What is responsible for the extinction of dinosaurs, asteroid impact or volcanic eruptions?

Approximately 65 million years ago, the Age of Reptiles suddenly ended with the extinction of dinosaurs. Barring a few winged species and sundry creatures here and there, all dinosaurs vanished forever. The suddenness of their disappearance suggests that they were wiped out by some worldwide natural disaster. What was the catastrophe?

The traditional view still held by some experts lays the blame on huge volcanic eruptions that started a few years before the giant reptiles died out. Basically, the Earth leaked lava and the major area of activity was Indian subcontinent. A million or so cubic kilometers of red hot lava flowed across what is now southern India. About 2.4 kilometers thick basalt deposit left behind came to be known as Deccan Trap. The large-scale volcanic activity could have caused a cloud of dust to envelop the globe, blocking out sunlight for months during which no photosynthesis was possible. The active volcanoes might have also released sulfur dioxide to cause strong acid rain which killed off whatever was left of the plant life. Cut off from their food supply most dinosaurs perished and later the settling volcanic ash buried them. A few winged dinosaurs survived because they happened to be sheltered in hollows of cliffs or rock piles.

The extinction theory that has gained wide acceptance goes like this: An asteroid, about 10 kilometers across, slammed the Earth 65 million years ago. The dust thrown into the atmosphere would have caused a long period of darkness and cold which had the same effects as those of volcanic eruptions. Plants, cut off from the sunlight, could not survive for more than a few days. Without plants, the herbivorous dinosaurs gradually died out and the carnivores soon afterward.

This line of thought was presented by the American scientist Luis Alvarez, who in the late 1970s discovered the evidence also: It was the chemical element iridium which is almost nonexistent in the Earth’s crust but 10,000 times more abundant in extraterrestrial rocks such as meteorites and asteroids. This element was incorporated into sediments laid down at the time dinosaurs perished. The asteroid theory was assumed significance as iridium anomalies have been found in nearly 100 places around the world since 1980.

Was it really an asteroid impact that killed off dinosaurs? If so, where is the actual crater which the violent impact must have created?

The crater had already been discovered 30 years earlier, but its existence was kept a commercial secret. Back in the 1950s the Mexican State Oil Company had detected a massive underground rounded structure beneath the Yucatan peninsula, extending up to the Gulf of Mexico. Company geologists thought it might contain oil, but test drills revealed only hard crystalline rock. The drill report was filed away and forgotten until 1991 when researchers got hold of the oil company’s data that described a crater about 200 kilometers across. This seemed to be the impact predicted by Luis Alvarez. Later, geological investigation proved it to be so. Because of this discovery, an asteroid impact is currently the most widely accepted explanation of dinosaurs’ extinction.

More reading:
Dinosaur (Wikipedia)
Extinction (Wikipedia)

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