Sixty-six million years ago, an asteroid roughly 10 to 15 kilometres wide slammed into the coast of what is now Mexico's Yucatán Peninsula at high velocity and, in the words of the Natural History Museum's Paul Barrett, "effectively vaporised." The blast, heatwave and atmospheric debris that followed triggered a global chain of events — unstable climate, blocked sunlight and collapsed photosynthesis — that killed off the non-avian dinosaurs and around 75% of Earth's species. Yet not every huge impact produces such devastation. Understanding why Chicxulub did, while other catastrophes passed with barely a blip, is the key to understanding mass extinction itself.
What is a Bolide Impact?
A bolide is simply a large extraterrestrial body — an asteroid or comet — that strikes Earth. The most famous example is the object that carved the Chicxulub crater, centred on the Yucatán Peninsula. Though the asteroid itself was 10 to 15 kilometres wide, the violence of the collision excavated a crater some 150 kilometres in diameter, the second-largest impact crater known on the planet.
Scientists linked the crater to the extinction because the two coincide precisely in the rock record. As the Natural History Museum notes, the impact site is "exactly the same age as the extinction of the non-bird dinosaurs, which can be tracked in the rock record all around the world." The date has been refined by recent redating to 66.0 million years ago. A further clue lies in iridium, one of Earth's rarest metals, which occurs far more abundantly in meteorites and is associated with extraterrestrial impacts. The original impact hypothesis was proposed in 1980 by a University of California, Berkeley team led by Nobel laureate physicist Luis Alvarez and his geologist son Walter.
The Chicxulub Impact: A Chain of Catastrophe
The extinction mechanism began with immediate, local devastation. Striking at enormous speed, the asteroid vaporised on impact, and "in the immediate area there was total devastation," Paul Barrett of the Natural History Museum explains. A huge blast wave and heatwave radiated outward, and the collision threw vast amounts of material high into the atmosphere.
That lofted debris was the true global killer. While the blast and heat destroyed everything within reach of the impact zone, the material suspended in the atmosphere spread worldwide, setting up the longer, darker phase of the catastrophe. The immediate effects, in other words, were only the opening act: the extinction of roughly 75% of Earth's animals, including the dinosaurs, required a planet-wide mechanism that could reach every ocean and continent.
