Influence of Pumice Texture on Magma Decompression Rate Estimates Using Bubble Number Density: Application to the Largest Known Plinian Eruption of Cerro Machin Volcano, Colombia

Bubble size and number in pumice is related to the decompression rate of magma during syneruptive ascent. Rapid magma decompression leads to a high degree of volatile supersaturation in the melt, which encourages the nucleation of many small bubbles and produces pumice with a high bubble number density (BND). Conversely, slow magma decompression allows bubbles to nucleate and grow at a low degree of volatile supersaturation, thereby producing pumice with low BND. For this reason, BND in pumice has been developed as a tool for estimating magma decompression rates during volcanic eruptions [1,2]. We have applied this technique to lapilli-sized pumice clasts from a dacitic pyroclastic fall deposit of the 3600 yr. BP Plinian eruption of Cerro Machin Volcano, Colombia. Nine thin sections were analyzed, all containing the mineral assemblage plagioclase-amphibole-quartz-biotite-oxides. Three different textures were identified: 1) white porphyritic clasts that have many large (~0.1 mm) coalesced vesicles; 2) gray porphyritic clasts that have many broken crystal fragments and fewer large vesicles compared to the white clasts; and 3) banded clasts combining the previous two textures. The presence of broken crystals and the rarity of large vesicles in the gray clasts may be a result of intense shearing against the conduit walls during ascent, while the presence of unbroken phenocrysts and large coalesced vesicles in the white clasts is suggestive of ascent in the central portion of the conduit [3]. We used the FOAMS application [2] to constrain BNDs from backscattered electron images of the pumices, from which we could estimate magma decompression rate [1]. BNDs in the pumices are dominated by the smallest bubbles (0.001 - 0.01 mm), and decompression rate estimates are the same within error for all textures: 0.52 + 0.3 MPa/s for the white pumice clasts; 0.55 + 0.1 MPa/s for the gray clasts; and 0.72 + 0.3 MPa/s for the banded clasts [2]. Our magma decompression rate estimates for Cerro Machin's 3600 yr. BP deposit are similar to other highly explosive dacitic eruptions such as the 1912 eruption of Novarupta (VEI:6), highlighting the hazard potential of this system [4].

[1] Toramaru, 2006 [2] Shea et al., 2010 [3] Polacci et al., 2001 [4] Cassidy et al., 2018