Taiwan's geography: a tilted fault block with its mountains to the east

How two plates built a tilted island

The island is young by geological standards. It formed about four to five million years ago where the Philippine Sea Plate and the Eurasian Plate converge: along most of the island the Eurasian Plate slides under the Philippine Sea Plate, and in the north-east the movement reverses and the Philippine Sea Plate goes under. The western side and much of the Central Range are built from sediment scraped off the descending edge of the Eurasian Plate.

What that produced is a single fault block tilted to the west. The eastern two-thirds is mountain, rising to fault scarps of 120 to 1,200 metres on the east face of the Central Range, and the western third falls away in hills, terraces and plains to the strait. Britannica describes the interior as sloping sharply down to the Pacific on the east and more gently towards the Taiwan Strait on the west. Raised coral reefs along the east coast and the frequency of earthquakes in the eastern rift valley indicate that the block is still rising.

The boundary is active. Taiwan records 15,000 to 18,000 earthquakes a year, of which 800 to 1,000 are felt. The Chi-Chi earthquake of 21 September 1999, magnitude 7.3 and centred in the middle of the island, killed more than 2,400 people. A magnitude 6.4 earthquake on 6 February 2016 killed 117 people in Tainan, most of them in the collapse of one 17-storey apartment building.

Where the mountains stand

Five ranges run the length of the island from north-north-east to south-south-west, roughly parallel to the east coast. Together they extend about 330 kilometres north to south and average about 80 kilometres across, and they carry more than two hundred peaks above 3,000 metres.

How the Central Range divides the island

The Central Mountain Range runs from Su'ao in the north-east to Eluanbi at the southern tip and is the island's principal watershed. Heavy rain has cut gorges and sharp valleys into its hard rock, and Taroko National Park, on its steep eastern side, takes in gorges cut by a fast river. To its north-west the Xueshan Range rises to 3,886 metres at Xueshan itself. To its south-west the Yushan Range carries the island's highest summit, Yushan, or Jade Mountain. Britannica gives Yushan as 3,997 metres; the record, the Foreign Ministry and the Wikipedia survey all give 3,952. West of the Yushan Range, across the valley of the Gaoping River, the Alishan Range reaches 2,663 metres.

Why the east coast holds few people

The East Coast Mountain Range, mostly sandstone and shale, runs from the mouth of the Hualien River south into Taitung County and reaches 1,682 metres at its highest. It is separated from the Central Range by the Huatung Valley, a narrow rift lying at about 120 metres. Apart from that valley the east coast leaves little room for settlement, and the only sizable plain on that side of the island is the Yilan Plain in the north-east.

Where the plains are and who lives there

About 23% of the land is alluvial plain and basin, watered by rivers coming down from the mountains, and over half of that lies in the Chianan Plain of the south-west. The Pingtung Plain, the Taichung Basin and the Taipei Basin are smaller. Between the plains and the mountains lie raised terraces of eroded material: the Linkou, Taoyuan and Dadu plateaus. The Wikipedia survey puts 90% of the population on the western plains, and Britannica describes the same side as holding most of the farmland. The Foreign Ministry's handbook of 2023 to 2024 gives agricultural land as 7,900 square kilometres.

The rivers are numerous for the island's size and almost all short. Most rise on the slopes of the Central Range; those running east are steeper and faster, and those running west spread into wide, shallow, silt-laden beds once they leave the mountains, which makes them hard to manage. The Zhuoshui in central Taiwan is the longest, at 186 kilometres. The Gaoping in the south has the largest basin. The Tamsui, which flows north past Taipei into the strait, is the only one Britannica describes as navigable. Irrigation and drainage channels connect many of them.

The harbours follow the same divide. Most of the good ones are on the west coast, among them Kaohsiung and Anping in Tainan in the south-west; the exceptions Britannica names are Su'ao Bay on the north-east coast and Keelung at the northern tip.

The volcanic soils were rich, but rain and long irrigation have leached them. Britannica records much of the soil as short of phosphorus and potash and in need of fertiliser, especially where the land is double cropped.

How the monsoon and the typhoons set the weather

The Tropic of Cancer crosses the middle of the island, and the climate is subtropical in the north and tropical in the south. The Department of Geography at National Taiwan Normal University puts the annual average temperature at about 22°C in the north and 24°C in the south; the north is coldest in February at about 15°C, the south in January at about 19°C. Snow falls on the high mountains. The warm Kuroshio Current flowing north past the east coast keeps the lowlands mild.

Where the rain falls

Rainfall averages more than 2,500 millimetres a year: Britannica gives about 2,590 and the Wikipedia survey about 2,600. It falls unevenly. Huoshaoliao, the wettest place on the island, receives about 6,700 millimetres a year, while the coast of the northern Chianan Plain and the Penghu islands receive less than 1,000. More falls in the mountains than on the plains and more on the east coast than the west. The monsoons divide the year: from October to March the north-east monsoon brings steady drizzle to the north while the south stays dry, and from May to October the south-west monsoon brings the south most of its year's rain. The summer monsoon accounts for 90% of annual rainfall in the south and 60% in the north.

When the typhoons come

Typhoons arrive from June to October, most often in August and September. The National Taiwan Normal University geographers count an average of 3.5 a year striking the island, and the Wikipedia survey about four direct hits. The British government's travel advice of 2 October 2026 gives July to September as the most active period, with frequency falling away in October. Most make landfall in the south-east and move north-west. The rain they bring causes floods and mudslides: Typhoon Morakot in 2009 left nearly 500 people dead or missing.

How the wind and the sun divide the island

The north-east monsoon blows from October to April, strongly enough that a windbreak forest has been planted along the northern coast, and it is stronger still over the strait, crossing Penghu at about ten metres a second for roughly a third of the year. Britannica attributes the thin tree cover on Penghu and the other small islands to persistent wind. Sunshine runs the other way from rain: the cloudy north around Keelung and Taipei gets little, while Tainan in the south-west receives about 2,649 hours a year, seven a day on average. The Wikipedia survey records that temperatures have risen by 1.4°C over the last hundred years and that the sea around Taiwan is rising at twice the global rate.

What grows in Taiwan's forests

Britannica records that the island was once almost wholly forested and that a little more than half of it still is. The vegetation changes with height. Bamboo, palms and tropical evergreens grow in the lowlands, subtropical evergreen forest including camphor laurel between about 600 and 1,800 metres, and conifers above about 2,300 metres. Most of the western plains and low hills were cleared for farming after Chinese settlers arrived in the 17th and 18th centuries. The mountain forests hold endemic trees, among them the Formosan cypress and the Taiwan fir.

The Foreign Ministry's handbook of 2023 to 2024 lists 121 species of mammals, 702 of birds, 118 of reptiles, 43 of amphibians and 2,956 of fish known on Taiwan, with 5,645 species of flowering plants. Several endemic animals have become extinct or endangered since industrialisation; the Formosan clouded leopard is probably extinct and the Formosan black bear is rare. The Formosan landlocked salmon, a fish believed to have been trapped in the cold mountain streams of central Taiwan when the sea fell during the last ice age, has its own refuge on the upper Dajia River in Shei-Pa National Park. The counts differ by source: Britannica gives about 45 indigenous mammal species and about 550 species and subspecies of birds sighted, of which some 85 are indigenous, and the Wikipedia survey gives more than 4,000 indigenous plant species, over 1,000 of them found nowhere else. The handbook gives about 20% of the land as protected, in nine national parks, one national nature park, 22 nature reserves, six forest reserves, 21 wildlife refuges and 39 major wildlife habitats.

Which islands the government holds and which it claims

The main island is 394 kilometres long and 144 kilometres wide and covers 35,808 square kilometres, about 99% of the land the Republic of China controls. The Taiwan Strait separates it from Fujian province. The Wikipedia survey of Taiwan's geography gives its width as 130 kilometres at the narrowest and 220 at the widest, and the article on the strait itself gives 126 at the narrowest. It lies on the continental shelf, almost entirely less than 150 metres deep, and has been a land bridge in glacial periods. To the south the Luzon Strait, about 250 kilometres wide, separates Taiwan from the Philippines.

Britannica counts 22 islands in the Taiwan group besides the main one and 64 in the Penghu archipelago. The smaller islands differ from the main one: Penghu is known for its columnar basalt, Green Island for marine hot springs along its shore, and Britannica describes the climate of the offshore islands as closer to that of coastal south-east China. The coastline of everything the Republic of China holds measures 1,566.3 kilometres, and it claims an exclusive economic zone of 83,231 square kilometres.

Pratas and Taiping are administered by the Republic of China and are not part of the Taiwanese archipelago. Beyond what it holds the government claims more. It claims the islands it calls Diaoyutai, north-east of Taiwan, which Japan administers as the Senkaku Islands and the People's Republic also claims as Diaoyu. In the South China Sea its claim to the Paracels overlaps those of the People's Republic and Vietnam, and its claim to the Spratlys overlaps those of the People's Republic, Vietnam, Malaysia and the Philippines. The Foreign Ministry's position is that Taiping meets the definition of an island in Article 121 of the UN Convention on the Law of the Sea. This catalogue records the claims and takes no position on any of them.