Mafic Plinian eruptions: Is fast ascent required?

Abstract

It has been hypothesized that for a Plinian eruption of mafic magma to occur, that magma must ascend rapidly from the chamber to cause it to fragment into a jet containing juvenile and nonjuvenile tephra. To determine how fast mafic Plinian magmas need to travel to the level of fragmentation, a number of decompression experiments were carried out on two hydrous mafic magmas, and the results are compared to the products of two well‐documented mafic Plinian eruptions: the basaltic andesite Fontana eruption of Masaya (Nicaragua) and the hawaiite 122 B.C. eruption of Etna (Italy). Comparison of natural and experimental textures shows that the Fontana eruption can be replicated in the lab at decompression rates between 0.1 MPa s−1 and 0.2 MPa s−1. This decompression rate is faster than any previously determined experimentally rate for more silicic eruptions. The hawaiite was unable to be reproduced in the lab. The natural groundmass is highly crystalline, which would have raised the viscosity of the initial melt by 1–2 orders of magnitude, which may not be enough to cause fragmentation.

Magma fragmentation in highly explosive basaltic eruptions induced by rapid crystallization

“…2a-c), similar to the skeletal plagioclase and dendritic pyroxene crystals observed in the products of explosive basaltic Plinian eruptions [4][5][6][7][8][9][10] , for example, Etna 122 B.C. 4,5,9,10 (Fig. 2d).…”

“…1b) is observed to occur in ~180 s during our 4D experiments. These distinctive textures are also reported in products of the Fontana Lapilli (Nicaragua) [8][9][10] and 1886 Tarawera (New Zealand) eruptions 4,6,7 . Therefore, all the studied examples of basaltic Plinian deposits show features that are consistent with the textures produced in our experiments.…”

“…The large ∆T reached in a relatively short time during our in situ 4D crystallisation experiments produced distinctive skeletal plagioclase crystals with swallow-tail morphology and dendritic pyroxene ( Fig. 2a-c), similar to the skeletal plagioclase and dendritic pyroxene crystals observed in the products of explosive basaltic Plinian eruptions [4][5][6][7][8][9][10] , for example, Etna 122 B.C. 4,5,9,10 (Fig.…”

“…This process has been proposed to explain explosive basaltic Plinian eruptions, supported by evidence of high microlite contents in basaltic Plinian eruption products [4][5][6][7][8][9][10] . However, no mechanism for exceptionally fast crystallisation has been proposed.…”

“…4e). However, products erupted from basaltic Plinian eruptions are characterized by a small phenocryst content (<10 vol.%) 4,5,[7][8][9][10]30 . Therefore, our results highlight that it is the small τ (c) of syn-eruptive crystallisation that is the primary cause for an increase in the probability of magma fragmentation (Fig.…”