The K-T Extinction

Similar to the end of the Paleozoic era, the Mesozoic Era ended with the K-Pg Mass Extinction (previously known as the K-T Extinction) 66 million years ago. This extinction event was likely caused by a large bolide (an extraterrestrial impactor such as an asteroid, meteoroid, or comet) that collided with earth. Ninety percent of plankton species, 75% of plant species, and all the dinosaurs went extinct.

One of the strongest pieces of evidence comes from the element iridium. Quite rare on Earth and more common in meteorites, it has been found worldwide in higher concentrations at a particular layer of rock that formed at the time of the K-T boundary. Soon other scientists started to find evidence to back up the claim. Melted rock spheres, a special type of “shocked”  quartz called stishovite found only at impact sites, were found in many places worldwide. The huge impact created a strong thermal pulse that could be responsible for global forest fires, strong acid rains, a corresponding abundance of ferns, the first colonizing plants after a forest fire, enough debris thrown into the air to significantly cool temperatures afterward, and a 2-km high tsunami inferred from deposits found from Texas to Alabama.

Still, with all this evidence, one large piece remained missing: the crater where the bolide impacted. It was not until 1991 that the crater was confirmed using petroleum company geophysical data. Even though it is the third largest confirmed crater on Earth at roughly 180 km wide, the Chicxulub Crater was hard to find due to being partially underwater and partially obscured by the dense forest canopy of the Yucatan Peninsula. Coring of the center of the impact, called the peak ring, contained granite, indicating the impact was so powerful that it lifted basement sediment from the crust several miles toward the surface. In 2010, an international team of scientists reviewed 20 years of research and blamed the extinction on the impact.

With all of this information, it seems like the case would be closed. However, other events at this time could have partially aided the demise of so many organisms. For example, sea levels are known to slowly decrease during the K-T event, which is tied to marine extinctions. However, any study on gradual vs. sudden changes in the fossil record is flawed due to the incomplete nature of the fossil record. Another big event at this time was India’s Deccan Traps flood basalt volcanism. At over 1.3 million cubic kilometers of material, it was a large source of material hazardous to ecosystems at the time. It has been suggested as at least partially responsible for the extinction. Some have found the impact and eruptions too much of a coincidence and even linked the two.