Active volcanic systems have veins and act like valves. To understand the role of these hydrothermal manifestations, a study by INGV, published in Scientific Reports - Nature
Most active volcanoes have them. They are the veins, fractures filled with minerals deposited by fluids. Always little studied, these structures are able to provide valuable information on the evolution and long-term dynamics of volcanoes and hydrothermal systems. To say so a team of researchers from the National Institute of Geophysics and Volcanology (INGV) who analyzed these structures on the island of Lipari. Research (http://www.nature.com/articles/s41598-017-00230-8) was published in Scientific Reports – Nature.
“This work”, explains Guido Ventura, INGV researcher, “aims to understand how veins are formed and why. The data collected not only made it possible to reconstruct the history of these structures, but also to determine the variations in permeability and pressure in the crust, as well as the depth of the source of the fluids that deposited the hydrated calcium sulphate in the veins".
Volcanoes work like a valve and their activity is controlled not only by the rising magma. In fact, the fluids released in depth go back to the surface, fill the existing fractures, and then deposit minerals, 'sealing' the crust like a plug that prevents magma and other fluids from rising. This process repeats over time and is controlled by fluid composition and tectonics.
“A decrease in gas emissions on the surface”, continues the researcher, “is therefore not uniquely correlated to a decrease in the release of gas from the magma, but reflects a phase of pressurization of the volcanic system which could precede hydrothermal explosions and volcanic eruptions. This would lead to a re-interpretation of the monitoring data of active volcanoes such as our Campi Flegrei and Yellowstone in the United States ”.
Veins play a fundamental role in the dynamics of volcanic systems because they somehow 'seal' the crust, thus causing a reduction in permeability and uplift.
“This crust”, continues Ventura, “in fact prevents new fluids from rising, creating pockets of overpressure in depth. An increase in pressure could fracture the system, produce a hydrothermal explosion or even trigger a volcanic eruption by instantaneous decompression of the magma chamber. When the system is, on the other hand, now fractured, another cycle of deposition and vein formation has a new course".
The main implication of the study concerns the monitoring of active volcanic areas.
“As known”, adds the researcher, “a decrease in outgassing in an active volcanic area is generally interpreted as a cooling of the magma system or a decrease in the release of magma gases. However, the data show that this could, instead, mean a decrease in permeability due to the deposition of minerals in the fractures with a consequent increase in pressure. The system would find itself evolving towards a condition of greater instability which could herald a hydrothermal explosion or a volcanic eruption".
Therefore, the formation of the veins and the consequent reduction of crustal permeability would be controlled by the chemical composition of the aqueous solutions which deposit the minerals and by the tectonics which controls the orientation and the degree of interaction between the veins. "Hence the result that the dynamics of these areas is not only affected by volcanic processes but also by the efforts produced by larger-scale geodynamic processes", concludes Guido Ventura.
Extended
Vein networks affect the hydrothermal systems of many volcanoes, and variations in their arrangement may precede hydrothermal and volcanic eruptions. However, the long-term evolution of vein networks is often unknown because data is lacking. We analyze two gypsum-filled vein networks affecting the hydrothermal field of the active Lipari volcanic Island (Italy) to reconstruct the dynamics of the hydrothermal processes. The older network (E1) consists of sub-vertical, NS striking veins; the younger network (E2) consists of veins without a preferred strike and dip. E2 veins have larger aperture/length, fracture density, dilatancy, and finite extension than E1. The fluid overpressure of E2 is larger than that of E1 veins, whereas the hydraulic conductance is lower. The larger number of fracture intersections in E2 slows down the fluid movement, and favors fluid interference effects and pressurization. Depths of the E1 and E2 hydrothermal sources are 0.8 km and 4.6 km, respectively. The decrease in the fluid flux, depth of the hydrothermal source, and the pressurization increase in E2 are likely associated to a magma reservoir. The decrease of fluid discharge in hydrothermal fields may reflect pressurization at depth potentially preceding hydrothermal explosions. This has significant implications for the long-term monitoring strategy of volcanoes.
Photos 1 and 2. Outcrop of a sub-vertical gypsum vein of metric thickness (left) and an anastomised system of decimetre and centimeter veins (right). Both systems emerge in the western sector of Lipari in the Quattropani area, in the area known as Cava del Caolino
Photo 3. Gypsum crystals inside a decimetre vein, Lipari, Cava del Caolino area
Photo 4. Metric veins in the hydrothermal area, Lipari, Cava del Caolino area