|A tephra jet consisting of steam, hot water, black tephra, and molten fragments blasted into the air. USGS photo|
TEPHRA FALLING TO THE GROUND FROM VOLCANIC PLUMES is the subject of this week's Volcano Watch, the column by USGS Hawaiian Volcano Observatory scientists and affiliates. This column is written by Joanne Schmith, visiting scientist from the Carlsberg Foundation who wrote:
I think the Island of Hawaiʻi is one of the most magical places on Earth. You can literally see land in the making when one of the volcanoes is erupting. But the familiar sight of red-glowing lava flows makes it hard to imagine a time when eruptions were different, and to think that such a time may come again someday.
I go to the summit of Kīlauea most weeks to study the extent, thickness, and physical characteristics of a 400–500-year-old tephra deposit, the product of explosive eruptions and part of what is called the Keanakākoʻi Tephra. The total deposit is up to 10 meters (30 feet) thick along the southern wall of Kīlauea caldera, but it was created by fragment upon fragment of tephra falling to the ground from volcanic plumes rising out of the caldera.
| Tephra from unit D in Kīlauea south caldera wall; the lower portion of the tephra|
deposit has been smoothed with a scraper tool, making the surface look
different from the portion above. a) finely laminated ash layers as well as a fine ash
layer and a coarser layer with pumice grains, b) microscope photo of a dense, glassy ash
grain from the fine lamination layers, c) microscope photoof a small pumice grain
from the coarser layer, and d) well-preserved example of a fine ash
layer with accretionary lapilli. USGS photos by J. Schmith
The tephra deposit has many stories to tell. The lowest and therefore the oldest part contains thinly laminated coarse and fine layers of volcanic ash (particles up to 2 mm or 0.08 inches in size). The thin laminations tell me explosions were small, but frequent. I visualize a cone in the caldera that expels small plumes of ash only minutes apart with loud blasts.
I look closer at the tiny individual fragments that make up the layers. They are glassy and dense with straight sides. This is common for ash generated by explosions where hot magma comes into contact with water at the surface of the volcano. So maybe the vent was submerged under a crater lake? I visualize the cone barely poking above the water surface.
Above the laminations the deposit changes. Several centimeters (a couple of inches) of fine ash that clumps together in 3–5 mm (0.12–0.20 inches) spheres—accretionary lapilli! The finer size of the ash suggests that the explosions got more intense, tearing the magma apart more efficiently, and the clumping is often associated with wet air. I visualize a denser plume rising from the vent with a louder blast. Does the water vapor come from the crater lake that I was imagining? Or was it raining?
If I had been at this very spot at the time of the explosions, the air would have been thick with ash. Breathing would have been difficult, if not impossible. Later, the fine ash would have been picked up by the wind, scratching my eyes and irritating my nose and lungs. Little time elapsed between the smaller explosions and these more intense ones, because all the fine layers are intact and have not been disturbed by wind erosion, which would likely happen within hours or a few days at Kīlauea summit.
|Tephra deposits as thick as 11 m (36 ft) at summit of Kīlauea|
Volcano were erupted during an extended period that
included many explosive eruptions between about
1500 and 1800. USGS image
Pumice was long thought to only come out of highly explosive volcanoes like Mount St. Helens—but here it is at Kīlauea! Pumice grains are pieces of broken magma foam that forms by intensive bubbling of magmatic gas before eruption. Pumice is very different in nature from those dense fragments that make up the rest of the deposit, so why is it here?
Pumice can be generated in lava fountains, so maybe a fissure opened outside the crater lake? But then why are the dense grains getting larger at the same time? Was the magma rising faster through the water, resulting in less time for interaction? Or did Kīlauea supply more magma to the eruption generating a higher plume? I know I can’t answer those questions from this one location alone. I need to see how the different layers are distributed in the area around Kīlauea caldera.
So, after measuring, sampling, sketching, and making notes about the Keanakākoʻi Tephra deposit at this site, I continue my journey to other locations hoping to unlock more secrets of Kīlauea’s explosive past.
Volcano Activity Updates: Kīlauea volcano is erupting. Its USGS Volcano Alert level is at WATCH (https://www.usgs.gov/natural-hazards/volcano-hazards/about-alert-levels). Kīlauea updates are issued daily. Over the past week, lava has continued to erupt from the western vent within Halemaʻumaʻu crater. All lava is confined within Halemaʻumaʻu crater in Hawai‘i Volcanoes National Park. Sulfur dioxide emission rates remain elevated and were last measured at approximately 3,200 tonnes per day (t/d) on April 6, 2022. Seismicity is elevated but stable, with few earthquakes and ongoing volcanic tremor. Summit tiltmeters show several minor deflation and inflation trends over the past week. For more information on the current eruption of Kīlauea, see https://www.usgs.gov/volcanoes/kilauea/recent-eruption.
|Tephra, including rock fragments, tiny spheres,|
and shards of volcanic glass, that was erupted from
Halema‘uma‘u Crater,Kīlauea Volcano, Hawai‘i.
This past week, about 67 small-magnitude earthquakes were recorded below the summit and upper elevation flanks of Mauna Loa—the majority of these occurred at shallow depths less than 15 kilometers (9 miles) below sea level. Global Positioning System (GPS) measurements show low rates of ground deformation over the past week. Gas concentrations and fumarole temperatures at both the summit and at Sulphur Cone on the Southwest Rift Zone have remained stable over the past week. Webcams show no changes to the landscape. For more information on current monitoring of Mauna Loa, see: https://www.usgs.gov/volcanoes/mauna-loa/monitoring.
No felt earthquakes were reported in the Hawaiian Islands during the past week.
HVO continues to closely monitor Kīlauea’s ongoing eruption and Mauna Loa for any signs of increased activity.
Visit HVO’s website for past Volcano Watch articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake info, and more. Email questions to askHVO@usgs.gov.
CONTINUED USE OF POHAKULOA FOR MILITARY TRAINING is the subject of a call for public comment. The U.S. Army put out the request on Friday, with a commentary period running until June 7. The Army's Environmental Impact Statement can be read and comments submitted at tinyurl.com/mrf2a
The 65-year lease on 23,000 acres owned by State of Hawai'i, ends in 2029. It is part of 132,000 acrE used for live firing and maneuvers training. It is located in the interior of the island between Mauna Loa, Maunakea and Hualalai Volcano
A statement from the U.S. Army says, "Public involvement is a fundamental part of the National Environmental Policy Act (NEPA) and Hawai‘i Environmental Policy Act (HEPA) Environmental Impact Statement (EIS) process. The NEPA-HEPA EIS process ensures environmental concerns are given appropriate consideration in decision-making, along with economic and technical considerations. The Army wants and appreciates the community's participation in the EIS process for the Army Training Land Retention (ATLR) at Pōhakuloa Training Area (PTA) EIS."
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