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Geo-Joint: Chicxulub Asteroid and the End of the Big Dinosaurs

Posted on June 06 2017

The dinosaurs. What a crazy bunch, huh? Rule the world for millions of years and then check out in one fell swoop. But if things had been just a liiiiittle bit different, they mighta coulda kept their crown. Maybe. The story of the downfall of The Big Reptiles is pretty well known. Unlike the hapless mega-lizards in Fantasia, succumbing to the heat and dust of a thousand volcanoes blowing their tops, scientists are pretty sure the dinos fell to a different dust source. The timing of that deathly event is well established by the fossil record. The dinosaurs largely ceased to lay down their bones about 65 million years ago at the end of the Cretaceous Period. The border between rocks of the Cretaceous and the overlying Tertiary Period is called the K-T boundary—yeah, I know, Cretaceous is spelled with a “C”, but the “K” refers to the German “kreida” which means “clay,” the material for which the time period is named.

This thin, light-colored layer marks the K-T boundary and the end of the big dinosaurs.

At the K-T boundary, in places all over the world, there is a thin layer of iridium-bearing rock. Iridium is not at all common in earth rocks, but is abundant in meteorites and asteroids. Back in the 1980s, Louis Alvarez, physicist of UC Berkeley, and his geologist son Walter, came up with the theory that an asteroid about 4 to 9 miles in girth could have struck the Earth and made a mighty crater. It would have obliterated itself and created a dust cloud made from Earth rock and asteroid bits, including iridium, which would spread around the world. Of course, so massive a cloud could easily cause a “nuclear winter,” dropping temperatures worldwide. For lack of sun and heat, the vegetation of the earth would have largely died off, followed by most of the herbivores, and in turn, just about all of the carnivores, especially the big guys like T. rex. But the theory needed evidence of such a catastrophic collision to account for the iridium layer and to set the biological apocalypse in motion. The discovery of the Chicxulub meteorite crater just off the coast of the Yucatan Peninsula in Mexico in 1991 provided the very smoky gun. That crater, 110 miles across and at the time of impact more than eight miles deep, dates from precisely the same period that saw the dinosaurs snuffed. Nobody was there to see it, but all the lines of evidence converge at this explanation.

Detail of where the asteroid hit.

The dinosaurs had a roughly 180-million-year run, and it all ended thanks to the random arrival of a big rock. Actually, the relative sizes of the Earth and the asteroid have been compared to a bowling ball and a grain of sand, but that grain of sand was moving fast, and did one heck of a lot of damage. Just how much damage happens to have been dependent upon where the asteroid hit. Recently, researchers drilled into the strata under the Chicxulub region and found that it is rich in sulphur compounds. That explosively ejected mass apparently functioned quite effectively as a sunlight-reflecting material when the huge debris cloud enveloped the Earth. Bringing temperatures below freezing sent something like 70% of all life over the edge in probably a matter of weeks or months, though the darkness may have persisted for years. Smaller animals with diverse feeding abilities, which describes the early mammals, had a survival advantage and persisted.

How the Chicxulub asteroid may have looked from space as it arrived.

Scientists have raised a thought-provoking question regarding that scenario. What if the asteroid had been just a couple of seconds faster or slower? Instead of hitting the shallow sea where it landed, it might have gone into the deep ocean waters of the Atlantic or the Pacific. The tsunamis would have been that much worse, but the dust probably not nearly as bad. For lack of this sun-killing umbrella, most of the extinction wouldn’t have happened. Dinosaurs might have kept their prominence, forestalling the rise of the mammals, including you and me. It’s an interesting possibility, and quite likely one of the world’s key turning points.

There are other factors that influenced the story of the dinosaur’s endtime. Some have pointed out that dinos were past their prime by the end of the Cretaceous Period. They still had planetary dominion, but their species diversity was in decline for a few tens of millions of years before the final act. The reasons for this may include the breakup of Earth’s land masses, or some of that Fantasia scenario, after all. In the area of present-day India, a volcanic outpouring of gargantuan proportions went on for perhaps a million years spanning the K-T boundary. It left lava beds in an area called the Deccan Traps that tell the tale. Dust from these continuous eruptions may have started a slow decline in plant production that had its effects on the food chain. And there’s always the possibility that other smaller asteroids could have impacted the planet causing lesser ill effects that still wore the thunder lizards down. In time, those oversized reptiles would probably have declined through climate change or some other pressure. Us mammals would have had our turn, but we certainly got the chance to flourish after the Chicxulub asteroid cleared the decks and re-wrote the Story of Life.


Would you like to further explore the Yucatan, Land of the Maya and the Chicxulub impact crater? Get this beautiful wall map by National Geographic, available from! Click here:




Caption: Detail of where the asteroid hit.
Source: Wikimedia Commons: NASA/JPL-Caltech (Public domain)
Caption: This thin, light-colored layer marks the K-T boundary and the end of the big dinosaurs.
Source: Wikimedia Commons: USGS (Public domain)
Caption: How the Chicxulub asteroid may have looked from space as it arrived.
Source: Wikimedia Commons: Made by Fredrik/NASA (Public domain)

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