How did geoscientists figure out what killed the dinosaurs?

Dinosaur bones are found on every continent. Everywhere on earth, dinosaur fossils disappear at the end of the Cretaceous Period, 65 million years ago.

If you examine an outcrop of rocks dated around that age, you find a lot of fossils, including dinosaur bones, then farther up (going into more recent rocks), there are much less fossils and no dinosaur bones. That boundary used to be called the K/T Boundary for Cretaceous (spelled with a K in German)/Tertiary and is now commonly called the K/Pg Boundary for Cretaceous/Paleogene.

Only birds, descendants of dinosaurs, survived. Geoscientists, for many years, tried to unravel the potential causes of that extinction event. Some paleontologists argued that the number of species was gradually decreasing before the 65-million-year mark, or that the rapid rise of flowering plants could explain the disappearance, or that intense volcanism in India caused climate disruptions, without assuming that a catastrophic event had occurred.

Where the record of rocks is complete, as for example near Raton, N.M., you can walk to the outcrop and observe a dark layer that marks the boundary. That layer exists everywhere on earth. This would imply a global event.

Chemical testing of the layer in 1980 in Italy, then around the world, showed that the layer contained very high concentrations of iridium, a metal found in such high concentrations only in meteorites. Based on the iridium concentrations and the calculated total mass of iridium on earth, the meteorite was estimated to have been about 6 miles in diameter. Such an impact would create a crater about 120 miles in diameter.

In addition to the iridium signature as a meteorite event, geologists also found glassy fragments and especially striated quartz grains (called shocked quartz) that also confirmed a very strong impact. This meteorite theory was a widely debated idea at the time for all geologists.

One main challenge was: where was the impact? If the impact was in the ocean, we may never find it, as that ocean crust may have disappeared due to plate tectonics in the past 65 million years. The search was on. Many of the evidence from rocks was found in various outcrops in North America and especially around the Caribbeans, suggesting that the crater may have been in that region.

Several known craters in North America were studied as potential candidates but were found to be too old. Petroleum geologists working in Mexico, then pointed out that they knew of clues to a very large crater, of a size and age that would match the assumed impact: a series of deep springs (called cenotes) aligned in a semi-circular pattern on the Yucatan peninsula, and evidence in their exploratory oil wells of very unusual rocks including melted rocks and striated quartz grains.

The age of the crater also matched. These observations were overwhelming. The crater had been found! It straddles half of the eastern edge of the Yucatan, with the other half now submerged to the north. The crater is named after a small local fishing harbor named Chicxulub (pronounced chick-shoe-lube).

Further testing over the years confirmed that the signatures in Yucatan matched the age, assumed impact size and consequences from a meteor impact around 65 million years ago that occurred in shallow sea. The meteorite appears to have hit obliquely, creating an explosion of more than 100 cubic miles of rocks within one minute, creating a hole 20 miles deep.

The impact was shown to have generated tsunamis, global fires and years of dark skies that would have ended any plant growth, resulting in the death of dinosaurs and the extinction of most marine life.

Some birds survived; some very small mammals, maybe because they were nocturnal and therefore survived in the dark, eventually evolved ... into us, humans.

(Paul Parmentier, a certified professional geologist retired from California and living in Los Lunas, shares the rich geologic features in Valencia County. He earned a Bachelor of Science degree in geology from Belgium and a master’s degree in geochemistry from Japan. The Geologic Landscapes and Observations of Surrounding Nature of Valencia County are featured monthly.)