Recognizing long-runout pyroclastic flow deposits using paleomagnetism of ash

Quantifying the spread of >600 °C pyroclastic flows (more broadly termed pyroclastic density currents—PDCs) is important because they regularly cause major volcanic catastrophes. Far from volcanic flanks, non-welded PDC deposits can be difficult to distinguish from cold-emplaced volcano-sedimentary units. A key indicator of high temperature is the coherence of magnetic remanence among different lithic clasts in a deposit. In long-runout PDCs, distal deposits are dominated by ash particles (<2 mm diameter), often lacking clasts large enough for conventional paleomagnetic sampling. Here we demonstrate a method of consolidating and sampling oriented blocks of friable ash material with a strengthening compound. This method was used to show that a >25 km runout mass-flow deposit from the 2518-m-high Mt. Taranaki (New Zealand) was emplaced as a hot PDC, contrary to an earlier cold lahar interpretation. We corroborate the results from ash with data from clast samples at some sites and show that the matrix was emplaced at temperatures of at least 250 °C, while clasts were deposited at up to 410 °C. Our case-study raises concerns for hazard-identification at stratovolcanoes worldwide. In the Mt. Taranaki case we demonstrate that PDCs traveled >9 km farther than previously estimated—also well beyond the “normal” PDC hazard zones at stratovolcanoes (10 or 15 km from source). Thus, attention should be paid to deposits in the 15–25 km range in other volcanic settings, where large populations are potentially unaware of PDC risk.