The Impact Theory - Dinosaur Extinction
In the early 1980's Luis Alvarez proposed the theory of dinosaur extinction by an asteroid impact. His theory suggested that an asteroid 9 to 12 miles (15 to 19 kilometers) in across collided with earth at the end of the Cretaceous Period, about 65 million years ago.
This asteroid impact would create a dust cloud enveloping the globe for months, or even a year. This dust rose high into the atmosphere blocking the sun's rays, causing the earth to be engulfed in darkness.
Photosynthesis in the sea and on land ceased. This caused a domino effect on the pyramid food chain. Plants died or became dormant soon after, herbivores starved and die out, followed by the carnivores.
Some of the best supporting evidence is that high levels of the rare earth element iridium at the K-T boundary and the probable remains of an impact crater. Iridium is an element that is found on earth, especially deep in the earth, high levels of iridium are associated with extraterrestrial events.
Generally the greater the impact object, such as an asteroid, the greater in concentration of this iridium element. The remains of a probable impact structure in "Chicxulub", a crater approximately 110 miles across at the north tip of the Yucatan peninsula from at or near the K-T boundary, are strong evidence for an impact.
Neither high concentrations of iridium or a crater impact are direct evidence for specific causes of extinction at the K-T boundary.
The impact generated scenario that extremely high levels of catastrophic extinction across most environments is so large in magnitude that it is difficult to test. Portions of impact theory are more easily tested such as the sudden cold snap, highly acidic rain, or global wildfires using the known K-T vertebrate record.
It is argued that if tremendous amount of dust were ejected into the atmosphere after a large impact, darkness would not only affect photosynthesis but also produce extreme subzero temperatures. This is known as "impact winter".
Following the asteroid impact, ocean temperatures would decrease only a few degrees because the huge heat capacity of the oceans, but on the continents however, temperatures would last for up to 6 months. These subfreezing temperatures will last for about twice the time of darkness caused by the dust in the atmosphere.
This may explain how some small animals such as mammals and fishes in the oceans avoided extinction. Small animals on land could burrow and find shelter underground from these temperatures, and animals in the oceans were better protected than large animals exposed on land.