Deccan Traps
The Deccan Traps are a large igneous province of west-central India. They are one of the largest volcanic features on Earth. They consist of multiple layers of solidified flood basalt that together are more than thick, cover an area of , and have a volume of . Originally, the Deccan Traps may have covered , with a correspondingly larger original volume.
Etymology
The term "trap" has been used in geology since 1785–1795 for such rock formations. It is derived from the Swedish word for stairs and refers to the step-like hills forming the landscape of the region.History
The Deccan Traps began forming 66.25 million years ago, at the end of the Cretaceous period. The bulk of the volcanic eruption occurred at the Western Ghats some 66 million years ago. This series of eruptions may have lasted fewer than 30,000 years.The original area covered by the lava flows is estimated to have been as large as, approximately half the size of modern India. The Deccan Traps region was reduced to its current size by erosion and plate tectonics; the present area of directly observable lava flows is around.
Effect on mass extinctions and climate
The release of volcanic gases, particularly sulfur dioxide, during the formation of the traps contributed to climate change. Data points to an average drop in temperature of about in this period.Because of its magnitude, scientists have speculated that the gases released during the formation of the Deccan Traps played a major role in the Cretaceous–Paleogene extinction event. It has been theorized that sudden cooling due to sulfurous volcanic gases released by the formation of the traps and toxic gas emissions may have contributed significantly to the K–Pg, as well as other, mass extinctions. However, the current consensus among the scientific community is that the extinction was primarily triggered by the Chicxulub impact event in North America, which would have produced a sunlight-blocking dust cloud that killed much of the plant life and reduced global temperature.
Work published in 2014 by geologist Gerta Keller and others on the timing of the Deccan volcanism suggests the extinction may have been caused by both the volcanism and the impact event. This was followed by a similar study in 2015, both of which consider the hypothesis that the impact exacerbated or induced the Deccan volcanism, since the events occur at antipodes.
However, the impact theory is still the best supported and has been determined by various reviews to be the consensus view.
Petrology
Within the Deccan Traps at least 95% of the lavas are tholeiitic basalts. Other rock types present include: alkali basalt, nephelinite, lamprophyre, and carbonatite.Mantle xenoliths have been described from Kachchh and elsewhere in the western Deccan.
Fossils
The Deccan Traps are famous for the beds of fossils that have been found between layers of lava. Particularly well known species include the frog Oxyglossus pusillus of the Eocene of India and the toothed frog Indobatrachus, an early lineage of modern frogs, which is now placed in the Australian family Myobatrachidae. The Infratrappean Beds and Intertrappean Beds also contain fossil freshwater molluscs.Theories of formation
It is postulated that the Deccan Traps eruption was associated with a deep mantle plume. The area of long-term eruption, known as the Réunion hotspot, is suspected of both causing the Deccan Traps eruption and opening the rift that once separated the Seychelles plateau from India. Seafloor spreading at the boundary between the Indian and African Plates subsequently pushed India north over the plume, which now lies under Réunion island in the Indian Ocean, southwest of India. The mantle plume model has, however, been challenged.Data continues to emerge that support the plume model. The motion of the Indian tectonic plate and the eruptive history of the Deccan traps show strong correlations. Based on data from marine magnetic profiles, a pulse of unusually rapid plate motion began at the same time as the first pulse of Deccan flood basalts, which is dated at 67 million years ago. The spreading rate rapidly increased and reached a maximum at the same time as the peak basaltic eruptions. The spreading rate then dropped off, with the decrease occurring around 63 million years ago, by which time the main phase of Deccan volcanism ended. This correlation is seen as driven by plume dynamics.
The motions of the Indian and African plates have also been shown to be coupled, the common element being the position of these plates relative to the location of the Réunion plume head. The onset of accelerated motion of India coincides with a large slowing of the rate of counterclockwise rotation of Africa. The close correlations between the plate motions suggest that they were both driven by the force of the Réunion plume.
Suggested link to impact events
Chicxulub crater
There is some evidence to link the Deccan Traps eruption to the contemporaneous asteroid impact that created the nearly antipodal Chicxulub crater in the Mexican state of Yucatán. Although the Deccan Traps began erupting well before the impact, argon-argon dating suggests that the impact may have caused an increase in permeability that allowed magma to reach the surface and produced the most voluminous flows, accounting for around 70% of the volume. The combination of the asteroid impact and the resulting increase in eruptive volume may have been responsible for the mass extinctions that occurred at the time that separates the Cretaceous and Paleogene periods, known as the K–Pg boundary.A more recent discovery appears to demonstrate the scope of the destruction from the impact alone, however. In a March 2019 article in the Proceedings of the National Academy of Sciences, an international team of twelve scientists revealed the contents of the Tanis fossil site discovered near Bowman, North Dakota that appeared to show a devastating mass destruction of an ancient lake and its inhabitants at the time of the Chicxulub impact. In the paper, the group claims that the geology of the site is strewn with fossilized trees and remains of fish and other animals. The lead researcher, Robert A. DePalma of the University of Kansas, was quoted in the New York Times as stating that “ou would be blind to miss the carcasses sticking out... It is impossible to miss when you see the outcrop.” Evidence correlating this find to the Chicxulub impact included tektites bearing "the unique chemical signature of other tektites associated with the Chicxulub event" found in the gills of fish fossils and embedded in amber, an iridium-rich top layer that is considered another signature of the event, and an atypical lack of scavenging of the dead fish and animals that suggested few other species survived the event to feed off the mass death. The exact mechanism of the site's destruction has been debated as either an impact-caused tsunami or lake and river seiche activity triggered by post-impact earthquakes, though there has yet been no firm conclusion upon which researchers have settled.