A Sea Beneath the Land
Deep below the Khorat Plateau of northeastern Thailand lies a vast and ancient geological formation. During the Cretaceous period, around 100 million years ago, a shallow inland sea repeatedly evaporated under an arid climate. This process left behind thick layers of evaporite minerals, primarily halite (rock salt), anhydrite, and gypsum. These deposits form the Maha Sarakham Formation, which can be over 1,000 meters thick in the basin's center. The formation is composed of three main salt members (Lower, Middle, and Upper) separated by layers of red claystone and siltstone.
Over millions of years, younger sediments piled on top of the salt layers, increasing pressure and temperature. Rock salt is a peculiar material; under immense, prolonged pressure, it behaves like a very thick fluid. It is also less dense than the sedimentary rocks that buried it, such as sandstone and mudstone. This density difference creates buoyancy, causing the salt to slowly push its way upward through weaknesses and fractures in the overlying rock layers. This process, known as halokinesis or diapirism, has formed massive underground pillars and domes of salt. In some parts of the Khorat Plateau, the Lower Salt member has flowed and deformed significantly, creating these intrusive structures that can rise for several kilometers.
Domes of Wealth and Hazard
The salt domes of Chaiyaphum province are a major economic resource and a source of environmental challenges. The upper sections of these salt structures, particularly the Lower Salt member, contain valuable deposits of potash minerals. These include sylvite and carnallite, which are important components of agricultural fertilizers. The Khorat Plateau holds what may be one of the world's largest unexploited potash deposits, attracting significant mining interest. One of the largest salt mines in Thailand is located at Bamnet Narong in Chaiyaphum province.
The upward movement of salt has consequences for the surface environment. As the domes push closer to the ground, they can be dissolved by groundwater. This process contaminates the water and leads to widespread soil salinization, a serious agricultural problem in fifteen provinces across the region. In some cases, the dissolution of shallow salt can cause the overlying ground to collapse, forming large sinkholes. One such event in a nearby district created a pit 15 meters deep and the size of a football field. The very presence of these domes, sometimes indicated by reservoirs on the surface, shapes the region and the lives of the people who inhabit it.