Local Rocks & Geologic Time

As is the case with every other large landmass, the island of Taiwan is composed of millions of tons of different kinds of rocks, which are themselves composed of minerals. The island being right next to a plate boundary also allows for all three categories of rocks found here to be manipulated in many different ways. Geologic change takes millions of years, however, which is why the stratigraphic history of what rocks were formed when is crucial to geologic study. The main way specific time periods have been found in this location is through named formations which act as large indexes for when large amounts of a specific rock were formed at a certain time. The start of Taiwan can be pinpointed to a time between the late Paleozoic and Mesozoic eras, specifically through schist found in great abundance on the eastern side of the Taiwan Central Range (Taiwan Geological Features, 2023). These rocks were likely metamorphized early in the island’s life and then jutted upward to form the range due to the Eurasian Plate sliding under the Philippine Plate.  Next in the timeline, the first sandstones were deposited and metamorphized to quartzite during the Eocene age from the Tachiachi River, and more would soon arrive . Stretching from the Eocene all the way to the Miocene, the Hsitsun Formation followed by the Shuichangliu Formation would be created from sandstone and slate northwest of the Tachien sandstone (Chen, 2011). 

Nearing the end of the Miocene era, the creation of the relatively small Aoti Formation started the trend of the formation of shales on the developing island (Chang). From the end of the Miocene to the Pliocene era, the Paliwan, Lushan, and Nanchuang Formations were created in that order. Each consists mostly of fine-grained shale and sandstone, both components not being metamorphized showing the regions' younger ages (Lin). The next major formation to be created was the Cholan, which introduced the first mudstones to the island during the Pliocene and Pleistocene. The Toukoshan formation also consists of muddied shale and sandstone throughout as well as very thick layers of gravel (Kuo-Chen, 2018). In addition, remnants of volcanic rocks have also been found in the Pleistocene in specks around the island's waters. These rocks contain highly mafic material and are likely evidence that Taiwan's volcano remained active throughout its history. Nearing the very end of the Pleistocene, both Hengchun Limestone and various Terrace deposits were formed, the latter consisting of mostly gravel, laterite, and clay. Finally, in present-day times, large amounts of Alluvium are building up on the depositional west coast of Taiwan, away from the easter plate boundary. Alluvium consists mostly of sand, silt, gravel, and clay much like the Terraces, and most likely first started as a part of one of the previous formations made millions of years ago (Chen).