The extinction of the non-avian dinosaurs 66 million years ago marks the end of the Cretaceous Period and the beginning of the Paleogene Period, known as the Cretaceous-Paleogene (K-Pg) boundary event. This shift was triggered by the impact of an asteroid approximately 10 to 15 kilometers in diameter striking the Yucatán Peninsula in Mexico, forming the Chicxulub crater. The death of the dinosaurs was not instantaneous but a rapid, multi-stage process. It began with immediate physical forces and culminated in a protracted global ecological collapse that eradicated roughly 75% of all plant and animal species on Earth.
The Initial Impact and Physical Devastation
The immediate destruction occurred within the first hours of the impact. The energy release generated a massive shockwave and air blast, radiating outward with winds exceeding 1,000 kilometers per hour near the impact site. This scoured the landscape and shredded life across the Americas. Within minutes, the impact also triggered a seismic event estimated between Magnitude 9 and 11, causing widespread ground shaking and landslides thousands of kilometers away.
The colossal force of the impact excavated a transient cavity 30 kilometers deep and generated a megatsunami. Initial waves near the impact site may have reached heights of 1.5 kilometers. Within four hours, the tsunami spread out of the Gulf of Mexico into the Atlantic and Pacific oceans. Significant waves continued to cross the world’s oceans for up to 48 hours, scouring the seafloor and devastating coastlines globally.
As the impactor and surrounding rock vaporized, superheated ejecta launched high into the atmosphere. The friction from this material re-entering the atmosphere across the globe generated a thermal pulse, turning the sky into a global oven. This intense burst of heat, lasting for an hour or less, ignited widespread wildfires across every continent, killing any animal not sheltered underground or underwater.
The Secondary Kill Mechanism: Global Environmental Collapse
While the initial impact was devastating, most extinctions were caused by the subsequent global environmental collapse that unfolded over months to years. Firestorms and vaporized rock injected massive amounts of soot, dust, and sulfur aerosols into the stratosphere. This dense, high-altitude cloud shrouded the planet, plunging the Earth into a period of darkness known as “impact winter.”
The thick veil blocked nearly all incoming sunlight, causing a sustained global temperature drop estimated at 15 degrees Celsius. This cessation of photosynthesis, which may have lasted for nearly two years due to persistent silicate dust, was the mechanism for mass extinction. Without sunlight, plants died first, collapsing the food chain from the bottom up. Herbivores starved, followed by carnivores, including the non-avian dinosaurs.
The impact struck a region rich in anhydrite, a calcium sulfate mineral, releasing immense quantities of sulfur into the atmosphere. This vaporized sulfur mixed with water vapor to create sulfuric acid aerosols, leading to prolonged acid rain for months or years. This corrosive precipitation destroyed vegetation and acidified ocean surface waters, contributing to the mass die-off of marine life, such as plankton. Complete ecological recovery from the impact winter would take many decades, if not centuries.
Reading the Clock: Geological Evidence of the Timeline
Scientists can determine this precise timeline by reading the geological record, which acts as a clock for the catastrophe. The most famous marker is the Iridium layer, a thin band of clay found worldwide exactly at the K-Pg boundary. Iridium is rare in the Earth’s crust but abundant in asteroids, providing the definitive signature of the extraterrestrial impact.
Directly below or within the Iridium layer are deposits of microtektites, tiny spheres of rock that melted and solidified as they were ejected and rained down globally following the impact. These markers confirm the impact was a geologically instantaneous event that occurred 66.043 million years ago, a date fixed with high precision using radiometric dating techniques.
Immediately above the boundary layer, the fossil record often displays the “fern spike”—a temporary, dramatic increase in fern spores relative to the pollen of flowering plants. This indicates that complex forests and angiosperm-dominated ecosystems collapsed, allowing resilient, fast-reproducing ferns to rapidly colonize the barren, fire-swept landscapes. This period of fern dominance suggests the initial ecological devastation and recolonization phase lasted for hundreds to a thousand years.