The Deccan Traps are a large area of volcanic rock located in west-central India (17–24°N, 73–74°E). They are one of the largest volcanic regions on Earth, formed by lava flowing from many different volcanic areas. The Traps are made up of many layers of hardened basalt rock. 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). At one time, the Deccan Traps may have covered up to 1,500,000 square kilometers (600,000 square miles), with a larger volume of rock. These rocks lie on top of the Archean age Indian Shield, a type of ancient rock that the volcanic province likely passed through during eruptions. The region is often divided into four sections: the main Deccan, the Malwa Plateau, the Mandla Lobe, and the Saurashtran Plateau.
The volcanic 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 boundary between the Cretaceous and Paleogene geological periods. Some scientists once believed these eruptions were a major cause of the mass extinction event that occurred around the same time. However, research on the Chicxulub impact, which is now considered the main cause of the extinction, has led many to reject this idea. Some scientists still suggest the eruptions may have had a small effect, but most agree the Deccan eruptions likely played only a minor role or may have even reduced the impact of the Chicxulub event.
The Deccan Traps are believed to have formed mainly from the Réunion hotspot, a volcanic area that is still active today. This hotspot is responsible for creating 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." The term refers to hills that look like steps and shape the landscape of the area. The name "Deccan" comes from 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 rock layers may be covered by younger layers. Most of the volcanic activity happened in the Western Ghats between 66 and 65 million years ago, when lava flowed out through long cracks in the Earth’s surface. Scientists find it hard to determine the exact age of Deccan rocks because the time between eruptions may have been only a few thousand years, and dating tools are not precise enough to measure such short periods. 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 lava flows was once as large as 1.5 million square kilometers, about half the size of modern India. Over time, erosion and movement of Earth’s plates reduced the region to its current size. Today, the area where lava flows can be seen directly is about 500,000 square kilometers.
The Deccan Traps are divided into three sections: the Upper, Middle, and Lower traps. Earlier ideas suggested these sections marked important events in the volcanic process. However, scientists now believe these sections are more closely connected to ancient land shapes and their distance from the eruption sites.
Effect on mass extinctions and climate
Volcanic gases, especially sulfur dioxide, released during the formation of the Deccan Traps may have influenced climate change. During this time, global temperatures dropped by about 2°C (3.6°F) on average.
Some scientists, including Gerta Keller, have suggested that gases from the Deccan Traps might have played a major role in the Cretaceous–Paleogene (K–Pg) extinction event. They believe that sudden cooling from sulfurous volcanic gases and toxic emissions could have contributed to the mass extinction. However, most scientists today agree that the extinction was mainly caused by the Chicxulub impact event in North America. This event would have created a dust cloud that blocked sunlight, killing much plant life and lowering global temperatures (a period called an impact winter).
A 2014 study proposed that both the Deccan Traps and the Chicxulub impact may have caused the extinction. A similar study in 2015 suggested that the impact might have increased or triggered volcanic activity in the Deccan Traps, as the events occurred near opposite sides of Earth. However, a 2020 study questioned whether the Deccan Traps contributed to the extinction at all, noting that their eruptions might 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 quickly and simultaneously in both ocean and land environments, as would be expected from an impact, not from volcanic activity.
In 2019, a study published in Proceedings of the National Academy of Sciences described the Tanis fossil site in North Dakota. This site showed evidence of widespread destruction in an ancient lake and its life forms at the time of the Chicxulub impact. Fossils of trees, fish, and other animals were found, along with tektites (glass-like particles from impacts) in fish gills and amber. A layer rich in iridium, a sign of the impact, was also present. Researchers suggested the site was destroyed by either a tsunami or earthquake-triggered water movements, but the exact cause remains unclear.
A 2024 study analyzed chemical evidence in fossilized peat and found that the Deccan Traps caused long-term warming of about 3°C over 100,000 years near the end of the Maastrichtian period. There was also a brief 5°C temperature drop around 30,000 years before the K–Pg boundary. However, by the time of the K–Pg boundary, global temperatures had returned to earlier levels. This suggests the Deccan Traps were not the main cause of the extinction.
Petrology
In the Deccan Traps, more than 95% of the lava layers are made of tholeiitic basalts. The main minerals in these rocks include olivine, pyroxenes, plagioclase, and some iron-titanium-rich oxides. The magma that formed these rocks contains less than 7% magnesium oxide. However, many of these minerals are found in changed or altered forms. Other types of rocks found in the area include alkali basalt, nephelinite, lamprophyre, and carbonatite.
Mantle xenoliths, which are pieces of deep Earth rock brought to the surface, have been found in Kachchh (northwestern India) and other parts of the western Deccan region. These xenoliths include minerals like spinel lherzolite and pyroxenite.
The Deccan Traps have been classified in many ways, including three main layers based on their position in the rock record. Geochemically, the area can be divided into as many as eleven different rock formations. Many differences in the rock types are caused by varying amounts of contamination from the Earth's crust.
Fossils
The Deccan Traps are known for layers of rock that contain fossils found between layers of lava. Some well-known species include a frog called Oxyglossus pusillus (Owen) from the Eocene period in India and a toothed frog named Indobatrachus, which is an early group 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
Scientists suggest that the Deccan Traps eruption was connected to a deep mantle plume. Magmas from the main eruption phase often have high helium ratios, which are common in magmas from mantle plumes. The area where the eruption occurred over a long period, called the Réunion hotspot, is believed to have caused the Deccan Traps eruption and created the rift that separated the Mascarene Plateau from India. Thinning of the Earth’s crust in the region supports the idea of this rifting event and may have helped the mantle plume rise there. Later, seafloor spreading between the Indian and African Plates moved India northward over the plume, which is now located beneath Réunion Island in the Indian Ocean, southwest of India. However, the mantle plume model has been questioned by some scientists.
New data continues to support the plume model. The movement of the Indian tectonic plate and the timing of Deccan Traps eruptions are closely related. 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 increased rapidly, reaching its highest point when the largest basalt eruptions happened. The spreading rate then decreased around 63 million years ago, when the main phase of Deccan volcanism ended. This connection is thought to be caused by the movement of the mantle plume.
The movement of the Indian and African plates is also linked, with the Réunion plume’s position playing a key role. The sudden increase in India’s speed coincided with a slowdown in Africa’s counterclockwise rotation. The strong connection between these movements suggests both were influenced by the force of the Réunion plume.
When comparing the amounts of sodium, iron, and silicon in the Deccan Traps to other large volcanic regions, the Deccan Traps show the highest levels of melting, which supports a deep mantle plume origin. Olivine separated at depths close to the Moho, and gabbro separated about 6 kilometers below the surface. Features such as widespread faulting, frequent diking events, high heat flow, and areas of higher gravity suggest the Deccan Traps’ volcanic activity was linked to a triple junction that may have existed during the Late Cretaceous period, caused by a deep mantle plume. Not all diking events contributed significantly to the total volcanic flow. Some large dikes are hard to locate because they are near the west coast and likely lie underwater.
Suggested link to impact events
The Deccan Traps started erupting long before the asteroid impact. However, a 2015 study used argon–argon dating to suggest that the impact might have increased the ability of magma to reach the 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 flows could have caused the mass extinctions that marked the end of the Cretaceous period and the start of the Paleogene period, known as the K–Pg boundary. Others disagree, saying this idea is based on incomplete observations.
A geological structure on the seafloor off India’s west coast, 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 it is an impact crater claim it might have triggered the Deccan Traps and sped up the movement of the Indian plate during the early Paleogene. However, most Earth scientists currently believe this structure is not a real impact crater.