Write a brief account on the Deccan Flood Basalt Volcanism in India.

Geographical Extent and Topography

The Deccan Traps constitute a large igneous province spanning significant portions of peninsular India, predominantly across Maharashtra, Madhya Pradesh, and Gujarat, with extensions into parts of Telangana, Andhra Pradesh, and Karnataka. The term "Traps" is derived from the Swedish word "trappa," meaning "stair-step," referring to the distinctive terraced or stepped topography formed by the differential erosion of successive horizontal lava flows.

• Current Area: The observable lava flows today cover an area of about 500,000 square kilometers.
• Original Extent: Estimates suggest the original area covered by these lava flows could have been as vast as 1.5 million square kilometers, roughly half the size of modern India.
• Thickness: The basaltic layers can reach a composite thickness of more than 2,000 meters, particularly along the west coast near Mumbai, thinning considerably towards the east.

Timing and Duration of Volcanism

The Deccan volcanism occurred primarily during the Late Cretaceous to early Palaeogene epochs, with peak activity closely coinciding with the Cretaceous-Paleogene (K-Pg) boundary, approximately 66 million years ago. This immense eruptive phase spanned several hundred thousand years, possibly in pulses.

• Onset: Eruptions began around 66.3 million years ago.
• Peak Activity: The most voluminous eruptions occurred around 66 million years ago.
• Duration: The main eruptive phase is estimated to have lasted for about 750,000 years, though some studies suggest episodic activity from 69 Ma to 63 Ma.

Nature of Eruptions and Petrology

The Deccan Traps are characterized by flood basalt volcanism, which involved highly fluid lava erupting from long fissures in the Earth's crust rather than from central volcanic cones. This allowed lava to spread widely over great distances, forming extensive, flat-lying sheets.

• Fissure Eruptions: Magma erupted through linear cracks and fissures, leading to sheet-like deposition of lava.
• Petrology: At least 95% of the lavas are tholeiitic basalts, which are fine-grained and generally low in olivine. Other rock types, such as alkali basalt, nephelinite, lamprophyre, and carbonatite, are also present.
• Intertrappean Beds: During intervals between successive volcanic eruptions, freshwater lakes formed, leading to the deposition of sedimentary beds, known as Intertrappean beds, which contain fossil flora and fauna.

Theories of Formation: Mantle Plume Hypothesis

The most widely accepted theory for the formation of the Deccan Traps attributes it to a deep mantle plume, specifically the Réunion hotspot. As the Indian Plate drifted northward over this stationary hotspot, it caused extensive melting of the underlying mantle and massive lava extrusions.

• Réunion Hotspot: This still-active hotspot, now located beneath Réunion Island in the Indian Ocean, is believed to have been positioned beneath western India during the Deccan eruptions.
• Plate Tectonics: The movement of the Indian Plate over the hotspot, combined with regional crustal thinning and rifting (which also facilitated the separation of the Seychelles microcontinent from India), provided the mechanism for the large-scale volcanism.

Environmental and Climatic Impacts

The Deccan Flood Basalt Volcanism had profound and long-lasting environmental and climatic consequences, making it a significant subject in paleoclimatology and paleontology.

• Atmospheric Changes: Massive releases of volcanic gases, including carbon dioxide, sulfur dioxide, and aerosols, led to significant climatic shifts.
  • Sulfur dioxide caused episodes of global cooling (e.g., a drop of about 2°C) and acid rain.
  • Carbon dioxide contributed to long-term warming trends after the initial cooling phases.
• Mass Extinction Debate: The timing of the Deccan eruptions closely coincides with the Cretaceous-Paleogene (K-Pg) mass extinction event, which wiped out approximately 75% of Earth's species, including non-avian dinosaurs.
  • Initially, the eruptions were speculated to be a primary cause of the extinction.
  • The current scientific consensus largely attributes the K-Pg extinction primarily to the Chicxulub asteroid impact in Mexico.
  • However, many studies suggest that the Deccan volcanism exacerbated or contributed to the climatic stresses, potentially making ecosystems more vulnerable to the asteroid impact, or that the impact itself may have intensified Deccan volcanism.
• Soil Formation: The weathering of these basaltic rocks over millions of years has led to the formation of the highly fertile "Regur" or Black Soil, which is rich in iron, magnesium, and aluminum, and excellent for agriculture, particularly cotton cultivation.