Volcano Watch — How are lava measurements made?

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Kīlauea's ongoing eruption from the episode 51 vents resumed on Aug. 15 after a four-day pause in the activity and reoccupied the tube system down to the southern edge of the lava shield. Pahoehoe flows are advancing from the end of the tube to the southeast and southwest.

Kīlauea's ongoing eruption from the episode 51 vents resumed on Aug. 15 after a four-day pause in the activity and reoccupied the tube system down to the southern edge of the lava shield. Pahoehoe flows are advancing from the end of the tube to the southeast and southwest. The flows are confined to the area of earlier episode 51 flows and extend about 1.2 miles from the vent. Almost no activity is visible at the vents, because the lava is fed directly into the tube. The lava pond next to the 51 vents is empty, as it has been since early July. The lava pond within the Pu`u `O`o Crater is about 200 feet below the rim.

The eruption updates that appear regularly in this column are primarily the result of weekly trips to the eruptions site by geologists from the Hawaiian Volcano Observatory. The geologists who monitor the ongoing eruption assess the potential hazards to life and property and conduct research that will increase our understanding of how the volcano works. This research enables us to make more accurate predictions of future activity. In addition to disseminating information to the public, the media, and other government agencies, we report the results of our research in scientific journals.

On a typical trip to the eruption site, a crew of three geologists reaches the rim of the Pu`u `O`o Crater after a three-and-a-half hour hike. At Pu`u `O`o, we measure the depth to the surface of the lava pond, using a distance range-finder and inclinometer. We change the film in a time-lapse movie camera, perched on the rim, that exposes a frame every three minutes and gives us a continuous record of the pond's activity and changes in its level. From the crater's rim, the crew walks around the cone to the 51 vents. Aside from noting any changes at the vents, our main aim is to collect a molten sample of lava, either spatter or lava dipped from flows, as close to the vent as possible. When the 51 lava pond was still active, we could collect a sample on a hammer head thrown into the pond on the end of a length of steel cable. Currently, we must walk to the south across the new lava flows until we find an active flow to sample by the same means.

The samples are analyzed into a laboratory to determine the chemical composition of the lava and the temperature at which it erupted. Thin sections of the samples are prepared for microscopic study of the minerals, glass, and gas bubbles that make up the lava. These samples contribute to our long-term record of the changing conditions within the magma plumbing system of Kīlauea. Since 1986, the lava chemistry has remained relatively constant, and the eruptive temperature of the magma has slowly decreased. This tells us that the eruption is still tapping a single batch of magma that is slowly cooling. A sudden change in chemistry or increase in temperature would indicate that a new batch of magma was supplying the eruption, with the possible consequence of prolonging the eruption.

HVO geophysists make direct measurements of the volume of erupted lava at the 51 vents, using magnetic and electrical instruments that detect the lava as it flows through lava tubes. Monitoring eruptive volume is another means of gaining insight into the workings of the volcano. We were able to accurately predict the demise of the Kupaianaha vent in February 1992 by measuring its steadily declining output. During periods when lava flows are threatening inhabited areas, monitoring lava volume in the tube near the vent can help us to make short-term predictions of the activity at the flow front.

About once a month, we hire a commercial helicopter so that we can map and photograph the flows from the air and do surveying work that requires heavy instruments. The maps we make from the overflights are preliminary. For more precise maps, we work from vertical aerial photos, taken from an airplane flying at approximately 20,000 feet above the ground. Recently, we have begun using portable GPS (Global Positioning System) receivers to map the active lava flows as we walk around the margin of the flow field.

At present, no residential areas are immediately threatened by the eruption. The main concern now is for archeological sites within the National Park and for the native forest, which is vulnerable to forest fires set by the flows. We exchange information with the National Park Service on flow advance and keep Civil Defense informed. On days when we are not in the field, we rely on information that commercial helicopter pilots provide us on the location of the lava flows.