It's such a simple idea that it's surprising no one seems to have thought of it before. Did a massive asteroid or comet impact kick-start plate tectonics on Earth?

For much of our planet's history, rocky plates have been sliding over a viscous, molten mantle, colliding to build mountain ranges or spreading apart to make way for oceans. In a neat piece of recycling, an oceanic plate dives, or "subducts", underneath a less dense continental plate, and eventually melts into the hot mantle.

Geologists estimate that plate tectonics began during the Archean period, between 2.5 and 3.8 billion years ago - but they don't know what triggered it. Ancient Earth was too hot for the crust to solidify completely, and the lightest minerals would have floated to the surface over the entire planet, making the subduction of denser plate material unlikely.

Now Vicki Hansen of the University of Minnesota-Duluth thinks she has an explanation. If a huge comet or asteroid slammed into the Earth at a particularly weak spot, it may have punched through the soft crust, causing dense mantle to well up through it. With a higher melting point than the surrounding crust, this would have solidified earlier as it cooled, forming the first rigid tectonic plate (Geology, vol 35, p 1059).

If the impact was big enough and centred on a line of weakness caused by convection in the mantle stretching the crust, the upwelling of material would not be plugged. This would push the weak crust apart, forming a ridge that spreads along the line and eventually two plates on either side (see Diagram). As the spreading continued, the denser plates would crumple the impact-weakened crust until they reached the edge of the crater. At this point, the plates would sink beneath the undamaged crust - so kick-starting subduction. Hansen believes the plate boundaries thus created may have spread along lines of weakness across the globe, perhaps joining up with impact-induced boundaries elsewhere.

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"This is a good idea in lots of ways," says Peter Cawood of the University of Western Australia in Perth. "Subduction is not very easy to initiate, and this work provides a unique mechanism."

Hansen points out that the work is "a thought experiment, and nothing more", and that computer modelling will help shed light on the feasibility of her ideas. Still, she says it's odd that no one appears to have suggested this before, because the early solar system was filled with large asteroids and comets.

It's an important consideration, agrees Cawood. Like liquid water, tectonics helped provide the conditions for life, as it led to volcanism which in turn helped to make our atmosphere habitable. Planetary researchers have also floated the idea that an asteroid or comet impact may have led to a brief period of tectonic activity on Mars.