Volcano Watch — The Puhimau thermal area is gradually enlarging

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Little more than a mile down the Chain of Craters Road from Crater Rim Drive, a large tract of bare ground suddenly appears on the right, and curious travelers wonder why it is so open. But there's no place to stop, so they move on, and the Puhimau thermal area is quickly forgotten.

What is the Puhimau thermal area, why is it in the middle of thick forest, and what might it mean for the future?

The thermal area was discovered in early May 1938. Vegetation was freshly killed or dying because of the heat. In late May and early June, and again in August, swarms of earthquakes rocked the area, opening cracks and buckling the Chain of Craters Road near Devil's Throat.

Thomas Jaggar conjectured that a shallow intrusion of magma had taken place, causing the earthquakes and supplying heat to kill the vegetation. If so, the intrusion must have begun before 1938, for observers found dead trees that they felt had been killed "within the last year or two."

The area had a maximum soil temperature of 83 degrees C (182 degrees F) at its center, cooling to 27 degrees C (80 degrees F) at its periphery. This contrasted with the soil temperature in surrounding areas of about 19 degrees C (67 degrees F). Temperature measurements were continued until at least early 1939, showing no sign of cooling or enlargement of the hot area.

Today such a hot area would be detected quickly, because of the frequent overflights. In the 1930s, however, flights over the park were infrequent, and the area, southwest of the Chain of Craters Road, was screened from it by forest.

Little interest was shown in the area for 30 years. A geochemical examination of soil-gas samples in 1977 showed elevated levels of helium along fractures. Mercury was concentrated over the thermal area and interpreted to come from a cooling magma body, though more recent work suggests that it is derived by alteration of existing rocks underlying the thermal area. Soil temperatures were as high as 95 degrees C (203 degrees F) at a depth of 15 cm in the steaming part of the area.

Two geophysical studies in the mid-1980s corroborated the presence of a hot, perhaps partly molten, magma body at a depth of a few hundred meters. Recent work shows that the amounts of carbon dioxide and sulfur dioxide emitted from the thermal area are small.

Unknown at the time was that the thermal area had been enlarging, especially since the early 1960s. Comparison of aerial photographs shows that the area expanded from about 6 ha (15 acres) in 1945 to 6.4 ha (15.7 acres) in 1961. In the next 15 years, the area enlarged to about 10.8 ha (26.8 acres). By 1985 (the date of the most recent good aerial photos), the area had reached 11.8 ha (29 acres). The current area is close to 15 ha (37 acres).

The area became visible from the road in the late 1980s, in part because of a slight road realignment but mainly because the area was expanding northeastward. In the late 1990s, vegetation was being killed across the road, and today the tree and fern kill is evident on both sides of the road.

The expansion, particularly since the early 1960s, is hard to explain by a single magma intrusion in the mid-1930s. The area does not appear to be inflating, as it would if the intrusion were enlarging today. Possibly a shallow stream of magma into the east rift zone supplies heat to keep the area hot and expanding. Magma may be flushing through the intrusion detected geophysically. Nearly continuous magma flowage may have started in the early 1960s, when the east rift zone was reactivated after almost 40 years without eruption.

Many pit craters along the Chain of Craters have active or once-active thermal areas near them. The most famous was at Alo`i Crater (buried in 1970 by lava flows from Mauna Ulu); so much steam rose from thermal ground that the road was divided to prevent head-on crashes during white-outs. On this basis, a pit crater might form in the Puhimau thermal area at some time in the future.

Volcano Activity Update

Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. A new, long and narrow tongue of lava is visible down to below the 1050-foot elevation on Pulama pali. This surface flow is about 300-400 m (1000 ft) west of the main Mother's Day flow and is burning through the forest. Another prong of lava can be seen near the top of Pulama pali. Below the breakouts feeding these flows, lava remains confined in the tubes down to the sole ocean entry at West Highcastle.

The public should be aware that the ocean entry areas could collapse at any time, potentially generating large explosions in the process. The steam clouds rising from the entry areas are highly acidic and laced with glass particles. The National Park Service has erected a rope barricade to delineate the edge of the restricted area. Do not venture beyond this rope boundary onto the lava deltas and benches. Even the intervening beaches are susceptible to large waves suddenly generated during delta collapse; these beaches should be avoided.

Two earthquakes were reported felt during the past week. Residents of Glenwood felt an earthquake at 2:47 p.m. on February 1. The magnitude-2.5 earthquake was located 3 km (2 mi) southeast of Mt. View at a depth of 18 km (11 mi). On February 5 at 9:19 a.m., a magnitude-3.0 earthquake shook residents of Waimea and Pa`auilo. The earthquake was located 6 km (4 mi) northwest of Pohakuloa at a depth of 23 km (14 mi).

Mauna Loa is not erupting. The summit region continues to inflate, though the rate of inflation has slowed gradually during the past month or two. The earthquake activity is low, with only one earthquake located in the summit area during the last seven days.