Recent seismic activity in the region of Katla has led researchers to believe it will erupt again. In order to understand the potential hazards of new eruptions, researchers need to understand how these eruptions historically behaved. In their new paper titled Large Explosive Basaltic Eruptions at Katla volcano, Iceland: Fragmentation, grain size and eruption dynamics, Schmith et al. (2018) investigated why this volcano is so explosive. In order to do this, they used size, shape, and composition of the ash fall deposits from the eruptions in the 17th and 18th century.
Katla, one of most hazardous volcanoes in Iceland, is located in the southeastern part of the country (Fig. 1 & 2). The volcano lies 1450 m above sea level, and the caldera covers an area of nearly 110 km2. Katla is topped by the 230 m thick Mýrdalsjökull glacier which covers an area of ~ 600 km2, which melts and causes lahars and flooding during eruptions. Katla has a history of major explosive eruptions that occur on average about every 50 years. These eruptions are sometimes large enough that ash falls on mainland Europe, including during the 1625 and 1755 CE eruptions.
The 2010 Eyjafjallajökull eruption in Iceland was small compared to past eruptions of Katla, yet airspace in 20 countries was closed for 5 days to commercial air traffic, ~ 10 million travelers were affected, and the world’s average temperature fell by 1 ℃. A future Katla eruption is predicted to be ten times larger than the 2010 Eyjafjallajökull eruption. Heat from Katla causes glacial outburst floods when parts of the Mýrdalsjökull glacier melt and breach the crater of the volcano creating large lahars. During the last major eruption of Katla in 1918, the lahar produced was so large that it extended Iceland’s coastline in the region by 5 km (Katla’s Hazards, 2019).
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In order to understand the mechanisms of past eruptions and the extent of magma/water interactions, the researchers first turned to ash deposits. They evaluated the amounts of tachylite (heterogeneous glass), sideromelane (homogeneous glass), lithics (rock pieces), or crystals. By understanding the amounts of each of these are present in the ashfall, scientists can infer whether the explosivity of an eruption is more fragmentation/gas dissolution driven or driven by interaction of water and magma to cause explosive steam and lava eruptions.
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Figure 2: Volcanic ash from previous eruptions found within the Mýrdalsjökull ice. Figure from British Geological Survey (Katla volcano, 2019).
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Next, Schmith et al. (2018) determined the extent of fragmentation at the time of the eruption. As magma rises to the surface, gasses dissolved in the magma begin to come out of dissolution and form bubbles. As the magma rises further the bubbles expand more and more until there are more bubbles than magma, resulting in fragmentation. When the bubbles all finally pop, an explosive eruption occurs. This process is similar to opening a shaken bottle of soda. When you open the bottle the pressure is decreased and bubbles begin to form. The bubbles degassing rapidly cause the soda to explode. By understanding the fragmentation process of the Katla volcano, geologists can infer past and future eruption sizes.
Finally they determined the water/magma interaction of glacial meltwater of the 1625 and 1755 eruptive events. To this end they examined the size and sorting of the ash grains. A decrease in grain size and increase in the sorting of ash indicates a decrease in the ash column height. A decrease in grain size with a decrease in sorting of ash grains indicates increases in fragmentation intensity (Fig. 3).
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Figure 3: Plots showing grain size base on median size (φ) in each unit vs. sorting. Figure adapted from Schmith et al., 2018.
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Overall Schmith et al. (2018) found that both the 1625 and 1755 ash deposits were composed mainly of the two glasses, tachylite and sideromelane, followed by crystals and lithics (Fig. 4), which indicates that the eruptions were not water driven.
Schmith et al. (2018) found that 1625 CE eruption was a combination of magmatic degassing and water/magma interaction (phreatomagmatic) and that the water/magma interaction generally decreased over the course of the eruption. However, toward the end of the eruption, the water/magma interaction became the larger driver of the eruption over fragmentation.
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Figure 4: Components of ash fall deposits for the 1625 and 1755 Katla eruptions. Figure from Schmith et al. (2018).
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The 1755 CE eruption was similar to the 1625 eruption in that it was both a result of fragmentation and of water/magma interaction. However, in the case of this eruption the water/magma interaction was only dominant at the beginning of the eruption, and fragmentation dominated most of this eruption.
By understanding how water from the glacier interacts with lava and superheated material, scientists can use the data from this study in order to help predict potential mechanisms for future Katla eruptions. This will allow insight into how far the ash from an eruption might travel, and the extent of lahar flows and flooding. Understanding the dynamics of Katla and the Mýrdalsjökull glacier allows governments to plan for hazards resulting from a Katla eruption and put protections in place to help keep the residents of Iceland and mainland Europe safe.
References
Katla’s Hazards Katla Volcano, http://katlavolcano.weebly.com/katlas-hazards.html (accessed March 2019).
Katla volcano, Iceland | Volcanology | Icelandic volcanism | Our research | British Geological Survey (BGS), https://www.bgs.ac.uk/research/volcanoes/katla.html (accessed March 2019).
Schmith, J., Höskuldsson, Á., Holm, P.M., and Larsen, G., 2018, Large explosive basaltic eruptions at Katla volcano, Iceland: Fragmentation, grain size and eruption dynamics: Journal of Volcanology and Geothermal Research, v. 354, p. 140–152, doi:10.1016/j.jvolgeores.2018.01.024.
