K-T Event (Cretaceous-Tertiary Event)

The K–T event (Cretaceous–Tertiary event) refers to the mass extinction of the dinosaurs that took place approximately 65.0 to 65.5 million years ago (mya). In addition to the dinosaurs, most large land animals perished and an estimated 70% of species became extinct during this Cretaceous–Tertiary extinction. Controversy exists as to how long the mass extinctions occurred. Scientists disagree, some saying it only lasted hundreds or thousands of years, while others state that it remained around for millions of years.

In the early 1980s, an team of physicists and geologists—headed by American physicist Luis Alvarez, geologist Walter Alvarez, chemist Frank Asaro, and chemist Helen Michels—documented a band sedimentary rock in Italy that contained an unusually high level (hundreds of time greater than the concentrations normally found on Earth) of the rare metal iridium (usually found on Earth’s surface only as a result of meteor impacts). The scientists eventually argued that that the iridium layer was evidence of a large asteroid impact that spewed iridium contaminated dust into the atmosphere. Blown by global wind currents, the iridium eventually settled into the present thin sedimentary layer found at multiple sites around the world. Given the generalized dispersion of iridium, the researchers argued that the impact was large and violent enough to cause dust and debris particles to reach high enough levels that they seriously occluded light from the Sun for a large expanse of Earth.

The subsequent reduction in photosynthesis was sufficient to drastically reduce land plant population levels and eventually drive many plant species to extinction. The reduction in plant population levels also provided evolutionary pressure on species nutritionally dependent upon plant life. Large life forms with especially high–energy demands (e.g., dinosaurs)were especially sensitive to the depleted dietary base. The adverse consequences of population reductions and extinctions of plant–eating life forms then rippled through the ecological web and food chain—ultimately resulting in mass extinctions. Some groups that became totally extinct during this period are the ammonoids (which fed on plankton), rudists (a group of clams), inoceramids (an ancient scallop), mosasaurs (a giant lizard), and plesiosaurs (a large marine reptile).

Calculations of the amount of iridium required to produce the observable layer (on average about a centimeter thick) yield estimates indicating that the asteroid measured at least 6 mi (10 km) in diameter. The impact crater from such an asteroid could be 100 mi (161 km) or more in diameter. Such an impact would result in widespread firestorms, earthquakes, and tidal waves. Post–impact damage to Earth’s ecosystem occurred as dust, soot, and debris from the collision occluded the atmosphere to sunlight.

Based on petroleum exploration data, Canadian geologist Alan Hildebrand identified a major impact crater in the oceanic basin near what is now the Yucatan Peninsula of Mexico. The remains of the impact crater, termed the Chicxulub crater, measures more than 105 mi (170 km) in diameter. Argon dating places the Chicxulub impact at the expected Cretaceous–Tertiary geologic time boundary, approximately 65 mya.

Other geological markers are also indicative of a major asteroid impact approximately 65.0 to 65.5 million years age (e.g., the existence of shock quartz, ash, and soot in sedimentary layers dated to the K–T event). Tidal waves evidence surrounding the Gulf of Mexico basin also dates to 65 million years ago (mya).

Other scientists have argued that it was not a solitary impact—that alone caused the mass extinction evidenced by the fossil record. At end of the prior Cretaceous period and during the first half of the Tertiary period, Earth suffered a series of intense and large impacts. Geologists have documented more that 20 impact craters greater than 6.2 mi (10 km) in diameter that date to the late Cretaceous period. Large diameter impact craters were especially frequent during the last 25 million years of the Cretaceous period. (i.e., the Senonian epoch).

The extinction of the dinosaurs and many other large species allowed the rise of mammals as the dominant land species during the Cenozoic era.

See also Astronomy; Catastrophism; Evolution, evidence of; Historical geology.