The geology behind Disney/Pixar’s “The Good Dinosaur”

The trailer for the new Disney/Pixar movie “The Good Dinosaur” begins by asking the simple question, “What if the asteroid that destroyed the dinosaurs missed?”

As a geologist, my immediate answer to the question was, “Well, the dinosaurs still probably would have died out.”

Most public knowledge of the End-Cretaceous mass extinction 65 million years ago involves the massive 180 km-wide Chicxulub crater in the Yucatán Peninsula of Mexico, formed by the asteroid that the trailer mentions.

An extraterrestrial cause for the extinction was first proposed in 1980 by the father-son duo of Luis and Walter Alvarez after discovering an iridium anomaly in the Cretaceous-Paleogene (K-Pg) boundary clay layer in Italy. Iridium is not a very common element in the Earth’s crust, so they interpreted the abundance to be extraterrestrial in origin–likely from a meteorite impact.

Across the ocean in the Caribbean, tektites, shocked quartz and tsunami deposits had been found in the K-Pg boundary layer. All of these can be formed by a meteorite impact and would indicate a close source. The half-submerged Chicxulub crater–formed by an impactor 10 km-wide–was then found in the Yucatán and dated to ~65 million years old.

The source rocks that the impact hit were primarily carbonates and evaporites, which would have been vaporized, releasing large amounts
of carbon dioxide and sulfur dioxide into the atmosphere. Locally, the huge amount of water displaced by the impactor and pressure wave would have generated massive tsunamis.

However, the question is, was this meteorite impact the sole cause (or even the primary cause) of the End-Cretaceous mass extinction?

Right around the end of the Cretaceous period, there was a series of massive volcanic eruptions in India called the Deccan Trap flood basalts. These flood basalts are more than 2,000 m thick and currently cover an area over 500,000 km². As these volcanoes were erupting, they were spewing massive amounts of volcanic gasses into the air, particularly sulfur dioxide.

Back during the End-Permian mass extinction 250 million years ago–the most severe mass extinction in Earth’s history–there was also a series of flood basalt eruptions. These eruptions were from the Siberian Traps, the largest flood basalt event in history, which covered an area of 2 million km² and are assumed to be the main cause of the extinction. With the release of vast amounts of carbon dioxide and sulfur dioxide into the air, the oceans became increasingly warm, acidic, stratified and euxinic from decaying organic matter, killing off 95% of all marine families.

While the Deccan Traps weren’t as large as the Siberian Traps, they still likely produced similar devastating effects on the environment and fauna. Research groups, such as those led by Gerta Keller, have long argued that the Deccan Traps and not the Chicxulub impact were the main cause of the End-Cretaceous extinction.

So, to answer “The Good Dinosaur’s” question, even if the Chicxulub meteorite impact never happened, the Deccan Traps still would have produced disastrous environmental effects, most likely spelling the end for the dinosaurs.

There is something deeply poetic thinking about how one single day in geologic history could have so profoundly affected the course of life; but while such catastrophic geology can play an important role in Earth’s history, it sometime does take processes tens to hundreds of thousands (or even millions) of years to produce substantial disastrous effects.

However…

to further complicate things, there have been proposals that have directly linked Deccan volcanism to the Chicxulub crater. The impact may have exacerbated (or even initiated, depending on which age dates you believe) the volcanism, making for an impact-volcanism dependent relationship. In this case, maybe there wouldn’t actually have been an End-Cretaceous mass extinction.

However, however…

even if the dinosaurs never went extinct, there likely wouldn’t have been the ecological niche open that allowed mammals to evolve and diversify to their present day extent. Most mammals during the Cretaceous were small and rodent-sized (which would have been beneficial in surviving an extinction event), but could they have followed their evolutionary trajectory if dinosaurs were still around? And could humans still have begun evolving 6 million years ago? Unlikely.

The End-Cretaceous extinction is still a wonderfully contested event, so even if “The Good Dinosaur” does take a more simplistic view of the manner, it will likely only help continue the fire for the debate on what really did wipe out some of the largest creatures to ever walk the Earth.