The evidence for extensive volcanic eruptions comes from an unusual geologic formation in present-day India.
Evidence: Western India is home to the Deccan Traps--a huge, rugged plateau that formed when molten lava solidified and turned to rock. The Deccan Traps date back to around 66 million years ago, when magma from deep inside Earth erupted to the surface. In some parts of the Deccan Traps, the volcanic layers are more than two kilometers (1.2 miles) thick, making this the second-largest volcanic eruption ever on land.
Interpretation: Volcanic activity of this magnitude would have spewed out huge amounts of carbon dioxide into the atmosphere, causing greenhouse warming. The eruptions would have also caused levels of toxic gases like sulfur and chlorine to rise, resulting in acid rain and further damaging the global environment.
Conclusions: The Deccan Traps eruptions could have caused some global climate change. Ash and dust may have blocked sunlight, causing temperatures to drop. Over time, the climate would have warmed as a result of higher levels of greenhouse gases. Some researchers think that intense volcanism could have contributed to the dinosaurs' decline well before a comet or asteroid impact sealed their fate.
Solid as a Rock
This chunk of rock from the Deccan Traps in western India was once molten lava that erupted from deep inside Earth around 66 million years ago. Intense volcanic activity around this time may have caused long-term global warming by injecting large amounts of greenhouse gases into the atmosphere.
New Ideas: Tiny Clues
The molten lava, ash and toxic gases erupting from a volcano are deadly. But just how deadly? And how long do the effects last? Researchers have had difficulty measuring the extent of the damage caused by the Deccan Traps eruptions, which began well before the impact. But paleontologist Gerta Keller thinks foraminifera, microscopic marine organisms called "forams" for short, may provide an answer. Keller has identified a species of foram (image) that flourishes only after environmental disasters. This species appears in the fossil record in large numbers around the time of the Deccan Traps eruptions--and continues to thrive for hundreds of thousands of years, indicating that the eruptions may have had widespread ecological effects for a long time.
"Year Without A Summer"
In 1815, the volcano Tambora erupted in Indonesia. Considered the largest volcanic eruption in recorded history, Tambora led to the "year without a summer." A haze of tiny particles blocked sunlight and in 1816, frosts occurred in summer in the United States. Recently, the eruption of Mount Pinatubo in the Philippines lowered the average global temperature by a small but measurable amount (0.3°C) for two years.