Sedimentary rock hardens within the earth's crust as layers of matter accumulate and the pressure of the layers above...

Mehran March 3, 2016

Hi @Flavio, thanks for your question. The stimulus presents us with an argument, specifically, the geologists' theory that the layer of iridium-rich rock was caused by a meteorite's collision with the earth. The question stem asks us to weaken (counter) this claim.

Notice that this theory is not about dating prehistoric events generally. Rather, it is specifically about a potential connection between a meteorite collision and the presence of a iridium-rich rock layer.

Answer choice (C) is entirely irrelevant. It describes how layers of sedimentary rock are used to determine dates of prehistoric events, whether or not these layers contain iridium. Thus, it does nothing to counter the specific theory presented in the stimulus.

Contrast this with answer choice (D), which says there was a surge in volcanic activity around the time of the alleged meteorite collision. This is an alternative cause for the perceived effect — the volcanic activity led to high iridium-rich dust clouds, and that in turn led to the layer of iridium-rich rock. This answer choice thus weakens the theory presented in the stimulus.

Hope this helps! Please let us know if you have any additional questions.

Flavio March 3, 2016

Thanks for explaining it @Mehran. I can clearly see now that it's an alternate cause with answer choice D to weaken the geologists' theory.