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Sri Lanka THE PHYSICAL ENVIRONMENT
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Figure 3. Topography and Drainage, 1988
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Figure 4. Precipitation and Irrigation
Source: Based on information from "Agro-Bio-Environmental Chart of Sri Lanka,"
Tokyo: Resources Council, Science and Technology Agency, 1977.
Geology
More than 90 percent of Sri Lanka's surface lies on
Precambrian strata, some of it dating back 2 billion years. 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 continuing to advance slowly
to the present time. Sri Lanka experiences few 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 hills. 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
(see
fig. 1). 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.
Data as of October 1988
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