A coal seam in Romania that has been smoldering naturally for decades, slowly cooking the surrounding rock. The heat creates rare minerals that form only at high temperatures, turning an underground fire into an accidental geology experiment.
A fire beneath the moor
The Soos National Nature Reserve presents a prehistoric landscape. Situated on the bottom of a dried-up salt lake, its surface is a mix of vast peat bogs, salt marshes, and bubbling mud volcanoes called mofettas. These mofettas are vents that release nearly pure carbon dioxide, a remnant of the early Quaternary volcanic activity that shaped this region. A 2.2-kilometer wooden walkway allows visitors to cross the fragile landscape, winding past the churning mofettas and mineral springs.
Beneath this damp, carbonated surface, another geological process is at work. Deep within the earth, a layer of the region's Cypris claystone, a carbon-rich shale, is slowly burning. This natural coal seam fire is a form of spontaneous combustion, likely ignited when tectonic movements allowed oxygen to interact with the reactive rock deep underground. The fire smolders rather than rages, a slow-motion process that can persist for centuries. The carbon dioxide escaping from the mofettas at the surface is directly connected to this deep, persistent heat source, which consumes the carbon-rich rock and releases the gas.
A natural geochemical laboratory
The immense heat generated by the smoldering claystone transforms the subterranean environment into a high-temperature geochemical laboratory. This process, known as pyrometamorphism, "bakes" the surrounding sedimentary rocks at temperatures between 600 and 1,000°C. This intense, sustained heat is sufficient to alter the fundamental mineralogy of the rock, breaking down common minerals and recrystallizing them into new, often rare, forms.
This underground furnace forges a suite of high-temperature, low-pressure minerals. The baked shales, known as clinker, contain silicate minerals such as cordierite, mullite, and tridymite. Oxides like hematite and magnetite are also formed, which can alter the local magnetic field. Geologists studying these pyrometamorphic zones have even discovered minerals typically associated with meteorites, which require super-reduced conditions to form. The Slatina burning event turns the ground into a natural factory for geological rarities, all cooked by a fire that has been burning for an unknown length of time. The result is a landscape where prehistoric volcanic activity and a persistent underground fire create a complex geological area.
💡Fun Facts
The name "Soos" is from a local German dialect word meaning "moor" or "marsh".
The reserve's mofettas release carbon dioxide that is approximately 99% pure, a direct link to the area's volcanic past.
Some of the heat-altered, reddish clays are known as porcellanites because their texture and hardness resemble unglazed porcelain.
The reserve hosts a variety of salt-loving plants (halophytes) and around a hundred species of birds.