The island of Taiwan was formed approximately 4 to 5 million years ago at a convergent boundary between the Philippine Sea Plate and the Eurasian Plate. In a boundary running the length of the island and continuing southwards, the Eurasian Plate is sliding under the Philippine Sea Plate. In the northeast of the island, the Philippine Sea Plate slides under the Eurasian Plate. Most of the island comprises a huge fault block tilted to the west.
The island is active geologically, formed on a complex convergent boundary between the Yangtze Subplate of the Eurasian Plate to the west and north, the Okinawa Plate on the north-east, the Philippine Sea Plate on the east and south, and the Sunda Plate to the southwest. Subduction changes direction at Taiwan. The upper part of the crust on the island is primarily made up of a series of terranes, mostly old island arcs which have been forced together by the collision of the forerunners of the Eurasian Plate and the Philippine Sea Plate, which is moving to the northwest. These have been further uplifted as a result of the detachment of a portion of the Eurasian Plate as it was subducted beneath remnants of the Philippine Sea Plate, a process which left the crust under Taiwan more buoyant.
South of Taiwan, the Sunda Plate is subducting under the Philippine Sea Plate, forming the Luzon Volcanic Arc (including Green Island and Orchid Island). The east and south of the island are a complex system of belts formed by, and part of the zone of, active collision between the North Luzon Trough portion of the Luzon Volcanic Arc and the Eurasian Plate, where accreted portions of the Luzon Arc and Luzon forearc form the eastern Coastal Range and parallel inland Taitung Longitudinal Valley of Taiwan respectively.
From the northeast of Taiwan and continuing eastwards in the Ryukyu Volcanic Arc, the Philippine Sea Plate is subducting under the Okinawa Plate, forming the Ryukyu Volcanic Arc.
Terranes
The Hai'an Range or Coastal Range terrane belongs to the Luzon Volcanic Arc (呂宋島火山弧), and is on the Philippine Sea Plate (菲律賓海板塊). West of this is the Longitudinal Valley or Huatung Valley, an active depositional zone in a rift valley. West of this is the Eastern Central Range terrane, part of the continental base that has been upturned. The western side of the range is the Western Central Range terrane, which is metamorphosed earlier Tertiary clastic sediments. The Lishan Fault separates the Hsuehshan Range terrane Eocene to Oligocene sandstone and shale. The Chukou Fault (觸口斷層) provides the boundary for the Western Foothills terrane, which is shallow marine detritus from Miocene to Pleistocene. A deformation front forms the boundary with the undeformed coastal plains alluvium, which is still depositing.
The Eastern Central Range terrane can be divided into the Tailuko belt on the west with greenschist metamorphism and the Yuli belt on the east with blueschist metamorphism. The two metamorphic belts are possibly separated by a fault called Shoufeng fault, but this is unproven.
Central Mountains
The oldest exposed rocks are heavily metamorphosed and found in the Central Mountains. Poor quality fossil evidence suggests an origin in the Permian Period. They form a band of rock from Nan'ao in the north to inland of Jinfeng on the eastern flank of the Central Mountain Range. The band is 30 km wide in the north and narrows to the south. These earlier rocks are heavily metamorphosed. They also occur as a basement beneath the western part of the island. They make up the Danana'ao Group or Tananao Schist. This group consists of the Kainanagang Gneiss, Tailuge Marble, and Changchun Schist.
The Yuli Schist is Cretaceous in age and consists of black schist and some greenschist, glaucophane schist, and spotted slate. It is the easternmost part and extends from Chian southwards to the west of the Chihpen hot springs in a belt about 150 km long. Although the dark colour of the black schist is due to carbon, carbon is less than 2% of the content. The minerals found are quartz, mica, chlorite, albite, sphene, and graphite.
The Changchun Schist being mostly greenschist is found on the western side and forms thick beds. It is found along with smaller amounts of chert, and black schist. The rock is foliated dark green rock containing chlorite, epidote, quartz, calcite, biotite, albite and actinolite. They are derived from mafic volcanic rocks. These rocks can host copper sulfide ores.
Siliceous schists are coloured grey, they are metamorphosed sandstone, rich in quartz. Quartzite and chert bands can be found, and they are commonly associated with the black schist.