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

Volcano Watchers may pride themselves on having witnessed a variety of lava flows, but how many can say they’ve seen flowing molten sulfur? It happens at some volcanoes, including a rare occurrence perhaps 60 years ago at Sulphur Cone on Mauna Loa’s southwest rift zone.

Let’s first dispel any thought that sulfur flows are like the extensive pahoehoe and aa erupted along Kilauea’s east rift zone. Sulfur flows are small, ranging in area from one-tenth to a few hundred square meters (the area of a bandana to the area of a basketball court). And sulfur flows are thin, only a few centimeters (inches) or, rarely, tens of centimeters (a foot or so) thick.

But, like many liquids, molten sulfur possesses sufficient viscosity, or resistance to flowage, that it may form features similar to pahoehoe, such as ridges, toes, channels, levees, and lava tubes.

Another thing to clarify is the origin of the material in a sulfur flow. Molten or partially molten rock within the earth is generated at high temperature and pressure. However, neither condition is necessary to generate a sulfur flow.

Sulfur has a low melting temperature, about 112 degrees Celsius (234 degrees Fahrenheit), slightly above the boiling point of water. The earth’s surface reaches such a temperature near volcanic fissures or close to active vents, as shown by the areas of steaming ground often observed near hot volcanic features. So all that’s needed is a way to deposit sulfur and have it ready for melting when ground temperatures spike.

The depositional volcanic agent is the gas discharged from fumaroles, or gas-emitting vents, which are common on active volcanoes. A familiar example for Kilauea visitors is Sulphur Bank in Hawaii Volcanoes National Park. There the bright-yellow crystalline sulfur forms in two ways: by condensation of vaporous sulfur and also by a chemical reaction between sulfur-containing gases.

Although molten sulfur droplets have been observed at Sulphur Bank, the sulfur deposits and temperature are insufficient to form an actual sulfur flow.

So we turn to Sulphur Cone, a cinder-and-spatter cone situated on the southwest rift zone of Mauna Loa at an altitude of about 3,430 m (11,240 ft). When named in 1921, the cone was notable for the abundance of sulfur crystals blanketing it and the surrounding area, a consequence of extensive degassing from a fissure downslope from Mauna Loa’s summit.

This bloom of sulfur at the surface surely marked a subterranean setting where sulfur had crystallized in the voids and coated many fractures. No sulfur flows were recognized in the area when the cone was named in 1921, suggesting that the sulfur flow must be younger than that.

What could have melted all that sulfur to make a flow? One proposed mechanism is that heating along the 20-km-long (12-mile-long) fissure of Mauna Loa’s spectacular 1950 eruption substantially raised the ground temperature.

Once the temperature exceeded the 112-degree melting point, the accumulated sulfur could melt and issue from cracks—in this case, forming a flow ranging in thickness from 10 to 50 cm (4-20 in.), covering about 380 square meters (450 square yards).

Worth noting is a feature of sulfur flows, recently rediscovered, that was first described in the 1780s at the island of Vulcano, near Sicily (Italy). Molten sulfur, if hot enough, can ignite and burn as it flows. When it does, much of the evidence of its passage may go up in smoke, so to speak; the only remaining trace may be a track of sulfur blocks too large to be entirely consumed.

Thus, sulfur flows are truly anomalous. Typical molten lava flows and ejected spatter glow incandescently, but they don’t ignite and burn, despite the vision conveyed by popular terms, such as “fire fountain” or “curtain of fire.” Sulfur is a rare instance in our volcanic world where the fire of Dante’s Inferno does indeed play upon the surface of a molten flow.