Sri Lanka, formerly called Ceylon, is an island country in the Indian Ocean, southeast of the Indian subcontinent, in a strategic location near major sea lanes. The nation has a total area of , with of land and of water. Its coastline is long. The main island of Sri Lanka has an area of 64, 058 km2; it is the twenty-fifth largest island in the world by area, similar in size to Tasmania. Dozens of offshore islands account for the remaining 1552 km2 area. The largest offshore island, Jaffna Island is separated by Chundikulam lagoon and Mannar Island, is linked to Adam's Bridge, a land connection to the Indian mainland, which is now mostly submerged with only a chain of limestone shoals remaining above sea level. According to temple records, this natural causeway was formerly whole, but was breached by a violent storm (probably a cyclone) in 1480.. The formation is also known as Rama's Bridge, as according to Hindu mythology, it was constructed during the rule of Lord Rama.
Sri Lanka's climate includes tropical monsoons; the northeast monsoon (December to March), and the southwest monsoon (June to October). Its terrain is mostly low, flat to rolling plain, with the mountainous upcountry in the south-central interior. The highest point is Pidurutalagala at . Natural resources include limestone, graphite, mineral sands, gems, phosphates, and clay.
Geology More than 90% of Sri Lanka's surface lies on Precambrian strata, some of it dating back 2 billion years. The granulite facies rocks of the Highland Series (gneisses, sillimanite-graphite gneisses, quartzite, marbles, and some charnockites) make up most of the island and the amphibolite facies gneisses, granites, and granitic gneisses of the Vijayan Series occur in the eastern and southeastern lowlands. Jurassic sediments are present today in very small areas near the western coast and Miocene limestones underlie the northwestern part of the country and extend south in a relatively narrow belt along the west coast.Pathirana, H.D.N.C., 1980, Geology of Sri Lanka in relation to Plate Tectonics; L. Natn. Sci. Coun. Sri Lanka v. 8, p. 75-85 The metamorphic rock surface was created by the transformation of ancient sediments under intense heat and pressure during mountain-building processes. The theory of plate tectonics suggests that these rocks and related rocks forming most of south India were part of a single southern landmass called Gondwanaland. Beginning about 200 million years ago, forces within the Earth's mantle began to separate the lands of the Southern Hemisphere, and a crustal plate supporting both India and Sri Lanka moved toward the northeast. About 45 million years ago, the Indian plate collided with the Asian landmass, raising the Himalayas in northern India, and it continues to advance slowly to the present time. Sri Lanka does not experience earthquakes or major volcanic events because it rides on the center of the plate.
The island contains relatively limited strata of sedimentation surrounding its ancient uplands. Aside from recent deposits along river valleys, only two small fragments of Jurassic (140 to 190 million years ago) sediment occur in Puttalam District, while a more extensive belt of Miocene (5 to 20 million years ago) limestone is found along the northwest coast, overlain in many areas by Pleistocene (1 million years ago) deposits. The northwest coast is part of the deep Cauvery (Kaveri) River Basin of southeast India, which has been collecting sediments from the highlands of India and Sri Lanka since the breakup of Gondwanaland.
Topography
Extensive faulting and erosion over time have produced a wide range of topographic features. Three zones are distinguishable by elevation: the Central Highlands, the plains, and the coastal belt.
The south-central part of Sri Lanka—the rugged Central Highlands—is the heart of the country. The core of this area is a high plateau, running north–south for approximately 65 kilometers. This area includes Sri Lanka's highest mountains. (Pidurutalagala is the highest at 2,524 m) At the plateau's southern end, mountain ranges stretch 50 kilometers to the west toward Adam's Peak (2,243 meters) and 50 kilometers to the east toward Namunukula (2,036 m). Flanking the high central ridges are two lower plateaus. On the west is the Hatton Plateau, a deeply dissected series of ridges sloping downward toward the north. On the east, the Uva Basin consists of rolling hills covered with grasses, traversed by some deep valleys and gorges. To the north, separated from the main body of mountains and plateaus by broad valleys, lies the Knuckles Massif: steep escarpments, deep gorges, and peaks rising to more than 1,800 meters. South of Adam's Peak lie the parallel ridges of the Rakwana Hills, with several peaks over 1,400 meters. The land descends from the Central Highlands to a series of escarpments and ledges at 400 to 500 meters above sea level before sloping down toward the coastal plains.
Most of the island's surface consists of plains between 30 and 200 meters above sea level. In the southwest, ridges and valleys rise gradually to merge with the Central Highlands, giving a dissected appearance to the plain. Extensive erosion in this area has worn down the ridges and deposited rich soil for agriculture downstream. In the southeast, a red, lateritic soil covers relatively level ground that is studded with bare, monolithic hills. The transition from the plain to the Central Highlands is abrupt in the southeast, and the mountains appear to rise up like a wall. In the east and the north, the plain is flat, dissected by long, narrow ridges of granite running from the Central Highlands.
A coastal belt about thirty meters above sea level surrounds the island. Much of the coast consists of scenic sandy beaches indented by coastal lagoons. In the Jaffna Peninsula, limestone beds are exposed to the waves as low-lying cliffs in a few places. In the northeast and the southwest, where the coast cuts across the stratification of the crystalline rocks, rocky cliffs, bays, and offshore islands can be found; these conditions have created one of the world's best natural harbors at Trincomalee on the northeast coast, and a smaller rock harbor at Galle on the southwestern coast.
Sri Lanka's rivers rise in the Central Highlands and flow in a radial pattern toward the sea. Most of these rivers are short. There are 16 principal rivers longer than 100 kilometers in length, with twelve of them carrying about 75% of the mean river discharge in the entire country. The longest rivers are the Mahaweli Ganga (335 km) and the Malvathu River (170 km). In the highlands, river courses are frequently broken by discontinuities in the terrain, and where they encounter escarpments, numerous waterfalls and rapids have eroded a passage. Once they reach the plain, the rivers slow down and the waters meander across flood plains and deltas. The upper reaches of the rivers are wild and usually unnavigable, and the lower reaches are prone to seasonal flooding. Human intervention has altered the flows of some rivers in order to create hydroelectric, irrigation, and transportation projects. In the north, east, and southeast, the rivers feed numerous artificial lakes or reservoirs (tanks) that store water during the dry season. During the 1970s and 1980s, large-scale projects dammed the Mahaweli Ganga and neighboring streams to create large lakes along their courses. Several hundred kilometers of canals, most of which were built by the Dutch in the 18th century, link inland waterways in the southwestern part of Sri Lanka.
Climate
Sri Lanka's climate can be described as tropical, and quite hot. Its position between 5 and 10 north latitude endows the country with year-round warm weather, moderated by ocean winds and considerable moisture. The average temperature ranges from a low of in Nuwara Eliya in the Central Highlands (where frost may occur for several days in the winter) to a high of in Trincomalee on the northeast coast (where temperatures may reach ). The average yearly temperature for the country as a whole ranges from . Day and night temperatures may vary by . January is the coolest month, especially in the highlands, where overnight temperatures may fall to . May, the hottest period, precedes the summer monsoon rains.
The rainfall pattern is influenced by the monsoon winds of the Indian Ocean and Bay of Bengal and is marked by four seasons. The first is from mid-May to October, when winds originate in the southwest, bringing moisture from the Indian Ocean. When these winds encounter the slopes of the Central Highlands, they unload heavy rains on the mountain slopes and the southwestern sector of the island. Some of the windward slopes receive up to of rain per month, but the leeward slopes in the east and northeast receive little rain. The second season occurs in October and November, the intermonsoonal months. During this season, periodic squalls occur and sometimes tropical cyclones bring overcast skies and rains to the southwest, northeast, and eastern parts of the island. During the third season, December to March, monsoon winds come from the northeast, bringing moisture from the Bay of Bengal. The northeastern slopes of the mountains may be inundated with up to of rain during these months. Another intermonsoonal period occurs from March until mid-May, with light, variable winds and evening thundershowers.
An increase in average rainfall coupled with heavier rainfall events has resulted in recurrent flooding and related damages to infrastructure, utility supply and the urban economy. Integrating urban agriculture and forestry into climate change action plans: Lessons from Sri Lanka, Marielle Dubbeling, the RUAF Foundation, 2014
Humidity is typically higher in the southwest and mountainous areas and depends on the seasonal patterns of rainfall. At Colombo, for example, daytime humidity stays above 70% all year, rising to over 90% percent during the monsoon season in June. Anuradhapura experiences a daytime low of 60% during the intermonsoonal month of March, but a high of 79% during the November and December rains. In the highlands, Kandy's daytime humidity usually ranges between 70 and 79%.
Ecological zones
The pattern of life in Sri Lanka depends directly on the availability of rainwater. The mountains and the southwestern part of the country, known as the "wet zone," receive ample rainfall (an annual average of 2500 millimeters). Most of the southeast, east, and northern parts of the country comprise the "dry zone,” which receives between 1200 and 1900 mm of rain annually. Much of the rain in these areas falls from October to January; during the rest of the year there is very little precipitation, and all living creatures must conserve precious moisture. The arid northwest and southeast coasts receive the least amount of rain—600 to 1200 mm per year—concentrated within the short period of the winter monsoon.