The Himalayas stretch 2,400 kilometres across central Asia and are home to nine of the 10 highest mountains on Earth. They’ve risen over millions of years as the Indian tectonic plate crushed up into the Eurasian continent. But their age has had Earth scientists stumped, with estimates ranging from 30 to 60 million years.
So my collaborators and I tried to pinpoint the Himalayas’ date of birth by scrutinising the depths of the Indian Ocean.
The seafloor has mountains and valleys, volcanoes andcanyons. All these structures can teach us about the Earth’s tectonic evolution over the past 200 million years. We can map the seafloor by measuring the shape of the ocean surface. Seawater piles up over mountains, while over deep troughs the sea level falls. Sensitive satellite instruments called altimeters can measure these bumps and dips that are invisible to the naked eye.
By studying seafloor maps, we can trace the past movements of tectonic plates. When two plates separate new crust forms in the zone between them. And when two continental plates collide it can trigger a tectonic shake-up that reshapes the seafloor – even cracking off bits of neighbouring plates to form new ‘microplates’.
We think that is what happened 47 million years ago – our proposed birth date for the Himalayas. Using high-resolution maps we discovered a small oceanic plate, about the size of Tasmania, that snapped off the Antarctic Plate at that time. We named it the “Mammerickx Microplate”, after seafloor mapping pioneer Jacqueline Mammerickx.
How did the microplate form? Around 50 million years ago the Indian Plate was moving north from Antarctica at 15 centimetres per year – that is fast for plates. It hit the Eurasian Plate 47 million years ago, reshuffling plate boundaries and causing this piece of the Antarctic Plate to snap off.
Over the next couple of million years, that fragment was rotated by around 25 degrees, jostled by bigger plates around it. As new crust has formed to its south, it now sits in the central Indian Ocean. Its contours match up with the seafloor of the Antarctic Plate, where it was once attached.
The more microplates we can discover, the clearer our picture of how the surface of our planet has evolved will become.
Kara Matthews is a geoscientist at the University of Sydney.
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