Taiwan -- Geographical Features

As introduced by the Yearbook of the Republic of China:

Introduction

Off the eastern coast of Asia lie the mountainous island arcs of the Western Pacific. The island chain closest to the continent marks the edge of the Asiatic Continental Shelf. Taiwan, one of the islands of this chain, is the largest body of land between Japan and the Philippines.

The island of Taiwan is 394 km long, 144 km at its widest point, and shaped like a leaf. It is located between 21°53'50"and 25°18'20"N latitude and between 120°01'00"and 121°59'15"E longitude.

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, 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.

Mountain Ranges

Taiwan's five longitudinal mountain ranges occupy almost half of the island. As a group, they extend 330 km from north to south and average about 80 km from east to west. They include more than two hundred peaks with elevations of over 3,000 meters.

Central Mountains

The Central Mountains extend from Su-ao 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 mountains are 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 Mountains is the steepest mountain slope in Taiwan, with fault scarps ranging in height from 120 to 1,200 meters.

Syue Mountains

The Syue Mountains lie northwest of the Central Mountains, beginning at Sandiaojiao in the northeast and gaining elevation as it extends toward the southwest. Syue Mountain, the main peak, is 3,886 meters high.

Jade Mountains

The Jade Mountains run along the southwestern flank of the Central Mountains. They include the island's tallest peak, the 3,952-meter Jade Mountain.

Alishan Mountains

The Alishan Mountains lie west of the Jade Mountains, with major elevations between 1,000 and 2,000 meters. The main peak, Data Mountains, towers 2,663 meters.

East Coast Mountains

The East Coast Mountains extend from the mouth of the Hualien River in the north to Taitung County in the south, and chiefly consist of sandstone and shale. Although Singang Mountain, 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 Port 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.

Foothills

Taiwan's foothills are found in a narrow zone surrounding the Central Mountains. This zone, with an elevation of from 100 to 500 meters, is connected with the Central Mountains and linked with the tablelands in continuous slopes. Low hills with gentle slopes and longitudinal valleys woven with transverse gullies are characteristic topographic features of this zone, as are broad escarpments and short hogbacks formed on fault scarps or along rock formations.

Along the Central Mountains, the Keelung-Miaoli foothills and those extending from Chiayi to Pingtung are the broadest. The Keelung-Miaoli foothills start from the coast at Keelung and end south of Miaoli. The Chiayi foothills are located below Alishan (Ali Mountain), 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 Mountains are the Hengchun foothills that occupy most of the Hengchun Peninsula. The topography is downgraded on the eastern and western sides.

Terrace Tablelands

From the foothills, Taiwan's terrain is gradually reduced to tablelands from 100 to 500 meters in height. These thick deposits of well-rounded sandstone gravel are usually accumulations of eroded material washed down from higher areas, though some of the gravel beds may have been deposited near the sea and then raised into flat-topped tablelands by recent tilting. The broadest tableland is the one between Taoyuan and Hsinchu in northern Taiwan. Next in size are the Houli Terrace in Taichung, the Dadu Terrace and the Bagua Terrace in Changhua, and the Hengchun Terrace in southern Taiwan.

Coastal Plains, Basins, and Valleys

To the west, the physical character of Taiwan changes through the foothills zone to the alluvial plain. Topographically, the coastal plains and basins are monotonously flat, except near the foothills. All of the larger rivers running through the plains have their sources in the high mountains. Flowing out of the western foothills, these rivers diverge into a number of channels and meander to the ocean, forming large alluvial deltas. Many of these have been linked by irrigation and drainage canals.

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 Yilan 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.

Rivers

The Central Mountains are the major watershed for Taiwan's rivers and streams; thus, most rivers in Taiwan flow in either an easterly or westerly direction. They are short and steep, especially on the eastern side of the island, and become torrential during heavy rainstorms, carrying heavy loads of mud and silt. The riverbeds tend to be wide and shallow, making them difficult to manage and develop as water resources.

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).

Natural Vegetation and Soils

Because of Taiwan's location, plant species are diverse and abundant. The high altitude of the island's mountains provides climatic and vegetation zones ranging from subtropical to alpine. Except for the western coastal plain and the Penghu Islands, Taiwan was once entirely covered by forests. The forested area today is estimated at 1.9 million hectares.

Acacia trees are ubiquitous on lower hills. Bamboo groves and forests are found naturally in central and northern Taiwan, whereas in the south, most stands of bamboo are cultivated on farms. Outside of forests, bamboo is normally confined to relatively moist areas; thus, it can be cultivated almost anywhere in the Taiwan area.

The flora of Taiwan resembles that of China. A wide range of Asian subtropical species are found in the lowlands, and low altitude flora is closely related to that of China's southern provinces. Mountain flora is related to that of western China, and high alpine flora to that of the Himalayan region.

Soils vary in fertility. Many have lost their natural fertility after centuries of irrigation and heavy rainfall. In the north, the soils of arable land are primarily acid alluviums and diluvial latosols, some of which are residuals. In the southwest, where agricultural production is concentrated, most of the arable soils are alluviums of neutral to weak alkalinity and planosol-like alluviums. Upland soils of mountainous areas are mostly lithosols, which are usually thin and infertile.

Climate

Situated off the east coast of Asia and in the path of warm ocean currents, Taiwan has an oceanic and subtropical monsoon climate, conspicuously influenced by its topography. Summers are long and accompanied by high humidity, while winters are short and usually mild. In the coldest months, snow is visible on the peaks of high mountains. Frost is rare in the lowlands, where most of the population lives and works. In winter times, the mean monthly temperature ranges from 15 to 20 around Taiwan, while it is about 28 in summer season. The relative humidity averages about 80 percent.

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 changes in wind direction 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 period.

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.

Taiwan lies in the path of many tropical storms/typhoons in East Asia. With their violent winds and extremely heavy rainfall, these storms often cause severe damage, especially to crops. Nevertheless, they are one of the main sources of water in the Taiwan area. During a typhoon, windward mountain slopes may receive as much as 300 mm of rainfall in 24 hours. An average of three to four typhoons hit Taiwan every year, usually in July, August, or September. In 2002 and 2003, four tropical storms/typhoons -- Nakri, Sinlaku, Morakot, and Dujuan -- hit the Taiwan area. Among them Dujuan brought torrential rains, especially in Maobitou, which received 282 mm of rainfalls in a single hour.

The distribution of water resources in Taiwan is uneven, both in time and space. Rivers can be characterized as steep and rapid. These characteristics mean that in spite of Taiwan's abundant rainfall, water available for use per capita is low. According to a statistical analysis by the Water Resources Agency under the Ministry of Economic Affairs, the mean annual rainfall in the Taiwan area is 2,483 mm, based on data collected from 1949 to 2002 at 464 rainfall gauging stations. Rainfall is most abundant in the northern region, with the mean annual rainfall at 2,938 mm, followed by the central region at 2,121 mm, the southern region at 2,476 mm, and the eastern region at 2,545 mm. The southern area of Taiwan receives 90 percent of its rainfall between May and October. In the north, the seasonal distribution of precipitation is more even, with 60 percent falling between May and October. Throughout the entire Taiwan area, the driest months occur between November and February.

The government's policies for water resources emphasize exploration, river management, and damage mitigation. Five major projects have been implemented to manage rivers: (1) overall planning for the Keelung River, which meanders through the Taipei area; (2) improving drainage in the Southern Taiwan Science Park; (3) maintaining levee and local drainage along the coast; (4) enforcing the River Management Regulations to ensure river dredging; and (5) supervising local governments in removing dumps in regulated areas to make room for rivers.

At the end of 2003, drilling of the Keelung River Yuanshanzih Flood Diversion Tunnel was completed. It's expected that the flood diversion tunnel will be able to protect land and residents along the Keelung River during the flood season. After other related projects such as improvement of run-off channels, pumping stations, and improvement of bridges are concluded, the flood diversion project is expected to protect 713 hectares of land in Taipei County and 316 hectares of land in Keelung City.

Earthquakes

Taiwan has a high degree of seismic activity due to its location at the junction of the Manila Trench and the Ryukyu Trench along the west side of the Philippine Sea plate. The collision of the Philippine Sea plate and the Eurasia plate, which created the uplift of land that became Taiwan's four major mountain ranges, continues to push against each other. Most of the Taiwan area is under northwest-southeast compression, with a measured convergence rate of about eight cm per year.

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.

According to the final statistics released by the National Fire Administration of the Ministry of the Interior, the massive "921 Earthquake" caused the deaths of 2,415 people and injured 11,305. The greatest number of casualties was in Taichung County, with 1,175 dead and 6,190 injured.

On October 22, 1999, another major earthquake occurred 2.5 km northwest of Chiayi City at 10:19 a.m., registering 6.4 on the Richter scale. Although there were no deaths, 122 were injured, and ten buildings were severely damaged or collapsed. There were also 37 cases of gas leaks and 4 cases of fire.

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 2002, 28,907 earthquakes were detected, including 196 larger earthquakes reported by the Central Weather Bureau. Six earthquakes registering a magnitude larger than 6 on the Richter scale occurred in the offshore area of eastern Taiwan. On March 31, the biggest quake of the year hit the area off Hualien city, registering a magnitude of 6.8. Compared with the previous three years, seismic activity in western Taiwan (the epicenter for the Jiji earthquake) has been decreasing, while seismic activity in eastern Taiwan has remained active.

On December 10, 2003, an earthquake registering 6.6 struck Taitung County, followed by 186 aftershocks within three hours. In Taipei, the quake reached 3 on the Richter scale.