Last month’s devastating earthquakes in Nepal were caused by the collision of the Indian subcontinent crashing into and (partly) under the bulk of Asia. The Himalayas are being created by the collision of tectonic plates. So, where are the volcanoes? That’s what I was asked when a friend wanted me to explain the Katmandu disaster.
Well, there are no volcanoes in Nepal. Nepal has huge mountains, of course, but not every mountain in the world is caused by a volcano (only a small percentage are, actually) – and relatively few nasty earthquakes are triggered by volcanoes. The nearest volcanic activity to Katmandu is a thousand kilometres northwest, in western China, where an eruption last occurred over 60 years ago. The action in far western China has produced a few cones, but no volcanic mountains. No one was hurt when the 1951 Kunlun cone erupted. In fact, the eyewitnesses – a road crew working in the excruciatingly remote region – reportedly said, “What the heck was that?” when the 1951 volcano shook and smoked. Then they went back to work. Before that, the last Kunlun Volcanic Group eruption was in 1850. To say the place is volcanically quiet is an understatement. And these are the nearest to Nepal of any volcanoes. The Himalayas are not on fire.
There are no fire-spitting volcanoes in the Himalayas, yet most people think of volcanoes when they think of earthquakes, mountains, and (Opa!) breaking plates. A “ring of fire”, for example, surrounds the Pacific Ocean with, well, a ring of volcanic fire. But there’s the rub – an ocean of water lubricates the subduction zones surrounding the Pacific. Ironically, water allows the ring of fire to exist while the lack of an ocean of water in the landlocked Himalayas quenches any possible volcanic action there. When India and Asia collided 90 million years ago, the continental crust of the Indian tectonic plate sank under the Asian plate, deep into the Earth’s mantle, to a depth of 200 kilometres or more. It was a continent-on-continent convergence of rock. Sans water, except perhaps a bit that got mixed in when India first arrived from across the sea, from present-day Madagascar. But that was 90 million years before Nepal’s current borders were drawn on a map.
Why is water so important? Water decreases the melting point of rock. When water is conveyed into Earth’s deep nether places, solid rock down deep may melt. The making of a volcano requires runny magma – melted rock. Oceanic plates have water-saturated sediments and crust, at least in their shallower sections. In places like Chile, Tonga, and the American Pacific Northwest, those wet sediments and oceanic crust end up deeply buried where the water vaporizes, rises, melts overlying rock, and forms volcanic magma. In the Himalayas, water doesn’t entire the mix. Instead, continental crust collides and piles up into mountains. No water, no melted rocks, no magma, no volcanoes. Just deadly earthquakes instead.