TY - JOUR

T1 - Quantifying the Water‐to‐Melt Mass Ratio and Its Impact on Eruption Plumes During Explosive Hydromagmatic Eruptions

AU - Hajimirza, Sahand

AU - Jones, Thomas J.

AU - Moreland, William M.

AU - Gonnermann, Helge M.

AU - Thordarson, Thor

PY - 2022/5/31

Y1 - 2022/5/31

N2 - The interaction of magma with external water commonly enhances magma fragmentation through the conversion of thermal to mechanical energy and results in an increased production of fine-grained volcanic tephra. Magma-water interaction is thus of importance for hazard mitigation on both a local and a regional scales. The relative proportion of water that interacts with magma, quantified as the water-to-melt mass ratio, is thought to determine the efficiency of thermal to mechanical energy conversion, termed the fragmentation efficiency. Here, we analyze the pyroclast size distributions from the 10th century Eldgjá fissure eruption in Iceland, where parts of the fissure erupted subglacially and other erupted subaerially. The subglacially erupted magma passed through a column of glacial meltwater, resulting in a larger proportion of finer pyroclast sizes relative to the subaerially erupted, purely magmatic tephra. This finer grain size distribution has been attributed to quench-granulation induced by enhanced cooling upon interaction with external water. We hypothesize that the additional fragmentation (surface) energy required to produce the finer grained hydromagmatic deposits is due to the conversion of thermal to mechanical energy associated with the entrainment of water into the volcanic jet, as it passed through a column of subglacial melt water. Based on field and granulometry data, we estimate that the interaction of the volcanic jet with the meltwater provided an additional fragmentation energy of approximately 3–14 kJ per kg of pyroclasts. We numerically model the hydrofragmentation energy within a jet that passes through a layer of meltwater. We find that the water-to-melt mass ratio of entrained water required to produce the additional fragmentation energy is in the range of 1–2, which requires a minimum ice melting rate of 104 m3 s−1. Our simulation results show that the water-to-melt ratio is an important parameter that controls the ascent of plume in the atmosphere.

AB - The interaction of magma with external water commonly enhances magma fragmentation through the conversion of thermal to mechanical energy and results in an increased production of fine-grained volcanic tephra. Magma-water interaction is thus of importance for hazard mitigation on both a local and a regional scales. The relative proportion of water that interacts with magma, quantified as the water-to-melt mass ratio, is thought to determine the efficiency of thermal to mechanical energy conversion, termed the fragmentation efficiency. Here, we analyze the pyroclast size distributions from the 10th century Eldgjá fissure eruption in Iceland, where parts of the fissure erupted subglacially and other erupted subaerially. The subglacially erupted magma passed through a column of glacial meltwater, resulting in a larger proportion of finer pyroclast sizes relative to the subaerially erupted, purely magmatic tephra. This finer grain size distribution has been attributed to quench-granulation induced by enhanced cooling upon interaction with external water. We hypothesize that the additional fragmentation (surface) energy required to produce the finer grained hydromagmatic deposits is due to the conversion of thermal to mechanical energy associated with the entrainment of water into the volcanic jet, as it passed through a column of subglacial melt water. Based on field and granulometry data, we estimate that the interaction of the volcanic jet with the meltwater provided an additional fragmentation energy of approximately 3–14 kJ per kg of pyroclasts. We numerically model the hydrofragmentation energy within a jet that passes through a layer of meltwater. We find that the water-to-melt mass ratio of entrained water required to produce the additional fragmentation energy is in the range of 1–2, which requires a minimum ice melting rate of 104 m3 s−1. Our simulation results show that the water-to-melt ratio is an important parameter that controls the ascent of plume in the atmosphere.

KW - hydrofragmentation

KW - subglacial eruptions

KW - Eldgja eruption

KW - water-to-melt mass ratio

KW - magma-water interaction

U2 - 10.1029/2021gc010160

DO - 10.1029/2021gc010160

M3 - Journal article

VL - 23

JO - Geochemistry, Geophysics, Geosystems

JF - Geochemistry, Geophysics, Geosystems

SN - 1525-2027

IS - 5

M1 - e2021GC010160

ER -