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

Last week’s “Volcano Watch” summarized Kīlauea’s explosive eruptions of 1924 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.

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. 

Color photograph of scientist examining ash deposit in the field
A USGS Hawaiian Volcano Observatory geologist examines layers of ash deposited during Kīlauea’s 1924 explosions south of the summit caldera. USGS photo by J. Chang.

A few years ago, USGS Hawaiian Volcano Observatory (HVO) 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 which had lain largely undisturbed over the past 100 years.

During the 1924 eruption, ash fell as far away as Pāhala; today, it is only preserved within about 2 miles (3 km) of Halema‘uma‘u. It is thickest in the downwind direction (to the southwest), ranging from about 3 feet (1 m) to several inches (a few centimeters) thick. Blocks were also ejected during the eruption and weigh up to 8 tons (8,000 kg).

In the lab, we studied the samples of ash. We examined 200 grains ranging 0.2–0.4 inches (0.5–1.0 mm) 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 (called 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, HVO 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 both in their chemistry and their 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 that 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 that the once round bubbles were squished. The high-MgO group has no small crystals and have 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 HVO scientists are hard at work probing their secrets.

100 years have passed since the 1924 explosive eruptions at Kīlauea. However, we have only begun to scratch the surface on 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.

On Monday, May 20 at 7 p.m., and Tuesday, May 21 at 3 p.m., join Don Swanson, HVO geologist emeritus, and Ben Gaddis, HVO volunteer, as they describe the 1924 explosive eruption of Kīlauea in presentations at the Lyman Museum in Hilo. Admission to Lyman Museum programs is free to Museum members, and $3.00 for nonmembers. See here for more information: https://lymanmuseum.org/events/

A newly available 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.

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