Geology on the Up
By Stephen Luntz
Prof Jonathan Aitchison is rewriting the story of the world’s greatest mountain chain. Soon textbooks may need to follow.
School students’ introduction to plate tectonics usually involves learning that India and Asia collided 55 million years ago, producing the Himalayas. However, Aitchison has found marine rocks in Tibet that are only 35 million years old.
If confirmed, Aitchison’s findings will ripple through geology because the Himalayas are considered the template for understanding the events that drove the formation of many more ancient mountain ranges. “We’re getting an idea of the rates at which things happen,” says Aitchison. “That will have implications for lots of other examples.”
Aitchison says that when plate tectonics first became accepted, the date for the India/Asia collision was estimated at 45 million years ago. However, it was subsequently observed that India’s northward motion slowed substantially 10 million years earlier. “People put one and one together and got an unrelated number,” says Aitchison.
The India/Asia collision has been thought of as a simple story of two large continental plates coming together, making it a textbook favourite. However, Aitchison notes: “In between was what is known as the neo-Tethyian ocean. No one really thought about what that ocean was like. When we look at the western Pacific Ocean we see ocean space can be very complicated, for example with the Philippine Sea plate.”
Aitchison imagines Asia and India converging at rates double those seen in the world today, because each was colliding with the plate carrying the marine waters in between. The convergence slowed 55 million years ago because Asia stopped overriding the oceanic plate, but India kept moving north for another 20 million years before it finally hit Asia. Today India is still moving north, but Aitchison says the mountain range is now expanding to the east and west, and consequently not getting any higher.
When continental plates collide, some of the marine rocks in between are forced down into the Earth, but Aitchison says that “some are forced upwards. The top of Mt Everest is made of marine rocks.”
However, finding and dating these rocks along the Yarlung Tsangpo River on the Tibetan Plateau is no easy task. “You’ll be following a series of rocks getting younger and younger and then you come to a fault and lose them,” Aitchison says.
Radioisotopes only provide information on when volcanic components of the rocks were formed, so geologists need to use fossils to date the point where what was once part of the sea floor was laid down. Missing or mangled fossils can make this difficult.
Aitchison adds that research work in the Himalayas can be tricky, since the shortage of oxygen makes hiking a challenge. “We always spend several days in Lhasa where just walking up steps can be a challenge until your red blood count increases,” he says.
When he does reach the field it is to sites his students have been at for a month before. “The most interesting rocks they want to show me are always at the tops of mountains, not handy to the camp,” he says. Aitchison is not sure if this is just “Sod’s law” or if the students “just want to see how far they can make the old man walk”.
Aitchison once liked easing the strain at the plateau’s hot springs, but has recently discovered that a snake species has adapted to live in exactly this environment, ruining his pleasure.
The idea of studying mountains came easily to a boy growing up on the South Island of New Zealand, particularly after an inspiring teacher took his class hiking. However, at first Aitchison was more interested in climbing peaks than working out the forces that made them.
After completing his Honours degree at the University of Otago “sometime after India collided with Asia” Aitchison did his PhD at the University of New England studying what had once been the convergent margin of Gondwana in what is now northern NSW. “I studied radiolarians – little marine protozoans. When they die, skeletons settle to the sea floor. When abundant enough they become rock called chert that is often seen in Alpine belts.”
Aitchison’s doctoral radiolarians were Devonian but the techniques stood him in good stead when China started opening up to the world. “The Cultural Revolution meant China had missed the tectonic revolution and I realised there were all these opportunities to look at Chinese rocks.”
By then he was based at the University of Sydney (where he is now head of the School of Geosciences), and Aitchison grabbed the opportunity to work out of Hong Kong and then in Tibet. He’s very glad he did.
“The Himalayas are the best mountains chain in the world,” he says. It’s cool to be able to work on them.”