Basaltic Plinian eruptions at Las Sierras-Masaya volcano driven by cool storage of crystal-rich magmas

Published in Earth & Environment
Basaltic Plinian eruptions at Las Sierras-Masaya volcano driven by cool storage of crystal-rich magmas

 Volcanic eruptions have significant impacts on communities living within volcanic regions. Highly explosive volcanic eruptions can produce columns of gas and ash which reach 10s of km in height, injecting hazardous gases into the atmosphere and producing dangerous pyroclastic density currents and ash fall. Such large eruptions are termed ‘Plinian’ in nature, named after Pliny the Younger, who described the highly explosive 79 AD eruption of Vesuvius, Italy. Understanding the driving mechanisms which produce highly explosive and hazardous eruptions provides vital information for hazard assessments.

 Plinian eruptions are generally produced by silicic magmas rich in SiO2, where their composition produces a highly viscous magma that is resistant to flow and susceptible to fragmentation during magma ascent. Magma fragmentation during ascent is the process which transforms the magmatic mixture (liquid phase with entrained gas bubbles) into a gas phase which contains particles of the fragmented magma. It is therefore magma fragmentation which leads to an explosive eruption. Although rare in comparison, volcanic systems which produce lower viscosity basaltic magma have also produced highly explosive eruptions. For the 1886 AD eruption of Mt. Tarawera, New Zealand, eye-witness accounts recorded the development of the eruption and its impact. However, it is still unclear as to which conditions lead to the fragmentation and explosive eruption of low viscosity basaltic magma.

 Masaya is a basaltic volcano located in western Nicaragua, approximately 25 km away from the capital city of Managua. Today, Masaya is characterised by a lava lake at its summit and passively emits large amounts of volcanic gases (Figure 1) which cause significant damage to nearby coffee and fruit growing regions. However, Masaya also has a highly explosive past, producing several explosive eruptions which deposited ash over a large area, including the present-day location of Managua. Investigating the cause of the explosive transition at Masaya is therefore important for understanding the eruptive history of the volcano.

Figure 1: A photo of Masaya volcano, showing a degassing plume. Photo: Fabio Arzilli.
Figure 1: A photo of Masaya volcano, showing a degassing plume. Photo: Fabio Arzilli.

 In our study published in Communications Earth and Environment, we combined the analysis of natural samples from the Las Sierras-Masaya volcanic system with numerical modelling techniques, to investigate the likely conditions which resulted in two historical Plinian eruptions of this volcano, the Fontana Lapilli and Masaya Triple Layer eruptions, which occurred approximately 60,000 and 2,000 years ago respectively. As these volcanic eruptions occurred in the past, we can use the deposit left behind by the event (Figure 2) such as lapilli and ash (known collectively as tephra) to reconstruct the conditions which occurred prior to and during the eruption. 

Figure 2: A photo of the deposit produced by the highly explosive Fontana Lapilli eruption of Las Sierras-Masaya volcanic system. Photo: Fabio Arzilli.

 Crystals erupted and preserved in tephra contain information that can be used to reconstruct the processes which occur at depth within the magma reservoir. Tiny pockets of melt called melt inclusions are trapped within crystals during crystallisation, also trapping gases dissolved in the melt such as H2O and CO2, which can be used to investigate the pre-eruptive gas content of a historical volcanic eruption. In our study, we analysed the geochemistry and gas content of melt inclusions in samples of the Masaya eruptions, to reconstruct the pre-eruptive conditions of magma storage within the crust, such as temperature and pressure. In addition to providing a record of the magmatic conditions at depth, crystals also form during magma ascent. Crystallisation during ascent can cause significant changes to magma properties during ascent and may lead to its fragmentation and explosive eruption.

 We used the information obtained from natural samples in a numerical model, which can simulate the magma ascent path from the reservoir to the surface. Numerical models are a valuable tool which can be used to investigate the processes occurring at depth within the crust which we cannot observe. Numerical models are also holistic and allow us to account for and combine all the processes which influence the magma ascent path and the likelihood of an explosive eruption, such as crystallisation and degassing. Using our data on the Plinian eruptions of Masaya, we find that lower temperature magmas with higher amounts of crystals are more likely to produce a highly explosive eruption. Evidence for storage of these cooling, crystal-rich magmas in the crust was preserved in tephra samples, as enclaves which extracted cool, crystalline portions of the magma reservoir and were then erupted at the surface. The presence of crystalline enclaves in samples indicates that the magma body at depth contained a mixture of liquid and crystals, termed a ‘mush’. The presence of the mush provides important information on the nature of magma storage at depth and the potential triggering mechanism of an eruption.

 By comparing our results from Masaya with other volcanic systems, we find that pre-eruptive temperature and crystal content are important controls on the eruptive style of basaltic volcanoes. Cooling, crystal-rich basaltic magma bodies stored at shallow depth within the crust may therefore be susceptible to producing a future highly explosive eruption. Natural samples allow us to investigate the eruptive history of a volcano, and when combined with numerical models which can simulate volcanic processes, can be used to reconstruct past eruptions, providing vital information for hazard assessments today.

 Further information can be found in our paper ‘Basaltic Plinian eruptions at Las Sierras-Masaya volcano driven by cool storage of crystal-rich magmas’ at the link

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