Appearance
1.8 — Mountains, and the Making of the Himalaya
At the summit of Mount Everest, 8,849 metres above sea level, the rock is limestone containing the fossils of sea creatures. Crinoids, brachiopods, ostracods — animals that lived on a warm shallow sea floor. That rock, the Qomolangma Formation, is Ordovician, roughly 470 million years old, and it was laid down under water.
The highest point on Earth is a piece of ancient sea bed. Working out how it got there is the story of this page, and it is also the story of why a fifth of the human race lives where it lives.
The three ways to make a mountain
Fold and thrust — the collision mountains. Two continents meet, neither will subduct, and the crust between them has to go somewhere. It folds like a rug pushed against a wall, and it also breaks along low-angle faults that stack slices of crust on top of one another. Both processes thicken the crust enormously. The Himalaya, the Alps, the Zagros in Iran and the Appalachians in North America were all built this way.
Volcanic — the built-up mountains. Chapter 1.7 covers these. A stratovolcano is a mountain that was constructed from material erupted onto the surface rather than pushed up from below.
Fault-block — the pulled-apart mountains. Where crust is being stretched rather than squeezed, it breaks into blocks along steep faults, and some blocks drop while others stay high or tilt. The Basin and Range province of Nevada is the classic case, and the Vindhya and Satpura escarpments in central India and the ranges flanking the East African Rift owe much of their form to faulting.
One more thing raises land without any of the above, and it surprises people: erosion. Because crust floats on the mantle (Chapter 1.3), removing weight from the top lets the whole block bob up, the way a ship rises when cargo is unloaded. Strip a kilometre off a mountain range and the range rises by a substantial fraction of that kilometre. Erosion therefore does not simply flatten mountains — it also keeps pushing fresh deep rock up into view, which is why metamorphic rocks that formed 20 kilometres down are now at the surface in Scotland and Scandinavia.
The collision that made the Himalaya
Around 140 million years ago, India was attached to Antarctica and Africa, part of the southern supercontinent Gondwana, sitting well south of the equator. Then it broke away.
And then it did something no other continent has been observed to do: it sprinted. For tens of millions of years the Indian plate moved north at 15 to 20 centimetres a year — roughly four times typical plate speeds. Nobody is entirely certain why. The leading explanations are that the plate was unusually thin and light after the Réunion hotspot burned through its underside (the same hotspot that produced the Deccan Traps), so it had less resistance; and that it had an exceptionally long, cold, heavy slab of ocean floor sinking ahead of it, pulling hard. Probably both.
The ocean between India and Asia — geologists call it the Tethys — was consumed at a subduction zone under the southern edge of Asia. That subduction made a chain of volcanoes along what is now southern Tibet, exactly as Chapter 1.7 describes, and the granite roots of those dead volcanoes are still there.
Then the ocean ran out. Around 50 to 55 million years ago the leading edge of the Indian continent arrived at the trench. Continental crust is too buoyant to be dragged down. The subduction jammed, and India, with all the momentum of a continent, kept going.
It has since pushed roughly 2,000 kilometres further north. That distance had to be absorbed by deforming rock, and the ways it was absorbed are what you see today.
What the collision built
The mountains themselves are stacked slices. The sediments that had been sitting on the Tethys sea floor and along India's northern margin were scraped off and thrust southward over one another along a series of great faults — the Main Central Thrust, the Main Boundary Thrust, and at the very front the Main Frontal Thrust, which is still moving. Each thrust carries older rock over younger rock, which violates the ordinary law of superposition from Chapter 1.1 and is precisely how geologists recognise a thrust fault. The marine limestone on Everest's summit is one of those scraped-off slices, riding on top.
The crust beneath is doubled. Normal continental crust is about 35 kilometres thick. Under Tibet it is about 70 kilometres — India's crust has partly slid underneath Asia's, giving two thicknesses in one place. Seismic surveys image this directly.
And that is what holds Tibet up. The Tibetan Plateau is an area roughly the size of Western Europe standing at an average altitude of about 4,500 metres. It is not held up by any force pushing from below; it floats high because it is thick and light, exactly as an iceberg with a deep keel stands tall (Chapter 1.3's isostasy). The plateau is the largest and highest such feature on Earth and there is nothing else like it.
Asia also got out of the way sideways. Not all the shortening went into thickening. Large blocks of Asia have been squeezed eastward out of India's path along enormous strike-slip faults, a process called tectonic escape. This is why Southeast Asia's mountains and rivers all run north-south in a tight bundle, and why the crust of eastern Tibet and Yunnan is rotating.
It is still happening. GPS measurements show India moving north relative to stable Asia at about 4 to 5 centimetres per year, of which around 1.5 to 2 centimetres is taken up as shortening across the Himalaya. Everest and its neighbours are rising by a few millimetres a year, partly offset by erosion. A survey by India and Nepal in 2020 revised Everest's official height to 8,848.86 metres.
And it is dangerous. The strain being stored along the Himalayan front is enormous and it is released in great earthquakes: Kangra in 1905, Bihar–Nepal in 1934, Assam in 1950, Kashmir in 2005 with about 87,000 deaths, and Gorkha in Nepal in 2015 with about 9,000. Seismologists have noted a long stretch of the central Himalayan front — roughly between the 1905 and 1934 ruptures — that has not broken in a long time and has accumulated many metres of unreleased slip. Tens of millions of people live in the cities below it.
What the Himalaya did to the world
This is where a geology chapter turns into a history chapter, and the connections are not decoration — each one is a documented physical mechanism.
It created the monsoon. A mountain wall 2,400 kilometres long and 8 kilometres high, with a vast high plateau behind it, transformed the atmospheric circulation of Asia. The plateau heats strongly in summer and acts as a raised heat source in the middle of the atmosphere, strengthening the pressure difference that draws moist air off the Indian Ocean. The wall then forces that air upward, wringing the water out of it. Chapter 2.5 does the mechanism properly. The monsoon is not a minor weather feature; it delivers around 70 to 80 percent of India's annual rainfall in about four months, and Indian agriculture, and therefore Indian history, runs on it.
It made the rivers, and therefore the plains. The Indus, Ganges, Brahmaputra, Yangtze, Mekong, Salween and Irrawaddy all rise in or around the Himalaya and Tibet. Together their basins feed on the order of a quarter of humanity. The Indo-Gangetic plain is not ordinary land — it is a foreland basin, a trough pressed down in front of the mountains by their sheer weight, which has then been filled with up to several kilometres of sediment ground off the mountains and carried down. The most fertile large plain in the world is powdered Himalaya, and every civilisation in northern India from the Indus onward has lived on it.
It created a barrier and a set of gates. The Himalaya is effectively impassable for armies. Every land invasion of the Indian subcontinent for three thousand years — Persian, Greek, Kushan, Hun, Turkic, Mughal, Afghan — came through the passes of the northwest, the Khyber and the Bolan, because that is where the mountains are lower and the routes are usable. Chapter 6.1 builds Indian history on this fact, and Chapter 6.15 explains why the same doorway kept being used successfully.
It may have cooled the planet. This is a real hypothesis with real evidence, though not settled. Raising an enormous volume of fresh silicate rock into a warm, wet, monsoon-drenched climate massively increased chemical weathering — the reaction from Chapter 1.4 that consumes carbon dioxide. Add the burial of vast quantities of organic carbon in the sediment fans dumped into the Bay of Bengal, and you have two large carbon sinks switching on at once. The Earth has cooled substantially over the last 50 million years, ending in the current ice-age cycles, and the Himalaya's rise is one of the leading explanations.
Why mountains are not permanent
As soon as a mountain rises it is attacked. Higher ground means steeper slopes, more rainfall, freezing and thawing, faster rivers and glaciers — every erosional process intensifies with altitude. A mountain range is a balance between uplift pushing rock up and erosion taking it away, and when the uplift stops, erosion wins.
The Appalachians are the standing proof. They were built about 300 million years ago when North America collided with Africa during the assembly of Pangaea, and the metamorphic minerals in their rocks show they were once Himalayan in scale. Three hundred million years of weather has reduced them to rounded hills under 2,000 metres. The Aravalli range in Rajasthan is older still — one of the oldest fold mountain systems on Earth, with a history going back well over a billion years — and it is now a worn ridge a few hundred metres high running past Delhi.
Which tells you something about the Himalaya's future. It is young — 50 million years is nothing — which is exactly why it is jagged, unstable, prone to landslides, and full of narrow gorges rather than broad valleys. Give it a few hundred million years after India finally stops pushing and it will be Aravallis.
Where this shows up in your life
Water. Himalayan glaciers and snowmelt feed the dry-season flow of the Indus, Ganges and Brahmaputra. Around 240 million people live in the mountains themselves and something like 1.6 billion in the basins downstream. Glacier retreat there is not an abstract environmental concern; it is a question about the timing of river flow for a fifth of humanity, and Part 14 deals with it.
Hazard. Himalayan slopes are steep, young, fractured by faults and hit by intense monsoon rain, which makes them among the most landslide-prone terrain on Earth. Road cuts and unregulated construction make it worse. The 2013 Kedarnath disaster, in which a glacial lake burst and the flood killed several thousand people, and the 2021 Chamoli disaster, are the kind of event this geology produces.
Air travel and weather. Flights from India to Europe route around the plateau because of terrain and jet stream considerations, and the same plateau splits the winter jet stream into two branches — one of the reasons north India's winters are dry.
And a piece of everyday physics. Everest's summit air pressure is about a third of sea level's, so each breath delivers about a third the oxygen. Volume V's respiratory chapters explain what that does to a body; the geology page's contribution is that the mountain is that tall because two continents are still colliding beneath your feet at the speed a fingernail grows.
What the next page covers
Mountains go up. Everything then works to bring them down, and the debris has to go somewhere. Chapter 1.9 covers erosion and deposition — how a river actually carves a valley and why it meanders, what a glacier does that water cannot, how deserts and coasts are shaped, and how the material stripped off the world's mountains becomes the deltas and plains that most of humanity lives on.