The Ganga Geology - The Ganga: Water Use in the Indian Subcontinent - page 15

Chapter 3
The Ganga Geology
The Ganga Basin
According to the theory of plate tectonics, the crust of the earth is formed by a
number of relatively rigid plates. They shift and move because of their spread and
subduction of sea-floors. The continental mass broke up and re-formed several times
in 4 1 / 2 billion (4,500 million) years of the earth's history. In course of development,
the shape of the earth changed continuously, owing to various geological processes.
Some of these are so slow that it is practically impossible for anybody, within his
life-span, to detect them but when these occur across billions of years, they enor-
mously transform the earth's shape. Eruptions of volcanoes and quakes are of short
duration and their effects are directly observed. Geologists have established that in
the history of the earth's development, there have been numerous crucial changes of
its physiographic conditions, renewing the organic world.
In the early Triassic period, most of the earth's land formed a single continental
mass, called Pangaea; it was surrounded by one ocean, called Panthalassa. The last
continental break-up occurred about 200 million years ago when the plates began
moving in diverse directions. Pangaea first split into two masses - Laurasia and
Gondwana. Laurasia later broke into three - the westernmost mass featured North
America and the two eastern masses formed most of the Asian-European land mass.
The Asian mass is believed to be carried on two plates - the Eurasian and the East
Asian. In the Jurassic period, the Indian portion of the Gondwana mass split and
began to move north, toward Asia. Apparently, the Indian portion was on the same
plate as Australia and both moved north, as the plate shifted owing to the sea-floor
spread in the newly formed the Indian Ocean. It swung north fast and collided with
the East Asian and Eurasian plates in the Eocene period (54-38 million years ago).
This led to the rise of the Himalayas in the north and the Arakan Yomas on the east.
The Indian plate was subducted under the East Asian plate along the Himalayas but
along the Arakan Yomas, the two plates were connected by a transform fault, as
shown in a Fig. 3.1.
In the Oligocene period (40-30 million years ago) a part of the north-eastern
Indian mass fractured and sank below the sea-level. It was filled over the next 37
million years to form the Bengal Basin. Bangladesh was formed on a mass of sed-
iment, left by very old rocks of the Gondwana continent. On the two sides of the