CHARACTERIZATION OF HOLOCENE BASALTIC SPATTER CONES AT ASKJA, ICELAND

Askja Volcano, Iceland, is a product of unique tectonic activity where rifting and hot spot volcanism meet. Volcanic deposits at Askja before, during, and after the last glacial period help provide better understanding of the magma plumbing system over the last 70,000 years. Monitoring at Askja has shown recent uplift of 60 cm since August 2021 and suggests inflow of new magma (Icelandic Met Office, 2022). Previous studies have focused on glacial, and <4285 year old post-glacial lavas, but immediate post glacial eruptive vents have not been described in detail. These vents outcropped as lava flows, spatter ramparts, scoria cones and tuff rings. Part of this initial investigation characterizes six early Holocene vent structures of the dozens that outcrop at Askja. Here, we present detailed description of the vents’ stratigraphy, structure, and whole-rock geochemistry. Vents are characterized by topographic high points in a conical/semi-conical shape made of agglutinated spatter or scoria. Most all of the vents are clustered together in groups of multiple cones. Some vents display local collapse and exhibit overlapping cone structures. The Holocene vents at Askja range in size from 5-200 m in diameter and 1-40 m in height. The individual spatter clasts range in size from 5 cm to >1 m and scoria clasts range from <2.5-50 cm. Volcanic bombs at Askja have dominant fluidal textures with lesser bread crust textures and cauliflower bombs prevalent at a single vent. When present, lavas related to the vents are buried by ca. 4285 ka lavas. Plagioclase phenocrysts are present in the spatter at <5% of the total composition and a size of <1 mm. Whole-rock geochemistry of the cones at Askja were analyzed using XRF and show the eruptive material as basaltic in composition (49.84-51.82 wt% SiO₂), with ranges of 4.63-7.34 wt% MgO, 8.96-12.03 wt% CaO, and 2.16-2.88 wt% Na₂O. These compositions are similar to the other basaltic eruptive units at Askja. Fissure eruptions are the most common type of eruption at Askja; therefore, more comprehensive descriptions of the Holocene vents are necessary to better understand the magmatic plumbing system in the event of future eruptions.