USGS: “Low-angle aerial view of the Great Crack and surrounding 1823 lava flows (dark gray) along Kīlauea’s lower Southwest Rift Zone. The crack is about 50 feet (15 meters) wide in this area with similar but variable depth, depending on the amount of rock rubble filling the opening.” (USGS photo by D. Downs)

(BIVN) – Kīlauea is not erupting, and while the Hawaiʻi island volcano is currently exhibiting signs of elevated unrest, the number of earthquakes at the summit over the last few days has decreased relative to the past three weeks of increased seismic activity.

There is no indication of unusual activity beyond the summit along the East Rift Zone or Southwest Rift Zone, scientists say. When it comes to “unusual” activity, the U.S. Geological Survey in its weekly Volcano Watch article examines the volcanic events of 1823.

From this week’s Volcano Watch article written by USGS Hawaiian Volcano Observatory scientists and affiliates:

Last month a “Volcano Watch” article discussed the bicentennial of the first visit of westerners to Kīlauea caldera, led by English missionary William Ellis, in 1823. Ellis did not just visit the summit region; he had approached from Kaʻū, traveling along what eventually became known as Kīlauea’s Southwest Rift Zone (SWRZ).

Ellis first witnessed evidence of Kīlauea’s restlessness on the SWRZ, in the form of a vast, 4.8-square-mile (12.5 square-kilometer) lava flow that had erupted just a short time before.

The eruption was over by the time Ellis passed through the Keaīwa area, just east of the modern town of Pāhala, from July 30–31, 1823. The lava was still steaming, and in places too hot to walk on. Although lava flows can retain heat for many years after an eruption, it seems unlikely in this case, because the flows from this eruption are unusually thin: in most places, the thickness is less than 1 meter (yard).

Thinner lava flows dissipate heat much more quickly than thicker ones; therefore, we can assume that the flows visited by Ellis had been molten no more than a few weeks earlier. This assumption aligns with oral accounts from native Hawaiians in the surrounding area, who reported that the eruption had occurred less than a month prior.

Over the past century, Hawaiian Volcano Observatory (HVO) scientists have occasionally visited the Keaīwa area to study the eruptive features in detail. They have notably found pāhoehoe lava veneers lapping up the flanks of older cinder cones, several yards (meters) high, resulting in the name “Lava Plastered Cones” that appear on maps of the area.

The thin lava veneers on the cones are indicative of high flow velocities, enough to overcome topography standing in the eruption’s way. Think of a water slide with a jump at the end, where fast-moving water briefly flows uphill. At such high velocities, the entire flow would have been emplaced over just a handful of days. In combination with Ellis’s reports, this evidence suggests that the eruption most likely occurred in early to mid-July 1823.

Seeing that this eruption produced relatively thin and fast-moving pāhoehoe flows, we know that the lava had a low viscosity. Essentially, it spread across the preexisting ground surface like a coat of spilled paint, rather than sticky tar. But why was the viscosity so low? For most Hawaiian eruptions, such low-viscosity lava is only found very close to the eruptive vent, not over the whole flow field as in 1823.

The 1823 eruptive fissure holds clues that may help answer this question. This fissure—part of the feature famously known as the “Great Crack”—stretches 18 miles (12 kilometers) with widths up to 50 feet (15 meters). This width is especially impressive, as most Hawaiian fissures only widen to about 1 yard (meter). The 1823 fissure is also unusual in having no substantial spatter deposits along its length; it’s just a big crack where lava welled up from below and then flowed downhill.

It is likely that the Great Crack existed prior to 1823, but it was possibly cleaved more apart by the intrusion of a subsurface magmatic dike when the eruption started. Large blocks of older rock that dropped into the opening and were coated in 1823 lava offer possible evidence that the crack was widening during the eruption. Prolonged dilation may have prevented the eruption from localizing at one vent, thus influencing the character of the effused lava.

Another factor to consider is the location of the 1823 eruption, it occurred at a lower elevation along the SWRZ than any event since. The lower end of the fissure system was close enough to sea level that it produced small phreatic explosions, as magma interacted with shallow groundwater. One plausible explanation is that the low elevation eruption engaged an unusually strong pressure gradient in the volcanic plumbing system from Kīlauea’s summit, driving magma to the lower SWRZ with greater force and speed.

HVO scientists continue working to solve the mysteries of the 1823 Kīlauea Southwest Rift Zone eruption, but after two hundred years, some questions will inevitably remain unanswered. That is, until another similar eruption occurs that we can observe for ourselves — unlikely in our lifetimes, but generally if something has happened once on a volcano, it can happen again!