Introduction à l'hydrovolcanologie

Hydrovolcanology: an introduction

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Résumé

L'hydrovolcanologie profonde s'intéresse au rôle joué par la molécule d'eau (sous forme hydroxylée ou moléculaire) dans l'individualisation des magmas par fusion partielle au sein du manteau et/ou de la croûte terrestre, leur remontée dans la plomberie magmatique, jusqu'à l'exsolution-fragmentation grande responsable des dynamismes éruptifs explosifs. L'hydrovolcanologie superficielle couvre le champ des phénomènes paravolcaniques pré et postéruptifs : solfatares, fumerolles, sources chaudes, flux géothermiques, qui représentent un indicateur important en terme de prévisions volcanologiques. Elle permet de mieux comprendre la dynamique des lacs de cratère et des lacs de rift, souvent sursaturés en CO2 d'origine volcanique, susceptibles d'éruptions limniques. L'hydrovolcanologie présente une perspective nouvelle au sein des sciences hydrotechniques où l'eau est à la fois l'élément déclencheur, voire le catalyseur, d'éruptions explosives majeures, mais aussi un indicateur précieux en matière de prévision et de gestion du risque.

Abstract

Our knowledge of the physics of how volcanoes work has expended enormously over the past 20 years, as have our methods of studying volcanic processes. In seeking to understand volcanic behavior, volcanologists call on a diversity of physics subdisciplines, including fluid dynamics, thermodynamics, solid mechanics, hydrovolcanology, ballistics and acoustics, to name a few. Deep hydrovolcanology describes the water involvement in magma generation and segregation through partial melting into the Earth's crust and/or mantle, magma upward migration in the volcanic plumbing system and exsolution-fragmentation in the subsurface. As the magma rises towards the surface the confining pressure decreases, the volatiles gradually exsolve from the magma forming the gas bubbles which are distributed throughout the liquid. It is the connecting together of a network of these bubbles that ultimately causes the continuous body of liquid to break apart or fragment into a spray of droplets or clots suspended in the gas. In magmas, typically 95-99% of the 'mass' of material erupted is liquid rock - at most the gas accounts for only a few percent of the weight; but that small amount of gas represents a very large 'volume' as it expands to atmospheric pressure, and is fundamentally important in producing explosive eruptions. Continued rise of the magma leads to further exsolution of gas and growth of gas bubbles through diffusion, decompression and bubble coalescence. The relative importance of each process depends on the amount of volatiles (gas - mostly water) present in the magma, the magma composition and the magma rise speed. Surface hydrovolcanology is involved in pre and post-eruptive paravolcanic activity such as solfataras, fumaroles, hydrothermal heat and water fluxes. It also focuses on limnic eruptions, otherwise referred to as a lake overturns, a type of natural disaster in which dissolved carbon dioxide (CO2) suddenly erupts from deep volcanic lakes, suffocating wildlife, livestock and humans. Hydrovolcanology presents a new perspective within hydrotechnical sciences where water is a trigger in magma generation and segregation, magma rising and eruption style. Water appears to be a most valuable indicator for volcanic hazard assessment and mitigation, short-term eruption prediction, and volcanic risk management. Understanding the physical behavior of volcanoes is critical for assessing the hazards posed to the ever-increasing populations living in close proximity to active volcanoes, and thus for mitigating the risk posed by those hazards.