The geology of Thailand and the whole Indochinese Peninsula was already complex before the Indian subcontinent careened north-northeast across the Indian Ocean and collided with the underbelly of southern Asia. The Indochinese Peninsula is assembled from a number of terranes that accreted to southeast Asia, much as was the case with western North America. By the late Permian – about 250 million years ago – most of the pieces and parts had sutured to Asia to make roughly what is today the Indochinese Peninsula. It was only much later that the Indian Plate arrived and collided with Asia about 40-50 million years ago that things really changed.
The mountainous spine that extends from the southern tip of Thailand on the Maylasian Peninsula to the northern border with Myanmar and China is the product of that collision. The main force of the India-Asia collision fell further northwest, creating the Himalayan Mountains, among other things. But if you think of the India-Asia collision as a wedge striking a stump, the side pressure is what pushed up that long belt of mountains. Much of what got shoved up was old ocean bottom, limestone and shale, but much of that rock was altered by the pressures and temperatures generated by the collision, metamorphosing the limestone to marble and the shale to slate. in northern Thailand, partial subduction of the margins of the Indian Plate generated granite that was exposed by erosion as the collision lifted the area. In the few places the melted rock emerged as basalt.
That’s complex, but it’s only part of the geological meat grinder that is working on the indochinese Peninsula. The area is located at the zone of convergence between the east-southeast moving Eurasia Plate, the northeasterly moving Indian and Australian Plates and the east-northeast moving Philippine Plate.
Indochina and adjoining regions today are a complex collage of continental blocks, volcanic arcs, and suture zones that represent the closed remnants of ocean basins (including back-arc basins). All crunched between five sizable geologic plates.
Add 50 million years of erosion guided mostly north-south, including the Mekong River, and you have pretty much a geologic sampler of most of the kinds of geology and geomorphology found on the planet. That erosion explains why the southern third of Thailand is barely above sea level: it’s a basin filled with river mud. You also have an explanation for the improbable course of the Mekong River, which originates in Tibet, but winds through China and each of the five countries of Indochina en route to its mouth in southern Vietnam. And the explanation for the existence of the improbably long and narrow Malayan Peninsula, some 700 miles long, dangling from the end of southeast Asia like a gigantic appendix.
The limestone and marble are resistant to erosion, even the 40 million years of erosion since the mountain spine of western Thailand was uplifted.
Think of it this way: the compression by the India-Asia collision and the pressure from surrounding plates have uplifted that previously deposited limestone and sandstone. A few outcrops have been pushed up in the flat southern third, but it will remains as flat as you’d expect a 100 million year old outwash plain to be.
The net result is a wide variety of habitats, which means a wide variety of birds. Probably the most of any country in southeast Asia. In addition, Thailand is in the flyway for north-south migrating east Asia species. That would include the Arctic Warbler, a species that breeds in Alaska but winters in southeast Asia and the Philippines.
But it all starts with the geology.