The megatsunami caused by the asteroid that wiped out the dinosaurs was at least 10m high when it reached New Zealand, a computer simulation of the monster wave suggests, but it may have been much higher.
The study, conducted by a team of international scientists including Kiwi palaeontologist Dr Chris Hollis, showed a tsunami track that ran across the Pacific Ocean and hit New Zealand “at full force”.
Rocks around the New Zealand coastline played an important role in confirming the results produced by the simulation, which was the expertise provided by Hollis, an adjunct professor at Victoria University of Wellington.
The so-called Chicxulub asteroid, thought to be about 14km in diameter, struck in shallow water near the Yucatan peninsula in Mexico about 66 million years ago. That is the time of what is nowadays known as the Cretaceous-Paleogene (K-Pg) mass extinction event.
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The researchers calculated the asteroid was travelling at 12km a second at the moment of impact. At that time there was no land between North and South America, so the tsunami was able to sweep into the Pacific Ocean.
Modelling that assumed a seafloor depth of 1km showed a wave 4.5km high, 2½ minutes after impact.
After 10 minutes, by which time everything thrown into the air by the impact had fallen back to Earth, a 1.5km wave was left pushing through the deep ocean. That was known as a rim wave, and was 220km from the point of impact.
The Chicxulub tsunami approached most coastlines of the North Atlantic and South Pacific with waves of over 10m high and flow velocities in excess of 1m per second offshore, the paper said. The tsunami had been strong enough to scour the seafloor in those regions.
The tsunami created by the asteroid impact was up to 30,000 times more energetic than the Boxing Day 2004 Indian Ocean tsunami – one of the largest tsunami in the modern record.
A press release from the University of Michigan, which was heavily involved in the project, said the modelling was corroborated by the geological record.
Of special significance, according to the authors, were outcrops of the K-Pg boundary on the eastern shores of New Zealand’s North and South islands, which were more than 12,000km from the Yucatan impact site, the release said.
The heavily disturbed and incomplete New Zealand sediments, called olistostromal deposits, were originally thought to be the result of local tectonic activity.
But given the age of the deposits and their location directly in the modelled pathway of the Chicxulub impact tsunami, the research team suspected a different origin.
“We feel these deposits are recording the effects of the impact tsunami, and this is perhaps the most telling confirmation of the global significance of this event,” lead author Molly Range said.
What the New Zealand rocks show
Hollis said there was strong evidence that rocks at Tora on the Wairarapa coast were disrupted at the time of the K-Pg event, in a way consistent with a tsunami. “Massive blocks of rock have been torn up and shifted.”
While it was strong evidence of a tsunami, it was not possible to say conclusively that was the cause, Hollis said.
There were records of the K-Pg boundary from the sub-Antarctic Campbell Island, all the way through to East Cape in the North Island. But Tora was the only place where such dramatic indications of a tsunami had been found.
Some time periods were also missing from all the geological rock sequences in the North Island.
“We have wondered why they aren’t complete, and now that we have this theory of this tsunami impact hitting the east coast, it raises the possibility that they have all been affected by the tsunami,” Hollis said.
“We have thought it was possible the sea level fell at that time … but a tsunami event is a much more compelling idea.”
The megatsunami simulation helped everything fall into place.
“When you think about it, if you’re going to have a 14km diameter asteroid hit the Earth in a shallow sea, it’s obviously going to generate a tsunami.”
Hollis said the simulation showed “New Zealand would have borne the brunt of this massive tsunami” across the Pacific Ocean.
“New Zealand had a series of sedimentary basins, where sediment was being deposited, at that time, and we have a really good record of that time in Earth’s history.
“Now we have the evidence of the tsunami, we can put all of these records into a new context. It’s quite important in that respect.”
Despite that, it was hard to estimate the exact size of the tsunami when it hit New Zealand.
Although the records were good, they were patchy. Also, there wasn’t really a record of the east coast’s near shore, or beach deposits, or coastal land at that time.
Rocks found at Tora would have been semi-consolidated mud on the seafloor at the time of the tsunami. They had been “ripped up, twisted round and re-deposited”.
“I think they would have been pushed towards the shore and then brought back into the sea,” Hollis said. “That must have been a really massive force.”
While the paper had talked about 10m high waves that was just a minimum height. “It could have been much higher than that.”
Along with the big pieces of rock that had been dislodged, the outcrops at Tora also included round pebbles. That was evidence of something that had been sitting on a beach somewhere being swept out into the deep sea.
The paper on the simulation has been published in the journal AGU Advances.