cones and craters on mount pavagadh, deccan traps

Cones and craters on Mount Pavagadh, Deccan Traps:Rootless cones?

Hetu C Sheth∗, George Mathew, Kanchan Pande, Soumen Mallick andBalaram Jena

Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.∗e-mail: [email protected]

Rootless cones, also (erroneously) called pseudocraters, form due to explosions that ensue whena lava flow enters a surface water body, ice, or wet ground. They do not represent primary ventsconnected by vertical conduits to a subsurface magma source. Rootless cones in Iceland are wellstudied. Cones on Mars, morphologically very similar to Icelandic rootless cones, have also beensuggested to be rootless cones formed by explosive interaction between surface lava flows andground ice. We report here a group of gentle cones containing nearly circular craters from MountPavagadh, Deccan volcanic province, and suggest that they are rootless cones. They are very similarmorphologically to the rootless cones of the type locality of Myvatn in northeastern Iceland. A groupof three phreatomagmatic craters was reported in 1998 from near Jabalpur in the northeasternDeccan, and these were suggested to be eroded cinder cones. A recent geophysical study of theJabalpur craters does not support the possibility that they are located over volcanic vents. Theycould also be rootless cones. Many more probably exist in the Deccan, and volcanological studiesof the Deccan are clearly of value in understanding planetary basaltic volcanism.

1. Introduction: Mount Pavagadh,Gujarat

The Deccan Traps of India constitute one of thefinest and best-exposed continental flood basalt(CFB) provinces of the world, with a present-day areal extent of 5 × 105 km2 and an originalextent estimated to be at least thrice as much(e.g., Wadia 1975). This CFB province is madeup largely of subalkalic, tholeiitic basalt lavas,but strongly undersaturated-alkalic and felsic rocktypes, and regional dyke swarms of mafic and othercompositions, are exposed in parts of the provincesuch as the western Indian rifted margin and theNarmada-Satpura-Tapi rift zone (e.g., Mahoney1988). Pahoehoe basalt flows constitute a large vol-ume of the Deccan lavas (Keszthelyi et al 1999;Bondre et al 2004). Very few feeder dykes or erup-tive vents have however been identified in the Dec-can province.

Mount Pavagadh (829 m, figure 1), with oneof the holiest shrines of the Hindus at its sum-mit, is an imposing mountain in Gujarat madeup of Deccan lavas and surrounded by Precam-brian basement rocks. A modern-type geochem-ical, petrogenetic and stratigraphic study of thelava pile forming the mountain is lacking, but con-siderable petrological diversity is known: a thicksequence of mafic-intermediate lavas (mostly tra-chyandesite, basalt and some picrite/ankaramite)is capped by a thick rhyolitic flow (Chatterjee1961). The mountain is surrounded by smallerhills, some of which apparently represent individ-ual satellite vents. Rhyolite outcrops, and volcanicbreccia deposits with clear quaquaversal dips, areseen in these hills. Here we describe the shal-low cones with craters that are found on theuppermost mafic lava flow of Mount Pavagadh,below the summit rhyolite. We assume a generalfamiliarity on the part of the reader with basic

Keywords. Deccan volcanism; Iceland; Mars; Pavagadh; rootless cones; pseudocraters.

Proc. Indian Acad. Sci. (Earth Planet. Sci.), 113, No. 4, December 2004, pp. 831–838© Printed in India. 831

832 Hetu C Sheth et al

Figure 1. Topographic sketch-map of mount Pavagadh. Thick grey lines are near-vertical cliffs. The pilgrim route fromMachi to the summit is shown as a dashed line. The rootless cones shown are schematic only. Inset shows the main outcropof the Deccan flood basalt province in western and central India (shaded), and the locations of Pavagadh, Jabalpur, andBombay.

volcanic landforms and volcanological terminology,and recommend an informative and nicely illus-trated website managed by Vic Camp, located atwww.geology.sdsu.edu/how−volcanoes−work.

The cones are found on the northwestern sideof the Pavagadh plateau and have craters nestledwithin them (figures 1–3). The cones are nearlycircular and 50–80 m wide, and very shallow inrelation to their diameters (saucer-shaped), withraised rims that gently slope away from the craters.Although this shape is reminiscent of maars, theyare undoubtedly somewhat eroded down sinceeruption of the Deccan lavas 65–60 million yearsago. The ratio of the crater diameter to cone diame-ter is high (> 0.5), and the raised rims stand < 5 mabove the crater floors. During our first field visitin December 2003, after the monsoon season, thelargest crater contained a shallow lake, but the oth-ers were dry (figure 2). However, during our secondfield trip in June 2004, the lake in the large crater

had dried up (figure 3) and the crater floor exposedsoft reddish-brown soil. Indeed, without the craterlake these gentle cones with very shallow craterswould most likely have been missed by us, as theynearly resemble an ordinary erosional landscape.This must be why, to the best of our knowledge,they have gone unreported by previous workers.We propose that they are a volcanic landform sofar unidentified in India, namely, rootless cones.

2. What are rootless cones, andhow do they form?

Many volcanic terrains all over the globe containdeposits and landforms that indicate interactionof lava, or magma, with surface or near-surfacewater. Phreatic is the term used for steam explo-sions resulting from conversion of water to steamdue to magmatic heat, but if juvenile magma is

Pavagadh cones and craters 833

Figure 2. View of the cliffs and the plateau NW of the rhy-olite (see figure 1) showing a nearly circular, shallow craterlake at left, a large rootless cone (dry) at right, and a muchsmaller one between these two. Note how the low rim of thelarge, dry rootless cone gently slopes away from it. Bushesand small trees on the rim provide a scale. The cliff faceclearly exposes two distinct basalt flows. December 2003.

Figure 3. View (looking west) of the large crater with thedried-up shallow lake. The maximum depth of the waterwould have been about a metre only. People at left for scale.June 2004.

also thrown out of a volcanic vent besides steamand fragments of the country rock, the explosionsare described as phreatomagmatic. Sheridan andWohletz (1983) suggest hydrovolcanism as a gen-eral term for eruptive phenomena generated by theinteraction of magma or magmatic heat with anexternal source of water, such as a surface waterbody or an aquifer, and hydrovolcanic processesoccur at volcanoes of all sizes ranging from smallphreatic craters to huge calderas, and even withinhydrothermal zones related to plutons a few kilo-metres deep (Sheridan and Wohletz 1983).

Rootless cones are hydrovolcanic cones, madeup of spatter, scoria, and ash, that form in andrest directly on tube-fed, inflated pahoehoe lavaflows (for a model of formation of inflated pahoe-hoe flows, see Self et al 1997, 1998). Rootlesscones form when lava flows interact with surface

or near-surface water and the resulting mild explo-sions produce scoria and spatter that accumulateto form small cones around the explosion spots(Thorarinsson 1953). Typical locations where thishappens are stream or river beds, lakes, or water-saturated sediments. The typical feature of rootlesscones is internal stratification with reverse grading(coarser material toward the top), and the conesare capped with welded spatter, which sequenceindicates decreasing explosivity during the courseof their formation, arguably due to volatile deple-tion. A single cone is usually the result of sev-eral explosions attendant upon rapid groundwaterrecharge (Thordarson et al 1992; Thordarson andHoskuldsson 2002).

A rootless cone may form when lava flowingthrough an internal pathway through a lava flow(a lava tube) presses down into the underly-ing, lower-density, water-saturated sediment (fig-ure 4a). Cracks develop in the lower lava crust andbring about contact of and explosive interactionbetween the lava and the water-saturated sedi-ment. When an explosion occurs (figure 4b), fur-ther lava flow movement (and further rootless coneformation in the downflow direction) is halted, butmore rootless cones can form upflow, and along thenew lava tube network that progressively develops(Thordarson 2000; Thordarson and Hoskuldsson2002; Bruno et al 2004). Rootless cones, thus, arenot primary magmatic vents directly connected toa subsurface magma source, and for this reasonhave also been termed “pseudocraters”. The termis however, erroneous, as there is nothing “pseudo”about these craters which themselves are quite real(Th. Thordarson, pers. comm., 2004).

Thorarinsson (1953) was the first to showunequivocally that rootless cones in Iceland wereformed by explosive interaction between lava andsurface water, though the cones were describedby several workers before him. The mechanismproposed by him for rootless cone formation wasessentially a “static” one, in that it envisaged lavaadvance over a waterlogged substrate and vapour-ization of the water by purely conductive means.When the vapour pressure exceeded the overbur-den pressure, an explosion would result. The modelwas also based on simple cooling of an instanta-neously emplaced flow unit and did not considercontinued recharge of lava to the explosion site viapreferred pathways (such as lava tubes) in the flow,and repeated explosions. In contrast to the staticmodel, Thordarson (2000) proposed a “dynamic”heat transfer model which envisaged physical mix-ing of lava and substrate sediments, and resultedin highly efficient heat transfer and vapourizationof the water and thereby thorough fragmentationof the lava and ejection of the fragments out of theexplosion site. For this model to work, lava must


Figure 4. Proposed mechanism for rootless cone formation. (a) Lateral lava feeder (lava tube) supplies lava to the flowfront, and the lava presses into the waterlogged sediment and cracks develop in the basal crust of the tube. (b) Physicalmixing of the lava and waterlogged sediment causes a rootless eruption. (Simplified from Fagents and Thordarson (2004)and Bruno et al (2004))

continuously mix with the substrate, and for this,lava must be able to move continuously through theflow field and feed into the explosion site. This canbe the case for a tube-fed, inflating pahoehoe lavaflow (figure 5; see Fagents and Thordarson 2004).

3. Rootless cones in Iceland

Iceland is a large and entirely volcanic island,and the only emergent part of the global mid-ocean ridge system (figure 6). Icelandic rootlesscones typically exhibit a well-bedded basal unit ofglassy ash to lapilli scoria, grading upward into acrudely-bedded sequence of spatter-rich material.

Icelandic rootless cones also do not show any post-formational shearing, indicating that they wereformed on a stationary upper crust of the lava. Thismeans that lava was fed through internal path-ways or lava tubes to the site of explosive interac-tion (Thordarson 2000). Rootless cones from thetype locality around lake Myvatn in northeasternIceland (figure 6) look very similar to the Pava-gadh cones (see figures 1 and 5 of Greeley andFagents 2001, and p. 176 of Scarth and Tanguy2001). The Myvatn rootless cones are raised-rimdepressions ranging in diameter from a few metresto > 100 m and are as deep as 15 m (measuredfrom the rim crest). They are circular or ellip-tical in plan, and the craters are either centred


Figure 5. (a) A tube-fed lava flow partly covers a lake basin and thickens by inflation, and lava-water-sediment interactionat the flow front and along cracks in the tube floors produce several rootless eruptions. (b) A fully developed rootless conegroup eventually results. Simplified from Thordarson and Hoskuldsson (2002) and Fagents and Thordarson (2004).

Figure 6. Sketch map of Iceland showing the locations of rootless cone fields (open circles), after Greeley and Fagents(2001) and Fagents and Thordarson (2004). Dotted lines mark the neovolcanic rift zones: NVZ (Northern Volcanic Zone),WVZ (Western Volcanic Zone), EVZ (Eastern Volcanic Zone). White areas are ice caps.

within the cones or offset. Many of the cones andcraters stand “shoulder-to-shoulder” with overlap-ping ejecta deposits, which is also what we see atPavagadh.

One of us (HCS) has field-tripped in Iceland andseen a major rootless cone field (several hundredindividuals) near Thjorsardalur, south-central Ice-land (figure 6). These cones are quite different inform from the Myvatn and Pavagadh cones, but are

similar to those from the Alftaver district of south-eastern Iceland described by Greeley and Fagents(2001) (see their figure 8). The Thjorsardalur andAlftaver rootless cones are smaller cones (generally< 50 m in diameter) that are steeper and more con-ical, and have either very small craters at the topor no craters at all. Different rootless cone fieldscan have a great variety of size and shape of indi-vidual cones: the ratio of crater diameter to cone


diameter is variable, flanks of the cones can beconcave upward or convex upward, and a greatrange of cone spacing exists. These characteristics,according to Greeley and Fagents (2001), can berelated to the environment of their formation: theMyvatn rootless cones, with close spacing, formedin a lake basin with a presumably abundant sup-ply of water, whereas the much more widely spacedcones of the Alftaver district formed along val-leys and braided river systems where the supply ofwater was more limited.

4. Rootless cones on Mars

The planet Mars is very similar to Earth in manyrespects, and the Martian surface is rich in vol-canic landforms. Olympus Mons on Mars, with aheight of 25 km over the surrounding plains anda basal diameter of 600 km, is the largest vol-cano in the Solar System (e.g., Head and Coffin1997).

Lanagan et al (2001), based on high-resolutionMars Orbiter Camera (MOC) images, havereported the presence of clusters of small cones inseveral areas (the Cerberus plains, Marte Valles,and the Amazonis Planitia) near the MartianEquator. These cones are similar in size, morphol-ogy, and geological setting to the larger of the Ice-landic rootless cones, and therefore Lanagan et al(2001) have suggested that the Martian cones arerootless cones. They were able to rule out alter-native explanations such as impact craters or sed-imentary mud volcanoes for the Martian conesbased on several lines of evidence. For example,these cones clearly sit on the tops of lava surfaces,do not appear to have lavas issuing from them,and are grouped in clusters with no obvious align-ments. The cones range in size from 20 m to 300 min basal diameter. They have large summit craterswith diameters about half as wide as the bases. Thedimensions are thus similar to those of the largerand more explosive of the Icelandic rootless cones(less explosive cones have smaller and narrowersummit craters). The Pavagadh cones are closelysimilar, with crater diameters more than half aswide as the cone diameters, suggesting consider-able explosive power. The Martian cones form clus-ters and each cluster includes a few to hundreds ofcones, and there are no strong cone alignments sug-gestive of an underlying fissure vent. The Icelandicrootless cones also do not show preferred align-ments, and there is no evidence at Pavagadh eitherof any. The surface on which the Martian cones restis formed of platy-ridged lava, and this has beeninterpreted as rubbly pahoehoe which forms whenthe upper crust of inflated flow gets broken up andremobilized (Keszthelyi and Thordarson 2000).

Based on impact crater size-frequency relation-ships (Hartmann 1999), it is known that the sur-faces upon which the Martian cones rest are noolder than 10 million years. The profound impli-cations of this for planetary science are that equa-torial ground ice or ground water must have beenpresent near the Martian surface in its recent geo-logical history, and it may exist in the uppermostlayers of the Martian ground even today.

5. The Pavagadh cones: Whyrootless cones?

Two other types of volcanic cones or craters,besides rootless cones, are not eruptive vents. Lit-toral cones, one of these types, form when lavaflows enter the sea. They are well known fromHawaii (e.g., Macdonald et al 1983; Mattox andMangan 1997). The other type is pit craters whichare nearly circular or elliptical craters that formby subsidence. Pit craters are common on the BigIsland of Hawaii and are particularly well seenalong the Chain of Craters Road on the active vol-cano Kilauea. Up to about a kilometre in diame-ter, these form in the roofs of large lava tubes asthe magma flowing underneath dislodges the roofrocks. Usually no lava or pyroclastic material isemitted from the pit craters during their forma-tion, though surface lava flows have been seen toflow into already existing pit craters and make lavaponds (Macdonald et al 1983). The Pavagadh conesare obviously not littoral cones, and do not looklike pit craters either.

We believe that the Pavagadh cones are rootlesscones because:

• they are similar in size to rootless cones knownfrom Iceland and elsewhere;

• they have a strong morphological resemblanceto the rootless cones from the type locality ofMyvatn, northeastern Iceland;

• the various cones are overlapping, and stand“shoulder-to-shoulder” (figure 2);

• they have not produced lava flows themselves(volcanic cones that are eruptive vents, e.g., cin-der cones, often have lava flows issuing fromthem);

• they occur on a basalt flow, are surrounded bythe lava flow and are not deformed by motionsof the lava, i.e., they formed subsequent to thehost lava flow after its surface crust had frozen— obviously the lava that formed scoria by mix-ing with water was fed to the lava flow inter-nally, consistent with observations elsewhere inthe world;

• though the area left today after erosion through65–60 million years is relatively small, the


Pavagadh cones have an apparently random dis-tribution, similar to that known for Icelandic andMartian rootless cones (Bruno et al 2004; Lana-gan et al 2001).

There is no evidence for a sedimentary sub-strate of the topmost basalt lava flow (in whichthe cones occur) at Pavagadh; however, we envis-age that shallow lakes had formed on the imme-diately underlying flow. Subaqueous eruptions arewell known from the Deccan (e.g., pillow lava andhyaloclastite in Bombay, see Sukheswala and Pold-ervaart 1958; Sethna 1981). If we are correct in ouridentification of the Pavagadh cones and craters asrootless cones, then this is the first time they arebeing identified in Deccan geology and, as far aswe know, in Indian geology.

6. The Jabalpur craters, northeasternDeccan Traps: Rootless cones?

Srinivasan et al (1998) discovered a group of threenearly circular phreatomagmatic craters, close tothe village Barela 15 km SE of Jabalpur (figure 1).These craters have diameters of 60–80 m and verylow relief (only 1–2 m above the ground surface)and are covered by ∼ 1 m thick soil. The vents aresurrounded by compact basalt. The large crater onPavagadh (figure 3) entirely matches this descrip-tion (see their figure 2). The Jabalpur vents them-selves are filled with volcanic tuff (lapilli set incoarse to fine ash) below the soil, and the tuff isat least 2 m thick (base not reached during pit-ting). The lapilli are mainly basaltic glass, but fel-sic lava fragments are also found. The tephra showpoor sorting and bedding. We could not carry outtrenching or pitting on Pavagadh and hence do nothave a depth control, but the rim of one of thecraters is made up of spatter-like material.

Srinivasan et al (1998) considered these featuresas phreatomagmatic eruptive vents — either cin-der cones, tuff rings or maars. They stated thatthese centres could not be tuff rings because tuffrings are much larger, and the features could notbe maars because they do not contain accidentalxenoliths (of the basement rocks). They interpretedthese vents as cinder cones instead, and arguedthat the subdued relief may be the result of fluid-rich basaltic volcanism and post-eruption erosion.Their description of these features matches ourown observations at Pavagadh, i.e., we consider ithighly probable that the Jabalpur craters are root-less cones. This means, of course, that althoughphreatomagmatic, they are not eruptive vents asSrinivasan et al (1998) imagined them to be. Theabsence of xenoliths of crustal (or mantle) rocks inthe ejecta is therefore hardly surprising.

A new study of these Jabalpur craters has justbecome available: Srivastava et al (2004) havecarried out a gravity and magnetic survey ofthem and found a Bouguer anomaly variation of1.2 to 2 mGal in a 1.5-km-long profile and totalfield magnetic anomaly of the order of −900 to1400 nT. Srivastava et al (2004) find no evidencefor deep-seated high-density material underlyingthese craters, and no isolated or elliptical mag-netic anomalies. By using the Werner deconvolu-tion technique they obtain depths to the graniticbasement under the three craters as 51.7, 57.6 and75 m, and conclude that their gravity and magneticstudy does not support the view that these cratersare vents. We think that the rootless cone explana-tion for the Jabalpur craters satisfies all data andobservations.

7. Discussion and conclusions

What alternative interpretations of the Pavagadhcraters are possible? Earlier we considered thePavagadh cones to be not rootless cones but maars,because of their very low relief and saucer-likeshape, and because of the experience of one ofus (HCS) with the Thjorsardalur rootless conesin southern Iceland that are morphologically verydifferent. However, maars imply primary magma-feeding vents, and are rarely < 1 km wide (S. Self,pers. comm., 2004), and as noted, rootless coneshave a great variety of shapes and sizes. Some of thelargest rootless cones in Iceland, 500 m in diameter,have tuff cone morphology and are entirely madeup of ash to fine-grained lapilli deposits indicatinga high degree of magma fragmentation (i.e., effi-cient magma-water interaction) (Thordarson et al1992; Thordarson 2000; Thordarson, pers. comm.,2004).

S. Self (pers. comm.) also pointed out to usthe possibility that the Pavagadh craters may berelated to subsidence along circular-annular jointfractures in the basalt. Such structures are seen inthe Columbia River Basalt province of the north-western U.S.A. and are not well understood. How-ever, the Pavagadh craters are themselves parts ofshallow cones, and these cones match the charac-teristics of rootless cones so well that we cannotconceive of a better explanation.

As a final statement, we think that rootless conegroups should be abundant in the Deccan, givenits abundance of tube-fed, inflated pahoehoe lavaflows. Rootless cones are known from other floodbasalt provinces of the world, such as the ColumbiaRiver province (Thordarson and Self 1998). Giventhe fundamental role of basaltic volcanism in thechemical and thermal evolution of the terrestrialplanets (Basaltic Volcanism Study Project 1981),


volcanological studies of the Deccan should providevaluable insights into planetary development.

Acknowledgements

HCS, GM and KP thank the Industrial Researchand Consultancy Centre (IRCC), IIT Bombay, forsupporting this work. Sveinn Jakobsson and GillianFoulger led the field excursions in southern andsouth-central Iceland in which HCS participated asa delegate in the GSA Penrose Conference ‘PlumeIV: Beyond the Plume Hypothesis’ (2003). SarahFagents kindly supplied a preprint, and Steve Self,Ken Wohletz, and Sheila Seaman provided helpfulreviews and discussions on an earlier version of thismanuscript. Self’s very critical review of that ver-sion resulted in substantial modification and clar-ification of our ideas. Thor Thordarson providedsome very helpful comments on the revised version.

References

Basaltic Volcanism Study Project 1981 Basaltic Volcanismon the Terrestrial Planets (New York: Pergamon Press)

Bondre N R, Duraiswami R A and Dole G 2004 Morphol-ogy and emplacement of flows from the Deccan volcanicprovince, India; Bull. Volcanol. 66 29–45

Bruno B C, Fagents S A, Thordarson T and Baloga S M2004 Spatial analysis of rootless cone groups on Icelandand Mars (abstract); Lunar Planet. Sci. XXXV 1368

Chatterjee S C 1961 Petrology of the lavas of Pavagad Hill,Gujarat; J. Geol. Soc. India 2 61–77

Fagents S A and Thordarson Th 2004 Rootless volcaniccones in Iceland and on Mars. In: The Geology of Mars:Evidence from Earth-Based Analogues (eds) M G Chap-man and I P Skilling; Cambridge Univ Press (in press)

Greeley R and Fagents S A 2001 Icelandic pseudocraters asanalogs to some volcanic cones on Mars; J. Geophys. Res.106 20527–20546

Hartmann W K 1999 Martian cratering VI: crater countisochrons and evidence for recent volcanism from MarsGlobal Surveyor; Meteor. Planet. Sci. 34 167–177

Head J W and Coffin M F 1997 Large igneous provinces: aplanetary perspective. In: Large Igneous Provinces: Con-tinental, Oceanic, and Planetary Flood Volcanism (eds)J J Mahoney and M F Coffin, AGU Geophys. Monogr.100, 411–438

Keszthelyi L and Thordarson Th 2000 Rubbly pahoehoe: apreviously undescribed but widespread lava type transi-tional between aa and pahoehoe (abstract); GSA Abst.Progr. 32 7

Keszthelyi L, Self S and Thordarson Th 1999 Application ofrecent studies on the emplacement of basaltic lava flowsto the Deccan Traps. In: Deccan Volcanic Province (ed)K V Subbarao; Geol. Soc. India Mem. 43 pp 485–520

Lanagan P, McEwen A S, Keszthelyi L P, Thordarson Th2001 Rootless cones on Mars indicating the presence ofshallow equatorial ground ice in recent times; Geophys.Res. Lett. 28 2365–2367

Macdonald G A, Abbott A T and Peterson F L 1983 Volca-noes in the Sea: The Geology of Hawaii 2nd edn. (Hon-olulu: University of Hawaii Press) 517 pp

Mahoney J J 1988 Deccan Traps. In: Continental FloodBasalts (ed) J D Macdougall; (Dordrecht: Kluwer AcadPubl) 151–194

Mattox T N and Mangan M T 1997 Littoral hydrovolcanicexplosions: a case study of lava-seawater interaction atKilauea volcano; J. Volcanol. Geotherm. Res. 75 1–17

Scarth A and Tanguy J-C 2001 Volcanoes of Europe (OxfordUniv Press) 241 pp.

Self S, Thordarson Th and Keszthelyi L 1997 Emplace-ment of continental flood basalt lava flows. In: LargeIgneous Provinces: Continental, Oceanic, and PlanetaryFlood Volcanism (eds) J J Mahoney and M F Coffin, AGUGeophys. Monogr. 100 381–410

Self S, Keszthelyi L and Thordarson Th 1998 The impor-tance of pahoehoe; Ann. Rev. Earth Planet. Sci. 26 81–110

Sethna S F 1981 Geology around Bombay — some intrigu-ing problems. In: Deccan Volcanism (eds) K V Subbaraoand R N Sukheswala; Geol. Soc. India Mem. 3 pp 87–92

Sheridan M F and Wohletz K H 1983 Hydrovolcanism: basicconsiderations and review. In: Explosive Volcanism (eds)M F Sheridan and F Barberi, J. Volcanol. Geotherm. Res.17 1–29

Srinivasan R, Jaffri S H, Rao G V and Reddy G K 1998Phreatomagmatic eruptive centre from the Deccan Trapprovince, Jabalpur, central India; Curr. Sci. 74 787–790

Srivastava R P, Chattopadhyay S, Vedanti N and Dimri V P2004 Gravity and magnetic studies over the circular geo-morphic crater looking structures in the Narmada Traps,central India; J. Geol. Soc. India 64 97–102

Sukheswala R N and Poldervaart A 1958 Deccan basaltsof Bombay area, India; Bull. Geol. Soc. Am. 69 1475–1494

Thorarinsson S 1953 The Crater Groups in Iceland; Bull.Volcanol. 14 3–44

Thordarson Th 2000 Rootless eruptions and Cone Groups inIceland: products of authentic explosive water to magmainteractions (abstract), Volcano-Ice Interactions on Earthand Mars 48.

Thordarson Th and Self S 1998 The Roza Member,Columbia River Basalt Group: A gigantic pahoehoe lavaflow field formed by endogenous processes? J. Geophys.Res. 103 27411–27445

Thordarson Th and Hoskuldsson A 2002 Iceland; ClassicGeology in Europe 3 Terra Publishing, Harpenden, U.K.200 p

Thordarson Th, Morrissey M M, Larsen G and Cyrus-son H 1992 Origin of rootless cone complexes in S-Iceland.In: The 20th Nordic Geological Winter Meeting (eds)A Geirsdottir, H Norddahl and G Helgadottir, IcelandicGeoscience Society, Reykjavik, 169

Wadia D N 1975 The Geology of India. 4th edn. (New Delhi:Tata-McGraw Hill) 508 pp

cones and craters on mount pavagadh, deccan traps

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