Volcano Watch: New outcrops make good geology

  • D. SWANSON/U.S. Geological Survey This aerial view of the western part of Kilauea Volcano’s caldera was taken Aug. 6, 2018. The downdropped block is faulted about 120 m (400 feet) below the caldera floor. Many 19th century lava flows are exposed in the fault scarps. Halema‘uma‘u (not visible) is to the left of this photo.

A good field geologist is an opportunist.

Never content with what outcrops are available, she jumps at the chance to see another one, hoping it will provide a better understanding to some question about what happened in the past.

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But it isn’t every day new outcrops are created, and it’s rarer still when they are on the scale of those formed during the faulting of Kilauea Volcano’s caldera floor in summer 2018.

As Halema‘uma‘u sank and widened, its crater wall began to expose lava flows that formed during earlier eruptions and were covered by later flows. In particular, the north side of Halema‘uma‘u bites some 500 m (1640 ft) deeper than before, potentially making accessible lava flows that erupted and gradually filled the caldera early in the 19th century.

In addition, the faults that bound the downdropped sector of the caldera exhumed the south sulfur bank and exposed lava flows not previously observed by scientists.

Why are these new outcrops so important?

Aaron Pietruszka, USGS scientist and former University of Hawaii graduate student, and his UH adviser, Mike Garcia, discovered gradual changes in the chemical composition of Kilauea lava with time, starting before the 19th century and continuing to present. The number of lava flows that could be sampled from the 19th century itself was, however, very small.

Chemical and isotopic analysis of 19th century caldera fill exposed in the high wall of Halema‘uma‘u and adjacent faults will augment and refine their startling finding. It won’t be easy to do the sampling, though, because a lot of rocky rubble mantles much of the wall, possibly obscuring some flows.

But it needs to be done before lava returns to Halema‘uma‘u.

The faults cutting the caldera floor might reveal details about the 19th century caldera fill that have long eluded geologists. Maps and sketches of the floor made at various times show wide, multiple lava lakes and probably several editions of “black ledges” adjacent to the lakes.

How deep were those lakes? Were they merely shallow ponds and bays no more than a few tens of meters (yards) deep, or were one or more of them so deep they connected with the magma reservoir perhaps 1 km (0.6 mi) or more deep?

The new exposures provide cross-sections through some of the 19th century caldera fill. These cross-sections can be examined to look for the margins and floors of ancient lava lakes, spills from those lakes, eruption conduits and other features leading to a greater understanding of how the caldera was filled.

Thick explosive deposits formed between about 1500 and the early 1800s are exposed high on the south wall of Halema‘uma‘u, covered only by one or two younger lava flows. Binocular observations of the lower north wall of Halema‘uma‘u, several hundred meters (yards) below the caldera floor, have not seen any explosive deposits. Yet the explosive debris must have fallen into the caldera as well as around it.

If the binocular observations are correct, then the explosive deposits must be buried still deeper than the base of the north wall. That would imply the deposits formed when the caldera was very deep, as geologists hypothesized but never documented. This interpretation badly needs boots-on-the-ground checking because the binocular observations alone are inconclusive.

The news isn’t all good.

Shaking during the more than 60 large earthquakes last summer caused rockfalls along the west side of the caldera that buried at least one outcrop of explosive deposits more than 1,000 years old. Luckily, those deposits were already sampled.

For a tantalizing time, a much larger outcrop of these old deposits reappeared in the caldera wall as the caldera floor dropped; this outcrop is shown in a photograph taken a few years before the 1919 lava flow covered it. But gradually throughout the summer of 2018, this exhumed outcrop became stranded and inaccessible as the caldera floor sunk below it. Perhaps a way will eventually be found to study this superb outcrop.

Summer 2018 gave field-focused geologists lots to do and think about in Kilauea Volcano’s caldera. With new samples and firsthand observations, their work will build on what is already known or surmised and help us better understand the caldera and how its 19th century activity differed so much from that of the past 100 years.

Volcano activity updates

Kilauea is not erupting. Rates of seismicity, deformation and gas release have not changed significantly during the past week.

On March 26, the USGS Hawaiian Volcano Observatory lowered the Volcano Alert Level for Kilauea to Normal and the Aviation Color Code to Green. For definitions of USGS Volcano Alert Levels and Aviation Color Codes, visit https://volcanoes.usgs.gov/vhp/about_alerts.html.

Three earthquakes with three or more felt reports occurred in Hawaii during the past week: a magnitude-2.5 quake 51 km (32 mi) southwest of Kahalu‘u-Keauhou at a depth of 28 km (17 mi) at 10:04 p.m. March 26; a magnitude-3.0 quake 31 km (19 mi) southeast of Waimea at a depth of 17 km (11 mi) at 3:43 a.m. March 24; and a magnitude-3.5 quake 5 km (3 mi) south of Volcano at depth of 13 km (8 mi) at 11:13 p.m. March 23.

Deformation signals are consistent with refilling of Kilauea Volcano’s deep East Rift Zone magma reservoir. Sulfur dioxide emission rates on the ERZ and at Kilauea’s summit remain low and have been steady during the past several weeks.

Hazards still exist at the lower ERZ and summit of Kilauea. Residents and visitors near the 2018 fissures, lava flows and summit collapse area should heed Hawaii County Civil Defense and Hawaii Volcanoes National Park closures and warnings. HVO continues to closely monitor Kilauea for any sign of increased activity.

The USGS Volcano Alert level for Mauna Loa remains at Normal.

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Visit HVO’s website (https://volcanoes.usgs.gov/hvo) for past Volcano Watch articles, Kilauea and Mauna Loa updates, volcano photos, maps, recent earthquake info, and more. Call 808-967-8862 for weekly Kilauea updates. Email questions to askHVO@usgs.gov.

Volcano Watch (https://volcanoes.usgs.gov/hvo/hvo_volcano_watch.html) is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.

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