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The K-T boundary extinction event took place 65,000,000 years ago, and is merely one in a long line of mass extinction events. The chief reason for its fame is that it ushered out those perennially popular beasts, the dinosaurs. Many explanations have been given for this mass extinction, the most widely known being a massive impact caused by a bolide, or meteorite (meteorites are meteors which have impacted on the surface of a planet or other body). This idea has proved popular with television programme makers and the general public alike. This is probably because 1,000 years of C02 poisoning by wide-ranging volcano systems lacks the dramatic effect and neat ending produced by computer generated acts of planetary penetration.
What's in a Name?
K-T confusingly describes the change-over period between the Cretaceous and the Tertiary. The 'K' is Greek, representing the word Kreta, or chalk. The 'Cret' of Cretaceous also comes from the same word, indicating that there was a lot of chalk laid down during this period1. The term was coined to delineate between two periods, the delineation point being the extinction event. This was established well before the bolide impaction theory was aired, but mentioning the K-T event is now more or less synonymous with the bolide impact. It has, in fact, proved such an unshakable image that it will be quite a while before (in the public eye at least) any other theory will get a fair hearing.
Never Mind the Bolides
The bolide theory is very neat, and there is a large amount of evidence to back it up. There is a fine layer of clay that delineates the Cretaceous from the Tertiary - this sedimentary layer has been found all over the globe. Tests performed on this layer reveal the presence of an unusually high concentration of iridium. Iridium is not uncommon on Earth, but the concentrations found at the K-T boundary clay layer indicate an extra-terrestrial source. Put bluntly, the only bodies in which such large concentrations of iridium are found are bolides. This layer points to a single, global event during which this material was distributed widely. One explanation of this distribution is a massive impact. Other evidence comes in the form of distressed quartz. Distressed quartz develops unusual properties when subjected to extreme impaction. Many examples of distressed quartz have been found in the same layer as the iridium, as have microtectites (tiny balls of glass found near impact craters, another symptom of a bolide collision).
The question of cause and effect remains, though. Was the bolide the cause of the extinction event, or did it arrive, coincidentally, during an established extinction process? Simply because the two events occurred at the same time, it cannot be stated with any confidence that the bolide collision definitely caused the K-T extinction.
Chicxulub and the Effects of Such a Collision
Under the seas surrounding the Yucatan platform of Mexico lay a secret that was undiscovered for 65,000,000 years. Named the 'Chicxulub Crater' after the nearby village of Puerto Chixulub, the crater is all that survives of a massive bolide collision with the Earth. The impact of an item approximately 12.5 miles wide left a crater of between 90 and 125 miles across. It is posited that one of the initially devastating effects of such an oceanic impact would be a wall of water up to five miles high. The wall obviously wouldn't stay there for long - it would come crashing down and become an unstoppable tsunami, flooding the land for miles and miles around, and covering the previously fertile soil with salt - ruining the chances of anything growing back.
While this was going on, portions of the bolide and fragments of the Earth's crust would have been hurled into and out through the atmosphere - some so violently that they would have achieved a temporary orbit before raining down to Earth. Dust and soot would have filled the air and darkened the skies - smaller meteors and burning debris from the main site would also rain down. The heat generated by the impact would have started raging fire storms which would sweep across continents. An estimated 77,220 cubic miles of the Earth's crust was vaporised and ejected from the Chicxulub site.
What follows this event in the usual narratives is the tale of a slow death for the dinosaurs. There was no food, for in the ensuing darkness no photosynthesis could occur. Cold dinosaurs plodded around blindly, choking on dust and dying in their droves across this shattered and ruined planet. An estimated 70% of all species perished. The face of the Earth would never be the same again. But was it really so bad?
The Moth in the Ointment
There are several reasons to believe that the Chicxulub impact crater was not quite as devastating as popular science programmes would have you believe. The humble moth survived quite nicely. Moths breathe through 'spiracles', an apparatus which requires clean air. Most fly sprays work by clogging these spiracles, and suffocating the insect to death. If the air was choked with dust and soot, you would expect moths to be quite high on the extinction list. Second to this, moths survive by eating pollen. No pollen, no moths.
So a single season without flowering plants would spell the end for the moth. So due to its presence, we must concede that the rhythms of the flowering plants were not disturbed by this terrible event. It was commonly accepted that the dinosaurs died out because they were cold-blooded, and couldn't regulate their core temperatures effectively in the new darkness, but frogs and newts (both cold blooded) survived admirably, as did crocodiles and alligators. This may have something to do with their semi-aquatic nature. But the ammonites, which were wholly aquatic, died out with the ichthyosaurs, the mososaurs, and the plesiosaurs, to name but a few.
The Deccan Traps
The Deccan Traps are often cited as a rival to the bolide theory for the cause of dinosaur extinction. These were a range of active volcanoes covering approximately 200,000 square miles of India. They had been discharging poisonous gasses into the atmosphere for years, causing acid rains, generally depleting the biosphere and adding lethal cocktails of gasses to the atmosphere (not to mention large volumes of soot and dust).
Interestingly, most of the lava from the Deccan Traps was released between 60,000,000 and 65,000,000 years ago - almost dead on the K-T boundary. Whether the impact killed the dinosaurs outright, or tipped them over the edge to extinction is open for question. One thing is certain: as long as there were no witnesses, we'll never really know what happened.
Fossils of reptiles that survived the greatest extinction in the Earth's history suggest that the catastrophe had a far greater impact on ocean life than on land-dwellers.