Final answer:
Iridium is a rare metal found in higher concentrations in meteorites. It serves as an important geologic age marker due to its spike in sedimentary layers at the K-Pg boundary, which indicates a significant asteroid impact and mass extinction event.
Step-by-step explanation:
Iridium is a chemical element with the symbol Ir and atomic number 77. It is a rare metal that is found in higher concentrations in meteorites compared to terrestrial rocks. This is why iridium is an important geologic age marker.
In 1980, researchers discovered a spike in iridium concentration within sedimentary layers at the K-Pg boundary. This phenomenon was hypothesized to be caused by an asteroid impact, which resulted in the mass extinction event. The presence of high iridium concentrations at this boundary helps scientists determine the timing and impact of the event.