The Opala IV: An Explosive Caldera-Generating Super Eruption, which Was the Largest in Kamchatka in the Last 50000 Years

摘要

It is shown that the youngest (similar to 40000 C-14 years BP) caldera of the Opala caldera complex, the Opala IV, was formed by a catastrophic explosive supereruption, the largest in Kamchatka during the last 50000 years of the seven dated similar-type eruptions that occurred during the Late Pleistocene paroxysm of explosive volcanism. It is thought that the ejected volume was on the order of 250 km(3) of pyroclastic material. A smaller part of it went to form pumice-rich pyroclastic flows, with the greater part being transported as tephra. The principal axis of the ash fallout was oriented NNE (azimuth similar to 30 degrees) where the tephra thickness was 20-30 cm at a distance of 300-320 km from the eruptive center. The uncontaminated juvenile material is rhyolite, the concentration of SiO2 was 75-76%, the total alkali content 7.3-8.3%, and the K2O/Na2O ratio 0.83-0.96. It was concluded that no such eruption can occur in the Opala caldera complex in the future for hundreds or thousands of years.