text: science homework help? give four pieces of evidence that a large impact occurred at the cretaceous-tertiary boundary. sorry, this is kinda long but stick with me. i studied this at uni so its a really interesting topic for me. if you dont like it blame my lecturers.   70% of all life including the dinosaurs became extinct 65 million years ago at what is known as the k-t event. the current theory which is known to be the best explanation for this to happen is that a large meteor roughly 10 kilometres in diameter collided with the earth changing the climate at the time drastically as dust and ash clouded the sky blocking out the sun. at the same time fires raged around the planet as forests caught fire from the intense heat. now that thats all cleared up lets get down to it.  1) in 1990 a scientist called alan hildebrand was looking over some old geophysical data that had been recorded by a group of geophysicists searching for oil in the yucatan region of mexico. within the data he found evidence of what could have been an impact site. what he found was a ring structure 180km in diameter which was called chicxulub. the location of this structure was just off the northwest tip of the yucatan peninsula. the crater has been dated as being 65 million years old  2) in 1980 scientists called walter and luis alvarez wrote a paper on a 1cm layer of clay they had discovered in gubbio which is in italy at what was known to be the zone that marked the time the dinosaurs became extinct. this layer was found to be rich in a rare element called iridium ( ir ).   the presence of this element in much greater abundance than would be normally expected caused great confusion. most of the earth's store of ir has been concentrated into the centre of the planet. they found the iridium levels to be 160 times higher that would normally present. this much iridium could not have occured naturally. the only way this amount if ir could have occured is from an asteroid or comet.  3) quartz is a mineral made from silica ( pure sand ) and has a structure that can be deformed if it undergoes high pressures. the high velocity impact of a meteor would not only create extreme levels of heat, but also high pressures. the type of pressure applied by an impact causes what is known as shock metamorphism, this deforms the structure of the quartz and creates parallel laminar ripples. this form of quartz has been found around the world and is a very strong argument for the meteor in two respects:  -it is found all over the world, not just locally.  -no other process on the planet creates this type of quartz.   4) certain conditions favour certain elements and so if we examine the ratios of elements we can often identify a pattern that tells us what the conditions were. meteorites have this 'fingerprint' and a characteristic set of elemental ratios. the ratios of elements found in the boundary layers of the impact suggest that it came from outside the earth.  hope this helped