| med | large |	| med | large |
The warm grass-covered area in the upper east rift zone known as the Puhimau thermal area has enlarged noticeably since 1961 (compare photos above). The expanding area, marked by dead and dying trees, now extends east across the Chain of Craters Road. Most scientists agree about what caused the thermal area, but are unsure about what is causing the area to grow in size.

The Mystery of The Puhimau Thermal Area

First recognition of dead and dying vegetation

On May 28, 1938, the Hawaiian Volcano Observatory recorded an episode of shallow tremor and numerous felt earthquakes centered near Pauahi Crater. Similar episodes continued into June and occurred again in August of that year. Interpreting this burst of seismic activity, which he labeled the "Chain of Craters Crisis," Dr. Thomas Jaggar, volcanologist at the Observatory, wrote in the Volcano Letter of May 1938 that the above activity represented "the acceleration of a process long continued." Two signs of that process were the recurring buckling of the road near Devil's Throat, and the appearance of a "hot area," now called the Puhimau Thermal Area.

No one knows when the thermal area first appeared. In May 1938, Chief Ranger Joseph Christ first noticed an area of about 15 acres of dead and dying vegetation roughly halfway between Puhimau and Ko'oko'olau Craters, on the west side of the Chain of Craters Road. Making his own judgment about its age, Dr. Jaggar stated in the Volcano Letter cited above that "the new vapors have killed the vegetation within the last year or two." Conjecturing on its cause, he wrote in his June 1938 report to Park Superintendent Edward Wingate that "the hot spot that is killing the verdure indicates the presence of active magma below."

The Park quickly devised a response to the dying vegetation and, within a month, had outlined a systematic method to measure and monitor ground temperatures. A press release by the Superintendent that appeared in the September 9,1938, Honolulu Advertiser and in the Hilo Tribune Herald stated that ground temperatures in the area ranged from 80°F (27°C) on the periphery of the area to a maximum of 182°F (83°C) near the center. As a control, ground temperatures were measured at two check points one half mile and two miles, respectively, from the area, where there was known to be no underground heat. The temperature at each check point was a steady 67°F (19°C).

Thermal area increases in size beginning in early 1960s

The above study was continued for several months to determine whether or not the area of dying vegetation might increase in size. In the January-March 1939 edition of the Volcano Letter, Dr. Jaggar wrote that "Temperature measurements at the hot area in `Ohia forest near Kokoolau¹ Crater showed no change and there is no encroachment of solfataric action on the surrounding forest." The area appears to have remained at about 15 acres for over 20 years. However, Dr. Don Swanson, Scientist-in-Charge at HVO, has plotted the area of dying vegetation as indicated in a series of aerial photos taken between 1961 and 1985. Estimates from the photos show that in 1961 the Puhimau Thermal Area was about 16 acres. By 1976 it had increased to 25 acres and, by 1985 to 29 acres. Today trees are dying on the east side of the road, suggesting an increase in size since 1985.

Why is the thermal area getting larger?

What might be the cause of the increase in size in the last several decades? Don Swanson offers some possibilities (without implying the probability of any): additional intrusion in the immediate area; changes in the heat flow from the source due to increased cracking caused by earthquakes since 1961; a combination of factors associated with magma movement and eruptions in the vicinity, the greatest of which was that of Mauna Ulu, 1969-1974.

Various geophysical studies were made of the thermal area in the late 1970s and mid 1980s to determine the nature of the heat source. The results have been inconclusive. While the studies concur on the strong likelihood of "active magma below," as Jaggar put it, they do not agree on the dimensions or depth of the magma body. One study, done in 1983, concludes that the source of heat is probably a shallow magma intrusion that has begun to solidify on its outer edges while the core remains molten. The top of the intrusion is seen to be at shallow depth—500 feet or less (Dunn and Hardee, 1985). Another study, done a year later, using different methods, concludes the presence of an inclined thick dike running in a general northeast-southwest direction with a steeply dipping southeast edge and a more gently sloped northwest edge. The depth to the top of the dike is estimated to be 650 to almost 800 feet. On top of this is a layer between 65 and 130 feet thick that could be steam-saturated rock. At a depth of 1,300 feet or more, the implied dike appears to be 330 to 400 feet wide (Bartel and Jacobson, 1987).

Other characteristics of the thermal area

A geochemical study was done in 1977 that showed higher than usual concentrations of volatile (gaseous) materials, including mercury, helium, carbon dioxide, and various sulfur compounds rising from the ground in the thermal area, suggesting the presence of a body of magma below (Hinkle, 1978).

Based on the inconclusiveness of the above studies, and in the interests of geothermal research, the Sandia National Laboratories of New Mexico proposed in 1985 to drill an exploratory hole into the thermal area. Their particular interest in the area was that it was the only known shallow source of magmatic heat. The proposal was not accepted by the Park on ecological grounds (Tunison, 1999).

It has been suggested that the Puhimau Thermal Area is in the early stages of pit crater formation. This would require major withdrawal of magma from beneath the area. It was claimed as early as 1977 that arcuate (curved) cracks, marking the beginnings of the outline of a pit crater, had already begun to form. Such cracks have not been found in recent visits to the area.

While the thermal area in general appears barren, there are a few shallow-rooted native and non-native plants growing on the heated surface. Among them is the makole (Portulaca sclerocarpa), a low-growing succulent with tiny white flowers. This plant, now an endangered species, was fairly common in certain parts of the park, and perhaps at Puhimau, in the 1960s before alien grasses became so prominent (Pratt, 1998). Was it as common at Puhimau before the assumed intrusion as it is now, or has it colonized the area after the death of most other vegetation? This remains among the unanswered questions about the area.

So the mystery of the Puhimau Thermal Area includes questions about when it formed and what lies under it, about what grows in it, and what its future will be. The area has not been studied intensively in the past, nor are there present plans for such studies. The mystery remains.

¹The crater, now called Ko'oko'olau, was formerly called Koko'olau. The two names refer to the native plant Bidens hawaiensis, and are interchangeable when applied to the plant.

References

Dunn, J.C., and Hardee, H.C., 1985, Surface heat flow measurements at the Puhimau hot spot: Geophysics, v. 50, no. 7, p. 1108-1112.

Jaggar, T.A., 1938, Hawaiian Volcano Observatory Report for May 1938: The Volcano Letter, no. 459, p. 4.

____, 1939, Hawaiian Volcano Observatory Report for January-March 1939: The Volcano Letter, no. 463, p. 2.

Hinkle, M.E., 1978, Helium, mercury, sulfur compounds, and carbon dioxide in soil gases of the Puhimau Thermal Area, Hawaii Volcanoes National Park, Hawaii: U.S. Geological Survey Open- File Report 78-246, 14 p.

Hawaii National Park, 1938, Monthly reports of the Superintendent: Hawaii National Park, National Park Service, June and Oct.

Pratt, L.W., 1998, written communication

Stone, C.P., and Pratt, L.W., 1994, Hawai'i's plants and animals; biological sketches of Hawaii Volcanoes National Park: Hawaii Volcanoes National Park, Hawaii Natural History Association, p.169.

Swanson, D. A., 1998, [Puhimau Thermal Area]: U.S. Geological Survey, personal communication, Oct. Tunison, J.T., 1999, [Puhimau Thermal Area]: National Park Service, personal communication, Jan.