During the Phanerozoic Eon the Earth has suffered a series of major extinction events almost all of which are considered to have been triggered by continental flood basalt emplacement episodes, which released vast amounts of toxic gasses into the atmosphere. The exception to this is the End Cretaceous Extinction, which is largely considered to have been triggered by the impact of an extra-terrestrial object into what is now the Yucatán Peninsula of Mexico.
While this is a compelling story, it has faced a number of challenges from rival theories, the most notable of which is that there was a significant outpouring of flood basalts at the End of the Cretaceous, leading to the emplacement of the Deccan Traps Igneous Province in India, This, combined with the fact that we have now identified a number of other large impacts in the Phanerozoic rock record, none of which seem to have been associated with extinction events, presents a serious challenge to the Chicxulub Impact Theory. However, studies of the Deccan Traps Igneous Province have suggested that the majority of the basalt-emplacement happened slightly after the extinction event, implying that it cannot have been the cause.
In a paper published in the journal GSA Bulletin on 5 November 2025, Vivek Kale of the Advanced Center for Water Resources Development and Management, Devdutt Upasani and Madhu Rajput of the Department of Geology at Fergusson College, Gauri Dole of the Department of Environmental Science at Savitribai Phule Pune University, and Shilpa Patil Pillai of the Department of Earth and Climate Science at the Indian Institute of Science Education and Research Pune, present a re-evaluation of the contribution of the Deccan Traps volcanism to the End Cretaceous Extinction, based upon new geochronological studies of the Deccan Traps Igneous Province.
The Deccan Traps Igneous Province covers about 50 000 km² of Western and Central India, and extends westward beneath the Arabian Sea, where it is thought to cover a further area of about 38 000 km². It was produced by shield-volcano-like eruptions, which produced a series of radially overstepping basalt formations. Studies carried out in the 1980s suggested that the onset of the Deccan Traps volcanism coincided with the End Cretaceous Extinction Event, leading the majority of the geological community to conclude that it could not be responsible for the event, and even some suggestions that the volcanism might have been caused in some way by the Chicxulub Impact.
However, much of this earlier work was based upon chemostratigraphic correlation between different parts of the Deccan Traps, something which is now considered unreliable as it has been demonstrated that the Traps are in fact made up of a series of subprovinces, the Western, Satpura, Central, Malwa, Mandla, and Saurashtra, each with their own distinct volcanic history. Thus the work carried out in the 1980s appears to have been valid for parts of the Western Subprovince, but not necessarily for any of the other subprovinces.
Kale et al. combined chemostratigraphic methods with palaeomagnetism and studies of key fossils from sedimentary beds interspaced with the volcanic layers, with the aim of understanding the timing and eruptive history of each subprovince of the Deccan Traps. To achieve a high level of confidence, they carried out extensive fieldwork, preparing more than eighty stratigraphic logs.
Kale et al. recognised three phases of volcanic activity within the Deccan Traps deposits, phases which were consistently recovered using chemostratigraphic and palaeomagnetic methods. These were the Early Magmatic Phase, the Main Flood Basalt Eruptions, and the Late Volcanic Phase. Deposits associated with the Early Magmatic Phase outcrops in the Saurashtra Subprovince, along the Narmada Son Lineament Zone between the Bastar and Dharwar cratons, and comprises mixed mantle-derived volcanic sediments more than 67.0 million years old. The Late Volcanic Phase comprises intrusive volcanic material inserted into the main deposits after they had been emplaced. These outcrop in a small coastal strip around Mumbai, and have also been found offshore in the Laxmi Basin of the Arabian Sea. This Late Volcanic Phase material is about 63.0 million years old, making them coeval with the separation of the Seychelles from the Western Margin of the Indian Plate. There may also be some material associated with the Late Volcanic Phase at the top of the Amarkantak Group.
The Main phase therefore accounts for about 90% of the material which makes up the Deccan Traps Igneous Province. Furthermore, most of this material appears to have been produced within a period of less than 1 million years within Chron C29r, a period between two reversals of the Earth's magnetic field, which lasted from about 66.43 million years ago to about 65.8 million years ago (therefore spanning the current accepted boundary between the Cretaceous and the Tertiary, at 66.04 million years ago), and the following Chron C29n, which lasted from about 65.8 million years ago to about 64.745 million years ago.
The Western Subprovince hosts the thickest and (probably the) most continuous deposits of the Deccan Traps, as well as the largest single outcrop, at Kalsubai Peak, where a 1642 m continuous stack of basalts is exposed, and continues some way beneath the ground. These deposits are assigned to the Sahyadri Group, which is divided into the Wai, Lonavala and Kalsubai subgroups, with about 3000 m depth of basalt produced within chrons C29r and C29n. These deposits show few fossiliferous sedimentary layers, with scattered 'interflow horizons' (horizons marking time-gaps between episodes of basalt-deposition) of limited lateral extent, suggesting that gaps between eruptive episodes were localised and brief.
Kale et al.'s revised study places the boundary between chrons C29r and C29n at the base of the Mahabaleshwar Formation from the Wai Subgroup of the Sahyadri Group. This is marked by the widespread presence of giant-phenocryst basalt. The Purandargarh Formation, which underlies the Mahabaleshwar Formation, is calculated to date from the earliest part of the Danian Stage of the Palaeocene, i.e. immediately after the Cretaceous-Tertiary boundary.
The Poladpur Lavas, which underlie the Purandargarh Formation, have been suggested to be of latest Cretaceous origin, on the basis of dates obtained from zircons, but Kale et al. reject this, on the basis that zircons can survive at very high temperatures and are often reworked within volcanic deposits, and also classify the Poladpur Lavas as earliest Danian. Based upon a sediment layer with key Mammalian fossils exposed at Naskal in Telangana State, Kale et al. estimate that the lowest 50 m of the Poladpur Lavas were erupted within 100 000 years of the Cretaceous-Tertiary boundary. The underlying Lonavala and Kalsubai subgroups, therefore, must have erupted entirely within the Latest Cretaceous.
The Satpura Subprovince forms the second deepest sequence of the Deccan Traps, and has been divided into six formations, with two of these formations containing giant-phenocryst basalts, something which has led to the correlation of the entire subprovince with the Wei Subgroup of the Sahyadri Group. Kale et al. reject this analysis, assigning the entire sequence to the Maastrichtian (Latest Cretaceous) on the basis of fossil inclusions within sedimentary layers.
The Central Subprovince forms the northeastern part of the Deccan Plateau, with a diffuse boundary with the Western subprovince. The lavas of this the Mahur Formation, which form the base of this sequence, also contain a distinctive giant-phenocryst basalt layer, which had led to them being comparied to the Wei Subgroup, but Kale et al. again reject this, assigning this formation to the Maastrichtian on the basis of fossil inclusions. Instead, they place the Cretaceous-Tertiary boundary at the top of the Ajanta Formation, which overlies the Mahur Formation, on the basis of palynological evidence (fossil pollen). This places the final three formations of the Central Subprovince, the Chikhil, the Buldhana, and the Karanja, within the Early Danian.
The Malwa Subprovince sequence has also previously been assigned to the Wei Subgroup, although in this case because it contains a normal-reverse-normal palaeomagnetic sequence, which was thought to mark the Danian chrons C29r and C29n. However, a revised dating sequence suggests that this sequence contains some of the oldest rocks of the Deccan Traps, with only the final, Singachori Formation being Danian in age, while all the lower formations are Maastrichtian, based upon fossil evidence.
The Mandla Subprovince has proven much harder to establish a chronological sequence for, but appears to contain both some of the oldest and some of the youngest lavas of the Deccan Traps. However, the lowermost Mandla Formation and unclassified underlying beds contain sedimentary layers which produce fossils of Maastrichtian age, while the higher beds of the Multai, Amarwara, Khamla/Khampla, and Kuleru formations contain magnetic reversals and fossils of Palaeocene origin.
The Saurashtra Subprovince is the only part of the Deccan Traps where an iridium layer has been discovered within the Anjur Section, and used as an identifier for the Cretaceous-Tertiary boundary. However, the deposits which host this layer have been shown to belong to Magnetochron C28r, making them too young to be related to the End of the Cretaceous. Other deposits, from layers beneath those exposed at Anjur, have produced Dinosaur bones and nests, as well as other clearly Cretaceous fossils, indicating a Maastrichtian origin. These deposits are also cut through by a series of dykes which have yielded ages of between 66.06 and 62.4 million years. Given the lack of a clear chronological sequence for this group, Kale et al. do not attempt to calculate its full sequence, but for the sake of modelling assume that it contains 5% of the total volume of the Deccan Traps lavas, and that this can be divided equally between the Danian and the Maastrichtian.
Kale et al. also assume that 75% of the Deccan Traps basalts were erupted on land, with about 25% offshore. This gives a total volume of about 1.8 million km² of erupted lava (higher than any previous estimate) with the terrestrial deposits produced during the Maastrichtian and the Danian, while the offshore deposits are assumed to be entirely Danian in origin.
While Kale et al. produce a higher total volume estimate for the Deccan Traps Volcanism than previous studies, this is not a major increase, with most previous estimates being of a similar order of magnitude. However, by separating the Deccan Traps into a number of subprovinces and studying those individually, they do significantly re-estimate the amount of volcanism that occurred before the Cretaceous-Tertiary boundary.
Kale et al. estimate that early magmatism associated with the Deccan Traps were widely spaced across India in the Late Cretaceous, and probably associated with the Reunion Hotspot passing under part of the Indian Plate. Cainozoic Late Phase Magmatism occurred largely on the spreading western edge of the Indian Plate, and the associated shallow submarine shelf, with much of the material produced probably being better viewed as ocean/island basalt rather than continental flood basalt.
Between these two events, the Main Flood Basalt Eruptions produced about 1.2 million cubic kilometres of continental flood basalt (i.e. about 70% of the total volume of the Deccan Traps) within the last 300 000 years of the Cretaceous. This equates to an eruption rate of about 1000 km³ per year of basalt being produced during the Cretaceous portion of Chron C29r, falling to about 300 km³ per year during the Palaeocene portion of the chron.
The lethal impacts of flood basalts themselves are rather limited, with most Animals able to out-walk all but the fastest lava flows. Ash clouds associated with such eruptions are more dangerous, potentially smothering plant life far from the source. However, the real threat comes from the gasses such events produce, with large volumes of carbon dioxide, carbon monoxide, gaseous sulphur, chlorine, fluorine, mercury, and other potent toxins into the atmosphere.
Kale et al. estimate that the Main Flood Basalt Eruptions of the Deccan Traps would have produced over 6000 gigatonnes of carbon emissions, less than that produced by the Siberian Traps Basalts (associated with the End Permian Extinction) or the Central Atlantic Magmatic Province (associated with the End Triassic Extinction). However, around 4200 gigatonnes of this would have been produced within the last 300 000 years of the Maastrichtian (i.e. immediately before the Cretaceous-Tertiary boundary) with the remaining 1800 gigatonnes released over a longer period of time.
Fuethermore, Kale et al. conservatively estimate that 1300 gigatonnes of sulphur was released into the atmosphere during the Maastrichtian portion of Chron C29r, with about 200 gigatonnes being released during the Danian portion.
High mercury levels have been observed around the world towards the end of the Maastrichtian, something which has previously been linked to the onset of the Deccan Traps Volcanism, although why mercury should peak at this point was unclear. Kale et al.'s results suggest that this spike did in fact coincide with the main phase of Deccan Traps volcanism, although they do not attempt to calculate the volumes of mercury produced. Estimates of the volumes of chlorine and fluorine produced by the eruptions were also beyond the scope of the study, although these are also likely to have been substantial.
Emissions of carbon dioxide from volcanic sources are typically enriched in the isotope carbon¹³ compared to other sources, something which is often used to track volcanic activity in the sediment record. Kale et al. examined the ratios of carbon¹³ in terminal Cretaceous sequences from the South Atlantic, North Atlantic, and Spain (areas which would have been far from India at the time), and found a significant spike in carbon¹³ levels immediately before the Cretaceous-Tertiary boundary, something they do not believe is coincidental. They also not fluctuations in other isotopic proxies which begin 600 000-800 000 years before the boundary, approximately the time frame for the onset of the Satpura, Malwa, and Mandla subprovince eruptions.
Based upon this, Kale et al. conclude that the majority of the Deccan Traps Flood Basalts were produced in a short period of time in the terminal Cretaceous, causing an environmental collapse which was the main driver of the End Cretaceous Extinction. The Chicxulub Impact could potentially have caused a single large mortality event against this backdrop, but is unlikely to have been the main cause of the extinction. The delayed recovery of the biosphere seen in the Early Danian is unlikely to have been caused by a 'nuclear winter' triggered by the impact, but instead probably relates to the ongoing, albeit reduced, volcanic emissions coming from the Deccan Traps at this time.
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