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September 30, 1999

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

Eruptive behavior not cast in stone as flows slowly meander down slope

The last two editions of this column have dealt with the intrusion of magma into the upper east rift zone of Kilauea on September 12 and with the aftereffects of that event. The story continues to unfold this week, with the resumption of lava flows from Pu`u `O`o.

The intrusion diverted magma from the supply line feeding Pu`u `O`o, causing the flow activity between Pu`u `O`o and the sea to pause for 11.5 days. Kona residents enjoyed the rare vog-free skies, and some wishful thinkers got their hopes up that the eruption might be over. As we use the term, however, a "pause" does not mean that the eruption has come to a full stop, only that lava-flow production from Pu`u `O`o is temporarily suspended.

Ponded lava was present in the crater of Pu`u `O`o during the pause. The level of the pond slowly rose during the pause, like standing water in a pipe, as the supply line repressurized.

By September 23, the lava in the crater was at a level close to that of the flank vents on the southwest side of the Pu`u `O`o cone, and at midday lava slowly oozed into two collapse pits on the south and west sides of the cone. Within a few hours, lava was breaking out of skylights on the pre-existing lava tube, 2 km (1.2 mi) from the vent.

Late that afternoon, lava erupted from a new vent on the south side of the cone. The "mini-vent" formed a spatter cone that was 9 m (30 ft) high by September 25. The new vent fed small pahoehoe flows that poured into the numerous collapse pits and subsidence basins in this area. By the 27th, the mini-vent was only intermittently active, and it was inactive when observed on September 30.

Throughout last week, surface flows continued to leak from the tube on the gentle slope between the cone and the top of Pulama pali. Measurements showed that the lava tube was empty a short way down the pali, indicating that the tube was blocked near the top of the steep slope. Such blockages frequently occur during pauses, when the tube drains and the unsupported walls and roof collapse.

Normally, lava viewers at the coast can expect surface flows to provide a good show within a day or two after a pause ends. This time around, however, it was almost six days before a small `a`a flow was visible from the coast, and by morning the flow was inactive.

As this article went to bed on Thursday, the tube was reoccupied to the 530 m (1750 ft) elevation, and breakouts fed an active channelized `a`a flow that was about halfway down Pulama pali on the eastern edge of the old flowfield.

Eruption Update

No earthquakes were reported felt during the week ending on September 30.

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Updated: 4 Oct 1999