c. 70 Million BCE

The Himalayas

Throughout Earth’s history, collisions among the lithosphere’s several dozen major tectonic plates have been the primary agent of volcanism, mountain-building (orogeny), and continental growth. In particular, continent-to-continent collisions have been responsible for most of the world’s major orogenies, with compressed, folded, and crumpled-up crust producing spectacular chains of peaks like those in the ancient Appalachian, Ural, and Atlas ranges and in the modern Rockies, Alps, and Pyrénées mountains.

When oceanic crust runs into continental crust, the former almost always dives under (subducts) the latter, because oceanic crust is made of dense high-iron basaltic volcanic rocks, while continental crust is made of less-dense, high-silicon volcanic and sedimentary rocks. Subduction causes melting of the oceanic plate, leading to the creation of deep ocean tranches, tall volcanic peaks, and some uplift of the continent in general. But by plate-collision standards, such interactions are relatively gentle. Continental–continental collisions are more of a titanic clash of equals, with neither crustal plate compelled to subduct under the other. The result, especially in head-on plate collisions, is enormous compression and uplift of the crust in the collision zone. Voilá, the world’s tallest mountains are formed.

Indeed, that is precisely what is happening in Nepal and surroundings. Starting around 70 million years ago, the relatively small Indian continental plate, which had broken off the supercontinent of Pangea around 140 million years ago, began to head toward a head-on collision with the Eurasian continental plate. The continents began fully plowing into each other about 10–20 million years ago, and they continue to compress and crumple each other up to this day.

The result is a spectacularly tall, young, and pristine mountain range, the Himalayas, which includes a whopping 50 peaks above 23,600 feet (7,200 meters), including the world’s tallest peak, Mt. Everest (29,000 feet, or 8,850 meters). The specific mechanism for building the mountains is known as thrust-faulting—converging blocks of crust are continually thrust upward and stacked on top of each other. The forces are so extreme that they have lifted what were once seafloor deposits from far below sea level to the top of the world.

SEE ALSO Continental Crust (c. 4 Billion BCE), Plate Tectonics (c. 4–3 Billion BCE?), Roots of the Pyrénées (c. 500 Million BCE), The Appalachians (c. 480 Million BCE), The Rockies (c. 80 Million BCE), The Himalayas (c. 70 Millio

A view of the Himalayas, looking south from over the Tibetan Plateau, taken by astronauts onboard the International Space Station in 2004. Mt. Everest is in the middle of this scene.