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.
Magic Magma 