Paleomagnetic evidence for high‐temperature emplacement of the 1883 subaqueous pyroclastic flows from Krakatau Volcano, Indonesia

Abstract

The paroxysmal 1883 eruption of Krakatau volcano in Indonesia discharged at least 6.5 km3 (dense rock equivalent) of pyroclastic material into the shallow waters of the Sunda Straits within a 15‐km radius of the volcano. Progressive thermal demagnetization studies of individually oriented pumice clasts from a core sample of the submarine pyroclastic deposits show that 41 out of 47 clasts exhibit single‐component remanence with mean inclination of −2.4°. The partial thermoremanent magnetization components of both pumice and lithic clasts are well grouped in orientation, indicating that substantial cooling of clasts must have occurred following deposition. Estimated subaqueous emplacement temperature for such clasts is greater than 500°C. Rare two‐component lithic fragments exhibit inflection points on vector endpoint diagrams that mark the temperature below which the fragments acquired magnetization of similar orientation. These inflection points range from 350° to 550°C, indicating a minimum subaqueous emplacement temperature of 350°C. Paleomagnetic evidence for high‐emplacement temperature supports the hypothesis that proximal 1883 submarine pyroclastic deposits resulted from entrance of hot, subaerially generated pyroclastic flows into the sea. Similar deposits have been interpreted from the geologic record, but this is the first documented example of submarine pyroclastic flows from a historic eruption. The Krakatau deposits thus serve as an important modern analog for the study of pyroclastic flow/seawater interactions.