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<p>Maar eruptions are driven by the explosive interaction between magma and water. The pyroclastic material produced by these eruptions is a valuable resource for interpreting the eruption style, fragmentation dynamics, and deposition mechanisms. However, in order to best understand these deposits from unwitnessed eruptions, it is important to connect their characteristics to observations from witnessed or partially witnessed eruptions. Ukinrek Maars erupted over 11 days in March and April of 1977, making it one of four maars in recent history to be at least partially witnessed during its eruption. The eruption produced one short-lived fissure and two maar craters, generating over the course of the eruption a thick tephra blanket comprising material derived from magmatic, phreatomagmatic, and hybrid fragmentation styles. We present detailed ash characterization of 17 samples from these deposits, including grain size distribution, componentry, internal textures, surface features, and morphometry, based on a standardized method carried out on three specific size fractions (−2, +1, +4&#xa0;φ). From these data, we refine the existing understanding of fragmentation mechanisms over the course of the eruption. We use these well-constrained layers to compare ash characteristics across fragmentation styles and determine which features can best be used to differentiate fragmentation processes. We find that phreatomagmatic layers at Ukinrek are defined by high concentration of fine ash, high lithic content, blocky juvenile clasts, and high abundance of&#xa0;interactive particles. Phreatomagmatic juvenile particles differ from their magmatic counterparts in several morphometric parameters; overall, they are smoother and less complex particles. While this fits with the general findings from other studies, this study demonstrates the importance of the quantification of the features and the standardization of the measurements. Even though some size fractions capture the variance in individual features better than others, it is best to analyze all three size fractions called for in the standardized method in order to capture all the nuanced differences between layers and styles. These results offer a framework for identifying fragmentation mechanism built on some of the best-characterized maar deposits globally and can be used to better interpret the deposits of non-witnessed eruptions.</p>

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Phreatomagmatic fragmentation signature in tephra deposits from Ukinrek Maars, Alaska

  • 期刊论文

  • 发表日期：

    2026年7月8日

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  • Sophia Leiter,
  • Pierre-Simon Ross,
  • Michael H. Ort

摘要

Maar eruptions are driven by the explosive interaction between magma and water. The pyroclastic material produced by these eruptions is a valuable resource for interpreting the eruption style, fragmentation dynamics, and deposition mechanisms. However, in order to best understand these deposits from unwitnessed eruptions, it is important to connect their characteristics to observations from witnessed or partially witnessed eruptions. Ukinrek Maars erupted over 11 days in March and April of 1977, making it one of four maars in recent history to be at least partially witnessed during its eruption. The eruption produced one short-lived fissure and two maar craters, generating over the course of the eruption a thick tephra blanket comprising material derived from magmatic, phreatomagmatic, and hybrid fragmentation styles. We present detailed ash characterization of 17 samples from these deposits, including grain size distribution, componentry, internal textures, surface features, and morphometry, based on a standardized method carried out on three specific size fractions (−2, +1, +4 φ). From these data, we refine the existing understanding of fragmentation mechanisms over the course of the eruption. We use these well-constrained layers to compare ash characteristics across fragmentation styles and determine which features can best be used to differentiate fragmentation processes. We find that phreatomagmatic layers at Ukinrek are defined by high concentration of fine ash, high lithic content, blocky juvenile clasts, and high abundance of interactive particles. Phreatomagmatic juvenile particles differ from their magmatic counterparts in several morphometric parameters; overall, they are smoother and less complex particles. While this fits with the general findings from other studies, this study demonstrates the importance of the quantification of the features and the standardization of the measurements. Even though some size fractions capture the variance in individual features better than others, it is best to analyze all three size fractions called for in the standardized method in order to capture all the nuanced differences between layers and styles. These results offer a framework for identifying fragmentation mechanism built on some of the best-characterized maar deposits globally and can be used to better interpret the deposits of non-witnessed eruptions.