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Classroom demonstrations of volcanic processes are typically performed with low-temperature materials designed to exhibit behaviors conceptually analogous to magma. However, there are advantages to finding methods by which real magma can be created and used with audiences and students. Here, we describe a method by which magma can be made at home or in a classroom using a standard household microwave. We have tested a particular demonstration of how volatiles such as H2O exsolve from natural obsidian to form pumice and ash, a direct replica of the volcanic phenomenon that drives eruptions on Earth and other planets. To demonstrate this, we used three obsidian samples—from Mono Craters (U.S.A.), Arteni (Armenia), and Hrafntinnuhryggur (Iceland)—within an off-the-shelf ceramics microwave kiln in both a 700 and 1000 W microwave. We found that millimetric obsidian chips will vesiculate to form pumice-like textures at low microwave power or low initial H2O content and will fragment safely within the kiln at high microwave power or high initial H2O content. This can be used to supplement existing well-known demonstrations using low-temperature analogs for magma. The demonstration results in real products that can be examined just as a geologist would examine natural specimens. 

The Arteni Volcanic Complex lies within the Ararat Plain, within theAragats Volcanic Province (AVP) of western Armenia, and is noted for itsremarkable diversity of obsidian types. K-Ar dates show EarlyPleistocene ages of 1.26 to 1.60 Ma, while fission track dating yieldsslightly younger ages of 1.17 to 1.38 Ma. The Arteni Volcanic Complexwas formed through successive eruptions from two adjacent centers: MetsArteni and Pokr Arteni, both dome-shaped rhyolitic volcanoes. Thecomplex consists of rhyolitic domes, dome-coulees, necks, fissures,extrusions, and dikes—typical of high-viscosity silicic volcanism. The Arteni rhyolitic lava flow extends a remarkable 8 km from the summitarea of Mets Arteni. Covering an area of ~15 km², itsvolume is estimated at 0.6 km³. The unit consists of a basal layer ofpyroclastics and crystalline rhyolitic-dacitic rocks, followed bycrystal-clear obsidian, and capped by vesicular water-rich perlite. Theobsidian typically contains between 0.06 and 0.2 wt% H₂O, while theoverlying perlite contains up to 4 wt% H₂O. Although rhyolitic melts are usually highly viscous, the Arteni flow’sexcessive length suggests lower viscosity due to temperature,composition and water content during emplacement. Consequently, wemodeled viscosity using 850-970 °C obtained from similar rheologicalcalculations for the Dry Fountain obsidian complex located 22 km northof Yerevan, Armenia. We note that the upper perlite layers of theAragats flow contain 3.5–4 wt% H₂O. If the underlying obsidian formedby rapid degassing under lithostatic pressure applied by the overlyingperlite, the original melt likely had a water content of~3.8 wt%. For this water content and temperature, theviscosity of Arteni rhyolite was 3.86–4.79 log η (Pa·s), much lowerthan anhydrous obsidian (7.45–9.18 log η Pa·s). We suggest that thislow viscosity led to the exceptional length of the rhyolite flow due todepolymerization of melt structure as well as lack of phenocrysts. 

Ash-producing volcanic eruptions are a major natural hazard. The explosivity of these eruptions has been described for decades by a semi-quantitative “Volcanic Explosivity Index (VEI)” based on a number of interdependent factors (eruption duration, volume of ejected material, eruptive column height, tropospheric and stratospheric injection, and many qualitative descriptors), most of which could only be assessed after eruption had ceased. The utility of this semi-quantitative index has been questioned since then, yet its common usage persists. We suggest that if VEI is to be used in the future, it may as well be an index that can be determined in real time during an eruption, and one that strictly reflects eruption intensity. We find that the same value for the “traditional” Volcanic Explosivity Index of past eruptions can be estimated in real time during an eruption, based solely on eruption column height. This approach is not intended to replace the traditional VEI scale or supplant more detailed mechanistic approaches to mass eruption rate (MER); rather it can be used to provide a more immediate way of calculating the VEI and linking to MER, in real time while an eruption is taking place. As such, if the easily observed height can be measured, both MER and VEI can be estimated. This may facilitate emergency response by aviation, hazard management, and others to mitigate societal impact of hazardous eruptions. This scheme can be inverted to use volume-based VEI of ancient eruptions to roughly retrodict unobserved column height for key historic and pre-historic eruptions. Further, ash particle morphology reflects explosivity in terms of decompression rate at the vent of unobserved eruptions. With sufficiently rapid decompression, this could result in an observable bi-modal bubble size distribution. The morphology of ash particles reveals a threshold between VEI 3 and 4, above which there is sufficiently rapid decompression to trigger a second nucleation event near the vent. 

AGU poster by undergraduate student working in project group 

Graduate student presentation at AGU 

Poster presented by undergraduate student within our project research group 

Keynote lecture presented the Armenian National Academy of Sciences Conference commemorating its 90th anniversary. 

Lava flows have presented the greatest hazard to human property during the most recent eruptions of Hawaiian volcanoes, and lava fountains are a source of these lava flows. The height of Hawaiian lava fountains reflects the exsolved gas content of the magma that controls eruption intensity. However, fountain height is not always observed, so we sought a proxy to estimate fountain heights of eruptions that were older or otherwise unobserved. Here, methods are described to empirically derive a relationship between the modal diameter of vesicles within Pele’s tears and spheres and lava fountain height, using samples of Pele’s tears produced during the last eruptions of Kīlauea Iki (1959) and Mauna Ulu (1969). The tears used to develop these relationships were approximately 1 to 4 mm in diameter. Additionally, since lava fountains 50–580 m high were used, the relationships we describe may only describe lava fountains in this height range. The strongest empirical relation follows the trendline H_max= −2575d + 820, where H_maxis maximum lava fountain height and d is modal vesicle diameter. This empirical relationship may be applied to sub-Strombolian eruptions of tholeiite basalt that were not directly measured or observed to assess long-term shifts in lava fountain heights and thus the exsolved gas contents of a volcanic system. While the same conceptual framework can be applied beyond Hawai’i, the quantitative empirical relation may be slightly different in different systems, depending on total dissolved volatiles, magma chemistry and other factors.