Asteroid Evidence Rocks Paleontologists

Walter asked his father, Luis Alvarez, for his opinion on the layered rock. Luis happened to be a Nobel Prize-winning physi-

Fossils found in the bottom layers of sedimentary rock are older than those near the top. The oldest fossils on Earth are about 3.5 billion years old.

cist. His laboratory was testing new ways of dating rocks that contained rare elements.

The results were exciting and changed the paleontological world. The mysterious clay layer contained iridium. Iridium is an extremely rare element found in meteorites and asteroids from outer space.

Even better news was to come. The rock sample was 65 million years old, from the boundary between the Cretaceous and Triassic periods, also known as the K-T mass extinction. This iridium layer was found all over the world in the thin, clay layers of rocks dated to the K-T boundary. This evidence linked the time of the K-T mass extinction to these high iridium levels.

The amount of iridium in Alvarez's rock sample was measured. It was possible to estimate the size of the asteroid by knowing the amount of iridium. In 1980, the Alvarez team announced their startling news to the world. An asteroid about 6 miles (10 km) across had crashed into Earth about 65 million years ago. Was this the reason dinosaurs became extinct?

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