Earth's Crust Does the 'Caterpillar Walk'

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The geological mystery of how colliding tectonic plates can force some rocks very deep into the Earth, then pop them quickly back to the surface, may have been solved with what’s being called a “caterpillar walk.”

The classic way of thinking about colliding plates is that one plate is shoved under another and they just keep ramming towards one another. The plate that goes under eventually peels off into the mantle and is gone. But that doesn’t explain how rocks that get pulled down and altered by the pressure so they become loaded with ultra-high pressure minerals — like the metamorphic eclogite rock shown above in my hand and at right — get boosted back to the surface so quickly. We know it happens because these pressure-altered rocks are found at the surface.

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A team of European geologist have looked at the situation in the Aegean and have come up with a model that might explain it. They propose that when plates collided in the Aegean, the sinking plate, as it peeled away and sank into the mantle, sucked down some rocks with it that were then quickly (geologically speaking, which means over millions of years) caught up in the upward flow of material that filled the space of the sinking plate. An analogy, if I’m getting this right, might be the way water would rush in if you quickly separate your hands underwater. The authors of the paper, in the latest issue of Geology, made a movie of the process that you can see here.

Anyway, this upwards flow of rock created an area of the Earth’s crust that was no longer smashing together, but stretching out. That means there are lots of vertical faults allowing deep rocks to rise to the surface (in the same was that a row of tilted books on a book shelf eventually reveal lower and lower portions of books if you allow them to spread out over the length of the shelf). Add to that the normal erosion of rocks by weather, and you should be able to get eclogites up in record time.

The caterpillar part comes in when you follow the rocks in the collision zone. At first they are bunched up and pulled down into the collision zone, then they are sucked up again and spread out in the crust. Think caterpillar on the ceiling. First spread out flat, then bunched up (collision and pressurization of the rocks) then spread out again (and the rocks that were deep are now shallow).

And just in case anyone was wondering, the fact that this process brings up metamorphic rocks does not mean they are in way connected to the metamorphosis of a caterpillar into a butterfly. It’s just a fun semantic coincidence.

IMAGES:

(Top) Metamorphic eclogite rock. (Larry O’Hanlon)

(Side) Eclogite is a metamorphic rock with a mixture of garnet (almandine-pyrope) and a green matrix of sodium-rich pyroxene (omphacite), this one from Norway is 425 million years old. (Gerald & Buff Corsi/Corbis)