A landscape built from rust
The distinctive red earth of Nakhon Ratchasima Province is the product of an intense, prolonged weathering process called laterization. Occurring over millions of years in hot, wet tropical climates, this geological transformation leaches soluble minerals from parent rock. The alternating wet and dry seasons accelerate the chemical breakdown. Heavy rainfall dissolves and washes away elements like silica, sodium, potassium, and magnesium. This leaves behind high concentrations of less-soluble elements, primarily iron and aluminum. The rusty red color comes from iron(III) oxides, specifically the minerals goethite (α-FeO(OH)) and hematite (α-Fe2O3), which become dominant in the resulting soil.
The parent rocks for laterite can be igneous, metamorphic, or sedimentary. In the Khorat Plateau, the surface geology largely consists of Mesozoic sedimentary rocks like sandstone and siltstone. The laterization process transforms this bedrock into a thick, clay-rich soil profile that can be up to 30 meters deep. This soil, technically an oxisol, is often porous and contains iron oxides that can make up between 30% and 60% of its mass by weight. The main iron-bearing mineral formed is goethite, a common component of rust. The entire Khorat Plateau, a saucer-shaped region covering 155,000 square kilometers, shows evidence of this geological process.
An ancient and modern resource
When wet, laterite is soft enough to be cut with hand tools. Upon drying, it undergoes an irreversible hardening process as the iron salts lock into a rigid lattice structure. This property made laterite a valuable construction material throughout Southeast Asian history. Starting around 1000 CE, the Khmer Empire used laterite blocks extensively for the foundations and internal structures of temples, including the famous Angkor Wat. Many historical sites in Thailand, such as those from the Ayutthaya period (14th-16th centuries), also feature laterite in their construction, especially in high-load areas like the base and core of structures. Builders empirically understood that laterite had a greater load capacity than brick.
The high iron content also makes some laterite deposits a viable iron ore. Economically useful lateritic iron ore deposits are rare compared to other sources like banded iron formations, but they do exist. To be considered a viable ore for a blast furnace, the material typically needs to contain at least 55% iron. The iron smelting and working on the Khorat Plateau dates back to at least 600-400 BCE. While it's uncertain if laterite was the primary source for this early industry, its widespread availability and high iron concentration make it a probable candidate resource.