The Deccan Traps are a large volcanic area in west-central India (17–24°N, 73–74°E). They are one of the largest volcanic features on Earth, formed by lava from many different volcanic areas. The Traps are made of many layers of hardened lava, called flood basalt. Together, these layers are more than 2 kilometers (1.2 miles) thick, cover about 500,000 square kilometers (200,000 square miles), and have a total volume of about 1,000,000 cubic kilometers (200,000 cubic miles). Originally, the Deccan Traps may have covered about 1,500,000 square kilometers (600,000 square miles), with a larger volume. These layers sit on top of the Archean age Indian Shield, which is likely the type of rock the lava passed through during eruptions. The area is often divided into four subregions: the main Deccan, the Malwa Plateau, the Mandla Lobe, and the Saurashtran Plateau.
The eruptions happened over a time period of 600,000 to 800,000 years, between about 66.3 to 65.6 million years ago. This time period includes the Cretaceous–Paleogene boundary. Some scientists once thought the eruptions were a major cause of the Cretaceous–Paleogene mass extinction event, which happened around the same time. However, later research on the Chicxulub impact, now believed to be the main cause of the extinction, led to this theory being rejected. Some scientists still suggest the eruptions may have had a small effect, but most agree the Deccan eruptions had little or no major role in the extinction.
The Deccan Traps are believed to have been created mainly by the Réunion hotspot, which is still active today. This hotspot is responsible for forming the modern Mascarene Islands in the Indian Ocean.
Etymology
The word "trap" has been used in geology since 1785–1795 to describe certain rock formations. It comes from the Swedish word "trapp," which means "stairs." This term describes hills that look like steps and shape the landscape of the area. The name "Deccan" has roots in Sanskrit and means "southern."
History
The Deccan Traps started forming 66.25 million years ago, at the end of the Cretaceous period. Some of the oldest material may be found beneath younger material. Most of the volcanic activity happened in the Western Ghats between 66 and 65 million years ago, when lava flowed through cracks in the Earth's surface. It is hard to determine the exact age of Deccan rocks because the time between eruptions may have been only a few thousand years, and current dating methods cannot measure such short periods accurately. This also makes it difficult to calculate how fast magma moved underground. These eruptions may have lasted for less than 30,000 years.
The area covered by the lava flows was once as large as 1.5 million square kilometers, about half the size of modern India. Over time, erosion and the movement of Earth's plates reduced the region to its current size. Today, the area where lava flows can be directly seen is about 500,000 square kilometers.
The Deccan Traps are divided into three layers: the Upper, Middle, and Lower traps. Earlier ideas suggested these layers marked important events in the lava eruptions. However, scientists now believe these layers are more closely connected to the ancient land shapes and distance from the eruption sites.
Effect on mass extinctions and climate
The release of volcanic gases, especially sulfur dioxide, during the formation of the Deccan Traps may have helped cause changes in Earth's climate. Scientists recorded an average drop in temperature of about 2°C (3.6°F) during this time.
Some scientists, including Gerta Keller, have suggested that the gases released during the formation of the Deccan Traps may have played a major role in the Cretaceous–Paleogene (K–Pg) extinction event, which marked the end of the dinosaurs. It has been theorized that sudden cooling caused by sulfurous volcanic gases and toxic emissions from the traps may have contributed to the mass extinction. However, most scientists now agree that the extinction was mainly caused by the Chicxulub impact event in North America. This event would have created a cloud of dust that blocked sunlight, killing much of Earth's plant life and lowering global temperatures (a period called an impact winter).
A 2014 study proposed that both the volcanic activity and the impact event may have caused the extinction. A similar study in 2015 suggested that the impact might have made the Deccan volcanism worse or caused it, as the events occurred near opposite sides of the Earth. A 2020 study, however, questioned whether the Deccan Traps had any role in the extinction, stating that their eruptions may have even reduced the climate changes caused by the impact.
One reason scientists doubt the Deccan Traps as the main cause of the extinction is that the event happened very quickly and at the same time in both ocean and land environments, as would be expected from an impact, not from volcanic activity.
In 2019, scientists discovered a fossil site in North Dakota called Tanis, which showed signs of massive destruction linked to the Chicxulub impact. The site contained fossilized trees, fish, and other animals. Researchers found evidence such as tiny glass-like particles from the impact, a layer rich in the element iridium, and signs that few animals survived the event. Scientists are still debating whether the destruction was caused by a tsunami or shaking from earthquakes.
A 2024 study of chemical markers in ancient peat found that the Deccan Traps caused Earth to warm by about 3°C over 100,000 years and cool by about 5°C for less than 10,000 years before the K–Pg boundary. However, by the time of the K–Pg event, temperatures had returned to normal. This suggests the Deccan Traps were not the main cause of the extinction.
Petrology
The Deccan Traps contain at least 95% tholeiitic basalt lava. The main minerals found include olivine, pyroxenes, plagioclase, and certain iron and titanium-rich oxides. These magmas have less than 7% magnesium oxide (MgO). However, many of these minerals are observed as changed forms over time. Other rock types found in the area include alkali basalt, nephelinite, lamprophyre, and carbonatite.
Mantle xenoliths, which are fragments of deep Earth rock, have been found in Kachchh (northwestern India) and other areas of the western Deccan. These xenoliths include spinel lherzolite and pyroxenite.
The Deccan Traps have been grouped in many ways, including three main layers based on their position in the rock record. Geochemically, the region can be divided into as many as eleven different rock formations. Many differences in the rock types are due to varying levels of contamination from Earth’s crust during their formation.
Fossils
The Deccan Traps are known for layers of rock that contain fossils found between lava layers. Notable examples include the frog Oxyglossus pusillus (Owen) from the Eocene period in India and the toothed frog Indobatrachus, an early relative of modern frogs now classified in the Australian family Myobatrachidae. The Infratrappean Beds (Lameta Formation) and Intertrappean Beds also include fossils of freshwater mollusks.
Theories of formation
It is believed that the Deccan Traps eruption was linked to a deep mantle plume. Magmas from this eruption often have high levels of helium, which is a sign of origin from mantle plumes. The area where the eruption occurred over a long period, called the Réunion hotspot, is thought to have caused the Deccan Traps eruption and also created the rift that separated the Mascarene Plateau from India. Evidence of thinning Earth's crust supports the idea that this rifting happened and may have helped the plume rise in that area. Later, seafloor spreading between the Indian and African Plates moved India northward over the plume, which now lies beneath Réunion Island in the Indian Ocean, southwest of India. However, some scientists question the mantle plume theory.
New data continues to support the plume model. The movement of the Indian tectonic plate and the timing of the Deccan Traps eruptions are closely connected. Marine magnetic profiles show that a sudden increase in the speed of plate movement began at the same time as the first major eruptions of Deccan flood basalts, which occurred about 67 million years ago. The spreading rate of the ocean floor increased rapidly and peaked when the Deccan eruptions were strongest. Later, the spreading rate slowed around 63 million years ago, when the main volcanic activity ended. This connection is believed to be caused by the movement of the mantle plume.
The movement of the Indian and African plates is also linked, with both influenced by their positions relative to the Réunion plume. When India's movement sped up, Africa's counterclockwise rotation slowed. The close relationship between these plate movements suggests both were driven by the force of the Réunion plume.
When comparing the levels of certain elements, such as sodium, iron, and silicon, in the Deccan Traps to other major volcanic regions, the Deccan shows the most melting, which suggests a deep plume origin. Olivine appears to have separated at depths near the boundary between the Earth's crust and mantle, with further separation of gabbro about 6 kilometers below the surface. Features like widespread faulting, frequent diking events, high heat flow, and areas of higher gravity suggest that the volcanic phase of the Deccan Traps was connected to a triple junction, which may have existed during the Late Cretaceous and was caused by a deep mantle plume. Not all diking events contributed significantly to the total volcanic flow. Some of the largest dikes are difficult to locate because they are near the west coast and are now likely underwater.
Suggested link to impact events
The Deccan Traps started erupting long before the asteroid impact. However, a 2015 study using argon–argon dating suggested that the impact might have increased the ability of magma to reach Earth's surface, leading to the largest lava flows, which make up about 70% of the total volume. Some scientists believe that the combination of the asteroid impact and the increased lava flow may have caused the mass extinctions at the boundary between the Cretaceous and Paleogene periods, known as the K–Pg boundary. Others have questioned this idea, calling it "convenient interpretations based on superficial and cursory observations."
A geological structure on the seafloor off the west coast of India, called the Shiva crater, has been proposed as a possible impact site. It is dated to about 66 million years ago, which matches the timing of the Deccan Traps. Researchers who suggest the Shiva crater is an impact site claim it may have triggered the Deccan Traps and contributed to the movement of the Indian plate during the early Paleogene. However, the current scientific consensus is that this structure is not likely to be an actual impact crater.