Yucatan’s Greatest Hit

Another story about the Chicxulub crater?  It seems like more articles appear every day, and entire museum exhibits are devoted to it — the asteroid and the dinosaurs, a story now so familiar.  You know the images — dino glances up at a light in the sky; blast and blackout; dust clears; a new leaf sprouts up.

Wrong, or at least inadequate in so many ways.  The accounts give little impression of the event’s suddenness and power — almost inconceivable — and the planet-wide devastation that lasted many thousands of years.  Or of the great detective story about figuring it all out.

So, this is another try at telling the story.

A big sky-rock hit Yucatán.  How could that one event possibly cause vast changes across the whole Earth?  Let’s consider some details.

Global-scale changes usually happen slowly on our planet.  But on one ordinary day sixty-five million years ago, change came with brutal rapidity and horror.  On the north coast of Yucatán, a single event changed the world forever.

The world of that long-ago time, what geologists call the Cretaceous Period, would have seemed familiar to us in many ways.  Leafy trees and flowering plants were increasing amid warm forests of conifers and ferns.  The wildlife included small furry animals, fish, insects, and flying feathered creatures recognizable as birds.  The one large difference was the giant lizards dominating the scene.  Things had remained about the same for many millions of years, but suddenly everything changed catastrophically and without warning.

A medium-sized asteroid came in over the South Atlantic, flashed across the northern edge of South America, the Caribbean, and the future Riviera Maya, and struck the Yucatán coast from the southeast.  The rock was about six miles in diameter.  It was traveling at about 60,000 miles per hour — fifteen times faster than the highest-velocity artillery shell.  It weighed one thousand billion tons and carried ten thousand times the energy of today’s entire nuclear arsenal.  The point of impact was the present location of Chicxulub Puerto.

Yucatán’s Greatest Hit 2The Chicxulub impact craterGravity anomaly map, present coastline and cenotes marked in white(Source: Geological Survey of Canada)

On impact, the asteroid blasted a hole twenty-five miles deep through the peninsula’s ancient limestone, granite, and basalt basement rock.  In seconds, its enormous moving energy changed to heat, and the rock of the crater and the asteroid itself entirely vanished into a fireball of vaporized rock.  The crater, about fifty miles in diameter and lined with boiling melted rock, quickly slumped into a shallower crater 110 miles in diameter.  A gigantic rebound splash in the center rose twelve miles high, then collapsed within minutes to a 3,000-foot peak.  Superheated rock debris shot into the highest levels of the atmosphere.  A searing blast wave raced away from the impact in all directions, followed by a second explosive wave as the limestone decomposed into an immense burst of carbon dioxide.  Even a thousand miles away, the wind was stronger than the worst imaginable hurricane.

The immediate effect was total destruction of life out to several hundred miles from the impact.  The hot atmospheric shock waves and falling red-hot rock turned the air into an oven thousands of miles wide.  Continent-sized wildfires swept the Americas, depleting the oxygen and creating dense smoke and soot.  A tsunami hundreds of yards high swept the coasts and far inland.  Within hours, North America, Central America, and northern South America were reduced to desolate wasteland covered in dense smoke.  Some living creatures, sheltered under rocks or mud, for example, might have survived.

These were only the immediate, local effects.  The cloud of debris in the upper atmosphere rapidly spread around the world, and the ejected molten rock caused distant fires.  Even in the absence of actual fire, air temperatures worldwide rose to life-threatening levels.  Within a few days, a large fraction of the world’s biomass burned.  The seismic effect was greater than any earthquake on record, strong enough to trigger volcanic eruptions on the opposite side of the world.

All this happened rapidly, and as destructive as it was, worse was to come.  Dust, soot, and sulfurous clouds gradually filled the upper atmosphere, causing world-wide total darkness that lasted years or decades.  With sunlight blocked, the Earth’s surface rapidly cooled by about 20 F°.  When the skies slowly cleared, a greenhouse effect took over, caused by the massive injection of carbon dioxide into the atmosphere from limestone and fires.  A hot period then followed the cold, with temperatures warming perhaps as much as 50 F° on land.  This lasted thousands of years.  Acids created from the vaporized rock and superheated atmosphere fell in intense rain, killing animals and plants and leaching toxic elements from rocks.  Dust from the asteroid also spread nickel at levels toxic to most plants.

The impact of a large body into thick rock rich in carbonates and sulfates was unparalleled.  About seventy percent of all plant and animal species were lost forever, leaving no descendants.  The few survivors repopulated a new world in the period called the Paleogene (formerly the Tertiary).

Some small land animals survived, none larger than about fifty pounds — warm-blooded, burrowing, nocturnal, omnivorous, eaters of carrion and dead plant matter — that is, the mammals.  Some reptiles and amphibians — turtles, crocodiles, snakes, salamanders, frogs — made it through.  Of the dinosaurs, only one small branch lineage survived and adapted to the new conditions.  Today we call them birds.

Surviving seeds allowed some plants to come back, and ferns, with very resistant spores, flourished after the event.  Many aquatic animals vanished.  Shells dissolved in the acidified oceans, and the darkness caused the marine food chain to collapse.  Some fish did survive, deep-water residents that fed mostly on organic waste.  The oceans were almost sterilized and took at least half a million years to recover.

When the skies cleared and temperatures, acidity, and toxics eventually settled down, the survivors evolved rapidly to fill vacant ecological niches.  Mammals came to dominate the world, and they became us.

We know all this because the rocks remember.  The impact hypothesis was controversial when proposed in 1980, but research during the next decade supported it.  A thin layer of rock with an age corresponding to the extinctions is especially informative.  At many sites around the world, geologists found it contains high concentrations of the element iridium and crystals rich in nickel, both signatures of extraterrestrial origin.  The layer also has bits of limestone, melted into droplets of glass, with characteristics of rapid cooling in the upper atmosphere, showing the origin to be impact, not volcanic.  The impact was on land but close enough to the sea to create a tsunami, as shown by geological evidence in Texas, northern Mexico, Haiti, northeastern Brazil, and deep-sea Atlantic sediments.  The evidence accumulated, but the actual site of the hit remained elusive.

Oil exploration work had detected the Chicxulub crater the 1950s, but the discoverers thought it to be a buried volcano.  Further studies in the 1970s led to speculation about an impact crater, but it seemed too large for that to be possible.  The research remained buried in proprietary files of the Mexican oil company, Petróleos Mexicanos, or Pemex.  Geologists were finally allowed to publish a brief note in 1991.  The timing of the impact was right, and those tiny droplets of limestone glass matched the composition of Yucatecan rock.

The news hit geologists, well, like a meteor.

The crater lies half on land, half offshore.  It is 110 miles in diameter, extending out from Chicxulub to present Celestún on the Gulf coast, to near the towns of Maxcanú and Ticul south of Mérida, and a bit past Izamal and Dzilam de Bravo in the east.  It is the largest known impact the Earth received in the last two billion years.  For comparison, it is 150 times larger than the well-preserved and impressively huge Barringer Crater near Winslow, Arizona.

In March 2010 an international panel of forty-one scientists reviewed twenty years of scientific literature, ruled out other theories such as massive volcanic eruptions, and published an endorsement of the asteroid hypothesis.  The impact at Chicxulub was the cause of the great extinction event.  Case closed.

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Mérida’s Gran Museo del Mundo Maya has an extensive presentation on the Chicxulub event.  The film that accompanies it is imaginative but fails to give much impression of the terrifying velocity of the impact or its world-wide consequences, and it implies more rapid recovery than was the case.

Chicxulub Puerto, the actual ground zero, is a fishing village and part of the rapidly developing strip of beach houses east from Progreso.  It is a pleasant enough place to visit, though nothing of the long-ago impact is to be found.  The crater’s 3,000-foot central peak would be an interesting attraction.

The crater would doubtless be an amazing thing to see if it were not buried under half a mile of sediment.  Today it is visible in gravity and magnetic anomaly maps, not particularly convenient for nonspecialists.  However, impressive evidence of the asteroid’s impact exists in the ring of cenotes that traces an outline of the crater’s rim.  The fracturing caused by the impact intercepts the ground water in Yucatán’s porous rock, causing upwelling into the water-filled sinkholes called cenotes (from Mayan ts’ono’ot).

The Peninsula is pocked with thousands of cenotes, many not associated with the Chicxulub event, but the ones that ring the rim of the impact crater are clearly visible on any good map.

The ring pattern is most obvious in its western segment, a great arc from east of Kopomá, passing north of Muna, and on into the scrubland eastward from Mayapán.  In the village of Chocholá, just off the main highway between Mérida and Campeche, impact Cenote San Ignacio has a restaurant and other amenities.  Organized tours go to some of the larger, more beautiful cenotes in this area.  Many others are accessible only with difficulty, and finding them may require a local guide, good route-finding skills, and maybe a machete.

Farther east, the village of Cenotillo, accessible by paved road from Izamal, has a remarkable concentration of impact-induced cenotes, reportedly more than 150 of them within the municipio (county).  Another large concentration can be found at the northeastern end of the arc, north of the town of Buctzotz, located between Motul and Tizimín.  Much of this region is very remote, and the explorer would benefit from the services of local guides.