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Volcanowatch

September 25, 2003

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


Mixing magma on Kilauea's east rift zone

Good red wine improves with age. So, often, does research. With time, more information is gathered, more thought given, and more sophisticated analysis attempted. The end result may be not only greater accuracy but even elegance. Too often in our fast-paced world, pressures from many directions result in rushed interpretations lacking critical data or thorough evaluation. The wine is drunk too early; it may be palatable but not memorable.

A scientific paper about Kilauea was just published that took 6 years to mature, and its results are both memorable and elegant. Carl Thornber, former HVO staff member, has provided the best evidence yet for mixing of different batches of magma within the east rift zone.

Aided by 9 USGS colleagues (6 from HVO), Thornber not only proves that mixing took place in 1997 but knows the vintage of the magmas involved in the event. Though blending of wine varietals is anathema to connoisseurs, hybrid magma is viewed with delight by volcanologists, who appreciate the insights into underground processes provided by the mixture.

The story begins in October 1963. An eruption took place in Napau Crater. Not all the magma was erupted; some remained underground in a storage reservoir, where it slowly cooled and began to crystallize. Another eruption occurred in Napau in October 1968. It, too, left magma stored underground.

Fast forward to January 29, 1997. An eruption had been ongoing for 14 years, centered at Pu`u `O`o where lava was ponded in the crater. That evening, the lava pond abruptly drained, the crater floor dropped 150 m (500 feet), and the west wall of the cone collapsed. About 8 hours later, lava began erupting from fissures cutting Napau Crater, 3 km uprift from Pu`u `O`o.

An interpretation of these events was that the interior of the east rift zone abruptly opened, removing support for the crater floor. Lava drained from Pu`u `O`o and migrated west through the torn rift zone to the Napau area, where it erupted.

But something about this interpretation wasn't quite right. Thornber and colleagues found that the chemical and mineralogic composition of lava at Napau did not exactly match that of the lava from Pu`u `O`o.

Careful chemical analysis showed that the erupted lava could be explained by mixing lava from Pu`u `O`o with magma stored underground after the 1963 and 1968 eruptions. This discovery wasn't easy, because the chemical composition of the stored older magma changed as it cooled and minerals crystallized-similar to the way that cooling seawater changes composition when it crystallizes salt.

Thornber had to calculate how the stored magma had evolved since the 1960s before he could determine what liquid was available to mix with the Pu`u `O`o lava. This procedure itself is intricate, but the results showed that the older magma had cooled about 60 degrees C (140 degrees F) in the thirty-some years it was stored under Napau and isolated from its surroundings.

Using computer models, these compositions were then mixed with the Pu`u `O`o composition in various proportions to come up with the compositions actually observed in 1997. The final calculations show that the hybrid lava was incompletely mixed and consisted of 32 to 65 percent of the 1960's stored magma, depending on the sample.

The hybrid erupted lava also contains telltale minerals from the hotter Pu`u `O`o lava and the cooler, stored magma. By careful work, Thornber identified minerals that formed at temperatures hotter than the erupted hybrid; they came from Pu`u `O`o lava. He also found minerals that formed at cooler temperature and had been heated during mixing; they came from stored magma.

Mixing of magma is important at Kilauea. The rift zones provide opportunities to store magma for long periods of time. The odds are good that a large stored body will be intersected by fractures at some time before it totally solidifies. If magma is rising up the fracture, it will meet and mix with the stored magma to erupt hybrids.

The work by Thornber and colleagues is the most elegant study of mixed magma at Kilauea. It took time and thought to identify the varietals that blended to form the 1997 lava. In the end, good wine aged well.

Activity Update

Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Surface activity is visible on Pulama pali as well as the coastal plain. The Kohola arm of the Mother's Day flow, on the western flow field, has some incandescent patches high on Pulama pali, as well as some small, sluggish breakouts on the western side of coastal plain. The August 9 breakout, which starts high up the Mother's Day tube system on the east side of the flow field, has descended Pulama pali to the gentle slope below. Much of the August 9 flow on the pali is incandescent. No lava is entering the ocean.

Two small earthquakes were felt on the island during the past 7 days. One, of magnitude 2.3, was felt at Leilani Estates at 11:50 p.m. September 17; it was located 3 km (2 miles) northeast of `Opihikao at a depth of 6.3 km (3.9 miles). The other felt earthquake, with magnitude 2.4, shook Hawaiian Orchid Island Estates at 8:25 p.m. September 18. It occurred 9 km (6 miles) northwest of Ka`ena Point at a depth of 8 km (5 miles).

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains low, with no earthquakes located in the summit area during the last seven days.

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Updated: September 29, 2003 (pnf)