Influence of magma-water interactions on eruptive products at Poás volcano, Costa Rica

Abstract:

Magma-water interactions greatly influence the style of volcanic eruptions, from smaller phreatic eruptions deprived of juvenile magma, to phreato-plinian eruptions that are more explosive than their ‘water-free’ counterparts. Interactions with water seem to decrease the size of pyroclasts produced upon magma fragmentation, which can have an important impact on ash monitoring, dispersion modeling, and hazard assessment. The influence of a hydrothermal system on the eruptive behavior of a volcano is thus critical and is the topic of this study.

Poás volcano is located in the Central Cordillera of Costa Rica, and is characterized by continuous degassing as well as episodes of Strombolian to Vulcanian eruptions (e.g., 1910 and 1955) and sporadic periods of phreatic to phreatomagmatic eruptions (e.g., 1834, 1953, 2006). Poás volcano has a hyper-acid crater lake that is the surficial expression of a well-developed hydrothermal system. In 2017, a new eruptive episode initiated with changes in gas composition and an increase in seismic activity. Since 2017, Poás experienced phreatic, phreatomagmatic, and purely magmatic eruptive phases, which were carefully monitored (e.g. seismicity, observations, gas composition) and frequently sampled, mostly at a single location, making this sequence of events well suited to study the influence of water-magma interactions on eruptive products.

Here, we analyze the size and shape distributions of ash from phreatic, magmatic, and phreatomagmatic events, classified based on rapid componentry analyses of the deposits. For each event, we quantify the grain size distribution of ash particles down to ~ 1 micron and the shape distributions of all grains larger than ~ 100 microns, using particle size analyzers based on imagery and X-ray diffraction. Preliminary results show a smaller grain size of ash produced by phreatic eruptions compared with that produced by magmatic explosions. The grain size distribution of phreatomagmatic deposits is more variable. Variations in ash shape parameters such as solidity, convexity and aspect ratio between the different types of events, combined with grain size distribution differences, could help us to understand how the presence of a hydrothermal system and magma-water interactions modify the mechanism of magma fragmentation.