Revolution in thinking about Kīlauea's explosions comes to HVO: Part 1

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The past five years, especially the past 18 months, have seen profound changes in the way HVO scientists view Kīlauea's explosive potential.

This is a photo of Keanakako`i Ash with abundant accretionary lapilli, largest of which are about 3 mm in diameter.

This is a photo of Keanakako`i Ash with abundant accretionary lapilli, largest of which are about 3 mm in diameter.

(Public domain.)

Once considered a curiosity, explosions at Kīlauea are now recognized as an important type of eruption with significant hazard to people on the ground and in the air. How did this revolution in thinking come about?

Deposits of tephra (anything solid exploded by a volcano) known as the Keanakako`i Ash are a familiar sight at Kīlauea's summit, particularly southwest of the caldera, where the trade winds often blew ash during eruptions. Geologists thought that the ash formed during eruptions in, or within a few years before, 1790, when 80-800 people were killed. Some older explosion deposits, such as the Uwekahuna Ash, were known but had not been studied carefully.

In the late 1980s, Jocelyn McPhie, an Australian on a Fulbright fellowship at UH-Manoa, studied the Keanakako`i Ash and found two ash beds distributed southeastward, though most of the ash, slave to the trade wind, went southwestward. She had no time to continue her work, but her important discovery set the scene for what was to follow.

During field work in the late 1990s, Dick Fiske and Tim Rose of the Smithsonian Institution and Don Swanson of HVO came across older tephra southeast of the caldera, across the trade-wind direction. Further study found that it erupted during a remarkably powerful explosion at the summit and was strewn as far away as the south coast of Kīlauea. They found heavy rocks the size of golf balls at Halape, 16 km (9.5 miles) from the caldera, and 4.4 kg (almost 10 pounds) blocks 7 km (4 miles) from the vent. Why this explosion, and 2-3 others of similar age they found, were so large and spread tephra southeast of the caldera was a geologic mystery waiting to be unraveled. Charcoal found in the deposits, dated by the radiocarbon (14C) method, showed that the big explosion took place sometime in A.D. 800-1000.

The three geologists also discovered that running water had eroded deeply into older parts of the Keanakako`i Ash before younger tephra fell-curious, if the ash had really been deposited during one eruption or a brief series of eruptions. They soon found evidence for multiple eruptions instead: incipient soil horizons, water-cut erosional surfaces, buried Hawaiian structures built between explosions, and layers of pure ash interbedded in sand dunes.

How much time was recorded by these features? Charcoal to the rescue! In many places, the deposits yielded charcoal that testified to fires during the series of Keanakako`i eruptions. Tens of radiocarbon ages from the charcoal now tell a complex, as yet incomplete, story of multiple explosions between about 1490 and the early 1800s.

Armed with McPhie's results and the fact that the Keanakako`i was not just a one-shot 1790 event, the three geologists started to examine the southeast-directed deposits in some detail. During the past year, four, possibly five, tephra beds have been traced into areas northeast to southeast of Kīlauea's summit, completely at odds with distribution by trade winds. Volcanic ash fell as far away as Kalapana (and beyond the coastline). Another deposit covers Fern Forest, Eden Roc, and areas still farther northeast.

Such ash deposits from Kīlauea's summit demand westerly winds. Radiosonde data from weather balloons released twice daily by NOAA scientists in Hilo show the nearly ubiquitous presence of westerlies a few kilometers above the island. Often the base of the westerlies-called the jet stream-is at an elevation of about 5 km (16,000 feet), and the top, about 18 km (60,000 feet). Volcanic ash erupted to such elevations would blow eastward from the volcano, fanning northeast to southeast, depending on the precise direction of the wind. If ash went high into, or above, the westerlies, the extreme wind velocities--often more than 80 knots and sometimes more than 120 knots--would carry falling ash far downwind before it fell to the ground.

Next week's Volcano Watch provides a map showing where several tephra deposits fell and explores some of the consequences of powerful explosions from Kīlauea. For more information about the explosions, see the feature story on https://www.usgs.gov/observatories/hawaiian-volcano-observatory.

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Volcano Activity Update

During the past week, the number of earthquakes located beneath Kīlauea remains at levels typical of the current eruption. Inflation of the summit continues.

Eruptive activity at Pu`u `O`o also continues. On clear nights, glow is visible from several vents within the crater and on the southwest side of the cone. Lava is still flowing through the PKK lava tube from its source on the flank of Pu`u `O`o to the ocean, with scattered surface flows breaking out of the tube. Surface flows on the pali are visible at night (weather permitting) from the end of Chain of Craters Road.

As of January 19, lava is entering the ocean at East Lae`apuki, in Hawai`i Volcanoes National Park. The active lava bench continues to regrow following the major collapse of November 28. Access to the ocean entry and the surrounding area remains closed, due to significant hazards. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

There were two earthquakes beneath Hawai`i Island reported felt within the past week. A magnitude-3.6 earthquake occurred at 11:04 p.m. on Saturday, January 14, and was located 7 km (5 miles) southeast of Hualālai summit at a depth of 28 km (17 miles). A magnitude-4.7 earthquake occurred at 4:05 p.m. on Wednesday, January 18, and was located 16 km (10 miles) offshore and east of Na`alehu at a depth of 40 km (25 miles).

Mauna Loa is not erupting. During the past week, the count of earthquakes located beneath the volcano remains at low levels. Inflation continues, but at a slower rate since early October 2005.