The rocks of Thailand are diverse: sedimentary rocks in places metamorphosed; granite batholiths and stocks intruded into metamorphic and sedimentary materials; younger intermediate igneous rocks; and local plugs or flows of basalt. Many of the older rocks have been moderately or strongly folded or faulted. As a consequence, present landforms are the result of physiographic processes operating on structurally deformed rocks of diverse physical and chemical character in a humid tropical environment.
The granites and limestones are generally more resistant to erosion than the other sedimentary or metamorphic rocks. Granite forms many prominent elongated ridges or isolated mountains whose prevailing slopes are steeper than those common in temperate climates. The granites weather deeply in the tropical environment, and the fresh rock is generally overlaid by a mantle of soil and decomposed rock 50 feet or more thick. The landforms developed in limestone are the most spectacular features of the Thai landscape. Karst solution features are common in all of the areas where limestone outcrops.
Sandstone, shale, and their metamorphosed forms are the most important clastic rocks of Thailand. They are highly variable in their resistance to erosion. Extensive areas of these rocks have been eroded by stream and marine erosion, planated along the coasts of the Peninsula.