Author: Steingrimur Thorbjarnarson

Most geothermal activity in Bulgaria is concentrated in the southwest of the country. By comparing mantle structure with the distribution of geothermal fields, it is possible to construct a simple explanatory model.

The southwest region is influenced by two opposing forces: on one hand, the general tectonic drift of the Eurasian Plate, and on the other, the counteracting effect of a mantle convection roll beneath the area. This opposition leads to rifting, which explains the geothermal activity around Velingrad and other sites. The Struma Valley marks the western boundary of this zone and includes Sapareva Banya, the hottest hot spring in Europe. When a phenomenon is exceptional—such as being the hottest or largest—it suggests that special geological conditions must be present. In this case, the nearby subduction of the Adriatic Plate beneath the Balkan Peninsula likely alters the regional stress field, creating the unusual geothermal regime observed both along the dividing line above and across the wider area marked in red on the map.

The east–west axis of the Balkan Mountains and the geothermal utilization hub near Varna are also shown on the map. While Varna’s use of shallow heat sources can partly be explained by its dense population, its location is also significant in light of the mantle-flow analysis.

This situation can be compared to Iceland, where volcanic zones about 1.5° wide (east–west) form directly above distinct mantle convection rolls. The East Volcanic Zone in southern Iceland (SIVZ) is a rift system, pulled towards the NW by the movement of the North American Plate. At the same time, the convection roll beneath it drives mantle flow in the opposite direction, thereby causing the rifting. Similarly, west of the Struma Valley, a zone of the same width has developed, representing a rift zone of the same type as the SIVZ. The recurrence of such 1.5°-wide zones strongly suggests that their formation is governed by the dynamics of a convection-roll system.

For reference. How the volcanc zones of Iceland appear in context with the convection rolls system drawn underneath.

Looking at the modern GPS based map representing tectonic drift vectors, it looks like Africa and Eurasia are more and less drifting in a parallel way:

Looking at the Mediterranean and its surroundings, it is obvious that the African plate is being subducted below the Eurasian plate. The relative movement between those two plates consists therefore mainly of a northward component. The convergent boundaries are quite complex, as can be seen here:

This is a clip from the presentation found at: https://www.youtube.com/watch?v=cqK-CbuM3Eo (Geoscience Information for Teachers Workshop at EGU).

If we try to simplify this as much as possible, the two main features of the North Coast of Africa, and the Adriatic Sea should be looked at:

It is known that Italy has rotated and the Adriatic Sea has thereby been enclosed with slab reaching under both Italy and the Balkan Peninsula. The African Plate subducts below the Aegean Sea, Sicily and Turkey. Subduction from the north and the appearance of ocean floor due to spreading is omitted here. The lines resulting from the Convection Rolls Model can be seen, and of course the main purpose of this study is to compare that model with what is known about the geology of the area. For this short post, it is mainly pointed out that the North Coast of Africa coincides with the 32nd parallel where the convection rolls are aligned directly N-S. One aspect of that latitude is that two downwelling lines of two different layers coincide, and thereby also two upwelling lines of two layers coincide. This creates the opportunity for convergent boundaries to appear, if the downwelling part is the dominant factor (no-slip) and the upwelling part becomes neutral (slip). A consequence of this is that extension within the area is then mainly caused by the north component of drift of the Eurasian Tectonic Plate, as the subduction is mainly related to the conditions close to 32°N.

A famous eruption occurred in 1918 of Katla Volcano in South Iceland and triggered the largest flood known on Earth in recent times. Today, a river flows through the area, called Múlakvísl. The very root of that river is originated from an ice cave, known as the Katla Ice Cave in the tourist industry. This is the opening of the cave:

The location of the glacial toungue of Kötlujökull can be studied on this map:

Looking closer at Kötlujökull, we find the location of the cave opening:

The most interesting sites are there for a reason. In the case of Kötlujökull, the inner forces meet with the outer forces of snow, ice and water flow. Here the similar conditions for shaping Barnafoss, Geysir and Hekla are mentioned. Barnafoss is shown here below:

Systematic measurement and compilation of data form the foundation of scientific work. However, over time, such efforts can also foster new perspectives on existing information—perspectives that may lead to fresh discoveries and innovation. Geothermal exploration relies on a range of data: knowledge of fractures, heat gradients, aquifers, thermal conductivity, surface water chemistry, and more. Based on this information, a well is drilled, and if conditions are favorable, hot water can be extracted from the ground.

The Convection Rolls Model offers an additional, indirect method to complement these approaches. By understanding tectonic drift vectors and recognizing that the boundaries between mantle convection rolls also influence divisions within the overlying tectonic plates, we gain a new framework for selecting promising geothermal sites. Iceland provides a compelling case study.

The map below shows the distribution of high- and low-temperature geothermal areas in Iceland. High-temperature zones are typically located near the boundaries of convection rolls, with a strong spatial correlation. In contrast, low-temperature zones tend to cluster within defined polygonal regions, also showing resemblance with the convection roll structure.

Map from Náttúrufræðistofnun Íslands, (Icelandic Institute of Natural History).

To identify new geothermal sites, a logical first step is to explore the intersections of convection roll boundary lines. Next, examining the distribution of known geothermal sites within the defined polygons may reveal consistent patterns — patterns that could guide the discovery of additional sites. However, this approach should be used in conjunction with established geological exploration methods to minimize the risk of error. The map, along with its scientific foundation, serves as a complementary tool to enhance the efficiency of land-based geothermal prospecting.