The Cretaceous-Paleogene (K-Pg) mass extinction event, which occurred approximately 66 million years ago, represents one of the most profound biological crises in Earth’s history. This catastrophe brought an abrupt end to the Mesozoic Era, leading to the sudden disappearance of roughly 75% of all plant and animal species on the planet. The loss was most famously characterized by the extinction of all non-avian dinosaurs, which had dominated terrestrial ecosystems for over 160 million years. Modern scientific understanding has largely converged on a singular cause for this global die-off.
The Asteroid Impact Hypothesis
The prevailing scientific explanation points to the impact of a massive asteroid or comet as the primary trigger for the K-Pg extinction. This theory, initially proposed in 1980, gained substantial support with the discovery of the physical impact site. The object that struck Earth is estimated to have been between 10 and 15 kilometers (6 to 9 miles) wide.
The collision released energy equivalent to approximately 100 million megatons of TNT, instantly creating a massive geological structure. This structure is the Chicxulub crater, a buried impact site located beneath the Yucatán Peninsula in Mexico, with its center partially offshore in the Gulf of Mexico. The crater measures an estimated 180 to 200 kilometers (110 to 120 miles) in diameter, making it one of the largest impact structures on Earth.
Geophysicists first identified the structure in the late 1970s while searching for oil reserves, but its true nature was confirmed in the early 1990s. The discovery provided the necessary link, as the crater’s age precisely matched the 66-million-year-old extinction event. The consensus is that the devastation and climate disruption caused by this single impact were the drivers of the mass extinction.
Concrete Geological Evidence
The impact hypothesis is supported by a thin, globally distributed layer of sediment known as the K-Pg boundary layer, which contains distinct physical evidence. The most compelling proof is the Iridium anomaly, an unusually high concentration of the element Iridium within this layer. Iridium is extremely rare in Earth’s crust but is found in much higher concentrations in extraterrestrial objects.
The Iridium levels in this thin band of clay are up to 160 times higher than background levels, suggesting an extraterrestrial origin. Accompanying the Iridium are widespread grains of shocked quartz, which are mineral crystals deformed by the extreme pressure of an impact event. These grains exhibit unique features created only by the shockwaves generated during a massive collision.
Further evidence includes small, glass-like spheres called tektites, formed when rock is vaporized and rapidly cooled as it is ejected from the impact site. These tektites were transported globally before settling back to Earth, forming part of the K-Pg boundary deposit. The presence of these three markers—Iridium, shocked quartz, and tektites—in a single layer confirms a massive impact event occurred exactly at the time of the extinction.
Concurrent Environmental Stressors
While the Chicxulub impact was the immediate cause, the environment was already under pressure from Earth’s internal processes. Massive volcanic activity, particularly the formation of the Deccan Traps in India, occurred around the same time as the extinction event. This immense volcanic province erupted vast quantities of lava, estimated at over a million cubic kilometers.
The main phase of these Deccan eruptions began roughly 250,000 years before the K-Pg boundary and continued for hundreds of thousands of years. This sustained volcanism released enormous amounts of gases, including sulfur dioxide and carbon dioxide, into the atmosphere. The sulfur dioxide led to short-term cooling and acid rain, while the carbon dioxide contributed to long-term global warming, destabilizing the climate and ocean chemistry.
This long-term environmental degradation likely stressed many ecosystems, making life forms more vulnerable to the sudden shock of the asteroid impact. Although volcanism is not considered the primary cause of the mass extinction, the pre-existing climate shifts and ocean acidification it caused are viewed as a contributing factor. The combination of a stressed planet and a sudden impact proved lethal to a majority of species.
The Mechanism of Mass Extinction
The impact initiated a rapid sequence of environmental collapses that spread across the globe. Immediately following the impact, a massive plume of superheated material was ejected, causing firestorms that ignited forests across the continents. The collision vaporized the carbonate and sulfate-rich rocks of the Yucatán Peninsula, injecting vast quantities of dust, soot, and sulfur aerosols into the stratosphere.
This atmospheric material blocked out the sun globally, leading to a prolonged period of darkness and cooling, often referred to as “impact winter.” The sudden drop in sunlight halted photosynthesis, causing a catastrophic collapse of the plant base of the food chain. Without plants, herbivores starved, and subsequently, the large carnivores that preyed on them also perished.
The vaporization of sulfur-rich rock also created immense clouds of sulfur aerosols, which fell as highly potent sulfuric acid rain. This acid rain severely damaged vegetation and acidified the surface waters of oceans and lakes, further devastating ecosystems. The large, non-avian dinosaurs were disproportionately affected because their size and high metabolic demands required a stable food supply that vanished almost overnight.