The disappearance of non-avian dinosaurs 66 million years ago remains one of Earth’s most significant and debated geological events. This catastrophic period, known as the Cretaceous-Paleogene (K-Pg) extinction, reshaped life on the planet. It ended the Mesozoic Era, an age dominated by vast reptiles, and ushered in the Cenozoic Era, marking the rise of mammals. The extinction wiped out an estimated 75% of all plant and animal species, highlighting the profound impact rapid environmental changes can have on global ecosystems.
The Impact Event: The Dominant Theory
The most widely accepted scientific theory attributes the K-Pg extinction to the catastrophic impact of a massive asteroid. This extraterrestrial object, 10 to 15 kilometers (6 to 9 miles) in diameter, struck Earth at about 20 kilometers per second (12 mi/s). The impact site, now known as the Chicxulub crater, is buried beneath the Yucatán Peninsula in Mexico.
The immediate aftermath generated an explosion equivalent to 100 teratons of TNT. This force created megatsunamis over 100 meters (330 feet) tall, surging far inland, eroding coastlines and depositing debris. The impact also produced powerful shockwaves and air blasts, with winds exceeding 1,000 kilometers per hour near the impact zone, shredding vegetation and scouring the landscape.
As debris re-entered the atmosphere, it heated the air to extreme temperatures, igniting widespread wildfires across the globe. Soot from these fires, combined with billions of tons of sulfur and dust, formed a dense global cloud that obscured the sun. This prolonged darkness, lasting at least a decade, initiated an “impact winter,” dropping global temperatures and halting photosynthesis. The collapse of plant life led to a rapid ecological collapse, exacerbated by acid rain that damaged vegetation and acidified freshwater bodies.
Volcanic Activity: A Co-Conspirator?
While the asteroid impact is the leading explanation, some scientists propose that massive volcanic eruptions, particularly those forming the Deccan Traps in west-central India, played a contributing role in the K-Pg extinction. These immense volcanic outpourings, occurring around the same geological period, covered 1.5 million square kilometers (0.58 million sq mi) with basalt layers over 2 kilometers thick. The main phase of these eruptions coincided with the end of the Cretaceous.
The Deccan Traps released vast quantities of gases, including carbon dioxide and sulfur dioxide, into the atmosphere. Such emissions could have triggered climate shifts, initially causing a “volcanic winter” due to ash and sulfur aerosols blocking sunlight, followed by long-term global warming from increased greenhouse gases. Sulfur dioxide would have also led to widespread acid rain and ocean acidification, further stressing ecosystems.
The scientific community debates the exact role of Deccan volcanism. Some research suggests these eruptions acted as a long-term stressor, weakening global ecosystems before the asteroid impact delivered the final blow. However, recent studies, including climate modeling, indicate volcanic carbon emissions may not have been a major driver of the extinction, as warming from the Deccan Traps had lessened by the time of the asteroid impact. Some analyses suggest a portion of the Deccan Traps’ heightened activity occurred after the K-Pg boundary, limiting its direct role in the extinction.
Reading the Earth’s Story: Evidence and Reconstruction
Scientists reconstruct dinosaur extinction events through geological and paleontological clues found worldwide. A thin, distinct sediment layer, the K-Pg boundary, marks the extinction event in rock formations globally. This boundary is dated to 66.043 million years ago.
A key piece of evidence is the “iridium anomaly,” a high concentration of the rare element iridium found within the K-Pg boundary clay. Iridium is scarce in Earth’s crust but more abundant in asteroids, suggesting an extraterrestrial impact. This discovery, made by Luis and Walter Alvarez in 1980, was foundational to the asteroid impact hypothesis.
Further evidence includes shocked quartz and tektites. Shocked quartz grains, characterized by distinctive deformation lamellae, form under extreme pressures from meteorite impacts or nuclear explosions. Tektites, small glassy spherules of molten rock, are also found in K-Pg sediments globally. The discovery and dating of the Chicxulub impact crater, a massive 200-kilometer (120-mile) structure, provides definitive proof of the impact.
The fossil record offers direct biological evidence, showing an abrupt, global disappearance of non-avian dinosaurs at the K-Pg boundary. While debate exists whether dinosaur populations were already in decline, current evidence suggests continued diversification until the impact event. The extinction also affected most other large tetrapods, plant communities, and marine life, highlighting its global reach.