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In this article we will discuss about:- 1. Definition and Distribution of Deccan Traps 2. Origin of Deccan Traps 3. Petrology 4. Structural Features 5. Classification 6. Life 7. Age.
Definition and Distribution of Deccan Traps:
Deccan Traps have been defined as the greatest volcanic formation of the Indian subcontinent that consists of congealed lava flows covering an area of more than 400,000 square kilometers, with a thickness estimated at about 3000 metres. These volcanic rocks occupy greater parts of Kutch and Kathiawar in Gujarat and are also spread over Andhra Pradesh, Madhya Pradesh and Maharashtra. After the Archean Rocks, these (Deccan Traps) may be described as the most extensive geological formations of Peninsular India.
Origin of Deccan Traps:
From a general study of the Deccan Traps, the following broad conclusions can be drawn about their mode of eruption:
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(a) They were erupted in a sub-aerial environment (and not subaqueous, that is under water). This is suggested by their relation with the older rocks.
(b) Eruptions of the lava took place along linear fissures, forming fissure-type of eruption and not from any central cone-type volcanoes. This is explained by numerous flows and their remarkably consistent horizontal (or nearly so) attitude.
(c) The lava when erupted was in a highly liquid form, which is indicated by the long distances over which the individual flows could spread. This fact also suggests that the lava might have been super-heated to reach that state of mobility.
(d) A large number of dykes that have been found occurring intersecting many lava flows might be quite younger to those flows, but some such dykes might have also been the feeder dykes for the volcanic eruptions.
(e) The lava flows, especially after the beginning and before the close of activity, passed through definite periods of no volcanic activity. During such periods streams and lakes appeared on the cooled and congealed volcanic land forms here and there and both animal and plant life domiciled these areas. This is indicated by the occurrence of inter-trappean layers found in the lower and upper flows and their fossil contents.
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Petrology of Deccan Traps:
The most important rock of Deccan Trap is Basalt, of augite-basalt type, with a grayish green colour. It often shows vesicular and amygdaloidal textures. Acidic volcanic rocks like rhyolites, granophyres and more basic rocks like gabbro and limburgite have also been found to form intrusions in some areas like Kutch and Bombay.
Microscopic analysis of Trap basalts show them as fine-grained basalts, remarkably free from olivine. Augite and felspars are the chief constituents whereas calcite, quartz, glauconite etc. are some of the common accessory minerals that commonly fill the cavities in the original basalts.
Structural Features of Deccan Traps:
Like a uniform mineralogical and chemical composition, traps display a remarkable uniform and consistent structure- they are layered and the layers are almost horizontal everywhere, average dip (angle of inclination) being between 5°-10°.
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The individual layers, called lava flows, are variable in thickness, which may be anything between 3 metres to 30 metres or even more. Compared to their thickness, the areal extent of the flows is often many times more, reaching 100 km in quite a few cases.
The successive flows are intervened at many places by non-volcanic sedimentary layers called the Inter-Trappean Beds. These indicate deposition in shallow water lakes and rivers that came into existence during the lull periods in volcanic activity of the age. The inter-trappean layers have yielded at places valuable fossils of animals and plants that could thrive during those periods of no volcanic activity. These fossils have been used in the interpretation of the age of the Traps.
Classification of Deccan Traps:
The Traps have been classified into three groups namely, Upper Traps, Middle Traps and Lower Traps.
Their main characters are summarized in the following table:
Life of Deccan Traps:
Many animal and plant fossils have been found from the inter-trappean beds in the lower and upper divisions of the Traps.
The Inter-trappean beds are composed chiefly of volcanic detritus, clay, impure limestones and similar material eroded from the congealed volcanic flows. Such beds have been studied at many places in Maharashtra, Madhya Pradesh and Gujarat. The inter-trappean beds appear in excellent sections in Malabar hills of Bombay where they are made up of dark shaded shales.
In the list of fossils yielded by Inter-trappean beds are included Cerithium, Potamides, Hydraspis, Rana and Paudina, all of which are fresh water or estuarine environment organisms. No fossils of marine life have been recovered from Deccan Traps.
Age of Deccan Traps:
There is as yet no broad agreement on the exact geological age of the Deccan Traps. Two positions have been suggested. According to the stratigraphic evidence, the Deccan Traps should be considered equivalent to Danian Stage of Upper Cretaceous. According to the second view, based primarily on fossil evidence, a Lower Eocene age is more appropriate.
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This is supported by recovery of some typical Tertiary system fossils of Palms and Fishes. Anyhow, considering the huge thickness of the Traps (more than 2000 m at places), it could be broadly stated that the activity resulting in the deposition of Deccan Traps started somewhere in the Upper Cretaceous and continued through the change of era right upto and even into the Lower Eocene period of Tertiary times.