The Cretaceous-Tertiary (KT) extinction event, also known as the KT boundary, was an extinction event that occurred about 65 million years ago, at the end of the Cretaceous period. This extinction event is best-known for the extinction of the dinosaurs, but many other forms of life perished as well; approximately 50% of all genera went extinct during this time. Many explanations for this event have been proposed, the most widely-accepted being an impact event.
In 1980, a team of researchers led by Nobel-prize-winning physicist Luis Alvarez, his son, geologist Walter Alvarez, and a group of colleagues discovered that fossilized sedimentary layers found all over the world at the Cretaceous-Tertiary boundary, 65.7 million years ago, contain a high proportion of iridium, which is relatively common in asteroids. The iridium concentration was almost two orders of magnitude greater than normal. The end of the Cretaceous coincided with the end of the dinosaurs and was in general a period of extraordinary mass extinction, leading to the Tertiary era, in which mammals began to predominate on Earth. The paper suggested that the dinosaurs had been killed off by the impact of a ten-kilometer-wide asteroid on Earth (see impact event).
Iridium is very rare on Earth's surface, but much more common in the Earth's interior as well as in extraterrestrial objects, such as asteroids and comets. Furthermore chromium isotopic anomalies are found in Cretaceous-Tertiary boundary which strongly supports the impact theory and suggests that the impactor must have been an asteroid or a comet composed of material similar to carbonaceous chondrites.
The resulting blast would have been hundreds of millions of times more devastating than the most powerful nuclear weapon ever detonated, may have created a hurricane of unimaginable fury, and certainly would have thrown massive amounts of dust and vapor into the upper atmosphere or even into space. The worldwide cloud would have choked off sunlight for years, resulting in a "long winter" that wiped out many existing species, as well as creating "acid rains" that would have inflicted further hardship on the environment.
Although further studies of the "Cretaceous-Tertiary" or "K-T" layer (geologists refer to the Cretaceous era as "K" because "C" is taken by the "Cambrian" era) consistently showed the excess of iridium, the idea that the dinosaurs had been exterminated by an asteroid remained a matter of controversy among geologists for over a decade.
One problem was that no known crater matched the event. This was not a lethal blow to the theory. Although the crater resulting from the impact would have been 150 to 200 kilometers in diameter, as mentioned in the previous section the Earth's geological processes tend to hide craters over time. Still, finding a crater would obviously have buttressed the "Alvarez hypothesis", as it came to be known. The discovery of a crater buried under Chicxulub in the Yucatan as well as various types of debris in North America have lent credibility to this theory (see Chicxulub Crater). Most paleontologists agree that an asteroid did hit the Earth 65 million years ago, but there is still disagreement over whether that impact caused the extinction.
A minority of scientists think the extensive volcanic activity in India known as the Deccan Traps may have been responsible for, or contributed to, the extinction. It has also been suggested that this is a secondary effect of the impact. However, paleontologists remained skeptical, as their reading of the fossil record suggested that the mass extinctions did not take place over a period as short as a few years, but instead occurred gradually over about ten million years. There was also a certain general distrust of a group of physicists intruding into their domain of expertise.
Luis Alvarez, who died in 1988, replied that paleontologists were being misled by sparse data. His assertion did not go over well at first, but later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as a drop in sea level and massive volcanic eruptions in India (Deccan Traps sequence), may have also contributed to the extinctions.
Skeptics remain. Although there is now general agreement that there was a huge impact at the end of the Cretaceous that led to the iridium enrichment of the K-T boundary layer, remnants have been found of other impacts of the same order of magnitude that did not result in any mass extinctions, and in fact there is no clear linkage between an impact and any other incident of mass extinction.
One interesting note about the K-T Event is that most of the larger animals that survived were to some degree aquatic, implying that aquatic habitats may have remained more hospitable than land habitats.
The impact and volcanic theories can be labeled "fast extinction" theories. There are also a number of slow extinction theories. Studies of the diversity and population of species have shown that the dinosaurs were in decline for a period of about 10 million years before the asteroid hit. Slower mechanisms are needed to explain such extinctions. Climatic change, a change in Earth's magnetic field, and disease have all been suggested as possible slow extinction theories.
It is worth noting that the Cretaceous extinction is neither the only mass extinction in Earth's history, nor even the worst. Previous extinction events have included the Triassic-Jurassic extinction event and the Permian-Triassic extinction event, which is the largest extinction event ever recorded.