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DECCAN BASALT VOLCANISM
".. It all happened nearly 70 million years back.. The mighty Himalayas were just beginning to come up, the great dinosaurs that ruled the earth were disappearing and the crust of the earth in the Western India was fractured. This was a remarkable event - large quantities of molten rock (Lava) were poured out from these fractures filling up valleys and covering the hills. The eruption continued intermittently with periods of quiescence, building up a huge volcanic province..." (West, W.D. 1988)
The geological world identifies this volcanic province either as the "Deccan Volcanic Province" (DVP) or simply the "Deccan Traps" and recognizes it as one of the Large Igneous Provinces (LIPs) representing high magmatic fluxes involving large amount of thermal energy in short period of geological time. The term “Deccan Trap” was coined by W.H. Sykes in 1833 and it is derived from a Sanskrit word Dakshin meaning south or southern and a Swedish word Trapp/ Trappa meaning Stair. The term was aimed to describe the step like or terrace like topography peculiar to this terrain.
A view of Mahabaleshwar section showing typical "Trappean" topography
 
In Western and Central India, DVP is exposed mainly in the states of Maharashtra, Madhya Pradesh, Karnataka, Gujarat, and Andhra Pradesh and also has its nominal presence in southern parts of Uttar Pradesh and eastern parts of Rajasthan. Based on the area of occurrence, the Deccan Lavas have been further classified into four classes viz. Malwa Trap: occurring in Malwa region of Madhya Pradesh, Mandla Traps: occurring in Mandla region of Madhya Pradesh, Saurashtra Trap: occurring in Saurashtra region of Gujarat and Main Deccan plateau: occurring in States of Maharashtra, Karnataka and Andhra Pradesh.
Map showing distribution of Deccan lavas and associated intrusives in the Western and Central India
 
Today, the DVP occupies an area of over 5,00,000 sq. km however, original estimates of the extent of the lava pile prior to erosion and possible down-throw on the western side into the Arabian Sea are of the order of 1 to 1.5 million sq km. The lava pile has a maximum thickness of over 1.5 km in the western parts of India and it reduces to few tens of meters near exposed margins of the province. Kalsubai, a prominent peak in the western parts of Ahamadnagar district bordering Thane district of Maharashtra is the tallest peak of the province (>1640m).
The DVP is largely made of basaltic lava flows with subordinate volumes of intrusive bodies and relatively trivial proportion of pyroclastics. The lava flows are nearly horizontal over vast regions of the province but they assume measurable dips in the western parts of the province (near Mumbai) and in Satpura ranges. The average thickness of the individual lava flows is around 20 m and they can be traced for few to nearly hundred kilometers.
As far as the rock sequences hidden beneath the Deccan lavas are concerned, direct evidences are provided by the localities where the Deccan lavas are seen resting over these sequences. If continuation of the older sequences is assumed beneath the Deccan Trap then, a variety of rock types including granitic rocks, sedimentary rocks as well as metamorphic rocks of Achaean to Jurassic age can be presumed to underlie the DVP. Further, crustal fragments brought up by dykes also provide a ready-made window to peep into the rocks that underlie the DVP.
Close up of a granitic xenolith occurring in a dyke exposed near Nandurbar
 
A granitic xenolith in a tinguite dyke near Bhiwandi
 
Unfortunately, such xenolith-bearing dykes are very few in number and they have so far hinted very limited variation in the crustal portion that lie below the Deccan lavas. For areas that do not have any exposures of the older sequences, geophysical methods mainly seismic and gravity surveys are useful in exploring the possibilities, and indicate the presence of largely granitic terrain underneath most parts of the DVP. The only exceptions come from the Narmada-Tapi tectonic zone where, two nearly E-W trending sedimentary basins with more than few hundred metre thick sedimentary pile have been interpreted. Last but not the least, the strontium (Sr) as well as other radio-isotope analyses of Deccan lavas too indirectly suggest presence of granitic crust and its involvement in the Deccan volcanism.
In 19th century and early part of 20th century, the lava flows were thought to be without any significant minerals of economic importance. Therefore, the province attracted only limited attention of the geologists and many of the early descriptions are restricted to the physiographic and petrographic accounts of the basaltic rocks and cavity minerals occurring in them. In 1960s Dr. G.P.L. Walker visited the DVP and described the lava flows as compound (made of many flow units) and simple (made of single flow unit). Subsequently, owing to the similarities seen between the recent eruptions in Hawaii islands and the DVP, the Hawaiian terminology was also extended to describe the Deccan lava flows. Based on surface morphology, viscosity, rate of eruption and strain rate, the Hawaiian terminology divides the lava flow into two main classes viz. a`a and pahoehoe. Akin to the Hawaiian lava flows, the a`a flows in Deccan too have upper and lower surfaces composed of rubble blocks and flattened and irregular, larger but fewer vesicles in their core.
 
Angular rubble blocks occurring near the upper surface of a typical a`a flow exposed in Diveghat area, Pune district
 
A typical view of a Flow Top Breccia of a`a flow showing the presence of angular blocks set in a basaltic matrix. Location: Shindawane Ghat, Pune district
 
Similarly, pahoehoe flows of the DVP have smooth or ropy surfaces and their interiors are characterized by pipe amygdales/ vesicles towards the base, occasional mega-vesicles as well as vesicle cylinders in the core and vesicular crusts made of numerous, small nearly spherical vesicles. Many-a-times pahoehoe flows consist of smaller units/ lobes known as toes.
 
Angular rubble blocks occurring near the upper surface of a typical a`a flow exposed in Diveghat area, Pune district
Lava toes exposed in Girna River bed in Nasik district, Maharashtra
 
In addition to the two lava flow types described above, few Deccan Lava flows share characters of a`a and pahoehoe types and thus they have been rather loosely described as “mixed flows”. Based on the distribution of predominant lava flow types, the DVP can be divided into two parts viz. areas with dominantly compound/ pahoehoe flows and area with simple / a`a flows.
 
Distribution of predominantly pahoehoe /compound and a`a/ simple flows in the Deccan Volcanic Province
 
The presence of vesicles in the lava flows is due to escape of gases during the process of cooling and solidification. These vesicles are either empty or host many varieties of secondary minerals such as zeolites, quartz, calcite etc. in them
 
A large vesicle hosting a variety of secondary/ cavity minerals. Location: Tamhani Ghat
 
Irrespective of the flow types, the lavas show various types of joints and fractures. Many of these joints have resulted during cooling of hot lava.
 
Columnar joints in lava flow from the DVP. Location: Near Kolhapur
 
A planar view of the columnar joints in a Lava flow. Location: Ajanta
 
The fractures are largely ‘post-cooling’ in age and tectonic in origin. Many a times, these fracture zones guide the development of drainages and ridges and are traceable over long distances. As a result of water and wind action facilitated by the jointed characters of the rockmass as well as the process of scarp retreat, spectacular land forms such as potholes, spheroidal weathering, mesas, buttes, spinacles etc have been carved out from the lava pile.
Fracture zones cutting across the Deccan lava pile exposed in Malshej Ghat, Thane district
 
Fracture zones/ master joints cutting across the Deccan Trap escarpment in Kalsubai ranges, Ahamadnagar district, Maharashtra
 
Spheroidal weathering in a Deccan lava flow. Location: Katraj Ghat, Pune District
 
A series of mesas and buttes from Trimbakeshwar hill, Nasik district.
 
To an unaided eye, the lava flow is usually light to dark grey, fine to medium grained rock with aphyric (without identifiable minerals) to phyric (with identifiable minerals/phenocrysts) texture. Sometimes the rock contains crystals (mainly plagioclase feldspar-an aluminosilicate) as large as 2cm or even more and is called a "megacrystic basalt" or "Giant Plagioclase Basalt (GPB)".
 
A giant phenocrystic/ megacrystic basalt horizon, Thalghat, Thane district, Maharashtra
 
A giant phenocrystic/ megacrystic basalt from Dhadgaon, Nandurbar district, Maharashtra
 
Under microscope, the rock shows fine to medium grained, equigranular to inequigranular texture and is made of primary minerals such as plagioclase, intergranular, inter-sertal, ophitic, sub-ophitic, pophyritic etc.
 
The Deccan lavas are intruded by numerous dykes and sills that show preferred occurrence along the Narmada-Tapi tectonic zone and the Western Coast. While most of the dykes of the Narmada-Tapi region trend in ENE-WSW direction, the coastal dykes are almost N-S in trend.
 
False Colour Composite (FCC) showing linear ENE-WSW trending linear ridges formed by dykes from Tapi region
 
In the other parts of the province, especially the plateau region, the dykes are fewer in number and they lack any pronounced preferred orientation. The dykes usually make linear ridges that can be traced for a long distance, even upto 50 km or more. Occasionally, dykes are more eroded than the lava flows they intrude and thus give rise to linear depressions along their trend. Individual outcrops are often well jointed or bouldery. Compositionally as well as mineralogically the dyke rocks are similar to the Deccan lavas. However, they usually have relatively coarser grain-size than the lava flows.
 
A negative impression of a dyke formed due to its preferred/ higher erosion than the lava flows. Location: Sakri, Dhule district, Maharashtra
 
The pyroclastic component of the Deccan Province is represented by bright red to green to khaki horizons that occur between successive lava flows at many places.
 
A red pyroclastic "interflow horizon" from Panhala road section, Kolhapur district, Maharashtra
 
Photomicrograph of a typical mineral assemblage and textural features in a red pyroclastic "interflow horizon"
 
These ‘inter-flow horizons’ are also known in the geological literature as ‘bole beds’. They are few cm to a couple of metre in thickness and can be traced for few metres to many kilometres. They generally have a sharp contact with the upper flow and a gradational contact with the lower flow. The association of these horizons is more common with the a’a/simple flows than the pahoehoe/compound flows. These horizons have glass shards as well as fresh phenocrysts in them and their texture is typically pyroclastics.
 
An Intertrappean bed (White coloured) topped and bottomed by Deccan lava flows. Location: Rajahmundri, Andhra Pradesh
Besides the three important components described above, a number of sedimentary horizons are intimately associated with the DVP in the states of Andhra Pradesh, Maharshtra, Madhya Pradesh, Rajasthan, Gujarat and Karnataka. Based on whether they occur immediately below or are within the successive lava flows, these horizons are known as infra- or inter-trappeans respectively. Although over 150 localities of infra- and inter-trappean beds are known, the restricted occurrence of these horizons near the fringes of the Deccan Province is noteworthy. These sediments consist of sandstone, siltstone, shale, limestone and chert and contain numerous plant and animal fossils. The dinosaurian remains are restricted to the infra-trappeans whereas the inter-trappeans have first reliable record of Angiosperms in them. The fossils yield Maastrichtian to Paleocene age.
 
Fossil assemblage from an infra-trappean bed. Location: Rajahmundri, Andhra Pradesh
 
Historically, based on presence or absence of sedimentary horizons occurring within the lava pile (inter-trappean beds) the DVP was divided into three parts viz. upper, middle and lower. However, this scheme of classification is no longer in vogue. Now-a-days, the lava pile has been divided into a number of individual flows/flow groups and their long-distance correlation has been attempted at many places. This has been done using field expressions, chemical compositional variations as well as palaeomagnetic signatures. The presence of GPBs at various levels has been extremely useful in tracing the lava flow packages over long distances especially in the western part of the province. The lava sequence has now been divided into various formations and groups and a workable stratigraphic framework for different parts of the Province has been established. Unfortunately, the studies so far have not permitted the erection of unique/unified stratigraphic column for the entire province as a whole.
Nevertheless, the present state of knowledge on the distribution of various formations has brought out southerly as well as easterly younging lava succession in the Province. This generalized disposition of the lava flows can be easily appreciated if one tries to understand the breakage and northerly migration of Indian Plate from the Gondwanaland. This event can be traced back to over hundred million years and has ultimately resulted in the collision of Indian Plate with the Eurasian Plate and rising of the mighty Himalayas. It is postulated that nearly halfway through its journey (i.e. around 65 million years) from the southern hemisphere to its present position, the Indian Plate passed over Reunion hotspot. As a result, large volume of lava flows preceded by the alkaline magmatism in Saurashtra was poured out together making up the Deccan Volcanic Province. Few years ago, this stupendous volcanic event was thought to have taken over 30 million years. However, the present 39Ar/40Ar dating of the Deccan lavas suggests a time span of not more than 5 million years for the volcanism. It has a peak activity near 65 million years ago that is at the Cretaceous-Tertiary (K-T) boundary. This coincides with the extinction of the mighty dinosaurs from the face of the Earth.
Even though, the role of a hotspot (or alternatively mantle upwelling through rifts) is widely accepted to be the reason for Deccan volcanism, the mode of transportation of liquid lavas over a large distance without freezing enroute is not yet fully understood. This is due to the fact that the undisputable evidences of sites of eruption are still illusive and the fracture fillings (dykes) earlier thought to be feeders to the Volcanic Province, are now largely shown to be shallow level (hypabyssal) intrusions.
Recent studies in active volcanic fields (for example, Hawaii) have provided certain clues that can explain the transportation of lava flows to great length even at a very gentle slope. In Hawaii, such effective lava transportation system comprise of channeled lava flows, lava flows in lava tubes as well as inflation of lava sheets (like water filling in a balloon). Out of these different transportation systems, few field evidences of possible lava channels and inflated lava sheets have already been found and few others are being scrutinized for their proper identification in the Deccan Province.
 
A sinus lava channel at Gunjale, Ahamadnagar district, Maharashtra
Although the DVP does not host any conventional mineral deposits, the lava flows have been proved to be ideal sites for Buddhist and Jain monuments that date from 2000 BC to 500 AD. The strategic importance of the hillocks was well understood and exploited by the Yadava as well as Maratha kings to build more than 350 forts in different parts of their kingdoms. Besides, in a number of temple complexes constructed in different parts of Maharashtra large blocks of basalt have been used as building stones. It is interesting to note that the compound/pahoehoe flow-types were preferred by the sculptors and architects over the a`a flows. In recent past, the blocks of Deccan basalts and associated rocks have also been used by the European rulers to build majestic buildings like Gateway of India, Victoria Terminus etc.
 
A panoramic view of Ajanta caves, carved out of a compound flow.
Kailash temple, Ellora Caves, a marvelous rock cut architecture in a compound flow.
Now-a-days, the cavity minerals hosted by the lava flows have attracted the attention of mineral collectors and many of these minerals have been displayed in museums world over.
 
Apophylite from Rahuri, Ahamadnagar
 
Blue cavenzite and heulandite, Pune
 
Green apophylite and pink stilbite
 
Source: Central Region, GSI

 
 
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