Emplacement temperatures of the Chachimbiro pyroclastic blast from paleomagnetic data

Abstract:

Volcanic blasts are highly destructive, laterally directed explosions resulting in pyroclastic density currents (PDCs). These super-heated mixtures of gas and rocks flowing downslope at high speed are among the most hazardous volcanic events. Knowing the emplacement temperature, it helps to assess the hazard that PDCs represent. There are two rock magnetic methods that can be used for this. The first is by looking at the blocking temperature of the thermoremanent magnetization and the second is the repeatability of thermomagnetic behavior. In northern Ecuador, a 650 m wide and 225 m high satellite lava dome of Chachimbiro volcano experienced such a violent laterally directed explosion at ~3640–3510 BC. The resulting PDC covered an area of 62 km2, with an estimated volume of 55 x 106 m3, with the thickest parts of the deposit displaying as much as 15 m. Here we present the emplacement temperatures of the Chachimbiro blast and the factors controlling this temperature based on the analysis of >80 samples from 6 locations; their distances varying between 1.8 km to 6.7 km away from the source. Our preliminary rock magnetic results indicate low titanium Ti-magnetite as the main magnetization carrier; maghemite being present in trace amounts. The overall temperatures vary from 250°C to 450°C depending on the clast size and type. For example a 1.2 m diameter block gave a temperature of 250°C, 8 cm from the surface of the block. This implies that this block was part of the cooled carapace of the dome, suggesting that the PDC material have had a minimum temperature of 250°C at this location (~1.8 km distance). In general, our results suggest a minimum temperature of ~250°C, with a large portion of the juvenile clasts having temperatures up to about ~450°C. This work highlights the usefulness of paleomagnetism and rock magnetism to evaluate the emplacement temperatures of PDCs, thereby allowing to better assess the risk associated.