April 24, 2024

The Discovery of the Chicxulub Crater

65 million years ago, the earth had a very bad day. This was the end of the age of the dinosaurs, and the key to what happened was found in the hidden Chicxulub Crater, in Mexico.

King of the dinosaurs, the T Rex

From a modern perspective, it is almost impossible to imagine how long the dinosaurs ruled the earth.

Consider human history.

All of recorded history – World War II, the Industrial Revolution, The Bronze Age, The Romans, the Egyptian Pharaohs, EVERYTHING that we have a written record of - occurred in the last 5 000 years. Modern style civilisation – towns, animal husbandry, crop cultivation – goes back a little further, commencing around 12 000 years ago.

Dinosaurs, on the other hand, were the dominant life forms on earth for 185 million years. That is, enough time to allow all of modern human civilisation to arise and develop 15 000 times over. It is incomprehensible stretch of time.

The reign of the dinosaurs is known as the Mesozoic era.

The Mesozoic Era began around 250 million years ago, when reptiles became the dominant animal class on Earth.

The planet was warmer and wetter than it is today, and much of the land was either swampy, or lush jungle, to which reptiles were ideally suited. Over the following millennia, dinosaurs thrived and evolved into a diverse variety of species.

The most well known dinosaurs are the giant land animals that appeared towards the end of the Mesozoic; Brontosaurus, Stegosaurus and the famous predator Tyrannosaurus Rex among them. But dinosaurs came in a range of sizes, from very large to very small, and were prevalent across all terrain types, and throughout the ocean.

A fossilised Stegosaurus

Of course, no one has ever seen a dinosaur; we know of their existence only from the fossil record.

As dinosaurs died, their bodies decomposed, slowly dissolving to nothing. But occasionally, dinosaur remains would become covered by sediment and preserved in he ground; their organic chemistry replaced gradually by hardier minerals, that could then survive for many millions of years.

These preserved remains have been unearthed throughout human history, but for a long time their origin was unknown. It was only in the 1830s that scientists began to suspect that these strange objects could be the remnants of ancient animals, long extinct.

As more, and more complete, fossils were unearthed, this idea became widely accepted. Palaeontology, the study of the fossil record to learn about Earth’s ancient past, became an established science.

Charles Darwin

But while this occurred little thought was given to why the dinosaurs were extinct.

The Theory of Natural Selection, formulated by Charles Darwin between 1836 and 1853, suggested that all animals will die out on a long enough time line, replaced by different species, better adapted to an ever changing environment.

As scientists of the 19th century came to grips with the existence of dinosaurs, they also assumed that they had lived so long ago that they had gradually died out and been replaced.

Walter Alvarez

Walter Alvarez was a young American geologist from Berkeley, California.

In the late 1960’s, as a post graduate student, he began work on a long term project, studying the development of the Apennine Mountains in Italy. The Apennine’s are geologically significant as there are several areas where the rock strata – the layers of rock from one era to the next – are exposed, and can be examined alongside one another.

Alvarez had relatively simple goals for his study; by investigating the rock strata, the differences from one layer to the other, he hoped to learn how and when the mountain range had evolved.

In one particularly fruitful area, the Bottacionne Gorge near the town of Gubbio, Alvraez found rock strata stretching back hundreds of millions of years, deep into the age of the dinosaurs.

Alvarez had found a time machine, of sorts, that enabled him to look deep into the Earth’s history.

Rock strata near Gubbio, Italy. The white layers show plentiful fossils, the darker layer, almost none.

The rock samples Alvarez took from this area were rich in fossils.

Not large skeletal fossils, but much smaller remains, left by tiny plankton that had thrived in the ocean millions of years beforehand. This showed that the Apennine’s had once been part of the sea bed, before being pushed upward by continental drift.

While the fossils were of only passing interest to a geologist, as Alvarez continued his work, he noticed something strange. The older rock strata showed a very high, and consistent, number of these small plankton fossils; the more recent rock strata showed almost none.

Between these concentration levels was a very distinct boundary; a thin, pronounced layer of black clay, with lots of fossils below it, and very few above.

And it was this mysterious black line, an inch thick, that would provide the key clue to the dinosaur’s extinction.


Intrigued, in spite of himself, Alvarez sent samples from the clay layer, and the rocks immediately above and below, for spectral analysis. This would show how old the samples were, and give a break down of their chemical composition.

All of the samples were dated to around 65 million ago. The rock samples were pelagic limestone, consistent with the general makeup of the Apennines.

But the clay layer showed something unexpected; very high traces of a metallic element called Iridium. Iridium is almost unknown on Earth, but is sometimes detected in minute traces, around 0.1 parts per billion.

The Apennine sample contained Iridium levels 90 times higher than this, at around 9 parts per billion. Multiple samples were tested, and all showed the same result.
Where had all that Iridium come from?

The Asteroid belt, rich in Irridium

While Iridium is not common on Earth, it has been detected in higher quantities elsewhere.

Meteorite samples, from asteroid impacts, often show high levels of Iridium, indicating that it is present in the asteroids and comets that roam our solar system.

The Iridium results from the clay layer, combined with the dramatic reduction in fossils in the rock layers above it, lead Alvarez to a radical new theory; what if the dinosaurs had not died out gradually, over a long period of time; what if, instead, they had been wiped out in a sudden cataclysmic event?

In 1977, after ten years of research, Alvarez, working with a small team, now theorised that a large meteor, or comet, had hit the earth 65 million years ago, and damaged the planet’s environment so badly that a mass extinction was triggered. A big enough impact event, would shower the earth in Iridium, explaining the black clay layer.

Conclusively proving this theory, was the difficult part.

The cliffs of Stevns Klint, Denmark

Alvarez now needed additional confirmation from other locations.

If an impact event had occurred, the Iridium layer should be present more or less globally. But intact, readily available rock strata like the ones in the Apennines are rare.

Slowly, other sites did come to light. The cliffs of Stevns Klint, near Copenhagen in Denmark, was one example. Some sites in New Zealand were identified, and a core sample taken from the floor the Pacific Ocean also showed the appropriate strata.

All of the samples from these new sites showed the high concentration of Iridium Alvarez had discovered. His theory was close to confirmed.

But there was one piece still missing. Where was the impact crater?

Meteor Crater, Arizona

Earth is dotted with a few well known meteorite craters.

There is Meteor Crater in Arizona, and Wolf Creek in outback Western Australia, both of which are about 1 kilometre across, and both caused by impacts from objects measured in the tens of metres.

But an impact large enough to destroy the dinosaurs would had to have come from something much larger; a meteor, or comet, approximately 10 kilometres wide, the size of San Francisco. The crater left by such a huge strike would be 200 – 300 kilometres wide.

In other words, hard to miss. But, as Alvarez put together is impact extinction theory in the 1970s, no crater of this size was known.

Gravity map of the Yucatan Penninsula, showing the circular anomaly

Glen Penfield was a geophysicist working for a state run oil company in Mexico.

In 1978, he was conducting a magnetic survey of the Yucatan Peninsula, looking for geologic anomalies that may indicate the presence of oil.

Similar to Alvarez, as he began to analyse his data, he noticed something strange. Buried deep below the surface of the Gulf of Mexico, was a gravity anomaly that traced a huge curve in the rock; it looked exactly like part of the edge of an enormous circle, hundreds of kilometres across.

Size and location of the buried crater, known as Chicxulub

Penfield found earlier gravity surveys from the 1960s, from slightly different locations, and by combining these was able to piece together a gravity map that seemed to clearly depict an enormous impact crater, buried below Mexico.

But his employer did not want to release the results of their geologic surveys, citing commercial confidence, and so this evidence would remain unseen for a decade.
Finally, in 1990, a journalist working for the Houston Chronicle got hold of the story, and Penfield’s evidence, and the hidden crater, was revealed to the world.

Artists impression of the crater

Alvarez now seemed to have final, conclusive, evidence, to support his theory.

65 million years ago, the earth had a very bad day: A rock the size of San Francisco hit the Yucatan Peninsula, carving out a crater 40 km deep, and 300 km wide.

‘Out to a few hundred kilometres from ground zero, no living thing could have survived.


Animals living just over the horizon would have witnessed a flash of light, and then one last moment of calm. Then, as the ground began to shake uncontrollably, the sky grew more and more intensely red, growing hotter and hotter.


Entire forests would have ignited, and continent sized wildfires swept across the land. A gigantic tsunami, perhaps a kilometre high, spread outward from the Gulf of Mexico at terrific speed.

Within a few hours of impact, most of Mexico and the United States would have been reduced to a desolate wasteland.’


- Walter Alverez, ‘T Rex and the Crater of Doom’

Following this cataclysm, the planet as plunged into a kind of long, cool winter.

Dust choked the earth’s atmosphere, blocking out sunlight and killing off many plant species. This caused a chain reaction; as plant eating animals died out, so did the predators that hunted then. The lack of sunlight also did for many ocean borne species, explaining the sudden drop off in the plankton fossils that Alvarez had observed.

Small, rodent like mammals were among the survivors of the K-T extinction

Around 75% of all plant and animal species are thought to have gone extinct during this period, one of six mass extinctions known to pre-history, and the most recent.

Survivors of this event, known as the K-T extinction, included small, furry mammals, a tiny part of the earth’s ecosystem to this point. Their very distant offspring, much altered by evolution, would dramatically later the earth in a different way.


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