11-6 Improving Askja Volcano, Iceland’s Eruptive Chronology with Mapping and Whole Rock Geochemistry

Session: Using Volcanic Deposits to Help Us Understand Volcanic and Magmatic Processes

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Abstract:

Askja volcano has produced historic basaltic fissure eruptions and caldera-forming rhyolitic explosive eruptions. In 2021 the caldera started inflating, a reversal from the previous forty years of subsidence. This change in deformation necessitates a more comprehensive analysis of the eruptive history of Askja incorporating previous mapping, distal tephrochronology, and recent field work to constrain the relative frequency of the eruptive styles to support ongoing observations.

The proximal record of Askja is constrained to be over 70,000 years old, with a tephra record potentially up to 200,000 years. From 70-14 ka Askja was mostly covered by glacial ice producing basaltic pillow lavas and stacked tindars. Post ice, the most abundant products at Askja are basaltic, interrupted by caldera forming rhyolitic eruptions estimated to have produced 0.3 km3 in 1875 and 1.5 km3 DRE ~10,800 bp.

Mapping at Askja has identified two rhyolite lavas, two rhyodacite lavas, two rhyolitic pumices, and some xenoliths of rhyolite pumice in 29 ka glacial deposits. Only two rhyolitic eruptions have been previously characterized: the historic 1875 and the 10,800 bp pumice also recognized as the Askja S tephra distally.

Whole rock major and trace element geochemistry has correlated two proximal rhyolitic lavas with explosive eruptions. Distal tephra deposits indicate additional explosive rhyolite eruptions at 2 and 9 ka. The edifice has four currently recognized calderas, but the older two are not currently correlated with explosive eruptive deposits. It is worth noting that the volume of the youngest calderas far exceeds the volume of erupted material they are associated with, so calderas alone may not indicate an explosive eruption. The two rhyodacitic lavas have not yet been correlated with distal deposits. If the proximal rhyodacites, glacial aged rhyolitic xenoliths, and younger distal rhyolitic tephras are all separate events, Askja has experienced seven high silica eruptions in the last 30 ka resulting in a recurrence interval of ca. 4,300 years.

These high silica eruptions occur between hundreds of small (<0.1 km3) volume basaltic eruptions that built the glaciovolcanic edifice and have filled in the 10,800 bp caldera since formation. Although the most likely future eruption based on frequency of past occurrence is a basaltic fissure eruption, this integrated eruptive history reveals more silicic eruptions than previously recognized.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6000

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