Asteroid showers Earth in molten rock
A massive metal-containing rock larger than Mount Everest collides with Earth at a speed of 100,000km/h. The strike liquifies the ground beneath into a porridge, and Earth is showered in molten rock.
Abright flash overloads the retina of an unsuspecting three-horned dinosaur as it stares down the river banks towards the inland sea. Triceratops blinks slowly, and raises its head as the earth starts to shake beneath its feet. A huge black cloud rises slowly above the horizon, and quake builds upon quake until trees start falling and the animals, whether tiny or titanic, scatter, run and fall in panic.
Then comes a moment of quiet. As the shaking subsides the dinosaur gets up cautiously, its widely set eyes still peering towards the sea. In the distance, the water has risen into a monster wave. The animal turns to retreat, but feels a sudden burning impact on its scaly back. Then another, followed by a third. Tiny red-hot balls of glass are raining down around the dinosaur, and the trees still standing are bursting into flames. Triceratops takes off at its top speed, but doesn't take notice of the lie of the land, the river banks rising higher on each side. From the eerie silence of the sea comes the roar of approaching water, and the great beast is swept away.
Later, when the water retreats, the land is strewn with drowned and injured animals entangled in a morass of tree trunks and ocean algae. A few fish still struggle for their lives in pockets of dirty water. But then another wave floods the coast, burying these remains of disaster under a thick layer of mud.
There they remained, until 66 million years later when a team of American scientists removed that particular layer of ancient mud under what had become North Dakota, USA, revealing for the first time what happened during the first fateful minutes after the Earth was struck by a huge asteroid.
Father and son identify iridium
The dinosaurs ruled the world for 160 million years, only to suddenly disappear almost completely, leaving behind them only the few small, feathered animals that would evolve into modern birds. The cause of this sudden extinction was uncertain for many years and debated over more than a century, with theories including shrinking brains, disease pandemics, and lack of sexual drive.
By the 1970s, scientists were starting to favour the theory that the disaster was caused by intense volcanic eruptions. But this theory was challenged in 1980 by a group of US scientists headed by father and son Luis and Walter Alvarez.
Walter was a geologist, and had gathered a collection of samples taken from a layer of red clay in Italy formed at the time of the dinosaurs’ disappearance. Back in the US, he was assisted in a close examination of the samples by his father – a physicist and Nobel Prize laureate for his invention of a hydrogen bubble chamber. Together they were trying to determine the age difference between the top and bottom layers of clay. But instead they discovered something much more interesting.
The clay layer included unusually high quantities of iridium, an element which is rare on Earth, but sometimes exists in large quantities in asteroids. The iridium levels were a clear clue to the cause of the yet-undetermined catastrophe of that era, but they knew they needed to confirm that the iridium was not merely local to Italy. So they travelled to Denmark, and to New Zealand, gathering more clay layers from the same crucial time period. Sure enough, the iridium levels were high all over.
It was a clincher. A massive asteroid had struck the Earth, and had left its evidence throughout the world. Such an impact could well have caused the observed extinction events. And as they came to their realisation and imagined the extent of the disaster, father Luis Alverez must have felt a shadow from his past. Decades earlier, on 6 August 1945, he had flown closely behind the plane that dropped the nuclear bomb on Hiroshima, his task being to observe what happened. Now here was another explosion 66 million years earlier, and the asteroid that had hit Earth was billions of times more forceful than that Hiroshima bomb.
The asteroid theory attracted attention – but also scepticism. And crucially, where was the crater for this supposed epoch-ending impact? It took another 10 years for scientists to find the answer. On the edge of the Yucatan peninsula in Mexico, they discovered a huge crater with a diameter of 180km, and its estimated formation date was a perfect match. Here, then, was the impact that destroyed the dinosaurs.
Volcanoes covered India in lava
The debate about the dinosaurs’ extinction didn’t end there. Many scientists contended that even if the world had been hit by a major asteroid, dinosaurs were on their way out before the impact anyway. And they already had a candidate for the cause.
The Deccan Traps in India are one of the largest volcanic features on the planet, an igneous geological formation created by activity during the late dinosaur era. Scientists have discovered a two-kilometre-thick layer of ancient lava covering an area of about 500,000km , formed when the volcanoes of the area began to erupt some 300,000 years before the asteroid hit. They undoubtedly influenced the fauna of the time, and global temperatures apparently fluctuated during the eruptions.
The question is exactly how much the eruptions influenced life on Earth. Some scientists believe the eruptions were responsible for the majority of the mass extinctions from this time; others believe that the eruptions only had a limited effect, or that the effects combined with those of the asteroid on the other side of the world.
One problem is the sequence of events. The Deccan volcanoes started to erupt before the impact, but the biggest eruptions might have happened afterwards. Other analyses also differ in indicating that the dinosaurs had been severely weakened before the impact, while others consider the dinosaurs to have been in perfect shape until the impact.
The disagreements are partly due to a lack of widespread data. Scientists are familiar with finds from only a few places that correspond to this final era of the dinosaurs – and none of these have yielded fossils from the time close to the impact.
A new discovery has changed that. In a remote area of northern USA, scientists have uncovered a prehistoric version of Pompeii, with animals preserved as they died their violent deaths only a few minutes after the asteroid struck.
Something fishy in North Dakota
Palaeontologist Robert DePalma from the Palm Beach Museum of Natural History in Florida was not overly enthusiastic when in 2012 a private collector told him about fossil fish near Bowman, North Dakota. The town is located close to the fossil-rich Hell Creek Formation that stretches through Montana, North Dakota, South Dakota, and Wyoming – a barren and deserted area known as the ‘Badlands’, with clay-rich soil and rocks which have been eroded by wind and water over millions of years so that in many areas their layers are exposed like sections of a sponge cake. And some of these layers are ripe with dinosaur fossils.
The fossil collector had given up extracting the fragile fish fossils, and offered to let Robert DePalma take over the site. DePalma agreed to take a look, and found some nice fish fossils in what he thought must be a prehistoric lake.
But there was
something odd about the fish he extracted. Many had small, round stones in their gills. The palaeontologist recognised the stones as tektites – gravel-sized glass pearls, the remains of molten rock balls falling to Earth from clouds of molten silicate droplets ejected under the pressure of an asteroid impact. And such tektites were already well-established as evidence of the Gulf of Mexico impact 66 million years ago.
He also found small pieces of quartz which demonstrated clear evidence of rocks having been subjected to extreme pressure.
DePalma realised that he might have uncovered a preserved ‘crime scene’ from the very doomsday of the dinosaurs. He set out to excavate the layers with greater enthusiasm, renting the excavation site from the local land owner, a cattle breeder. It became clear from the deposits that this had not been a lake, but rather a river area that had been flooded. Layer by layer he uncovered a chaotic death scene with wreckage from ocean, fresh water, and land. Using a chisel and a paint brush, he liberated fragments of ammonites, shelled octopuses and algae side by side with incinerated tree trunks, branches and roots tangled up with freshwater fish and the bones of terrestrial animals, including the Triceratops dinosaur from the beginning of our story. This was a mass grave of plants and animals from terrestrial, fresh water and sea environments, all buried on the same day 66 million years ago. Like a detective, DePalma began to piece together what had happened.
He named the place Tanis after an ancient Egyptian royal city, and in 2019 finally shared his discoveries with the world. His studies provide a rare insight into the last fateful minutes of the dinosaurs.