Let's start off with an understanding that subduction is a pretty complex thing.  As complex as it is, we butted up with the concept in both our water cycle and carbon cycle topic pages and have been wanting to dive into it a bit more both times.  In researching the topic, a lot of the literature out there isn't exactly in layman's terms, so we'll do what we can to present what subduction is without oversimplifying the process.  With that said...

What if we told you the Earth was a giant robot?

By KDS4444 - Own work, CC BY-SA 4.0, Link

Layers of the Earth

Okay okay, before we unravel that question, it's probably worth reviewing a bit about how the Earth is constructed.  Our planet has 3 major layers: the crust, mantle, and core.  The mantle and core are comprised of 2 subsections, each having outer and inner layers.  The inner core is solid, making up a majority of the Earth's elemental composition and the outer core is liquid in nature.  Going up a layer we have the lower mantle which transitions into the upper mantle.  At the outermost parts of the upper mantle, we hit the Earth's crust.

It's at this meeting point between mantle and crust that we'll be exploring subduction.  What makes this phenomenon possible is the brittleness of the mantle where it meets with the crust.  If there wasn't a transitory section there, it'd likely be that the crust would melt under the pressure and temperature of the mantle.  This space between mantle and crust is characteristically unstable, yet stable enough for our planet to have a layer of crust!

What is Subduction

​So now we know where subduction is happening: between the mantle and crust.  We know that the mantle is ridiculously hot and pressurized, but there's a transitory layer where it meets the crust such that the crust isn't just consumed by the mantle.

The Earth is made of 3 primary layers: the crust, mantle, and core.

​Subduction is a process that occurs where the crust and mantle meet.

Or is it?

​The way the Earth's crust is formed is in sections known as plates.  You might remember learning about this concept as tectonic plates.  You might also have learned that these plate tectonics are what form our continents and what makes them move.  This is true, and the reason that happens is subduction!

Where tectonic plates meet, one of the oceanic plates will always be pulled into the mantle.  That's subduction!

Where these plates meet are called subduction zones.  Since the Earth's crust isn't one contiguous piece of land, how they meet is integral to how the planet functions.  In all of these zones, one piece of the plates is pushed under the plate it is butting up against.  If you remember our review of the Earth's structure, where does that mean the crust is being pushed into?  That's right, the mantle!

In these acute angles where the crust enters the mantle under adjacent plates, the basaltic composition of the oceanic lithosphere (the plate that is under the ocean), undergoes a slow change in composition to eclogite, a metamorphic mineral.  The characteristics of this rock give the ocean plate a negative buoyancy, or a downward push, that lets the crust "slide" into the mantle. 

Roles in Water & Carbon Cycles

Subduction is how the water and carbon cycles interact with the mantle.  The ocean plate's mineral composition is highly hydrous, or made of elements that break down into water.  Under the temperature and pressure of the mantle, these minerals are transformed into water, though in a supercritical state.  In this state, the water hits temperatures and pressures that can cause the subducted crust to melt into magma, which in turn form diapirs.  These diapirs push the magma up to the continental crust.  When these magma highways find brittle parts of the crust, they can push up through the plate, forming various thermal features, such as volcanoes.  These features are how water can be cycled back out of the mantle.

The carbon content of the ocean crust is also recycled into the mantle at a certain depth.  While this deep cycle isn't particularly well-defined, we do know that carbon can enter a gaseous state when cycled through the mantle.  Piggy-backing off of the thermal features created in the deep water cycle, carbon can also find its way out of the mantle in the same manner.

​While water and carbon are cycled into the mantle, we don't generally consider this a part of the traditional cycles that take place above the lithosphere.  When elements find their way into the mantle, they aren't able to be interacted with by forces above the mantle.  As such, the elements don't cycle out of the mantle without the mantle giving it back on its own.  To distinguish these parts of the traditional cycles, they are referred to as "deep" cycles.

Water that enters the mantle enters a supercritical state and creates "magma highways" called diapirs which may someday become volcanoes!

Impacts on the Planet

Since first realizing that subduction was a thing, humans have found that it is probably one of greatest potential threats to living things on the planet.  In stark contrast, we also understand that this process is likely one of the most important systems that allow the Earth to be...well...the Earth.

​We understand that subduction zones are some of the most dangerous places on the planet.  Not just at the point where the process occurs, but above the area and in the arc surrounding the area.  We mentioned that in cases where magma forms at the subduction zone, volcanoes can form, but other natural disasters like earthquakes and tsunamis result from subduction zone activity.  To date, the most widespread of these natural events have occurred over and as a direct result of subduction zones.

The Earth's crust essentially functions as an ever-moving gear that shapes the planet

We also understand that the Earth's continents formed because of these subduction zones.  Both as a result of volcanoes formed between the mantle and crust and by sediments of the subducting plate that build up on the continental plate.  If the planet's plates were locked or contiguous, the land would not shift.  Without subduction, carbon would not be cycled out of the lithosphere and atmosphere.  It also would not have cycled into those spheres, making life, in general, a lot rarer on the planet.

So, it's kind of hard to say that subduction has an "impact" on the planet.  More accurately, subduction is a part of our planet.  To circle back to the original question:

​What if we told you the Earth was a giant robot?

​​There may be quite a bit of truth in that.  The Earth's crust effectively functions as a giant set of gears, powering many of the cycles, systems, and the very formation of the planet we know and love today.  It's fascinating to think of the Earth as an organic titan whose forces really do create and connect everything about this planet we call home.  And, for the moment, it's letting us live here for a time.

~ And, as always, don't forget to keep wondering ~

Sources
* Stern, R. J., Subduction zones, Rev. Geophys., 40 ( 4), 1012, doi:10.1029/2001RG000108, 2002.

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