October 7, 1999

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

Half speed ahead, with no ocean entry

For the past two weeks, people have been asking, "Why isn't the eruption going at full speed?," and "Why isn't lava going into the ocean?"

The first question is the more difficult to answer. The eruption rate is now only about half or two-thirds what it was before September 12, when about two million cubic meters (2.6 million cubic yards) of magma intruded into the east rift zone and formed a dike between Pauahi Crater and Mauna Ulu. There were no surface flows for 11.5 days following the intrusion; we termed this period a "pause." Surface activity resumed on September 23, but it came back slowly and has been limping along ever since, even pausing again for 36 hours on October 3-5. The estimated lava output now hovers around 150,000-200,000 cubic meters (200,000-260,000 cubic yards) per day, whereas the output before the intrusion was generally 300,000-350,000 cubic meters (390,000-460,000 cubic yards) a day.

One possible reason is that the factory's supply to the wholesaler has been cut. That is, the rate of magma supplied from the mantle's factory may not be enough to keep Kilauea's warehouse reservoir stocked. But this seems unlikely, because the shelves are plenty full now, as shown by the inflation of Kilauea's summit since September 12. The summit recovered in 5-7 days after the September 12 deflation, consistent with the size of the dike and a daily supply rate of 285,000-400,000 cubic meters (375,000-525,000 cubic yards). Now the summit reservoir has more in it than before. So, we think that the rate of supply from the mantle has not dropped.

Sales are still down, however, possibly because the distribution system to the retailer, Pu`u `O`o, is not efficient. The pipe carrying magma from Kilauea's summit reservoir to Pu`u `O`o may be a bit clogged, so that less magma is being sent into the east rift zone now than before September 12. This would account for the current summit inflation; more is coming in than is going out.

Still another possibility is that the wholesaler's distribution system is working fine, but the retailer's sales force needs a kick in the pants. The east rift zone widened during the intrusion, perhaps making more room to store magma than before. More magma could be entering the rift zone from Kilauea's summit than is reaching the surface. Before September 12, there was an approximate balance between how much went into the rift zone and how much left it via the eruption. We are currently analyzing data to estimate how much room could have been made during the intrusion.

No matter what the reason, the downturn in eruptive activity probably results from some change within the volcano itself rather than in its connection to the Earth's mantle. Likely this is just a temporary situation. If we had to bet, we'd put our money on the eruption gaining full speed before long.

Why isn't lava going into the ocean? This reflects partly the slowed pace of activity but, more importantly, the relatively long pause that allowed the lava tube to cool. Rocks shrink when they cool, cracking and falling apart. Rocks from the walls and roof of the tube caved in, forming a dam. Once activity resumed, lava traveling in the tube was blocked and forced to rise through skylights to the surface. The dam above the top of Pulama pali has proved sturdy, so for nearly two weeks lava has spilled onto the surface there and hasn't flowed farther down the tube. It will take some time before this and other downstream dams are broken; indeed it may never happen. Perhaps a new tube will form within the currently active surface flows. Lava will probably once again reach the sea, but whether in an old or new tube system remains to be seen.

Eruption Update

As mentioned above, there was a 36-hour pause in surface flow activity of Kilauea Volcano during the past week. The `a`a flow that was reported last week to be half-way down Pulama pali made it to the base of the pali before stopping. As of late Thursday afternoon, multiple breakouts from the tube system occur between Pu`u `O`o and the 650-m (2150-ft) elevation, where a perched lava pond has formed. No lava was flowing over the pali.

A magnitude-3.3 earthquake at 11:16 a.m. on Sunday, October 3, was reported felt in the Glenwood area. The temblor was located 8 km (4.8 mi) north of Kilauea summit at a depth of 26 km (15.6 mi).