Date of Award
Dr. Marc-Antoine Longpré
Highly explosive, Plinian-type eruptions of basaltic magma are enigmatic because basaltic melts have lower viscosities than more silicic melts which have traditionally been considered essential for such explosive eruption style. Masaya volcano, Nicaragua, is one of the few basaltic volcanoes to have generated a wide range of eruptive styles, from effusive lava lake activity to voluminous Plinian fall deposits. This volcano thus offers a unique case study to better constrain ideas on basaltic eruption style controls.
We analyze olivine-, plagioclase- and clinopyroxene-hosted melt inclusions as well as matrix glasses from lava lake (LL) ejecta and two Plinian tephra deposits—the 2.1 ka Masaya Triple Layer (MTL) and the 1.9 ka Ticuantepe Lapilli (TIL)—for volatiles (CO2, H2O, S, Cl, F), major, and trace elements, to test whether pre-eruptive volatile contents and degassing history may be linked to eruptive style. All samples display a relatively narrow and largely overlapping basaltic–basaltic andesitic compositional range (50–54 wt.% SiO2, 3–6 wt.% MgO). However, lava lake and Plinian samples show systematic differences in pre-eruptive volatile contents, forming distinct groups with mean H2O contents of 0.6 wt.% in LL, 1.1 wt.% in MTL, and 1.9 wt.% in TIL. Together, these groups generate broad positive correlations between S, Cl and H2O concentrations, with maximum values reaching 920 ppm, 1300 ppm and 2.3 wt.%, respectively, which are low compared to typical Central American arc magmas. Overall, calculated Psat are low, mainly less than 100 MPa, and temperature estimates, 1020–1170°C, suggest melt viscosities of 102–103 Pa·s. These observations, coupled with evidence for Cl exsolution during second-boiling, demonstrate that regardless of eruption style, all Masaya magmas undergo variable, but extensive, pre-eruptive degassing in a shallow (less than 4 km) reservoir.
Pre-eruptive volatile contents are not the culprit for explosivity at Masaya. Our results instead suggest a top-down control on eruptive style, whereby temporary sealing of the conduit may instigate a transition to explosive behavior. When the seal eventually breaks from degassing-induced pressurization, rapid magma ascent could yield high degrees of undercooling, triggering rapid microlite growth, increasing viscosity and explosive potential. There may thus be a thin line between open-conduit conditions and Plinian eruptions at Masaya.
Hlinka, Lisa, "Top–Down Control on Eruptive Style at Masaya Volcano Inferred from Melt Composition" (2020). CUNY Academic Works.