Hot clasts and cold blasts: thermal heterogeneity in boiling-over pyroclastic density currents

Abstract

Abstract Partial thermal remanent magnetization data from clasts in pyroclastic density current (PDC) deposits provide information on the emplacement temperatures of both lithic and juvenile magmatic clasts contained in the deposits. We collected palaeomagnetic data from clasts in PDC deposits emplaced during historical eruptions of two volcanoes in Ecuador, the 2006 eruption at Tungurahua and the 1877 eruption at Cotopaxi. These eruptions were characterized by emplacement of PDCs mainly related to boiling-over activity. The deposits of these eruptions are similar and are characterized by cauliflower-textured juvenile scoria clasts up to 1 m in diameter and a diverse assemblage of lithic clasts surrounded by an unwelded ashy matrix. On the basis of progressive thermal demagnetization experiments, we infer that emplacement temperatures for most of the lithic clasts in PDC deposits are below 90 °C. In contrast, palaeomagnetic data from juvenile clasts from the same deposits provide emplacement temperatures higher than 540 °C. These data indicate the PDC were thermally heterogeneous over short length scales (decimetres) also after deposition. We hypothesize that PDCs emplaced by the boiling-over mechanism cool quickly owing to atmosphere entrainment, causing the juvenile clasts to form a rind that retains heat and that also prevents lithic clasts from appreciable heating. Several deposits on Cotopaxi, despite being morphologically similar to the PDC deposits, contain both cold lithic and juvenile clasts, which we interpret to be lahar deposits formed by PDCs travelling across glacial ice and snow. Rare deposits containing both hot lithic and hot juvenile clasts are classified as well-mixed, hot PDCs, and were erupted during a more energetic phase at Tungurahua.