Why is a high concentration of iridium in geological layers considered evidence of an asteroid impact?

• A. Iridium is commonly produced by volcanic eruptions, indicating increased volcanic activity.
• B. Iridium forms naturally in sedimentary rocks through biological processes.
• C. Iridium is a byproduct of atmospheric reactions with oxygen.
• D. Iridium is rare in Earth's crust but abundant in asteroids, so its presence suggests extraterrestrial material was deposited.

Answer: (D)

Verified step by step guidance

1. Understand that iridium is a rare element in Earth's crust but is found in much higher concentrations in extraterrestrial objects like asteroids and meteorites.
2. Recognize that geological layers with unusually high iridium concentrations are unusual because typical Earth processes, such as volcanic eruptions or biological activity, do not produce significant amounts of iridium.
3. Learn that when an asteroid impacts Earth, it deposits extraterrestrial material, including iridium, into the geological layers at the impact site.
4. Connect the presence of a high concentration of iridium in a specific geological layer to the hypothesis that an asteroid impact occurred at the time that layer was formed.
5. Conclude that the iridium anomaly serves as a geochemical marker indicating extraterrestrial material deposition, supporting the asteroid impact theory for events like the mass extinction at the Cretaceous-Paleogene boundary.