Summary: You recall that the supercontinent Pangaea began to have some cracks in it almost as soon as it was assembled back in the Carboniferous and Permian. The break-up really got underway in the Jurassic and Cretaceous, and in particular, India separated from Africa and from Australia-Antarctica. India began to move northward, opening what is now the Indian Ocean and closing the most recent incarnation of the Tethys Ocean, sometimes called the Neotethys, in Late Cretaceous time. India’s northward drift from the old East Gondwana was remarkably fast, at rates close to 20 centimeters, 8 inches, per year. That’s not quite ten times the more typical rate of sea-floor spreading. Sea-floor spreading rates are affected by the geometry of mid-ocean ridges and by the chemistry of the basaltic material brought up – sometimes it is somewhat denser than normal, or less dense. It may have something to do with the strength of a mantle heat plume, and spreading rates are definitely not uniform all along a spreading center at a mid-ocean ridge. In the case of India, differential rates meant that as the Indian block moved, it rotated a bit. I don’t think we have a clear idea of why India moved so quickly. But we do have a pretty clear idea of what happened when it reached another strong continent. The Himalayas happened. India probably began to collide with Eurasia about 55 million years ago, about the boundary between the Paleocene and Eocene epochs. One line of evidence for this is the change in the spreading rate – it slowed dramatically from almost 20 cm per year to more like 4 or 5 cm per year. The collision may have slowed it down. The collision is still going on, 55 million years later. from USGSWhen continents collide, their relatively low density, compared to oceanic crust, means that they do not subduct beneath each other, at least not much. But the crust does shorten, as the margins of the two colliding continents together with whatever was between them get scrunched together into huge folds and uplifts – and there is some pushing down, too, not truly subduction, but the crust on one continent may at least flex down under the weight of all the squeezed-together stuff. In India, that downward flexing created the Ganges Plain in northern India, just south of the Himalayan Mountain front. The India-Eurasia collision is so intense that the force is breaking the continent at great distances from the point of impact. Much of southeast Asia is being pushed out from between the two colliding continents, and 1000 kilometers or more inland, tension has broken the crust of Asia to form the Lake Baikal rift in Siberia. The high mountain relief of the Himalayas is the result of this squeezing. Present-day measurements of the rate of uplift, the highest on earth at about 10 cm per year in places, show that the collision is still continuing, and the earthquakes in the region are of course evidence for it as well. The Himalaya also has one of the highest erosion rates on earth, no surprise, at 5 to 10 cm per year. So erosion is pretty close to keeping pace with the uplift. Map of Alpine Orogeny elements by Woudlouper, used under Creative Commons license. India may have been the speediest part of Gondwana to charge headlong into Eurasia, but it wasn’t the only part. The northeastern prong of Africa, today’s Arabian Peninsula, was toward the narrow, western end of the Tethys Ocean. Tethys had already been narrowed somewhat by the movement of the Cimmeride continental blocks across it – we talked about that back in the Permian and again in the Triassic. The Cimmerian blocks had amalgamated to the southern margin of Eurasia to form much of what is Iran, Afghanistan, and Pakistan today. Some other Cimmerian blocks were probably caught in the big squeeze between India and Asia, and are now somewhere within the Himalayas. The parts further west, Iran and vicinity, were collided with by the Arabian edge of Africa, and that collision is s
