View north-northeast across Kilauea's summit caldera with Halemaumau crater to the left of the photograph. (Photo courtesy Hawaiian Volcano Observatory)

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

It seems that something new and different is always cooking at Kilauea. In December a change in the aroma coming from Pele’s kitchen was noticed by the HVO and National Park staffs, along with nearby residents. The biting, choking odor of sulfur dioxide (SO2) from Halemaumau that had inundated Volcano Village and the Golf Course subdivision during trade wind disruptions started to change. The pervasive odor of rotten eggs or sewer gas that tainted the air was caused by an increasing appearance of hydrogen sulfide (H2S.)

The presence of H2S at Kilauea’s summit is not unusual in itself. Visitors to Sulphur Bank, near Kilauea Visitor Center, have long experienced the characteristic mix of odors. As Mark Twain quipped during his 1866 visit to Kilauea, “The smell of sulphur is strong, but not unpleasant to a sinner.”

Twainian humor aside, H2S gas, like SO2, can be hazardous at concentrations greater than 5-10 parts per million (ppm). Eye irritation and headaches are frequently reported symptoms. The human nose –very sensitive to H2S when first smelled — is able to detect it at concentrations well below 1 part per 1 million. Unlike SO2, however, olfactory fatigue causes a loss of odor sensitivity upon prolonged exposure. The state hosts a 1-hour ambient air standard for H2S of 0.025 ppm. California enacted a similarly protective standard in 1969, targeted at decreasing odor annoyance. The World Health Organization established a 24-hour average guideline of 0.15 ppm intended to protect human health.

H2S is formed at Sulphur Bank when SO2 escaping from Kilauea’s magma chamber rises and combines with ground water. The resulting chemical reaction, called hydrolysis, produces a mixture of SO2 and H2S at the surface. The amount of gas released is very small, and when the emissions are blown toward Halemaumau, they tend to affect only a small section of Crater Rim Drive.

Apart from the background Sulphur Bank gas emissions, HVO scientists have been watching carefully for appreciable H2S coming from the Overlook Vent at Halemaumau since the eruption began last March. A substantial increase in this gas, along with other eruptive indicators, might signal that a large amount of water was coming into close proximity of red-hot magma. The subsurface encounter of abundant groundwater and magma was the suspected culprit causing the explosive eruptions of 1924.

The lack of significant H2S presence at Halemaumau through late November reflected hot, dry conditions within the vent. And SO2 emissions remained high, as well, despite significant collapses of the vent rim and walls in September and October that added large amounts of rubble to the conduit bringing gas and other eruptive products to the surface.

This situation began to change in early December, when further collapses widened the vent rim to football playing-field size–100 m (110 yards) Accompanying the vent-widening was some seasonally notable rainfall. The gage at HVO recorded over a foot of rain in December, much of it during a single storm late in the month.

Hot, collapsed rim and wall rock accumulating in the vent, combined with the prodigious rainfall, provided good conditions to produce H2S from SO2 within the conduit. By mid-December, the vent had cooled somewhat, and the ratio of H2S to SO2 in the air near Halemaumau had climbed to higher levels than seen previously.

Despite the seemingly favorable H2S-producing conditions, the total emissions, while significant, were still less than one-tenth those of SO2. HVO staff, however, watched the situation unfold with great interest, wondering what might happen next.

Pele’s SO2 exhalations were not to be dampened by a little rainfall, though. By mid-January, the rains had tapered off, and magma heat and gas from below the rubble in the vent appeared to burn through. By late January, faint glow was once again seen in the vent, and the presence of H2S declined to near background as the vent heated up and dried out. And so, for now, it seems that hot liquid rock is once again what’s cooking at Kilauea’s summit kitchen. Ah, pungent sinner soup! Can you smell it? Can you taste it?