How Islands Came to Rise Atop the Himalayas

Oct. 5, 2008 — -- Home to hundreds of the world's loftiest mountains, including Everest, the Himalayas take their name from a Sanskrit word meaning "the Abode of Snow." But parts of these lofty mountains were once sun-kissed isles glistening in a now-vanished sea.

That's according to an upcoming study that highlights this geological reminder of impermanence on Earth.

"The islands are actually tipped on their sides," says the University of Houston's Shuhab Khan, lead author of the study scheduled for release in the October Geological Society of America Bulletin. "So you can walk — it's incredible — from the bedrock to the seafloor sediment of these islands."

What scraped the islands off the seafloor and lifted them up to the top of northern Pakistan was the collision between the continental crusts of India and Asia. Khan and his colleagues show the collision that built the Himalayas came 50 million years ago in their analysis, which ties together satellite maps, field geology, mineralogy, chemistry and magnetic dating data from the Kohistan-Ladakh bloc, the one-time archipelago dating to the time of the dinosaurs.

The collision of India with Asia continues to this day, with the Indian subcontinent essentially diving beneath the Asian one, lifting up the Tibetan plateau. Geologists have long agreed the Kohistan bloc islands represent the contact point between the two continents, but until now they weren't certain how they made their journey to the Himalayas.

To answer the question, Khan and his colleagues travelled to Northwestern Pakistan, examining rocks from volcanic mountains bordering the Kohistan bloc and from the bloc itself. Magnetic studies of volcanic rocks reveal the compass orientation of metals inside them when they were last molten, which in turn tells geologists how close the rocks were to the Earth's magnetic north pole when they erupted from volcanoes.

For the Kohistan rocks, the magnetic results indicate the islands were on or near the equator, basking in the long-gone Neotethys Ocean, when something crashed into them. Since Asia's southern continental crust edge starts 1,800 miles north of the equator, that means it must have been India doing the bulldozing. Dating of zircon stone spat out by these eruptions set the date of the island's demise at 61 million years ago.

So the islands packed onto the leading edge of India and then, as shown by more zircons from the northern side of the bloc, slammed into Asia about 50 million years ago. Because the islands were made of lighter crust material, they didn't descend under Tibet but tumbled sideways as they stayed on the surface, sandwiched between a big rock and a hard place.

"Both sides of the bloc are known for world-class gemstones, which is the last evidence of the collision, in a way," says Khan. Rubies found north of the former island bloc are colored red by chromium from the former seafloor. Emeralds found south are colored green by beryllium once trapped in sea water. "They are famous, and they are still being mined today," Khan says.