Question

In 1980, scientists found that a certain layer of rock contained more than usual amounts of clay with the element iridium. Scientists determined that the rock in this layer was about 65 million years old. An image of this rock layer is shown below.


Image courtesy of USGS. Knife shown for scale.


The scientists have also found the following three pieces of evidence.
1. Evidence of dinosaur fossils were found in rock layers older than the iridium layer.
2. Evidence of dinosaur fossils were NOT found in rock layers younger than the iridium layer.
3. Iridium is not found in high concentrations in Earth layers but is abundant in asteroids.
What does this information most likely show about Earth's history and dinosaurs?
A.
Iridium was produced on Earth's surface by sedimentation and killed off the dinosaurs.
B.
A large asteroid collided with Earth and killed off the dinosaurs.
C.
Iridium was produced on Earth's surface by sedimentation and helped dinosaurs thrive on Earth.
D.
A large asteroid collided with Earth and helped dinosaurs thrive on Earth.

Answer 1

B. A large asteroid collided with Earth and killed off the dinosaurs.

Explanation: The global iridium spike at the ~65 million-year-old layer (rare on Earth but common in asteroids) marks the impact event at the K–Pg boundary. Dinosaur fossils occur below but not above this layer, indicating a sudden mass extinction coincident with the asteroid impact.