Life cycle of volcanic ash: field observations and processes

DOMINGUEZ, L.; BONADONNA, C.; FORTE, P.; MINGARI, L.; CIONI, R.; BRAN, D.; PANEBIANCO, J.E.

Montreal

Congreso; 27th IUGG General Assembly; 2019

IUGG

Although volcanic eruptions are often short-lived and represent a small period in the total history of a volcano, the life cycle of volcanic ash is a continuous process due to the large availability of loose material combined with erosive processes. In fact, the reworking and secondary deposition of volcanic material represent an important phenomenon that influence the geomorphology of volcanic regions. We present a detailed physical characterization of the deposit, transport and deposition mechanisms associated with wind-remobilisation of the 2011 Cordón Caulle (Chile) eruption. The fine-rich tephra layer of the primary deposit has been remobilised since shortly after its deposition. We combine field observations, grain size and shape analysis with atmospheric parameters and surface conditions to investigate wind-remobilisation processes in arid and semiarid regions of the Patagonian steppe. Two main types of deposits have been identified: i) deposits associated with roughness elements (e.g. vegetation and rocks), and, ii) deposits associated with pre-existing dunes or analogous erodible bedforms. Grain size analysis show that wind is capable to remobilise particles of a very specific range of sizes, from <0.4 to 500 µm, with median sizes of 25 to 92µm and typical modes between 30 to 92µm. Sedimentary structures and morphology of the deposits observed suggest that saltation and creep are the most likely transport mechanisms. The chronological evolution of mass flux from 2011 to 2015 is mostly controlled by the amount of available fresh ash, specific meteorological conditions, such as wind and soil moisture, and compaction of the deposit with time.