Deccan Traps

The Deccan Traps shown as dark purple spot on the geologic map of India
The Western Ghats at Matheran in Maharashtra

The Deccan Traps are a large igneous province located on the Deccan Plateau of west-central India (17°–24°N, 73°–74°E) and one of the largest volcanic features on Earth. They consist of multiple layers of solidified flood basalt that together are more than 2,000 m (6,600 ft) thick, cover an area of c. 500,000 km2 (200,000 sq mi),[1] and have a volume of c. 1,000,000 km3 (200,000 cu mi).[2] Originally, the Deccan Traps may have covered c. 1,500,000 km2 (600,000 sq mi),[3] with a correspondingly larger original volume.

Etymology

The term "trap" has been used in geology since 1785–95 for such rock formations. It is derived from the Scandinavian word for stairs ("trappa") and refers to the step-like hills forming the landscape of the region.[4]

History

The Deccan Traps began forming 66.25 million years ago,[3] 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 less than 30,000 years in total.[5]

The original area covered by the lava flows is estimated to have been as large as 1.5 million km², 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 500,000 km2 (200,000 sq mi).

Effect on mass extinctions and climate

The release of volcanic gases, particularly sulphur dioxide, during the formation of the traps contributed to contemporary climate change. Data points to an average drop in temperature of 2 °C in this period.[6]

Because of its magnitude, scientists formerly speculated that the gases released during the formation of the Deccan Traps played a role in the Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction).[7] It was theorized that sudden cooling due to sulfurous volcanic gases released by the formation of the traps and localised gas concentrations may have contributed significantly to the K–Pg, as well as other mass extinctions.[8] However, the current consensus among the scientific community is that the extinction was 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, called an impact winter).[9]

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.[10][11] This was followed by a similar study in 2015.[12][13]

Chemical composition

Crystals of epistilbite and calcite in a vug in Deccan Traps basalt lava from Jalgaon District, Maharashtra

Within the Deccan Traps at least 95% of the lavas are tholeiitic basalts. However, other rock types do occur:

Mantle xenoliths have been described from Kachchh (northwestern India) 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 (Owen) 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.[14][15] The infratrappean and intertrappean beds also contain fossil freshwater mollusks.[16]

Theories of formation

It is postulated that the Deccan Traps eruption was associated with a deep mantle plume. The area of long-term eruption (the hotspot), 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.[17]

Data continue to emerge which supports 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 begins 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.[18]

The Indian and African plates' motions have also been shown to be coupled, with 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.[18]

There is some evidence to link the Deccan Traps eruption to the asteroid impact which created the 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.[19] 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 geological structure exists in the sea floor off the west coast of India that has been suggested as a possible impact crater, in this context called the Shiva crater. It has also been dated at approximately 66 million years ago, potentially matching the Deccan traps. The researchers claiming that this feature is an impact crater suggest that the impact may have been the triggering event for the Deccan Traps as well as contributing to the acceleration of the Indian plate in the early Paleogene.[20] However, the current consensus in the Earth science community is that this feature is unlikely to be an actual impact crater.[21][22]

See also

References

  1. ↑ Singh, R. N.; Gupta, K. R. (1994). "Workshop yields new insight into volcanism at Deccan Traps, India". Eos. 75: 356. doi:10.1029/94EO01005.
  2. ↑ Dessert, Céline; Dupréa, Bernard; Françoisa, Louis M.; Schotta, Jacques; Gaillardet, Jérôme; Chakrapani, Govind; Bajpai, Sujit (2001). "Erosion of Deccan Traps determined by river geochemistry: impact on the global climate and the 87Sr/86Sr ratio of seawater". Earth and Planetary Science Letters. 188: 461.
  3. 1 2 https://newsoffice.mit.edu/2014/volcanic-eruption-dinosaur-extinction-1211 What really killed the dinosaurs? Before an asteroid wiped out the dinosaurs, Earth experienced a short burst of intense volcanism." Jennifer Chu | "MIT News Office," 11 December 2014
  4. ↑ Trap at dictionary.reference.com
  5. ↑ "India's Smoking Gun: Dino-killing Eruptions." ScienceDaily, 10 August 2005.
  6. ↑ Royer, D. L., Berner, R. A., Montañez, I. P., Tabor, N. J., Beerling, D. J. (2004). "CO2 as a primary driver of Phanerozoic climate" (PDF). GSA Today. 14 (3): 4–10. ISSN 1052-5173. doi:10.1130/1052-5173(2004)014<4:CAAPDO>2.0.CO;2.
  7. ↑ Courtillot, Vincent (1990). "A Volcanic Eruption" (PDF). Scientific American. 263: 85–92.
  8. ↑ Beardsley, Tim (1988). "Star-Struck?" (PDF). Scientific American. 258: 37–40.
  9. ↑ Schulte, Peter; et al. (5 March 2010). "The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary". Science. AAAS. 327 (5970): 1214–1218. Bibcode:2010Sci...327.1214S. ISSN 1095-9203. PMID 20203042. doi:10.1126/science.1177265. Retrieved 5 March 2010.
  10. ↑ Keller, G., Deccan volcanism, the Chicxulub impact, and the end-Cretaceous mass extinction: Coincidence? Cause and effect?, in Volcanism, Impacts, and Mass Extinctions: Causes and Effects, GSA Special Paper 505, Pp. 29-55, 2014 abstract
  11. ↑ Schoene, B., et al., U-Pb geochronology of the Deccan Traps and relation to the end-Cretaceous mass extinction, Science, 12 Nov. 2014, abstract
  12. ↑ "State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact". Science. 2 October 2015. Retrieved 2 October 2015.
  13. ↑ "Asteroid that killed dinosaurs also intensified volcanic eruptions - study". The Guardian. 2 October 2015. Retrieved 2 October 2015.
  14. ↑ Noble, Gladwyn Kingsley. "The Fossil Frogs of the Intertrappean Beds of Bombay, India". American Museum of Natural History. 401: 1930.
  15. ↑ http://tolweb.org/Myobatrachinae/16946
  16. ↑ Hartman, J.H., Mohabey, D.M., Bingle, M., Scholz, H., Bajpai, S., and Sharma, R., 2006, Initial survivorship of nonmarine molluscan faunas in end-Cretaceous Deccan intertrappean strata, India: Geological Society of America (annual meeting, Philadelphia) Abstracts with Programs, v. 38, no. 7, p. 143.
  17. ↑ Sheth, Hetu C. "The Deccan Beyond the Plume Hypothesis." MantlePlumes.org, 2006.
  18. 1 2 S.C. Cande & D.R. Stegman; Indian and African plate motions driven by the push force of the Réunion plume head; Nature; Volume 475; pp. 47–52; (7 July 2011) doi:10.1038/nature10174
  19. ↑ Richards, Mark A.; Alvarez, Walter; Self, Stephen; Karlstrom, Leif; Renne, Paul R.; Manga, Michael; Sprain, Courtney J.; Smit, Jan; Vanderkluysen, Loÿc; Gibson, Sally A. "Triggering of the largest Deccan eruptions by the Chicxulub impact". Geological Society of America Bulletin. 127 (11-12): 1507–1520. doi:10.1130/B31167.1.
  20. ↑ Chatterjee, Sankar. "The Shiva Crater: Implications for Deccan Volcanism, India-Seychelles Rifting, Dinosaur Extinction, and Petroleum Entrapment at the KT Boundary." Paper No. 60-8, Seattle Annual Meeting, November 2003.
  21. ↑ Mullen, Leslie (2 November 2004). "Shiva: Another K-Pg Impact?". Spacedaily.com. Retrieved 20 February 2008. - original article at source
  22. ↑ Moskowitz, Clara (18 October 2009). "New Dino-destroying Theory Fuels Hot Debate". space.com.
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