Volcano Watch — As lava slows, diversion of Etna discussed

The episode 51 vents on the west flank of Pu`u `O`o were active from April 23 at about 11:00 a.m. until April 28 at 11:30 a.m., when the eruption stopped once again. Since then, the lava lake inside Pu`u `O`o vent has risen from about 140 to between 120 and 130 feet below the rim of the crater.

During this pause and the previous one from April 19 to 23, the number of earthquakes in the upper East Rift Zone and beneath the summit has increased. These earthquakes result from deformation caused by storage of magma within the upper East Rift Zone.

Reports on Friday that lava was cascading over the Hilina Pali were false; a large fire, unrelated to the eruption, was burning in Hawai`i National Park.

Because the eruption has been so uneventful this week, the remainder of the column will address the question of lava diversion as it applies to Mount Etna in southern Italy.

During the last several weeks, a major eruption on Mount Etna threatened Zafferana Etnea, a town of about 7,000 people. The map shows the location of the town on the east-southeast slopes of Mount Etna and the distribution of flows from the 1989 to 1992 eruptions.

The Italian government attempted to stop the advance of the flows before they reached the town by building five separate earthen barriers across the projected path of the flow. The main earthen barrier, about 65 feet tall and built on January 3, 1992, was overtopped on April 8. The barrier was apparently undermined by the lava, and a vent-like structure formed just below the barrier.

The remaining barriers were also emplaced perpendicular to the projected flow direction. The uppermost three barriers, each 10 to 16 feet tall, were built on April 10 and 11, 1992, and were buried and overtopped sequentially within a few days. The lowest barrier was about 33 feet tall and was built on April 12 and overtopped on April 13. Each of these barriers in the path of the flow served as "speed bumps" in slowing the flow, which then resumed its advance after overtopping the barrier.

In certain circumstances, such a slowing of the flow could prove critical in averting disaster. For example, if eruption stops, then the barriers serve a real function in having slowed down the flow. However, if the eruption continues, such efforts are futile.

A second technique was employed to try to stop the flow. The vent was bombarded with large blocks of cement in an attempt to plug up the main channel and divert the lava into other flows. The main flow did overtop the channel near the vent, thereby making a new, adjacent flow, which stopped near the outskirts of town. However, it is not clear that the bombardment caused this important shift that eventually spared the town. During the 1984 eruption on Mauna Loa, the advancing `a`a flow stagnated above Hilo, and the upslope lava channel then overflowed and formed a new flow adjacent to the original one. This process occurred several times, and none of the subparallel flows advanced into Hilo. It seems most likely that the same thing occurred at Etna. This process limits the advance of `a`a flows and serves as a natural barrier to advancing `a`a flows.

There is a major problem with diverting flows in Italy, where the town is scattered along the road at one altitude: the lava diverted from one populated area will necessarily flow into an adjacent populated area. This is why the Italians attempted to stop, rather than divert, the flow.

This same situation occurs in Hawai`i where development commonly occurs along the coast or along a highway that is parallel to the coast. In most, but not all, situations, one can only try to slow down the advance of the flow and hope that the eruption ends because diversion will simply send the flow into an adjacent populated area.

The eruption at Etna continues, so Zafferana Etnea may have been spared only for the time being. There were numerous times during the Pu`u `O`o-Kupaianaha eruption when it appeared that houses were spared, only to have the continuing eruption eventually destroy them.