Playing Fetch with the Earth
Subduction zones cause volcanoes and earthquakes, and we can learn about them by studying what's brought back to the surface.
If you threw a rock into an active subduction zone, perhaps
the Atacama Trench off the coast of Chile, it is going to be transported tens
to hundreds of kilometers beneath your feet. Why? Because the thin veneer on
the surface of our planet, known as the crust, is moving. In some areas, known
as ridges, the crust is separating and magma is moving in to fill the gap
(check out The Interior of the Earth is Balloons! for more on this). In other
areas, such as the Atacama Trench, the plates are moving towards and into each
other. Where oceanic crust converges with continental crust, a subduction zone,
the oceanic crust is subducted, or driven beneath, the continental crust.
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| Cartoon diagram depicting an oceanic-continental subduction zone |
Most of the material in these subduction zones is moving down
into the Earth, but it doesn’t always stay there. If you’re lucky, Earth with
play fetch with the rock you threw into the Atacama Trench and, after a couple
million years, bring your rock back to the surface. In Geology, this is known as exhumation, and
the rocks that are fetched provide us with a lot of information about what
happens during subduction.
Now, if you somehow manage to live long enough for your rock to be exhumed and have the urge to go search for it, don’t expect it to look the same as it did when you threw it. It was under a lot of heat and pressure in that subduction zone, and I think we all know from (Insert Disney Star who went off the deep end here) what happens when something is under a lot of pressure: it changes, sometimes radically. Geologist call this metamorphism, and you can use the changes made to your rock to understand what happened to it while it was in Earth’s slobbery mouth.
“But Kayleigh,” you may ask, “why do I care about the Earth
‘playing fetch’ with some rocks?” Good question! In fact, you should care about
it quite a bit, especially if you live in Japan, or Chile, or even Northern
California. While these rocks are being subducted and exhumed, they cause some
serious problems for us at Earth’s surface; a large proportion of volcanoes,
earthquakes, and tsunamis can be attributed to subduction zones. By learning
what happened to your rock between the time you threw it into the Atacama
Trench and when you picked it back up a couple million years later, you have a
chance to understand the processes within the subduction zone that generated
natural disasters at the surface.
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| Look at all of those earthquakes and volcanoes caused by the subduction zone under Japan! Since we don''t have a way to view the rocks causing these earthquakes and volcanoes directly in the subduction zone, we have to look at the rocks that have been exhumed. |
Image sources:
Subduction Zone Cartoon 1 | Young dog playing fetch | Old dog retrieving tennis balls | Atacama Trench | Garnet amphibolite block | Japan Trench
