K–T extinction

The Cretaceous–Paleogene (K–Pg) extinction event, also known as the Cretaceous–Tertiary (K–T) extinction, was a mass extinction of some three-quarters of the plant and animal species on Earth that occurred over a geologically short period of time, approximately 66 million years ago. With the exception of some ectothermic species such as the leatherback sea turtle and crocodiles, no tetrapods weighing more than 25 kilograms (55 lb) survived. It marked the end of the Cretaceous period and with it, the entire Mesozoic Era, opening the Cenozoic Era that continues today.

In the geologic record, the K–Pg event is marked by a thin layer of sediment called the K–Pg boundary, which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows high levels of the metal iridium, which is rare in the Earth's crust, but abundant in asteroids.

As originally proposed in 1980 by a team of scientists led by Luis Alvarez and Walter Alvarez, it is now generally thought that the K–Pg extinction was caused by a massive comet or asteroid impact, estimated to be 10 km (6.2 mi) to 15 km (9.3 mi) wide, 66 million years ago and its catastrophic effects on the global environment, including a lingering impact winter that made it impossible for plants and plankton to carry out photosynthesis. The impact hypothesis, also known as the Alvarez hypothesis, was bolstered by the discovery of the 180-kilometre-wide (112 mi) Chicxulub crater in the Gulf of Mexico in the early 1990s, which provided conclusive evidence that the K–Pg boundary clay represented debris from an asteroid impact. The fact that the extinctions occurred at the same time as the impact provides strong situational evidence that the K–Pg extinction was caused by the asteroid. Possibly, it was accelerated by the creation of the Deccan Traps; however, some scientists maintain the extinction was caused or exacerbated by other factors, such as volcanic eruptions,climate change, or sea level change, separately or together. A 2016 drilling project into the Chicxulub peak ring confirmed that the peak ring comprised granite ejected within minutes from deep in the earth (rather than usual seafloor rock), and evidence of colossal seawater movement directly afterwards (from layered sand deposits). Crucially, the cores also showed a nearly complete absence of gypsum, the usual sea floor rock in the region, which is sulfate-containing; this would have been vaporized and dispersed as an aerosol into the atmosphere, providing evidence of a probable link between the impact and a global scale of longer-term effects on the climate and food chain.

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