Steam Vents and Sulphur Banks

Steam rising from the forested edge of a volcanic crater
Wahinekapu (NPS Photo/J. Wei)
Fumes rising from rocks covered with white crystals
Haʻakulamanu (Sulphur Banks)

NPS Photo/D. Boyle

At the summit of Kīlauea, two of the most popular sights involve different substances rising up from cracks in the earth.

At Wahinekapu (Steaming Bluff), you can feel hot water vapor as it billows from the ground in steam vents. This striking phenomenon is created as ground water seeps down to rocks heated by magma deep underground. The rocks are so hot that it vaporizes the water, returning it to the surface as steam.

A short distance away, at Ha'akulamanu (Sulphur Banks), volcanic gases seep out of the ground, along with ground water steam. These fumes can be amazingly hot. In 1922, scientists drilled two holes to measure underground heat in the area. Temperature measurements remained constant at 205° F (96° C) down to 50 feet (15 m), the maximum depth drilled. Fumes emitted here include sulfur dioxide (SO2), which smells like a struck match, and hydrogen sulfide (H2S), the gas that smells like rotten eggs. These two gases react chemically to produce pure sulfur, a yellow mineral known to Hawaiians as kūkaepele, the waste of Pele.
Comparison of four different minerals: sulphur, gypsum, opal (milky glaze), and hematite
Can you spot these four minerals at Haʻakulamanu?

Minerals at Haʻakulamanu
Given enough time, water can dissolve rock. Adding heat and acid to water speeds up the process. At Haʻakulamanu (Sulphur Banks), all three ingredients— heat, water, and acid—are plentiful. As hot water vapor and acidic gases rise to the surface, they “cook” the pre-existing rock. This reaction causes chemical and textural changes in the minerals that make up the rock. Scientists call this process hydrothermal alteration, derived from the Greek words for water (hydro) and heat (therme).

Is it "sulphur" or" sulfur"?
Both are correct. Sulphur Banks, as named by visitors in the early 1800s, is spelled according to British usage. Sulfur is the spelling used in the United States.

Why do some points release potentially dangerous volcanic gases, while others only exude mere water vapor?

It all depends on the depth of the crack. Shallow earth cracks emit only steam, as they are not deep enough for volcanic gases to seep up. However, deep faults and fractures that extend down to magma allow gases a pathway to the surface, as at Ha'akulamanu.

Illustration showing cracks in the crust of the earth and fumes rising
Illustration of steam vents and sulphur banks at the Kīlauea summit. Shallow cracks allow the release of water vapor and create steam vents. Deeper fractures may allow the transport of volcanic gases, and create sulphur deposits (NPS Graphic)

“The smell of sulfur is strong, but not unpleasant to a sinner.”

—Mark Twain about the summit of Kīlauea, 1866

Last updated: October 28, 2020

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