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February 25, 1999

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


Tiny eruptions

Some readers will know that the largest eruption in the world during the 20th century took place in Alaska in 1912, producing the Valley of Ten Thousand Smokes near Mount Katmai (13 cubic kilometers; 3.1 cubic miles). These same readers may also know that the second largest eruption of this century formed the caldera at Mount Pinatubo, Philippines, in 1991 (5.3 cubic kilometers; 1.3 cubic miles).

But who can say what the smallest eruption of the 20th century was? Though we can't be sure, a tiny eruption in Iceland in 1977 may qualify. An eruption of Kilauea some time between 1974 and March 1982 is a close second. A third eruption at Kilauea in the 19th or perhaps late 18th century is an additional example of what you seldom hear about: tiny, even teensy, eruptions.

On September 8, 1977, lava actually erupted out of a geothermal borehole at Namafjall, Iceland. This weird event was observed to last only 15-25 minutes and produced about 1.2 cubic meters (1.6 cubic yards). You could almost haul away this basalt in your pick-up! In a way, though, this eruption shouldn't count, because it is doubtful if it would have occurred without human intervention in the form of the borehole.

Probably the most recent of the tiny eruptions broke out some time after 1974 and before March 1982, when ex-HVO geologist Norm Banks discovered the evidence. It took place from three closely spaced vents along cracks 5 km (3 miles) down the east rift zone from the top of Mauna Ulu. Three diminutive flows were erupted, at most 3 m long by 5 m wide by 0.15 m thick (10 feet long by 16 feet wide by 6 inches thick). The total volume of lava is less than 3 cubic meters (4 cubic yards), which would easily fit in an average dump truck!

This puny event probably took place on the dark and stormy night of March 10-11, 1980. At that time, HVO recorded numerous earthquakes and tremor coming from the area. Cracks opened between the eventual eruption site and the summit of Mauna Ulu, as well as near Pauahi Crater and across the Chain of Craters Road at the junction to `Ainahou Ranch. The event was interpreted as an intrusion of magma into the area. No newly erupted lava was noted in the days following the intrusion, but the eruption had likely occurred at the same time as the intrusion. Probably one reason the eruption went unobserved was heavy rain. In March 1980, 179.6 cm (70.7 inches) of rain fell at National Park headquarters!

A larger but still tiny eruption took place in the southwest rift zone some time after the last major explosion of Kilauea in about 1790. A wee cone, only 6 m (20 feet) high, was built about 900 m (0.5 mile) southwest of Pu`u Koa`e; the cone overlies all of the older explosive deposits. This cone went unrecognized until geologists stumbled across it in 1995. It represents the only eruption after 1790 in this part of the southwest rift zone. The lava pad surrounding it is no more than 15 m (50 feet) wide, and the total volume of lava in the undated eruption is probably less than 25 cubic meters (33 cubic yards). It wouldn't take many dump trucks to remove it.

Tiny eruptions generally don't impact society much, but their occurrences keep volcanologists on their toes. Such eruptions tell us that supposed periods of quiet might not be so quiet after all. They are proof that magma can sometimes arrive at the ground surface and erupt before observations can be made. Probably eruptions will always be accompanied by earthquakes, but we may still miss tiny outbreaks of lava that occur during periods of bad weather.

Eruption Update

Lava continued to erupt from Pu`u `O`o and flow through a network of tubes from the vent to the sea. No surface flows from breakouts of the tube system were observed on the coastal flats. Lava is entering the ocean near Kamokuna and enlarging the bench. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the new land. Especially vigorous explosions took place on Thursday and formed a new littoral cone about 15 meters (50 feet) high. The steam clouds are highly acidic and laced with glass particles.

No felt earthquakes were reported during the week ending on February 25.

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Updated: 05 Mar 1999