Field geologist and Earth Sciences Professor Mike Searle shares his expertise on the shifting plates beneath our feet
Colliding Continents: A Geological Exploration of the Himalaya, Karakoram & Tibet is the outcome of more than 30 years of research by Mike Searle, currently Professor of Earth Sciences at University of Oxford. Combining geological fieldwork with mountaineering expeditions and unforeseen adventures, the author has produced a seminal narrative on the geological history of one of the most active collision zones on earth. Stretching from the Arabian Sea along the great mountain ranges of Pakistan, India, Nepal and Bhutan, then south through Myanmar and Thailand to the great volcanic arc of the Andaman Islands, Sumatra and Java, this zone’s story features some of the highest mountains in the world, spectacular geology, earthquakes and tsunamis. An interview:
At 55 mm per year, the ongoing collision between the Indian and Asian plate is currently the fastest one on earth. What is the impact?
India collided with Asia 50 million years ago, and the whole Himalayas are a product of the shortening and thickening of the crust. It is why the deep rocks are coming up as high mountains. The Indian plate is actually subducting [or diving] under Tibet. And it’s the Indian lower crust that is holding up the southern part of Tibet.
You call 2004’s Boxing Day earthquake, which occurred in this collision zone, ‘the deadliest natural disaster in recorded history’. Why is that?
The Sumatra earthquake was the second largest earthquake since instrumental records began, at a magnitude 9 to 9.3 [on the Richter Scale]. The ground surface was elevated as much as 11 metres at the epicentre, which set off the tsunami and killed people all around the Indian Ocean. It brought home to geologists how devastating earthquakes could be.
[To quote from the book: ‘the force of the earthquake perceptibly shifted the earth’s axis, raised the sea level globally and speeded earth’s rotation. The total energy released from the earthquake was estimated to be 65,000 times the energy of the Hiroshima atomic bomb. The tsunami wave height reached approximately 10-15 meters along the coast of Sumatra, and 24 hours after the earthquake, it swept all the way back to Indonesia, having encircled the entire planet.’]
Can we also attribute earthquakes as distant as Bhuj and Latur to this collision?
The collision is the fastest convergence of two continental plates that we know of today. Whether the Bhuj earthquake was also a result of the flexure [or bending] of the Indian plate diving down is a [subject of much] debate. It almost certainly has to do with the collision because nothing else is happening in India. People still don’t know quite why these big earthquakes occur so far south. Substantial earthquakes are happening a long way from the Himalayas, which is why one has to be careful about where nuclear power stations or dams are set up.
According to Colliding Continents, the Indian plate has some of the youngest and oldest rocks in the world. How did that happen?
We have found granites that formed as recently as 750,000 years ago on the Nanga Parbat in Pakistan. They were created at depths of 15 to 20 km beneath the surface and are now sitting 7,000 metres above sea level. This is astounding. It is the highest exhumation [or erosion] rate of rocks anywhere in the world, more than 11-13 mm per year.
[In contrast], the rocks found on the Indian plate, especially the Aravalli range and South and Central India, cover the last 3 billion years and are some of the oldest continental rocks we know of on the planet. Originally, India was joined to South Africa and Antarctica, and all those are very old and stable continents. They belong to the Pre-Cambrian period.
Rocks from the summit of Everest still preserve remnants of sea lilies that lived in a tropical sea around 400 million years ago, around the time the collision of continental plates led to the closing of the Tethyan Sea. Where else can we find remnants of the Tethyan Sea?
The northern Himalayas—that is, Ladakh, Zanskar, Lahaul and Spiti—were the shallow seas of Tethys, and one finds fossils deposited in the sediments of the seabed [there]. Spiti is the main area for finding Jurassic ammonites, [an extinct group of marine invertebrates] and you also get Shaligrams in Pokhara, Nepal. The mountains of Zanskar are made of limestone, which also preserves fossils.
You have studied the Siachen glacier from both sides; what do you make of it?
There is a very big fault—the Karakoram Fault—that runs through the Nubra valley [where the glacier is located]. The Siachen glacier has eroded down to the fault. There’s a lot of discussion on how important these faults are to the evolution of Tibet and the Himalayas, which is why the glacier is important to geologists.
The Siachen glacier is also a high altitude desert. It is completely desolate and far too cold to support human habitation. You have all these people living in army tents up there just to say it’s occupied. The armies are camped on either side of a 6,000-metre high valley. In the middle is a huge mountain range, the Baltoro Kangri range, which cannot be crossed. It’s on the border with Ladakh and the upper part of Baltoro.
We went right up to the frontline on the Baltoro side where [the Pakistani army] used to fire guns and try to get over the ridge. But they never went; they landed up in ice fields and set off avalanches.
Are we engaging in potentially dangerous activities in geologically sensitive zones?
Yes. It is plain stupid to build a dam in an earthquake zone. There was one planned on a faultline in the Chenab valley. I don’t think it was ever commissioned because they realised it was breaking even before they built it. It is a classic example of a government going ahead with a project without consulting a geologist.
There was also a huge dam built on the Indus in Pakistan that was silted up within 10 years. Dams have a short lifespan. It is much better to make smaller, hydro-electric type generation projects.
The Three Gorges dam on the Yangtze river in China is another example of a massive amount of construction for [what is] probably short-term benefit. China has also built a city called Kanding in Western Yunnan on the most active fault of Tibet. Three earthquakes of magnitudes 8 have occurred there in the past 80 years. It’s just a matter of time before the whole city is destroyed.
We also know there is a big seismic gap in Nepal where there hasn’t been a big earthquake for nearly 70 to 90 years.
There are three ranges in Tibet the size of the Alps. We have no idea what’s in them. Almost no geological work has been done. So we know more about the moon than we know about eastern Tibet It is roughly centred at Kathmandu. Even a small earthquake could bring the city down, which is built on an old lake. As soon as you propagate a seismic wave, the sediment will flux and turn into jelly. Kathmandu is a disaster waiting to happen, like the Mexico City earthquake. It wasn’t a big earthquake but it flattened the city because it was built on old lake sediment.
You mention that Burma was a continent between Asia and India, and Kohistan (now in Pakistan) an arc of islands that got squeezed in between by the collision of plates.
Yes. Burma was probably a small continent between India and Asia, which collided with Asia first, then India. Kohistan was a small island arc, probably the size of Japan, sitting in the ocean in between. As India moved north, Kohistan collided with both Asia and India, closing the ocean.
According to your book, the Indus River predates the collision, whereas the Ganges is relatively young…
Tibet was always high, even before India collided with Asia. So the Indus, Sutlej, Brahmaputra were all initiated before the Himalayas rose. And as the mountains continue to rise, the rivers cut down deeper into them.
The Ganges is younger. It was initiated during the rise of the Himalayas. We know it was initiated probably 35 million years ago, possibly a little [longer]. It’s now the main river source for draining the whole central and eastern Himalayas. It takes sediment down to the Bay of Bengal. The 20 kilometres of sediment material that is found offshore from Kolkata has been eroded from the Himalayas.
The course of rivers has changed over time and you suggest a model to predict the future. What can we predict?
It is fairly easy to speculate, for we are now sure that the original drainage from Tibet actually flowed down the Yangtze to the Yellow Sea. The Yangtze River was progressively captured by the Red River in Vietnam, then by Mekong, then by Irrawaddy, and now Brahmaputra. These rivers are all cutting back and capturing drainage. If you look at a map of Nepal, the Arun valley is very close to capturing the Yarlung Tsangpo (known in India as Brahmaputra). When that happens, the whole drainage will come down from Nepal instead of great gorges of Namcha Barwa [mountains].
You write of the sea floor spreading. That is, new land being created in the Andaman Sea…
The Andaman Islands are the northern continuation of the Java Sumatra arc, which have huge volcanoes like Mount Semeru, Krakatoa, Merapi in the subduction zone. We have two volcanoes in the Andamans also—Barren Island and Narcondam Island—behind which new ocean crust is forming, just like it is in the Atlantic. But it’s small at the moment. It’s linked in to the Sagaing fault that comes down through Burma.
You have also studied Mount Kailash…
Kailash is geologically very important because all the three great antecedent rivers of Asia—Indus, Yarlung Tsangpo-Brahmaputra, Sutlej—arise from that region.
In the book, you write of several episodes of extortion by Maoists on trekking routes in Nepal, most of whom you describe as people ‘who had no idea who Mao Tse-Tung was, or what he stood for’. You also write of your interactions with the various armies of the countries you worked in. At present, what is the most difficult area to work as a geologist?
I would say Northwest Pakistan and Afghanistan, purely as a result of the Western invasion. They’ve created ten times more problems than there were to start with. The people who support these invasions are people who don’t know the local population and culture, especially the local religion. I’ve been travelling through Pakistan for 20 years and met some of the friendliest, most hospitable, nicest people around. And when you read stories about Pakistan in the Western press, it couldn’t be more wrong. The Taliban people are completely crazy, but they have risen through sheer ignorance. That’s why these problems will only ever be solved by education, not military means.
What are the biggest unresolved questions about the continental collision?
There are all sorts. One is: What happens to the whole of Asia? Tibet is a huge region of high and thick crust. There are three mountain ranges in east Tibet that are the size of the European Alps. We have no idea what’s in them. Almost no geological work has been done there. So we know more about the moon than we know about eastern Tibet.
In the past, scientific breakthroughs had arrived in the form of the theory of evolution and continental drift. What, in your opinion, will be the next breakthrough?
I think the next breakthrough will be on continents—how they behave. During the 1960s and 70s, plate tectonics was the biggest revolution in the earth sciences. Scientists used it to look at earthquakes, they could map the age of the ocean floor and they could see how the continents moved around the globe driven by plate tectonics. Plate tectonics undoubtedly work in the oceans 99 per cent of the time. In the continents, it works for maybe 50 per cent of places, especially old ones like Africa, North America, South America and East of the Andes.
But for the mountain belts—the Andes, the Himalayas and Tibet—plate tectonics don’t work. Deformations and earthquakes are not restricted to the boundaries [of plates], they are occurring all over the place. To understand this, you have to look at rocks from the deep crust, how they formed over time, how they were buried and exhumed. I think the next big revolution is happening right now, in this very field.