With a total area of nearly 36,000 sq. km, Taiwan is separated from China by the Taiwan Strait, which is about 220 km at its widest point and 130 km at its narrowest. The island is almost equidistant from Shanghai and Hong Kong.

The surface geology of the island varies in age from very recent alluvial deposits to early sedimentary and crystalline rocks. The structure is formed by a tilted fault block running roughly northeast to southwest along the entire length. The steep slope of this tilted block faces east and the rock mass slopes more gently to the west. This block is composed primarily of old rocks, some of which have been subjected to heat and pressure. Only one-third of the land area is arable. The mountains are mostly forested, with some minerals, chiefly coal, at the northern end.

On the east coast, the mountains rise steeply from the Pacific. To the west, the level sediments lie just below the surface of the sea. As a result, river deposits have filled the shallow waters and extended the land 15 to 30 km westward from the foothills, giving Taiwan a larger proportion of useful level land than either Japan or the Philippines. Natural resources and agricultural potential make this coastal plain of great importance.

The shoreline of Taiwan is relatively smooth and unbroken with a total length of 1,566 km (including the Penghu Islands). Off the southern end of the island lie small areas of coral reefs, which have built up along the island's shores.

The most important feature of Taiwan's topography is the central range of high mountains running from the northeast corner to the southern tip of the island. Steep mountains over 1,000 meters high constitute about 31 percent of the island's land area; hills and terraces between 100 and 1,000 meters above sea level make up 38 percent; and alluvial plains below 100 meters in elevation, where most communities, farming activities, and industries are concentrated, account for the remaining 31 percent. Based on differences in elevation, relative relief character of rock formations, and structural patterns, the island can be divided physiographically into five major divisions: mountain ranges, volcanic mountains, foothills, tablelands, and coastal plains and basins.

Central Range
The Central Range extends from Suao in the north to Eluanbi in the south, forming a ridge of high mountains and serving as the island's major watershed for rivers and streams. The range is predominantly composed of hard rock formations resistant to weathering and erosion, although heavy rainfall has deeply scarred the sides with gorges and sharp valleys. The relative relief of the terrain is usually extensive, and the forest-clad mountains with their extreme ruggedness are almost impenetrable. The east side of the Central Range is the steepest mountain slope in Taiwan, with fault scarps ranging in height from 120 to 1,200 meters.

Mount Snow Range
The Mount Snow Range lies northwest of the Central Range, beginning at Sandiao Jiao in the northeast and gaining elevation as it extends toward the southwest. Mount Snow, the main peak, is 3,884 meters high.

Mount Jade Range
The Mount Jade Range runs along the southwestern flank of the Central Range. It includes the island's tallest peak, the 3,952-meter Mount Jade.

Mount Ali Range
The Mount Ali Range lies west of the Mount Jade Range, with major elevations between 1,000 and 2,000 meters. The main peak, Mount Ta, towers 2,484 meters.

East Coastal Range
The East Coastal Range extends from the mouth of the Hualien River in the north to Taitung County in the south, and chiefly consists of sandstone and shale. Although Mount Singang, the highest peak, reaches an elevation of 1,682 meters, most of the range is composed of large hills. Small streams have developed on the flanks, but only one large river cuts across the range. Badlands are located at the western foot of the range, where the ground water level is the lowest and rock formations are the least resistant to weathering. Raised coral reefs along the east coast and the frequent occurrences of earthquakes in the rift valley indicate that the fault block is still rising.

Volcanic Mountains
Although igneous rocks are not commonly found in Taiwan, smaller outcroppings of extrusive bodies are scattered over the island, representing at least five periods of igneous activity.

The Datun mountain area is a prominent group of volcanic peaks lying at the promontory between Keelung Harbor and the Danshuei River and overlooking the Taipei metropolitan area. The entire area is covered by lava that poured out of the volcanic craters, which now stand as conical notches of over 1,000 meters. The area is unique for its hot springs and fumaroles.

Along the Central Range, the Keelung-Miaoli foothills and those extending from Chia-I to Pingtung are the broadest. The Keelung-Miaoli foothills start from the coast at Keelung and end south of Miaoli. The Chia-I foothills are located below Mount Ali, with its northern border on the Jhuoshuei River and southern border between Kaohsiung and Pingtung. There is a shallow-faulted region between these foothills and the Fongyuan foothills, extending from Fongyuan, just north of Taichung, to Nantou , some distance to the south. This is the widest section of western foothills in Taiwan. It is intersected by three rivers: the Dajia, Dadu, and Jhuoshuei. Included in this region is the Sun Moon Lake Basin, which lies about 765 meters above sea level and forms a graben basin. At the southern flank of the Central Range are the Hengchun foothills that occupy most of the Hengchun Peninsula. The topography is downgraded on the eastern and western sides.

The coastal plains are generally covered with gravel, sand, and clay, with an average slope of between half a meter and one kilometer. Slopes are gentle enough to eliminate the need for major terracing and are rarely subject to serious soil erosion. The western edge of the plain, where it meets the Taiwan Strait, is marked by wide tidal flats, and the coast is swampy. Shore currents have built up a series of spits and offshore bars, with many lagoons formed by shoreward shifting of the sandbars.

The Jianan Plain is the broadest in southwestern Taiwan, extending from Changhua to Kaohsiung. It is about 180 km long and 43 km wide at its broadest point, making up more than 12 percent of the total land area of Taiwan. Next largest are the Pingtung Plain and the Ilan Plain. Finally, there are two major basins, the Taipei Basin and the Taichung Basin.

The East Longitudinal Valley is an extremely narrow fault valley in proportion to its length. It has a general elevation of about 120 meters above sea level and dips slightly toward the east. Coalescing alluvial fans have developed at the foot of both sides, and the river beds are filled with gravel. Due to repeated movements along the fault line and frequent shocks, subordinate watersheds have developed in the valley.

Taiwan has 151 rivers and streams. The Jhuoshuei River is the longest (186 km), while the Gaoping River has the largest drainage basin (3,257 sq. km).

Climate

Taiwan is in the trade wind belt of the planetary wind system and is greatly affected by the seasonal exchange of air masses between the continent and the ocean. Besides location and topography, the winter (northeast) and summer (southwest) monsoons are the main factors controlling the climate of Taiwan.

The different directions of the winter and summer monsoons cause seasonal distribution of rainfall in northern Taiwan to be different from that in the south. The northeast monsoon in the winter lasts about six months from October to late March and brings steady rain to the windward (northeast) side of the island. The central and southern parts of the island, however, are on the leeward side of the northeast monsoon; thus, they have sunny winters, with less than 30 percent of their annual precipitation falling at this time.

The annual "plum rain" season in May and June brings a lot of precipitation. In the summer, the southwest monsoon prevails for about five months, beginning in early May and ending in late September. During this period, southern Taiwan usually has wet weather, while northern Taiwan is relatively dry. The moisture, carried by the southwest monsoon and local terrestrial winds, falls largely in convectional form. Thundershowers and typhoons bring Taiwan heavy rainfall during the summer months.

The largest earthquakes in the past 100 years include a 7.1 magnitude temblor that killed more than 3,250 people in 1935; a 6.8 magnitude quake on November 14, 1986, which killed 15 and injured 44; and a powerful and devastating earthquake that struck at 1:47 a.m. on September 21, 1999, toppling high-rise buildings, damaging roads and bridges, and severing powerlines across the island. This last quake registered a magnitude of 7.3 on the Richter scale and had its epicenter at Jiji township in Nantou County. More than 1,300 aftershocks were reported by the morning of September 22, with the strongest registering a magnitude of 6.8 in central Taiwan.

The release of tectonic energy in the aftershock area of the "921 Earthquake" continued in 2000. Three strong aftershocks with magnitudes of 5.3, 6.7, and 6.1 were reported in the area on May 17, June 11, and July 29, respectively. In addition, a strong earthquake with a magnitude of 6.2 occurred to the north of Hualien in eastern Taiwan on September 10.

In 2001, 16,244 earthquakes were detected and 136 larger earthquakes were felt and reported by the Central Weather Bureau. Three earthquakes registering a magnitude larger than 6 on the Richter scale occurred on the offshore area of eastern Taiwan. On December 18, the biggest quake of the year hit in the offshore area of Hualien City, registering a magnitude of 6.8 but causing no damage. Compared with the previous two years, seismic activity in western Taiwan (the epicenter for the Jiji earthquake) has been decreasing, while seismic activity in eastern Taiwan has been quite active.