 took place after November 1962, 14 in the 1960s and 1970s! These figures don't include the many inferred intrusions-magma movements without eruptions--recorded instrumentally in the segment during the past 50 years.

In contrast, only four east-rift eruptions started outside this segment since 1840. An unconfirmed 5th eruption occurred off Kumukahi in 1924.

Thus, 84 percent of the 25 confirmed east-rift eruptions since 1840 started within only 19 percent of the on-land area of the east rift zone and 8 percent of its total length.

Two factors, working together, help explain such a concentration of eruptions in the Hiiaka-Napau segment.

First, the segment is nearer the summit magma reservoir than is most of the rift zone. More magma moves into this segment than into any other segment, so that more eruptions are likely to take place there.

Curiously, though, a 4-km- (2.5-mile-) section of the upper Chain of Craters above Hiiaka lacks eruptions since 1840 (or had one, if the July 1974 extrusion is counted as a rift, not a caldera, eruption). On probability grounds alone, this section should be an eruption hotbed, but it's not.

The second factor is that the Hiiaka-Napau segment lies along a prominent bend in the east rift zone, which changes direction from southeast near the summit to east-northeast beyond Makaopuhi and Napau. What causes this nearly right-angle bend? Does it influence the location of eruptions?

All eruptive fissures and ground cracks in the east rift zone trend east-northeast, even those between the summit and Hiiaka. Magma travels down that section almost at right angles to cracks at the surface! These cracks continue westward beyond the bend to form the Koae fault system, one of the most active fault systems in the world.

The cracks are perpendicular to the south-southeast direction that Kīlauea's south flank moves as it is pushed by intrusions and pulled by gravity. The orientation of most of the east rift zone reflects this south-flank motion.

P But why does the upper Chain of Craters trend southeastward? One explanation is that the rift zone has migrated southward, following the south flank, from its original starting point east of the summit. To maintain its connection to the summit magma reservoir, the migrating rift zone would necessarily develop a bend. Southward migration is consistent with the concentration of most recent eruptive fissures near the southern edge of the rift zone and is supported by some geophysical (gravity) evidence.

The bend is the most complex structure on Kīlauea. One can easily imagine that magma has difficulty passing through it, often erupting or staying underground without advancing farther down rift. By this thinking, the Hiiaka-Napau segment is the most active part of the east rift zone because of the bend. And, in a feedback loop, the more magma that intrudes or erupts in the bend, the more the south flank moves, and the sharper the bend becomes, making it more conducive to trapping magma.

For more discussion of the east rift zone, come to the After Dark in the Park lecture on Tuesday night, January 16, 2008 at the Hawaii Volcanoes National Park Visitor's Center starting at 7 p.m.

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### Volcano Activity Update

Kīlauea summit and Puu Oo continued to deflate. Seismic tremor levels at the summit are elevated but are still at low levels. Summit sulfur dioxide emissions have also increased. Earthquakes were located mostly beneath Halemaumau Crater and the south flank faults.

On July 21, 2007, lava began erupting from a set of fissures on the east flank of Pu'u 'O'o. Eruptive activity soon stabilized at fissure D, 2.3 kilometers (1.4 mi) northeast of Pu'u 'O'o. For most of last Fall, this lava was directed entirely into a perched lava channel, consisting of separate pools of molten lava separated by bridges of cooled lava. At dawn on November 21, lava began to erupt directly from fissure D, outside of the perched channel, creating the Thanksgiving Eve breakout (TEB) flow. As of December 27, the lava supply to the original perched channel had been completely redirected through this new outlet, cutting off supply to the eastward tube which had been feeding flows in the vicinity of Pu'u Kia'i through much of November.

The TEB flow has continued to build itself vertically and laterally, as a series of low shields, over the last several weeks. By early this past week, the TEB lava building these shields had not advanced significantly and the flow front was still only about 2 km (1.2 mi) southeast of fissure D. By early Wednesday morning, however, the front of the southeast-most shield had collapsed, allowing a large volume of lava to surge out and form a rapidly moving aa flow. This flow had advanced nearly a mile and a half by late Thursday morning, and was continuing to move southeast toward the top of the Royal Gardens subdivision at a steady pace of about 80 m/hr (260 ft/hr).

Minor incandescence in Pu'u 'O'o has been observed a few times since early December, but has otherwise been absent since Aug. 31. As in years past, Pu'u 'O'o likely is serving as a large chimney, beneath which lava is stored briefly and degassed substantially enroute to the erupting fissure. Sloughing of Pu'u 'O'o into its own crater since late August has left numerous fresh cracks on the north rim and south flank of the cone.

Vent areas are hazardous. Access to the eruption site, in the Puu Kahaualea Natural Area Reserve, is closed (http://www.state.hi.us/dlnr/chair/pio/HtmlNR/07-N076.htm).

One earthquake beneath Hawaii Island was reported felt within the past week. A magnitude-2.8 earthquake occurred at 5:09 p.m. H.s.t. on Tuesday, January 8, 2008, and was located 16 km (10 miles) southwest of Kawaihae at a depth of 11 km (7 miles).

Mauna Loa is not erupting. One earthquake was located beneath the summit. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates. The Mauna Loa webcam radio transmitter is buried in snow and is not yet operational.

Visit our Web site for daily Kīlauea eruption updates and nearly real-time Hawai`i earthquake information. Kīlauea daily update summaries are also available by phone at (808) 967-8862.