SOURCE # 1 : What Killed the Dinosaurs? - an online resource accessed at http://www.pbs.org/wgbh/evolution/extinction/dinosaurs/low_bandwidth.html
@2001 WGBH Educational Foundation and Clear Blue Sky Productions, Inc. All Rights Reserved
Hypothesis: Asteroid Impact
Did a collision with a giant asteroid or comet change the shape of life on Earth forever? It is widely agreed that such an object -- 10 kilometers across -- struck just off the coast of the Yucatan peninsula 65 million years ago. According to scientists who maintain that dinosaur extinction came quickly, the impact must have spelled the cataclysmic end. For months, scientists conclude, dense clouds of dust blocked the sun's rays, darkening and chilling Earth to deadly levels for most plants and, in turn, many animals. Then, when the dust finally settled, greenhouse gases created by the impact caused temperatures to skyrocket above pre-impact levels. In just a few years, according to this hypothesis, these frigid and sweltering climatic extremes caused the extinction of not just the dinosaurs, but of up to 70 percent of all plants and animals living at the time.
Impact Crater This 150-kilometer-wide crater lies just off the Yucatan peninsula. Scientists calculate that it was blasted into Earth by a 10-kilometer-wide asteroid or comet traveling 30 kilometers per second -- 150 times faster than a jet airliner. Scientists have concluded that the impact that created this crater occurred 65 million years ago. The date corresponds perfectly to the date of the dinosaur extinction.
Rare Metal The metal iridium, which is similar to platinum, is very rare on Earth's surface but is more common in asteroids and in molten rock deep within the planet. Scientists have discovered levels of iridium 30 times greater than average in the Cretaceous/Tertiary (KT) boundary, the layer of sedimentary rock laid down at the time of the dinosaur extinction.
Melted Rock These pieces of once-molten rock, called impact ejecta, are evidence of an explosion powerful enough to instantly melt bedrock and propel it more than a hundred miles from its origin. Ranging in size from large chunks to tiny beads, impact ejecta are common at or near the Cretaceous/Tertiary (KT) boundary, the geological layer that defines the dinosaur extinction.
Fractured Crystals These crystals, often called "shocked quartz," show a distinctive pattern of fracturing caused by high-energy impacts or explosions. Some scientists maintain that the fracture pattern in these quartz crystals could only have been caused by a massive asteroid or comet impact. The pattern is prevalent in quartz found at or near the Cretaceous/Tertiary (KT) boundary, the geological layer deposited at the time of the extinction.
Fossil Record A gradual decline in the number of dinosaur species would likely mirror an equally gradual cause of their ultimate extinction. Conversely, a sudden "now you see them, now you don't" end to the dinosaurs implies a catastrophic cause. Depending on location and interpretation, the fossil record seems to say different things. Some paleontologists see evidence in the fossil record that dinosaurs were doing quite well prior to the end of the Cretaceous -- that they were in no way declining in abundance when the impact occurred.
SOURCE # 2 : Guide to the Dinosaurs of Colorado? - an online resource accessed at http://webspinners.com/dlblanc/paleo/dino-colo/index.php
A resource of the Morrison Museum in Colorado Written by Donald L. Blanchard @2004
Currently the most popular theory is that an asteroid or giant meteor struck the Earth at the end of the Cretaceous Period, causing a drastic and devastating global climate change.
Serendipity Strikes: Geologist Walter Alvarez was familiar with the Fish Clay sediments there. Interested in determining the span of time over which the clay sediments were deposited, he analyzed the sediments for trace elements left by accumulations of cosmic debris. This debris contains unusual concentrations of certain platinum-group rare earths, notably iridium, which are otherwise very rare in the Earth's crust. One could argue that the "iridium spike" represented a period where the rate of clay deposition was drastically slowed, allowing more time for cosmic debris to accumulate. To account for the measured iridium concentrations, several million years would have had to elapse. But the maximum time interval for accumulation of the clay layer was already bounded by other constraints .
Additional evidence of a meteor impact was also discovered, in the form of 'microtektites,' small, spherical particles of molten ejecta with a distinctive fracture pattern. Microtektites are normally associated only with the most violent of explosions, such as occur when a giant meteor strikes the Earth. Microtektites have been found at many, but not all, of the boundary clay deposits in various parts of the world.
Supporting Arguments: The Giant Meteor Impact theory meets many of the criteria for a successful extinction theory, and its incredible popularity among the scientific community attests to its success. It satisfactorily explains the K/T mass extinction event, including why some species were extirpated while others survived. A predicted consequence of a giant meteor impact is that immense quantities of dust and aerosols would be thrown up into the atmosphere, darkening the sky for many months, blocking out the Sun and causing something like the "Nuclear Winter" scenario predicted as the aftermath of an all-out nuclear war.
Weaknesses of the Theory: Despite its popularity, the Meteor Impact theory is not without its problems. Debate between experts continues over fallout patterns, the size versus the length of time particles could remain airborne and how far they could disperse before settling, and the predictions of what effect how much protracted darkness would really have on various ecosystems. The whole "Nuclear Winter" hypothesis is based on extrapolations from various atom bomb tests and volcanic explosions, and the data thus derived remains debated and unproven.
Aside from these technical objections, several other assumptions of the theory have been challenged. Iridium and the other platinum-group rare earths are rare in the Earth's crust because they are siderophilic, or iron-loving. Siderophiles are thought to be much more common in the Earth's interior, which is composed largely of iron and its associated elements. But material from the Earth's interior is just as likely to be injected into the atmosphere as is extraterrestrial material -- from volcanic eruptions. Iridium in measurably concentrations has been detected from the eruptions of the volcano Kilauea in Hawaii. Now it is known that for about half a million years, spanning the Cretaceous/Tertiary boundary, one of the greatest volcanic eruptions of all time was going on, forming the Deccan Traps of western India. The Deccan Traps are basaltic deposits, as are the eruptions of Kilauea, and it is quite possible that volcanics could be the source of the Iridium Spike.
The Meteor Impact theory also fails to explain the perceived gradual die of of foraminiferans and dinosaurs. It postulates a very sudden dieoff, striking down whole lineages of organisms in their prime. Most paleontologists reject this claim. While most of the scientific community heartily embraces the theory, the majority of paleontologists reject it. Many paleontologists are willing to believe that a meteor impact could have occurred, but don't accept that it caused the extinctions. It could, they maintain, have been the last straw that finished off an already dying breed. Many question that a meteor impact ever occurred at all.
Also, the theory does not address the last condition for a "really good" extinction theory; that it explain ALL mass extinctions, not just the K/T event. Search though they might, no advocate of the meteor impact theory has been able to find an iridium spike associated with any other mass extinction event.