|WikiProject Geology||(Rated Start-class, Low-importance)|
This came up in discussions at Katie. I have to hand a copy of The Great Extinction; What Killed the Dinosaurs and Devastated the Earth, by Michael Allaby and James Lovelock. (see ISBN 038518011X) which goes into some detail about the Alvarez hypothesis on the effect of a large lump hitting the Earth. On impact with the sea, rock and the sea where they met would be dissociated into atoms, stripped of electrons to form an extremely hot plasma cloud, a very dense gas which would then rise, not through convection but because there was nowhere else to go, forming a fireball carrying between 6,000 and 60,000 billion tonnes of matter into the atmosphere at about escape velocity so that some could have gone into orbit. The hot, dense, plasma would have been almost disc shaped, rising extremely quickly as a very wide barrel of fire. That, of course, is just the start. It's an ancient source, from the days before macs roamed the earth, but on that basis the image at Katie seems pretty reasonable. The info's not here, let me know if you'd like to add to it to expand the article ... dave souza, talk 09:09, 14 March 2008 (UTC)
In Organic Bio, I was told this is a theory, not a hypothesis. Im gonna go with my professor and say wikipedia is wrong... — Preceding unsigned comment added by 126.96.36.199 (talk) 16:19, 1 March 2013 (UTC)
Reason for Lack of Iridium in Earth's Crust
The pages states: "Iridium is extremely rare in the Earth's crust because it is very dense, and therefore most of it sank into the Earth's core while the earth was still molten."
This is a common misconception. Iridium is rare because it preferentially segregated into a phase (molten nickel-iron) that was dense, not because iridium itself is dense. Uranium, one of the densest elements, is enriched in the Earth's crust relative to the Earth as a whole, because the element tends to move into the low density silicate minerals of the continental crust. 188.8.131.52 (talk) 20:48, 6 December 2012 (UTC)
I suggest adding: Raup, David M. The Nemesis Affair: A Story of the Death of Dinosaurs and the Ways of Science. W. W. Norton & Company; revised enlarged edition 1999, original edition 1986. 184.108.40.206 (talk) 18:30, 15 March 2013 (UTC)
"The isotopic ratio of iridium in asteroids is similar to that of the Cretaceous–Paleogene boundary layer but significantly different from the ratio in the Earth's crust."
Look, I have a question here but no answer. The sentence above is what I was taught in Science at school forty years ago. That is, the extraterrestrial nature is not due to either the presence of iridium or the amount of it; rather, it is indeed the isotope ratio that marks it as different. The problem is, from all that i have read in Wikipedia on this and related topics, this article here is the only one that says it. Pretty well all other articles either suggest, or state outright, that it is the sudden abundance of iridium that is the evidence of the ET impact.
Where to go from here? Is this still seen to be correct? The isotopic composition is the most suggestive of ET origin? And to a more practical point, should this (possibly very fine) point be propagated in other pages? — Preceding unsigned comment added by 220.127.116.11 (talk) 10:12, 18 June 2014 (UTC)