INTRODUCTION

Volcanic eruptions constitute one of the more hazardous natural disasters for humans. The magma melt that is rising to the ground surface (lava) causes great damage to the environment. Lava that is being ejected from a vent at a great velocity occasionally reaches velocities of about 100 km/h on Hawaii) or ash plumes produced by explosions burn out or shower ash and volcanic bombs on everything around the volcano (Figs. 1, 2).

Fig. 1.
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Vegetation burning under the impact of a lava flow discharged by Kilauea Volcano, Hawaii. Photograph from the web.

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Fig. 2.
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Buildings on La Palma Island, Canaries under ash and volcanic bombs ejected by Cumbre Vieja Volcano. Photograph from the web.

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Basic lavas are liquid, and are less subject to explosion. At the same time we can observe that their explosive activity causes great damage to the environment (see Fig. 2).

However, active volcanoes that discharge silicic products pose a still greater hazard.

When cooled, volcanic ejecta become rocks with certain features of their external appearance, namely, structure. Structure is defined as the external appearance of a rock due to the positions and sizes of its constituent components. It bears information relevant to the understanding both of the processes involved in the rise of magma melt onto the Earth’s surface and of its impact on the environment.

STATEMENT OF THE PROBLEM

The goal of the present paper is to use Late Cenozoic silicic volcanic rocks in the Caucasus to demonstrate the need for studying the structure peculiarities of volcanic rocks discharged by “dormant” or “sleeping” volcanoes to predict the character of hazard that can arise during a possible resumption of their activity.

THE DATA SET

We report multiyear observations of structure for volcanogenic rocks in various orogenic volcano-plutonic belts of different ages. Our case involves observations of structure in the Caucasian Late Cenozoic silicic rocks that compose Mount Elbrus and adjacent areas, as well as the environs of Mount Aragats in Armenia.

THE RESULTS

Observations of structure in silicic lavas and ignimbrites provide evidence of a great role of volatiles in their formation, which make the melt mobile. At the same time, rock structure can frequently tell us about a rapid, or even catastrophic, separation of volatiles as the melt emerges at the ground surface. Silicic rocks are defined as those whose concentration of silica is above 64 wt % (Petrograficheskii …, 2009).

THE STRUCTURES OF SILICIC VOLCANICS

Silicic lavas are usually thought to be viscous and unable to move over great distances. However, such lavas frequently exhibit well-defined banding due to the presence of fine fluidity in the shape of thin continuous bands that differ in color and chemical composition (Kurchavov, 2009, 2010, 2022). This occurs in volcanics of different ages in many regions (Fig. 3).

Fig. 3.
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Fluidal banded silicic lavas in various regions. (a) Paleogene rhyolites in the upper reaches of the Partizanskaya River, southern Sikhote Alin; (b, c) Permian rhyolites in Karkaralinsk Area, central Kazakhstan; (d) Quaternary trachydacites on the southeastern slope of Elbrus. Photographed by A.M. Kurchavov.

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The structures of the Elbrus silicic lavas

The trachydacites that compose the southeastern slope of Mount Elbrus have fine-fluidal structure (see Figs. 3d, 4). Such a banded (fluidal) structure provides evidence of a high mobility (fluidity) of the melt because of its saturation with volatile components. However, this fluidal structure can give way to brecciated structures over short distances (see Fig. 4).

Fig. 4.
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Banded fluidal structure in silicic sheets that passes to brecciated structure. Trachydacites, Mount Elbrus, southeastern slope near altitude 3000 m. Photographed by A.M. Kurchavov.

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This alternation of structures shows that volatiles are released very rapidly from the melt, making it viscous instantly, resulting in a rock that has the appearance of lava breccia with chaotically located fragments (see Fig. 4) or with flattened fragments making the rock look as ignimbrite (Fig. 5). However, such flattened fragments have no endocontact zone that is so characteristic for ignimbrite fiamme (Kurchavov, 2009) (Fig. 6, bottom photographs).

Fig. 5.
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The transition of fluidal lava to brecciated lava of the “ignimbrite-like” appearance. Trachydacites on Mount Elbrus, southeastern slope. Photographed by A.M. Kurchavov. (a) a sharp change from fluidal lava to brecciated lava with flattened lava fragments that are oriented in the plane of fluidity; (b) flattened lava fragments in an oriented arrangement.

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These changes in the structure appearance of Elbrus cooled lava provide evidence of volatiles being released from the melt near the vent. A rapid release of considerable volumes of volatile components from a melt poses a high degree of threat to every living thing near the volcano and at large distances from it.

The structure of Caucasian ignimbrites

Ignimbrite generation poses still greater threat to the environment. Ignimbrites are rocks with numerous oriented flame-like segregations (fiamme) whose edges are split like a flame immersed in a complex-structured vitreous basis (see Figs. 6, 7).

Fig. 6.
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Ignimbrites in the village of Artik area, Armenia. The two bottom photographs show ignimbrite fiamme with a well-pronounced endocontact zone of greater porosity, but having a more vitreous matrix: on left is a longitudinal cross section of the fiamme, on right is a transverse cross section. Photographed by A.M. Kurchavov.

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Fig. 7.
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North Caucasian ignimbrites. (1, 2) Lower Chegem massif: (1) a quarry near the summit of Mount Khora-Khora on the left bank of the Baksan River near the village of Zayukovo, (2) Baksan right bank near the village of Zayukovo; (3, 4) Upper Chegem massif.

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The appearance of the flattened segregations (fiamme) and their parallel location provide evidence of flow in the magma melt that was saturated with volatile components and propagating at the ground surface as boiling milk might do.

Ignimbrites are formed from gas-charged melts, being accompanied by high-temperature scorching gas-charged liquid clouds. Examples of terrifying impact on the environment are well-known eruptions of Mont Pelée on Martinique Island or Krakatau in the Sunda Strait, Indonesia. However, pyroclastic flows at lower temperatures also affect the environment in a catastrophic way. Examples are furnished by eruptions of Bezymianny and Shiveluch in Kamchatka (Girina, 1996; Girina and Rumyantseva, 1993).

CONCLUSIONS

The presence of great amounts of volatiles in a primary magma melt makes for its high mobility, while the generation of rocks itself is accompanied by intensive release of volatile components. Among these we note carbon dioxide, chlorine, fluorine, and many other volatile substances. When interacting with water, as is frequently the case during glacial melting, they aggravate the catastrophic impact on the environment and all living organisms around.

It should be emphasized that the Elbrus volcanic area has widely abundant lavas and ignimbrites which typically contain high concentrations of volatile components in the parent melt. This should be borne in mind when making predictions of impacts on the environment, when the volcano of interest should resume activity.

We thus see that the study of structure in the rocks that have already been formed during earlier phases of volcanic activity is important, not only for understanding the process of melt generation and the ways that volcanic products find to be discharged onto the ground surface, but also for predicting the character of impact on the environment should magmatic activity be resumed.