Hawaiian Volcano Observatory


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September 7, 2000

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

Large rocks in strange places

Sometimes in science you find something that theory says you shouldn't. Then your pulse starts racing. Are your observations or calculations wrong? Is the theory wrong? Or is there a middle ground, in which observations can be fit to theory if both are tweaked a little?

In late August two volcanologists from the USGS (one from HVO) and one from the Smithsonian Institution were faced with this dilemma. We don't yet know what the final outcome will be, but we're pretty excited.

As this column has previously noted, Kilauea has exploded more often than many people think. Ongoing research at HVO is trying to learn as much as possible about these explosions, for flying rocks are clearly hazardous events that will impact the public.

We went looking for rocks that were thrown out of the volcano about 1,000 years ago. Work in past months had shown that a wide variety of material was exploded from Kilauea then, mostly cherry-sized scoria and other fine-grained material. However, several times explosions were apparently more violent or powerful. They ejected large, heavy rocks, much like those from the 1924 explosions that litter the surface around Halema`uma`u today. The question we asked was how far out were such rocks thrown?

Using a hand-held GPS unit, we established a grid between the Hilina Pali Road and the `Ainahou Ranch Road. The grid is about 700 m (2,300 feet) on a side. At each node of the grid, we spent a total of 18 minutes looking for rocks on the surface-6 minutes per person with a full crew, and 9 minutes with only two of us. We selected the largest 10 rocks we could find during the search. It was like an Easter egg hunt, except the rocks can't be eaten and Nature put them there.

We were searching for the largest rocks we could find. We were not interested in those that broke off the surface of the lava flow beneath our feet, but in those that were clearly foreign-that reached their resting place by flying through the air ballistically. After a little practice, recognizing the ballistics became a simple matter. Some of the rocks are even coarse-grained gabbro, which cooled and crystallized underground before being blasted out.

What we found surprised, even shocked, us. Rather than seeing few, if any, large rocks so far from the caldera, we found lots. And some were very large.

At a distance of 10 km (6 miles) from the summit, we found one rock (a gabbro) weighing 1292 g (2 lbs 13 oz.). At 7.9 km (4.7 miles) from the summit, we found another weighing 1998 g (4 lbs 5 oz.). Many others weigh 100 g (3.5 oz.) or more.

When we compared our findings with theoretical models of how far such large rocks could have been thrown from a volcanic vent, we found that we were observing the impossible. The models simply say no dice, it can't be done. Even if we assume that the source for the rocks was on the east rift zone, say near Pauahi Crater or Mauna Ulu, the distance of more than 5.25 km (3.2 miles) is still too great for the models to accept. But, models or no models, the rocks traveled through the air to get where we found them-and that has to be explained.

We think we are on to something. Kilauea has likely had explosions that were either more powerful, or of a different type, than existing theoretical models can explain.

Before you toss out theory, all steps in the observation and interpretation process must be checked and double checked. We are doing that now. Explosions of such unusual power or type are significant; we can leave no stone unturned (pun intended) in trying to determine their nature and cause.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week, following a brief pause two weeks ago. Lava is erupting from Pu`u `O`o and flowing through the old tube system for 1.5 km (0.9 mi) to the southeast. Breakouts from the old tube system at the 2,300-ft elevation feed two diverging flows. One flow is to the southwest and extends down to the 1900-ft elevation. The second flow is to the southeast and extends 3.6 km (2.2 mi) down to the 1700-ft elevation.

Residents of Pahala felt an earthquake at 7:06 p.m. on September 3. The magnitude-3.4 earthquake was located 4 km (2.4 mi) south of Pahala at a depth of 11 km (6.6 mi).

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Updated: September 11, 2000