I was looking at the geology subreddit the other day and someone asked an intriguing question: Why aren’t the Himalayas volcanic? They are the the biggest mountains on the planet and stretch for thousands of kilometers, but no volcanoes can be found. This may seem strange to many people because there are mountain ranges that are full of volcanoes – the Andes and the Cascades for example. Why are they different?
The Himalayas are huge, running over 1,400 miles from end to end. The multitude of mountains over 25,000 feet in this range is astounding, with 18 full peaks reading this elevation. However, none of these mountains are volcanic. Instead, they are built by the collision of two tectonic plates that lasts for tens of millions of years.
A diagram of the collision of the Indian subcontinent with Eurasia. Credit: US Geological Survey.
The Indian subcontinent and Eurasia are at odds with each other. India moved steadily north, and when it hit Eurasia, things took a turn for the worse. Both plates are continental, so they are made of relatively low density rocks like granite and sandstone. They are also tens of kilometers thick, so when India hit Eurasia, it was like a car crashing into a giant wall.
That collision in progress that’s what makes mountains climb. The crumpling and stacking of the two continents as this tectonic energy smashes them together keeps the earth’s surface moving faster than all the forces of weathering and erosion can bring it down. With this you get towering peaks like Everest and K2.
A view of the Andes in northern Chile from the space shuttle Endeavour. Credit: NASA.
Just like the Himalayas, the Andes are a chain of large mountains. The Andes are actually a much longer chain, running 5,500 miles along the west coast of South America. The mountains themselves are not as high as the peaks of the Himalayas. The highest Andean mountain is Aconcagua (which, by the way, is not volcanic) which comes in at around 22,800 feet, which is over 7,000 feet shorter than Everest. That being said, there are dozens of mountains in the Andes that exceed 20,000 feet.
Unlike the Himalayas, two continents do not collide to form the Andes. Instead, the South American continent interacts with oceanic tectonic plates that are denser and much thinner than continental plates. This means that when they collide, instead of the two plates collapsing, the oceanic plate gets stuck under the thicker, less dense continent. There is still stress that can cause the South American continent to wrinkle a bit and thicken, raising that land surface, but nothing like the Himalayas.
Andean tectonics. Credit: Wikipedia.
The consequence of this type of collision is that the oceanic plate plunges into the warm mantle of the Earth. Now the mantle is not a sea of molten rock but it is hot, but not hot enough to melt. However, if you add water to these mantle rocks, their melting point drops much like how water melts at a lower temperature when you add salt.
The oceanic plate, with all the oceanic sediments and weathered basalt, is full of water. So as it goes down and warms up, it releases that water into the mantle under the Andes and all of a sudden you have magma. This magma rises to create the thousands of volcanoes that litter the Andes from Colombia to Tierra del Fuego.
The tectonic engine
We call the style of tectonic interaction in the Himalayas a continental collision, which is a fairly intuitive name. Because no plates are pushed back into the mantle to supply water, little or no magma forms and you have no volcanoes.
The Andes are what we call a subduction zone, which is a much less intuitive name. However, what you can try to remember is that submarines go underwater like an oceanic plate under a continent. The melting happens thanks to all the water released and you get volcanoes.
These two boundaries converge as the plates move towards each other, so beyond mountain building, the Andes and Himalayas are also at the center of earthquakes. Interestingly, although you might think that two continents mixing together would cause bigger earthquakes, that’s the subduction zones that hold the record for the largest known earthquakes.
Now, there are still other ways to form mountains, but at least when it comes to the largest ranges on Earth today, it’s plate collisions that create them. However, not all collisions are the same leading to the giant, non-volcanic Himalayas and the less giant, but highly volcanic Andes.