Iridium anomaly

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The term iridium anomaly commonly refers to an unusual abundance of the chemical element iridium in a layer of rock strata at the Cretaceous–Paleogene (K-Pg) boundary. The unusually high concentration of a rare metal like iridium is often taken as evidence for an extraterrestrial impact event.

Anomaly Characteristics[edit]

The type locality of this iridium anomaly is near Raton, New Mexico.

Iridium is a very rare element in the Earth's crust, but is found in anomalously high concentrations (around 100 times greater than normal) in a thin worldwide layer of clay marking the boundary between the Cretaceous and Paleogene periods, 66 million years ago. This boundary is marked by a major extinction event, including that of the dinosaurs along with about 70% of all other species. The clay layer also contains small grains of shocked quartz and, in some places, small weathered glass beads thought to be tektites.[1]

Impact Theory[edit]

A team consisting of the physicist Luis Alvarez, his son, geologist Walter Alvarez, and chemists Frank Asaro and Helen Vaughn Michel were the first to link the extinction to an extraterrestrial impact event based on the observation that iridium is much more abundant in meteorites than it is on Earth.[2] This theory was later substantiated by other evidence, including the eventual discovery of the impact crater, known as Chicxulub, on the Yucatan Peninsula in Mexico.

See also[edit]


  1. ^ Hildebrand, Penfield, et al.
  2. ^ Alvarez, L.W.; Alvarez, W.; Asaro, F; Michel, H.V. (1980). "Extraterrestrial Cause for the Cretaceous-Tertiary Extinction". Science. 208: 1095-1108.
  • Graup, G., Spettel, B., Herm, D., Weidlich, K. F. (1989): Mineralogy and phase-chemistry of an Ir-enriched pre-K/T layer from the Lattengebirge, Bavarian Alps, and significance for the KTB problem, in Earth Planet. Sci. Lett., Vol. 95, Amsterdam, S.271–290 Abstract
  • Kiesling W., Claeys P. (2001) A Geographic Database Approach to the KT Boundary, in Buffetaut E., Koeberl C. (Eds.): Geological and Biological Effects of Impact Events, Springer. ISBN 3-540-42286-2.
  • Shukolyukov A., Lugmair G.W. (1998) Isotopic Evidence for the Cretaceous-Tertiary Impactor and Its Type, Science 282, 927–929.

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