c. 65 Million BCE

Dinosaur-Killing Impact

The role of large impacts in catastrophically changing Earth’s climate and biosphere was not fully appreciated until smoking-gun evidence was found that the impact of a large asteroid with Earth was probably responsible for causing the extinction of the dinosaurs and many other species about 65 million years ago, at the boundary between the Cretaceous and Paleogene geologic periods. The key was the discovery of a thin global layer of sediment enriched in the rare element iridium. Iridium is a heavy metal in the platinum family that often bonds with iron in rocks and minerals. Most of Earth’s heavy metals sank into the deep mantle and core when Earth was forming, so a globally distributed iridium-rich deposit in the crust is quite an anomaly. Geologists hypothesized that the iridium came from a large metal-bearing asteroid that impacted Earth and vaporized, dramatically changing the climate and wreaking havoc on most plant and animal species.

The impact would have lifted vaporized rock and dust into the atmosphere and set off large-scale fires that filled the sky with soot and smoke, blotting out the Sun and lowering global surface temperatures for years. While the effect on Earth’s life was not as large as during the Permian-Triassic extinction of 252 million years ago, species such as the ancient dinosaurs that ultimately depended on sunlight and photosynthesis to nurture the base of their food chain were still decimated. Some species, such as mammals and birds capable of burrowing or subsisting on insects, carrion, or other non-plant food-chain staples, weren’t driven to extinction by the event, however.

The idea that the ancient dinosaurs were killed off by an asteroid impact, and that other large impact events could also have led to mass extinctions at other times in Earth’s history, is a hypothesis that is constantly being tested. Other geologic and climatic effects, such as dramatic changes in atmospheric oxygen, large sea-level changes, or massive eruptions of volcanic rock and gases, have also occurred throughout Earth’s history, and sometimes around the same times as hypothesized extinction-level impact events. Thus, multiple events that seem to have been conspiring may have contributed to the environmental conditions that led to Earth’s major mass extinctions.

SEE ALSO Earth’s Core Forms (c. 4.54 Billion BCE), Late Heavy Bombardment (c. 4.1 Billion BCE), Cambrian Explosion (c. 550 Million BCE), The Great Dying (c. 252 Million BCE), Arizona Impact (c. 50,000 BCE), The Tunguska Explosion (1908), Extinction Impact Hypothesis (1980)

Artist’s concept of a large asteroid crashing into the Earth marks the precise moment of the end of the Cretaceous and the beginning of the Paleogene (formerly Tertiary) period of geologic history, about 65 million years ago.