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November 18, 1999

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Quiet in Kilauea's caldera, eruption on the east rift zone

Here's a question for you trivia aficionados. Since written records began in 1823, what is the longest period of time without an eruption in Kilauea's caldera? Close your eyes if you want to guess. For the rest of us, the answer is nearly 18 years-6,471 days, to be exact. From 1934 to 1952, the caldera and its pit crater, Halema`uma`u, lay fallow, as did all of Kilauea. Volcanologists at HVO must have been gnashing their teeth, wondering if one of the world's most active volcanoes would retain that claim. During that same time, Mauna Loa erupted five times, but Kilauea frustrated and puzzled volcanologists with its inactivity-particularly in the caldera, which had hosted an almost continuously active lava lake in the 19th and early 20th centuries.

The caldera is now in its second-longest repose since 1823. As of today (November 21), 6,265 days have passed since the last eruption on the caldera floor on September 25, 1982. If the quiet continues, the record will be surpassed on June 13, 2000. Any bettors?

Kilauea may have snubbed the caldera, but it has been favoring the east rift zone since January 3, 1983, when the Pu`u `O`o-Kupaianaha eruption began. What has happened to the volcano to enable such a long-lasting rift eruption while the caldera has remained dark?

To tackle that question, we need to realize the distribution of past eruptions that built Kilauea. More eruptions have taken place in the summit area than in any other small area on the volcano. Eruptions built the volcano higher there than elsewhere, because they were concentrated in one place, directly above the conduit supplying magma from the earth's mantle. The shortest way to get to the surface is up, into the caldera.

Most of Kilauea's lava, however, erupts outside the summit area, along the east rift zone and the smaller southwest rift zone. These eruptions do not concentrate in one small area to build a high, durable edifice as at the summit. Instead, they are spread along the rift zones, which are 120 km (75 miles) and 30 km (18 miles) long, respectively. So, the rift zones are lower than the summit despite having more eruptions along them.

In this light, what has been happening since 1983 is expectable. Most of Kilauea's magma erupts along the east rift zone, and that is just what has been taking place in the ongoing eruption.

But why is the east rift zone now such a magma hog, not allowing the summit any of the action? There is no simple, obvious answer, besides the trite one that magma coming up from the mantle finds it easier to move an extra 15 km (9 miles) into the rift zone than to rise directly into the caldera.

One idea is that the magnitude-7.2 earthquake in 1975 opened the east rift zone and the conduit leading down from the summit area. But if so, why did two eruptions take place in the caldera in 1982? Another idea is that the east rift zone is spreading more rapidly now than before 1983, keeping one or more conduits open and making it easier for magma to enter the rift zone. Whether that is true can be debated and is likely wrong; even if right, the hypothesis begs the question of why spreading should be faster.

Whatever is the reason, the ongoing eruption is probably responsible for the dearth of activity in the caldera. The supply of magma to the volcano is being roughly balanced by the amount erupted from Pu`u `O`o, and there is no overabundance that needs to come out somewhere else, such as the caldera. It is anybody's guess as to how long this situation will continue.

Eruption Update

A 67-hour pause in lava flow activity of Kilauea Volcano illustrated the sporadic output of lava during the past week. Lava stopped flowing outside of Pu`u `O`o from about 3:30 p.m. on Thursday, November 11 to 10:30 a.m. on November 14. Lava is again flowing from Pu`u `O`o and reoccupies the tube to the top of Pulama pali where several perched lava ponds are located. A breakout from the tube system fed a slow-moving `a`a flow heading toward the southeast. As of November 18, there were no flows descending Pulama pali and no active flows on the coastal plains.

There were no earthquakes reported felt during the week ending on November 18.

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Updated: 5 Jan 2000