Pros for Impact Theory
by far, the most popular theory for the last 25 years.
widely accepted by the scientific community.
effectively explains all phenomenon regarding the event interpreted from the geological record.
explains why some flourishing species went extinct quickly and others were hardly affected at all.
secondary events in response to the impact would have wreck havoc too: such as dust and gases ejected into the atmosphere would have caused climatic changes and artificial winter conditions, changing environments for months, perhaps years.
dust would have blocked out the sun leading to several months of darkening, which would wreck havoc on photosynthesizing plankton effectively destroying ocean food chains. On land, the protracted stages of darkness would have halted plant growth which would have killed off herbivores.
added destruction would have been caused by tsunamis, earthquakes and other impact related phenomenon.
Cons for Impact Theory
many organisms started to decline or went extinct much before the impact.
many lineages do not end abruptly; they go extinct gradually. Some even extend into the Tertiary before going extinct.
impact would have caused earthquakes measuring 15 on the Richter Scale. Undisturbed deep sea sediments show no signs of such an earthquake.
much of the bolide impact also supports the mass volcanism theory.
recently, Dr. Gerta Keller at Princeton University has proved that the Chicxulub Crater predates the K/T boundary by 300,000 years. This proves that the impact could not have been the sole cause for the extinction.
much of the events that the impact would have causes are purely theoretical (especially ones dealing with how long and how much the dust would have changed the climates.
iridium can also be brought up through volcanism. Hawaiian eruptions have been recently shown to bring Ir from the mantle to the surface.
only stands to explain the K/T extinction. Many people believe a universal extinction theory will explain all the major extinctions.
Another note about climate: during the Cretaceous, the Earth's climate was far different than at any other time in geologic history. There were essentially no freezing temperatures anywhere on the earth - not even at the poles during the winter. Currently, the cold currents in the bottom of our oceans have a temperature of close to 4 degrees C; in the K, it ran close to 17 degrees Celsius. Many of the typical flora and fauna of tropical areas were found in high paleolatitudes (even as far as 70 degrees). If warming had resulted in creating these temperatures at the poles then the temperatures in the tropics would be unbearable for life. Hence, what we though to be an invariant temperature gradient from the poles to the tropics did not exist during the Cretaceous. How can a meteorite impact cause such climate changes? And cause them to last for 65 Ma.