The shape of the lava: tell me how you are and I'll tell you where you will arrive

The shape of the lava: tell me how you are and I'll tell you where you will arrive

Author: Etna Moving Admin | Date: 29/05/2019

The shape of the lava: tell me how you are and I'll tell you where you will arrive

The shape of the lava: tell me how you are and I'll tell you where you will arrive

by Sonia Calvari

The surface of a lava field can be considered as the pages of a book. If we know how to read, then we have the fundamental tool to understand the meaning of the words written in its pages. Similarly, if we know how to recognize the shapes we observe on the surface of lava flows and how they are produced, we can go back to the processes that formed those structures. We will thus be able to foresee the dangers that we will have to face when new castings form from a volcano, will expand and flow on its flanks. This knowledge aims to provide useful tools to protect populations, inhabited areas and infrastructures from the advance of lava flows. A work recently published in Annals of Geophysics (Calvari S., Understanding Basaltic Lava Flow Morphologies and Structures for Hazard Assessment, 2018, 61, doi: 10.4401 / ag-8048) illustrates these aspects.

As has been observed in Japan and the Hawaiian Islands, lava flows can have very high initial speeds and reach 30-50 kilometers per hour. At Etna, these speeds are rarely reached, so usually, at the time when a flow forms, the population has time to leave without problems (Figure 1).

Figure 1 - Examples of typical castings of the Etna volcano (A) and the Kilauea volcano (B) on the island of Hawaii. The two types of castings, called aa and pahoehoe, respectively, have very different speeds and progression modes. Please refer to the original work Calvari (2018) for more details.

Among other things, the destructive potential of lava flows is rapidly reduced as the eruption proceeds and away from the eruptive vents, mainly due to surface cooling. The loss of heat from the surface of the lava flows, which have initial temperatures above 1000 ° C, leads to a rapid loss of mobility. In fact, an external plastic casing is formed, that is able to deform, albeit slowly, and then solid, which finally manages to contain and stop the flow itself. All this occurs if a lava flow is free to expand on the ground (Figure 2), and if over time the feeding from the effusive mouth is exhausted.

Figure 2 - Examples of castings characteristic of the initial phases of very rapid advance eruptions. (A) Lava tracimation from the brink of the Southeast Crater during the initial phases of the eruption of Etna 1989. (B) Lava flow expanding from the eruptive fissure on the northern flank of Etna during the 2002-2003 eruption, while the eruptive fissure was still spreading, producing fountains of lava and clouds of ash (visible in the background. Foto INGV. (C) Castings of the Kilauea, island of Hawaii, which form an expanse in which it is possible to distinguish the individual lobes that compose the lava field.

If fluid lava continues to come out of the eruptive mouth and gradually accumulates under the solid crust, the conditions for the formation of lava tunnels may occur (Figure 3). These are solid structures that surround the lava flows, keeping their internal heat almost unchanged, and which have the effect of silently and often transporting a large amount of lava towards the most distant areas of the lava field without being visible on the surface. , Allowing the casting to expand much more than if it had been free to flow on the surface while cooling. It is as if the lava travels on the subway, which, by avoiding surface traffic, allows people to reach points very far away from the city quickly, while buses or machines that move to the surface must slow down continuously.

Figure 3 - (A) Collapse well above one of the main lava tunnels that fed lava effusion on the island of Fogo (Cape Verde) during the 2014-2015 eruption. (B) Collapse shaft showing the incandescent lava flow that flows inside one of the active lava tunnels during the effusive eruption of 2003 at the Kilauea volcano (island of Hawaii).

In the last decades, studies on lava flows have allowed to associate to each surface structure of casting different modalities and speed of formation, which involve different degrees of danger. Furthermore, these studies have made it possible to establish empirical formulas to calculate the maximum length that a lava flow can reach up to the time of its exhaustion, using measurements of the maximum lava flow emitted by the eruptive vent during the initial phases of its opening, usually also the most vigorous ones.

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ach of the nations that must frequently face the advance of lava flows has developed its own strategies to mitigate the problems associated with the flow of lavas. On the island of Hawaii, for example, in which the Kilauea volcano erupted in 1983 giving rise to an eruptive activity that continued on and off until last year, the Hawaii Volcanological Observatory uses maps in which the lines of maximum slope of the ground indicate the path that will be used by the expanding lava flows. The Etneo Observatory, on the other hand, uses an automatic system based on the satellite detection of hot spots on the surface of the volcano from which flows can be formed. The system, once identified the origin point of a flow, proceeds to the calculation of the effusive rate, and inserts this parameter in a software of simulation of the castings. This allows to predict in a short time the size and the extension of the lava field that will be formed.

The experience gained in well-monitored volcanic areas for many years has allowed us to develop strategies that can also be used in other volcanoes where monitoring is more sporadic or even non-existent. Studying these structures in depth will allow us to better face any new eruptive emergencies.

https://ingvvulcani.wordpress.com/2019/05/22/la-forma-della-lava-dimmi-come-sei-e-ti-diro-dove-arriverai/