Evolution of the Krafla Central Volcano, North Volcanic Zone, Iceland

The Krafla central volcano forms a low, broad shield c. 25 km in diameter astride the on land extension of the Mid-Atlantic Ridge in northern Iceland, with an 8 by 10 km caldera in its center. Recent volcanic activity (including the 1724-1729 Myvatn and 1975-1984 Krafla fires, respectively) and an extensive high temperature geothermal area under the summit region (340 deg C at 2 km depth) attest to the active nature of the volcanic system. The purpose of this study was to use high precision Ar/Ar techniques to document the temporal evolution of this important natural resource in Iceland (including a geothermal-powered 60 MW of generated electricity). Sixteen samples were chosen for initial analysis. Eight (4 of 10 basalts, 4 of 5 rhyolites) yielded useful age information, i.e., better than 1% precision on rhyolite and 3-10 k.y. precision basalt, and span the entire stratigraphic succession at Krafla. The ages show for the first time that (1) an Icelandic central volcano evolves in less than c. 250 k.y., (2) the caldera collapse and welded ash-fall tuff formed at c. 110 ka, and (3) the complicated glacial-interglacial volcanic stratigraphy of the system is consistent with known climatic records. However, the ages of several tall (600-800 m), subglacially erupted rhyolite domes suggest that the ice thickness around Krafla at c. 90 ka was significantly greater than previously thought. Also, the central caldera collapse feature has undergone 1-2 km of extension since formation, suggesting that 50-100% of the full spreading rate of the Mid-Atlantic ridge system has been accommodated by the Krafla system alone on that segment of the Northern Volcanic Zone. Improvement on these results will probably require chemical treatment on the unsuccessfully-dated samples, to help remove the affects of atmospheric contamination and low-temperature alteration.