Presentation by Tamara Carley.
What lies beneath retreating ice? Nunataks reveal a young, diverse, history of rhyolitic volcanism at Þórðarhyrnaon the southern margin of Vatnajökull ice cap, Iceland
Iceland is famous for its volcanoes, glaciers, and the complicated relationship between the two. In the present study, rapidly retreating ice on the southern margin of Vatnajökull creates an opportunity to probe the eruptive history and magmatic origins of Þórðarhyrna (anglicized as “Thordarhyrna”) using increasingly—and in some cases, newly—exposed nunataks. Þórðarhyrna is located in the highly active Eastern Volcanic Zone, between Grímsvötn to the northeast and Laki to the southwest. Historically quiescent and largely obscured by ice, Þórðarhyrna has escaped the attention received by other volcanoes in Iceland’s neovolcanic zones, especially its famous, basalt-erupting, neighbors. The significant concentration of rhyolite and the diversity of effusive and explosive textures in Þórðarhyrna’s sparse, subaerial, exposures of rock is striking. Mineral assemblages suggest that multiple, discrete, batches of rhyolite contributed to the emplacement of the edifice emerging from the ice. Zircon trace elements and O isotopes suggest a history of extensive, low-temperature, fractional crystallization dominating rhyolitic petrogenesis at Þórðarhyrna. Zircon U-Th disequilibrium dates reveal crystallization and subsequent rhyolite eruption occurred as recently as the end of the Last Glacial Maximum. We explore the origin, evolution, and emplacement of Þórðarhyrna rhyolites in the context of a changing climate, past and present.