Editing Iridium (section)

=== Cretaceous–Paleogene boundary presence ===
[[File:K-T boundary.jpg|thumb|left|The red arrow points to the [[Cretaceous–Paleogene boundary]].|alt=A cliff with pronounced layered structure: yellow, gray, white, gray. A red arrow points between the yellow and gray layers.]]

{{Main|Cretaceous–Paleogene extinction event}}

The [[Cretaceous–Paleogene boundary]] of 66 million years ago, marking the temporal border between the [[Cretaceous]] and [[Paleogene]] periods of [[Geologic time scale|geological time]], was identified by a thin [[stratum]] of [[iridium anomaly|iridium-rich clay]].<ref name="Alvarez" /> A team led by [[Luis Walter Alvarez|Luis Alvarez]] proposed in 1980 an extraterrestrial origin for this iridium, attributing it to an [[asteroid]] or [[comet]] impact.<ref name="Alvarez">{{cite journal|title=Extraterrestrial cause for the Cretaceous–Tertiary extinction|author=Alvarez, L. W.|author-link=Luis Walter Alvarez|author2=Alvarez, W.|author3=Asaro, F.|author4=Michel, H. V.|s2cid=16017767|date=1980|journal=Science|volume=208|issue=4448|pages=1095–1108|doi=10.1126/science.208.4448.1095|pmid=17783054|bibcode = 1980Sci...208.1095A |url=http://chaos.swarthmore.edu/courses/soc26/bak-sneppan/13_alverez.pdf|citeseerx=10.1.1.126.8496}}</ref> Their theory, known as the [[Alvarez hypothesis]], is now widely accepted to explain the extinction of the non-avian dinosaurs. A large buried impact crater structure with an estimated age of about 66 million years was later identified under what is now the [[Yucatán Peninsula]] (the [[Chicxulub crater]]).<ref>{{cite journal |last=Hildebrand |first=A. R. |author2=Penfield, Glen T. |author3=Kring, David A. |author4=Pilkington, Mark |author5=Zanoguera, Antonio Camargo |author6=Jacobsen, Stein B. |author7= Boynton, William V. |title=Chicxulub Crater; a possible Cretaceous/Tertiary boundary impact crater on the Yucatan Peninsula, Mexico |date=1991 |volume=19 |issue=9 |journal=[[Geology (journal)|Geology]] |pages=867–871 |doi=10.1130/0091-7613(1991)019<0867:CCAPCT>2.3.CO;2 |bibcode=1991Geo....19..867H}}</ref><ref>{{cite book|author=Frankel, C.|title=The End of the Dinosaurs: Chicxulub Crater and Mass Extinctions|date=1999|publisher=Cambridge University Press|isbn=978-0-521-47447-4|oclc=40298401|url-access=registration |url=https://archive.org/details/endofdinosaursch00fran}}</ref> Dewey M. McLean and others argue that the iridium may have been of [[volcano|volcanic]] origin instead, because Earth's core is rich in iridium, and active volcanoes such as [[Piton de la Fournaise]], in the island of [[Réunion]], are still releasing iridium.<ref>{{cite book|title=The Cretaceous-Tertiary Event and Other Catastrophes in Earth History|author=Ryder, G.|author2=Fastovsky, D. E.|author3=Gartner, S.|publisher=Geological Society of America|date=1996|isbn=978-0-8137-2307-5|page=47}}</ref><ref>{{cite journal |author=Toutain, J.-P.|author2=Meyer, G.|date=1989|title=Iridium-Bearing Sublimates at a Hot-Spot Volcano (Piton De La Fournaise, Indian Ocean)|journal=Geophysical Research Letters|volume=16|issue=12|pages=1391–1394|doi=10.1029/GL016i012p01391|bibcode=1989GeoRL..16.1391T}}</ref>