Controversies in Science/What killed the dinosaurs/Experimental evidence that an asteroid impact led to the extinction of many species

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Did Asteroid kill the dinosaurs?
Upon impact, the asteroid exploded and sent a thick layer of dust and iridium into the atmosphere. This thick layer of smoke blocked out the sun and carried the iridium around the globe. There are more than 36 locations worldwide where iridium has been found. The black smoke blocked out the sun and destroyed all of the earth's vegetation.

This thick layer of dust made it impossible for the sun’s rays to reach the earth, which prevented photosynthesis from taking place and thus killed off all the plants. This started a chain of extinction. As the Herbivores began disappearing, the Carnivores were losing their food source. They too would fall victim to the lack of sunlight.

Scientists estimated that the dark period, without presence or warmth from the sun, wiped out the dinosaurs and vegetation within a 3-6 month period.

Methods
Scientists measured the abundance of Iridium by collecting rock samples. Iridium is very rare on earth, this is due to the heating of the earth which caused certain elements in the platinum group (Iridium) to "sink" past the observable layers where it would normally be expected to be found. Earth's natural process of forming the core effectively diminished the Iridium element from existing in abundant quantities.

In order to concentrate the iridium, we now use the trick of heating rock samples with molten iron, giving scientists a greater signal-to-noise ratio. .

Results
An asteroid hit the earth 65 million years ago at 25km/second. This covered the earth in a blanket of dust. Chemical analyses of this layer in the earth showed that it contained high amounts of iridium, which comes from outer space. This dust caused darkness for 3-6 months, blocking the process of photosynthesis, triggered the death of phytoplankton, caused the herbivore dinosaurs to die, and finally caused the carnivore dinosaurs to die. Further evidence shows dinosaur fossils 3.4 meters before the iridium layer, but not after this layer.