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In this article we will discuss about the composition of igneous rocks and its landforms.
Wide range of variations in the structure and composition of different types of igneous rocks largely controls the landscape characteristics of a given region. Basaltic plateaux and plains give birth to picturesque landscapes after continued weathering and erosion.
Very deep and long gorges and canyons have been formed by the source segments of the Sarswati (draining towards Arabian Sea) and the Krishna rivers (draining towards the Bay of Bengal) through their vigorous vertical erosion in the massive and thick basaltic covers of Mahabaleshwar plateau (about 100 km south-west of Pune).
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Similarly, Ullahas River has entrenched a very deep gorge in the basaltic plateau near Khandala (between Mumbai and Pune). The Yellowstone River has dug out a large canyon in the Columbian lava plateau of the U.S.A.
If the sills are intruded in the tilted or inclined sedimentary layers and if they are more resistant than the surrounding sedimentary rocks, the latter are enoded more than the former and thus resistant sills project above the general ground surface as cuestas and hogbacks (fig. 8.8).
Granitic rocks when subjected to exfoliation or onion weathering give birth to domeshaped landforms known as exfoliation domes. Several exfoliation domes of granite-gneisses are seen over the Ranchi plateau, for example, Kanke Dome near Ranchi city, a group of gneissic domes near Buti village (near Ranchi city).
Massive granitic batholiths, when exposed to the earth’s surface due to removal of superincumbent load of overlying rocks through continued erosion, become interesting landforms. These dome-shaped hills project above the general surface. Such exposed granite-gneissic domes are very often found on Ranchi plateau.
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The granitic batholiths were intruded in the Dharwarian sedimentaries during Archaean period. After a long period of prolonged subaerial erosion the Dharwarian sedimentaries have been removed and the batholiths, regionally known as Ranchi batholiths, have been exposed well above the ground surface (50 to 100 m from the ground surface).
Murha Pahar near Pithauria village, located to the north-west of Ranchi city, is a typical example of exposed grantic-gneissic batholithic domes. These exposed batholithic domes have suffered intense fracture because of the removal of superincumbent load of Dharwarian sedimentaries and hence resultant massive joints have been responsible for the development of different types of ‘tors’.
The differential erosion of the basaltic ‘cap rocks’ (fig. 8.9) produces interesting features like mesas and buttes. Mesa is a Spanish word meaning thereby a table. Mesas, in fact, is such a hill which is characterized by almost flat and regular top-surface but by very steep slopes (wail like) from all sides. When mesas are reduced in size due to continuous weathering and erosion, they are called buttes.
Mesas are locally called as ‘pats’ or ‘patland’ on the Chotanagpur plateau of Jharkhand. Jamira pat, Netarhat Pat, Bagru pat, Khamar pat, Raldami pat. Lota pat etc. are typical examples of lava-capped mesas of the western Chotanagpur High Lands. Mahabaleshwar plateau and Panchgani plateau (of the Western Ghats, Maharashtra) are characteristic representatives of well-developed basaltic mesas.
Grand Mesa and Raton Mesa of the state of Colorado, USA, are typical examples of extensive mesas. Grand Mesa rises more than 1500 m (5,000 feet) above the valley- floor of the Colorado and Gunnison rivers, Raton Mesa is more than 1824 m (6,000 feet) higher than the surrounding ground surface.
Sometimes magma is injected in a vertical columnar form in the sedimentary rocks. The upper portion of vertical column of magma appears as butte when the overlying rocks are eroded down. Such butte is called as ‘volcanic butte’ (fig. 8.10).
The granitic rocks having rectangular joint patterns are weathered and eroded along the interfaces of their joints and thus smaller tables or blocks are separated by the eroded narrow clefts developed along the joints. Such granitic topography develops retangular drainage pattern (fig. 8.11).
The igneous rocks having columnar joints give birth to hexagonal landforms after weathering and erosion (fig. 8.12).
Well jointed granitic rocks give birth to very peculiar landforms such as tors which are piles of broken and exposed masses of hard rocks particularly granites having a crown of rock-blocks of different sizes on the top and clitters (trains of blocks) on the sides. The rock- blocks, the main components of tors, may be cuboidal, rounded, angular etc. in shape.
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They may be posted at the top of the hills, on the flanks of the hills facing a river valley or on flat basal (fig. 8.13).
