Volcano Watch: A fresh look at Kīlauea’s 1924 explosive deposits

“Volcano Watch” is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates. This week’s article was written by HVO geologist Kendra J. Lynn and University of Hawaiʻi at Mānoa student Reed Mershon.

Last week’s “Volcano Watch” summarized the explosive 1924 eruptions of Kīlauea volcano on the Big Island and their impacts on communities. This week, we dig a little deeper and share new discoveries we are making by examining the ash deposited during these events.

A few years ago, Hawaiian Volcano Observatory geologists began studying the 1924 explosive deposits by conducting detailed field and laboratory studies. Around Halema‘uma‘u, we sampled and described these ash layers that had lain largely undisturbed throughout the past 100 years.

During the 1924 eruption, ash fell as far away as Pāhala; today, it is only preserved within about 2 miles of Halema‘uma‘u. It is thickest in the downwind direction (to the southwest), ranging from about 3 feet to several inches thick. Blocks were also ejected during the eruption and weigh up to 8 tons.

In the lab, we studied the samples of ash. We examined 200 grains ranging from 0.2 to 0.4 of an inch in size; each grain was classified according to its rock or mineral type. Typical components include older, “recycled” lavas, called lithic material, and fresh magma, or juvenile material.

Most of the 1924 ash layers we’ve studied have 95% or more lithic (recycled) material. This finding supports the classic interpretation that the 1924 eruptions were driven by water-rock interactions called phreatic explosions.

A surprising recent discovery was that many of the youngest layers in the 1924 deposits, from the later explosions, have up to 30% juvenile material, or fresh magma!

This finding is not consistent with the classic interpretation of steam driven explosions.

To learn more about the magma involved in the 1924 explosions, Hawaiian Volcano Observatory scientists have been collaborating with colleagues at the University of Hawai‘i at Mānoa. We have used a range of analytical techniques to study the compositions and textures of the 1924 juvenile material.

There are a few separate ash groups, distinct in their chemistry and textures.

To distinguish the different ash groups based on chemistry, geochemists use the magnesium oxide (MgO) content: the amount of MgO decreases as the magma cools, so we can use it as an analogue for temperature.

Almost like a chemical fingerprint of the history of the magma.

Most of the 1924 grains we looked at have MgO contents within the normal range we expect for lava erupted from Halema‘uma‘u. However, we’ve also observed two rarer groups of 1924 grains with higher amounts of MgO, likely from a hotter source material.

This suggests fresh batches of magma could have entered the magmatic system of Kīlauea during the 1924 explosions.

The different chemical groups of 1924 grains also have distinct textures, which we can see using a scanning electron microscope.

The lower-MgO group have lots of tiny crystals and very few vesicles (gas bubbles) in them. The middle-MgO group has few crystals and many vesicles that are ovals or other shapes indicating the once round bubbles were squished. The high-MgO group has no small crystals and circular vesicles.

These chemical and textural differences in the 1924 deposits show that three magma types can be distinguished in the 1924 explosions. From this, we can infer that at least three different magmas were interacting underneath Halema‘uma‘u prior to and/or during the 1924 explosive eruptions, and perhaps the mixing of these magmas could help explain why the eruptions were so explosive.

We also found olivine crystals, the very common green mineral you find in Hawaiian rocks, in the juvenile component of the 1924 eruptions. The olivine chemistry and textures vary widely, indicating multiple groups of minerals with different histories prior to eruption.

Many of the olivine crystals are zoned, with different chemistry in their centers compared to their rims, indicating that magmas were mixing just prior to eruption. There is much more to be learned by studying the olivine crystals, and Hawaiian Volcano Observatory scientists are hard at work probing their secrets.

One hundred years have passed since the 1924 explosive eruptions at Kīlauea. However, we have only begun to scratch the surface of what we can learn from the deposits of these explosions.

How did the magmas interact with each other? How long did they sit waiting in magma reservoirs and what happened to cause the explosions? We hope to answer these questions with our continued research.

A new video provides audio of Thomas Jaggar describing the 1924 explosive eruption of Kīlauea, with historical photos: “100 years ago at Kīlauea: The 1924 explosive eruption described by Thomas Jaggar.”

Volcano Activity Updates

Kīlauea is not erupting. Its USGS Volcano Alert level is Advisory.

Unrest that began April 27 continues beneath the upper East Rift Zone and the summit caldera south of Halemaʻumaʻu. During the past week, activity decreased slightly compared to the previous week.

Less than 200 events were detected per day, most magnitude-2 and smaller; depths remain concentrated between 1.2 and 3.1 miles beneath the surface.

Tiltmeters near Sand Hill and Uēkahuna bluff continued to record inflationary trends. Kīlauea’s summit region is pressurized, and changes could occur quickly moving forward.

See the information statement published May 2 for background information.

Mauna Loa is not erupting. Its USGS Volcano Alert Level is at Normal.

Webcams show no signs of activity on Mauna Loa. Summit seismicity has remained at low levels during the past month. Ground deformation indicates continuing slow inflation as magma replenishes the reservoir system following the 2022 eruption. SO2 emission rates are at background levels.

Four earthquakes were reported felt in the Hawaiian Islands during the past week:

• A magnitude-3.8 earthquake 3 miles south-southwest of Pāhala at a depth of 20 miles at 3:25 a.m. May 16.
• A magnitude-3.4 earthquake 8 miles east of Pāhala at a depth of 17 miles at 6:39 p.m. May 14.
• A magnitude-1.8 earthquake 0 miles north of Pāhala at a depth of 18 miles at 8:11 p.m. May 11.
• A magnitude-3.5 earthquake 13 miles south of Wai‘ōhinu at a depth of 4 miles at 7:59 p.m. May 11.

The Hawaiian Volcano Observatory continues to closely monitor Kīlauea and Mauna Loa.

Visit the observatory’s website for past “Volcano Watch” articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information and more. Email questions to askHVO@usgs.gov.