The short answer: 66 million years ago a rock about six miles across hit shallow sea off Mexico's Yucatan Peninsula at roughly twelve miles a second. Within seconds it dug a hole 60 miles wide. Within minutes the ground across North America rolled like water and a wave close to a mile high rose in the Gulf. Within half an hour the sky over much of the planet was burning with falling debris, and within weeks sulfur haze and fine dust had dimmed the Sun enough to stop most photosynthesis. About three quarters of all species died. The ones that lived were small, and they included the dinosaurs we now call birds.
You know the headline. Big rock, dinosaurs die, birds happen, roll credits. What nobody bothers to tell you is what actually happened, in order. So here it is, minute by minute, with where the evidence comes from. And not one creature on the planet looked up. Neither would you. You cannot find your own car in a car park.
The day before: a world in good working order
Montana and the Dakotas were a warm, humid coastal floodplain on the shore of an inland sea, with palm trees, crocodilians and about 75 inches of rain a year. Duck-billed hadrosaurs moved in herds. Triceratops was so common that in some rock beds it is the single most frequent large animal found. Tyrannosaurus had been running that landscape for more than a million years. This was not a world winding down. It had about a day left.
Sources: Hell Creek Formation; Triceratops
Four minutes out: how big and how fast was the asteroid?
About ten kilometers across, six miles. Drop it on London and it covers the whole city and sticks out of the top. It was moving at about twenty kilometers a second, some sixty times faster than a rifle bullet, so four minutes out it was already closer than New York is to London. And the sky looked completely normal. No warning shot, no rumble, no ominous music, because the universe does not do foreshadowing, you overcooked sausage. Somewhere a Triceratops was eating a fern and having, statistically, an excellent day.
Source: Chicxulub impactor
One second out: the atmosphere does nothing
The rock crossed the whole depth of the atmosphere, about eighty kilometers of air, in roughly a second. It did not slow down and it did not burn up. For one second there was a column of superheated air punched clean through the sky, and anything looking straight at it was already blind.
Impact: where did the asteroid hit, and why was the angle so bad?
It hit shallow water off what is now the Yucatan Peninsula in Mexico, at an angle of roughly 45 to 60 degrees from horizontal. That sounds like a detail. It is not. Straight down would have dug deeper and thrown out less. A grazing hit would have skipped much of the energy back into space. Modelling published in 2020 found that an angle in that range is the most efficient at digging up rock and throwing climate-changing gas into the sky. Of all the ways this could have gone, it went the worst one.
Sources: Collins et al., Nature Communications, 2020; Imperial College London
Plus one second: a hole deeper than the Himalayas are tall
The rock was vaporized, and so was a slab of the Earth's crust the size of a mountain range. The hole was about a hundred kilometers wide and, briefly, deeper than the Himalayas are tall. At those pressures rock flows like liquid, so the crater floor rebounded upward and froze as a ring of peaks that are still down there, buried. Drillers reached that peak ring in 2016. The energy released was, on the most-cited estimates, around a hundred million megatons. Every nuclear weapon ever built, set off at once, is a rounding error next to it.
Sources: Chicxulub crater; IODP Expedition 364, which drilled the peak ring
Plus ten seconds: what actually kills the planet goes up
Everything within about a thousand kilometers was simply gone. A shockwave of burning air moved outward faster than sound. And above the crater a column of vaporized rock was being flung clean out of the atmosphere. That is what kills the planet. Not the bang. The stuff that goes up, because everything that goes up has to come back down through the air. Even you could have worked that one out. Congratulations, you decorative gourd, you have done physics.
Plus two minutes: an earthquake with nothing to compare it to
The seismic wave reached the edges of the continent. Estimates put it at around magnitude eleven, beyond anything in recorded history. Fault lines slipped across thousands of kilometers and landslides collapsed into the sea. Somewhere in the middle of North America the ground was not shaking. It was rolling, in waves, like water.
Plus ten minutes: how big was the tsunami?
The impact had punched a hole in the ocean, and the ocean filled it back in. Modelling published in 2022 puts the first wave near the crater at about 1.5 kilometers high. Close to a mile. Not a mile long, a mile high. Read that again, slowly. It collapsed inward, rebounded outward and sent waves across the Atlantic and the Pacific that were still moving hours later. In Texas and Mexico there are beds of jumbled rubble hundreds of feet thick, laid down in an afternoon.
Sources: Range et al., AGU Advances, 2022; University of Michigan
Plus thirty minutes: the sky catches fire
All that vaporized rock came back down, re-entering the way a meteor does. Millions of them, everywhere, at once. The upper atmosphere glowed, and some models put the heat at the ground near oven temperatures for minutes to hours. How bad this got, and exactly where, is still argued about, and burrows, water and caves clearly sheltered some animals. But the broad picture is not in dispute. For a while the sky itself was a heat lamp, and it was pointed at everything.
Source: Cretaceous-Paleogene extinction event
Plus one hour: fish with impact glass in their gills, if it holds up
At a site called Tanis in the Hell Creek badlands of North Dakota, about 1,900 miles from the crater, fossil fish have been found with impact glass lodged in their gills. If that reading holds, they died within roughly an hour of the strike, buried by a surge of water thrown up a river valley by the seismic wave, long before any ocean wave could have arrived. That interpretation is contested, genuinely and by serious people, so it is said here up front. But if it holds, Tanis is not a fossil site. It is a security camera pointed at the worst hour in the history of life.
Sources: Science, astonishment and skepticism over Tanis; DePalma et al., PNAS, 2019
How do we know an asteroid killed the dinosaurs?
Good. Finally, one decent question. In 1980 the physicist Luis Alvarez and his geologist son Walter, with their colleagues, found a thin clay layer laid down at exactly the extinction and absurdly rich in iridium, a metal that is rare in the Earth's crust and common in asteroids. The same layer, with the same chemistry, turns up all over the world. They said something had hit us, and they were laughed at for about a decade. Then it emerged that geologists surveying for Mexico's state oil company had found a giant buried circular structure in the late 1970s. In 1991 it was tied to the extinction: a crater about 180 kilometers across, the right age, with shocked quartz and glass beads around it. Nobody was standing there with a stopwatch. That hour was reconstructed from a clay layer, a gravity map and some fish that never got to finish swimming.
Sources: Alvarez, Alvarez, Asaro and Michel, Science, 1980; Chicxulub crater
Plus one day: the real killer was the sulfur
The fires were out or dying, and the actual killer arrived. It was not fire and it was not water. The rock at the impact site was limestone and gypsum, rich in sulfur, and the impact turned hundreds of billions of tonnes of it into sulfate haze in the stratosphere. Seventy percent of this planet is ocean. It could have hit deep water. It found the gypsum instead. Sulfate high in the sky does two things: it blocks sunlight coming in, and it comes back down as acid. So when the rain returned, it did not help.
Sources: Chicxulub crater; Senel et al., Nature Geoscience, 2023, on the fine dust
Plus one week: the sun goes out, and rivers decide who lives
Sunlight at the surface dropped far enough that photosynthesis largely stopped. That is not a plant problem. Plants are the bottom of everything, and everything above them was now living on stored energy. Here is the detail that decided who lived. A forest runs on sunlight. A river does not. A river runs on rubbish: dead leaves, silt and anything that washed in and rotted. That food chain does not care whether the Sun is out, which is why the survivors are a suspiciously damp list. Crocodilians, turtles, freshwater fish. They did not outfight anybody. They were standing in the one pantry that stayed stocked.
Sources: Sheehan and Fastovsky, Geology, 1992; Cretaceous-Paleogene extinction event
Plus one month: the impact winter
Temperatures crashed. Estimates for the land surface run to a drop of well over ten degrees Celsius, eighteen Fahrenheit, and in places far more; one climate model has years below freezing. The plant-eaters went first, because there were no plants. Then the meat-eaters, because there were no plant-eaters. And the big ones were worst off every time, because being large means needing more, and needing more was the one thing nobody could do. No roaring, no last stand. I know that is not what you wanted, you great romantic lump. The most successful animals in the history of the planet got hungry, and then they got quiet.
Source: Brugger, Feulner and Petri, Geophysical Research Letters, 2017
Plus one year: how many species died?
The dust began to clear, and the carbon dioxide the impact released, no longer masked, started trapping heat. So the freezing was followed by warming, and the survivors of the cold had to survive the swing back. Roughly three quarters of all species on Earth did not. No purely land-living animal much over about 25 kilograms, around 55 pounds or a Labrador, made it through. Not one.
Source: Cretaceous-Paleogene extinction event
Plus one hundred years: the fern spike
Something strange shows up in the rock: ferns. In the pollen record right above the boundary, fern spores dominate everything. Ferns are what grows first after a volcano, a landslide or a fire. They are the weeds of catastrophe. Pull a core out of the ground and read it upward, and you get a normal world, then a thin layer of ash and glass, then ferns and almost nothing else. The whole planet grew the same scab, and you have walked past that fern in a garden center and thought nothing of it.
Sources: Fern spike; Vajda, Raine and Hollis, Science, 2001
Plus one thousand years: your ancestor ate seeds and hid
The great reptiles on land, the flying reptiles, and in the sea the big marine reptiles and the ammonites, were finished. What was left was small, and it burrowed, swam or ate seeds. Seeds matter more than you would think. A seed is a packed lunch that keeps, and in a world with nothing growing, the animal that can crack one open eats. Your ancestor was down there, about the size of a rat and terrified, and that fear is the only reason you exist. Not strength. Not intelligence. It hid in a hole and ate seeds.
Plus 300,000 years: Colorado is a palm forest
Fossils from Corral Bluffs near Colorado Springs show a palm forest within about 300,000 years of the impact, which in geological terms is the time it takes to answer an email. The mammals crawling out of the wreckage grew fast, from about a pound to over a hundred pounds within some 700,000 years. Colorado has its own entry in what every US state looked like before people. The planet never needed saving. It carried on without the previous tenants and rented the place out to whatever survived. It has done this five times, in the five mass extinctions, it will do it again, and it will not check whether you are finished.
Source: Denver Museum of Nature & Science, Rise of the Mammals (Lyson et al., Science, 2019)
Did the asteroid kill all the dinosaurs?
No. A handful of small, feathered, beaked, seed-eating, ground-dwelling dinosaurs made it through, not because they were strong but because they were small enough to hide and undemanding enough to eat rubbish. The tree-dwelling birds, whose forests had collapsed, did not. There are now about fifty billion wild birds, far more than there are people. The pigeon eyeing your sandwich is a dinosaur. The chicken in your fridge is a dinosaur. The thing that woke you at four in the morning is a dinosaur, descended from the only line that survived the worst day this planet has ever had, and it has chosen to spend that inheritance screaming outside your window at dawn. Nature has an outstanding sense of humor. Unlike you lot, it commits to the joke.
Sources: Field et al., Current Biology, 2018; Callaghan et al., PNAS, 2021
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Questions people ask
How big was the asteroid that killed the dinosaurs?
About ten kilometers, or six miles, across. It was traveling at about twenty kilometers a second and left the Chicxulub crater, about 180 kilometers wide, now buried under Mexico's Yucatan Peninsula and the Gulf.
Where did the asteroid hit?
In shallow sea off the Yucatan Peninsula in Mexico, near the modern town of Chicxulub. The rock there was limestone and gypsum, rich in sulfur, which made the impact far deadlier than a hit in deep ocean would have been.
Did the asteroid kill all the dinosaurs?
No. Birds are dinosaurs, and a few small, ground-dwelling, seed-eating bird lines survived. Every other dinosaur, and every purely land-living animal much bigger than about 25 kilograms, died out.
How do we know an asteroid caused the extinction?
A clay layer at the extinction boundary, found worldwide, is rich in iridium, a metal common in asteroids and rare in the Earth's crust. The Alvarez team proposed an impact in 1980, and in 1991 the buried Chicxulub crater, of the right age and surrounded by shocked quartz and impact glass, was tied to it.
How many species died?
Roughly three quarters of all species on Earth, including the large reptiles on land, in the air and in the sea, and the ammonites. Animals in rivers and freshwater, which run on decaying matter rather than living plants, came through far better.
Sources
Every claim above was sourced before the episode was narrated. The volcanic eruptions in India's Deccan Traps overlapped the extinction and their share of the blame is still argued about, so nothing here says the asteroid acted alone. The main papers:
- Alvarez, Alvarez, Asaro and Michel (1980). Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208.
- Collins et al. (2020). A steeply-inclined trajectory for the Chicxulub impact. Nature Communications 11.
- Range et al. (2022). The Chicxulub impact produced a powerful global tsunami. AGU Advances 3.
- DePalma et al. (2019). A seismically induced onshore surge deposit at the KPg boundary, North Dakota. PNAS 116.
- Brugger, Feulner and Petri (2017). Baby, it's cold outside: climate model simulations of the effects of the asteroid impact at the end of the Cretaceous. Geophysical Research Letters 44.
- Senel et al. (2023). Chicxulub impact winter sustained by fine silicate dust. Nature Geoscience 16.
- Sheehan and Fastovsky (1992). Major extinctions of land-dwelling vertebrates at the Cretaceous-Tertiary boundary, eastern Montana. Geology 20.
- Vajda, Raine and Hollis (2001). Indication of global deforestation at the Cretaceous-Tertiary boundary by New Zealand fern spike. Science 294.
- Lyson et al. (2019). Exceptional continental record of biotic recovery after the Cretaceous-Paleogene mass extinction. Science 366.
- Field et al. (2018). Early evolution of modern birds structured by global forest collapse at the end-Cretaceous mass extinction. Current Biology 28.
- Callaghan, Nakagawa and Cornwell (2021). Global abundance estimates for 9,700 bird species. PNAS 118.
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