Oceanic crust – that sinking feeling

Some rocks lead a quiet life. Stable parts of continental crust just sit there for billions of years, doing nothing. In the oceans things are much more dynamic. Live fast, die young, stay pretty is the motto of oceanic crust. It goes on one of the most amazing journeys rock can take. Along the way it affects well nigh everything in and on the planet. Let’s tag along.

Making the crust

Mantle material, usually made of a dark heavy rock called peridotite, is odd stuff. Whenever I’ve seen it on the surface it always looks out of place somehow, too homogeneous, too brown, too massive. Yet it forms over 80% of the earth’s volume – excepting the metallic core it is what earth is made of. Found everywhere immediately below the crust, from between 5 to 75 kilometers depth, it is very very hot (> 1000 °C) – hot enough so that it flows (slowly to us, but fast on geological timescales) but still remains solid. The top, “lithospheric”, layer of mantle has ‘frozen on’ to the crust, becoming part of a rigid plate. The rest makes up the “aesthenosphere”, where the mantle is constantly flowing and mixing, like hot soup in a saucepan.

Our little piece of oceanic crust is about to be formed at a mid-ocean ridge, where two plates move apart. This makes space that is filled by part of the hot mantle rising up – whereupon the reduction in pressure causes it to melt. Rock is made up lots of different minerals and when it melts it usually doesn’t melt completely and produces a magma that has a different composition than the original rock. Shallow melting of mantle material gives basaltic magma. This rises up and pools into magma chambers. Here it may cool to form coarse gabbro, or flow upwards through cracks to form basalt lava flows, or cool in the cracks to form a sheeted dyke complex. This creates a characteristic layered pattern in the crust.

All of this is taking place under water. A 2.5 km thick pile of water in fact. When magma reaches the surface, it flows out as lava but it cools extremely quickly and forms piles of distinctive pillow shapes. Water also flows down into the crust where it is heated up. When it comes back to the surface, at places called hydrothermal vents, it may form dramatic chimneys called black smokers, built up as minerals precipitate out of the suddenly cooled brine. Water is making important changes under the surface, changing the original igneous minerals into new ones, often by putting H2O into the mineral structure.  Incredibly, it seems there may be bacteria living in this hot wet rock deep below the surface.

That sinking feeling

As our new crust slowly drifts at finger-nail speed (5 cm/yr) away from the ridge, it cools and grows denser, causing the sea-bed to sink. Sediment builds up on top. We are a long way from land and the sediment tends to consist of dead things with great names falling from above – diatomaceous ooze, globigerina ooze, the Titanic. It depends.

After 50Ma (million years) our crust is denser than the underlying mantle. It would sink down into it, but it is part of a rigid plate so it can’t – until it reaches a subduction zone, that is. Subduction zones, usually associated with deep ocean trenches, are found around the world. They are doorways – a place where our crust leaves the surface and enters the interior of our planet.

As it moves down into the subduction zone, our crust is pushed down under another plate. It bends down and starts to sink into the mantle – the older the crust, the steeper the angle. Some of the sedimentary cover is scraped off, to form an accretionary wedge above the plate. All this scraping and bending is associated with earthquakes, some of the strongest ones known. The bending of the plate allows water to get into the mantle part of the plate, further changing its composition.

Subduction zones and mid-ocean ridges are linked. In them, creation and destruction is broadly balanced. Oceanic crust is created from the mantle and returns there to be destroyed. The balance is not perfect: 0ceanic crust starts of different in composition from the mantle and all that brine it interacted with caused many chemical changes. The crust returns changed and can’t just turn back into peridotite – it stays as something more interesting.

As our crust starts to sink deeper into the subduction zone, the pressure increases. A lot. Imagine the pressure of a kilometre of rock sitting on top of you (I bet you can’t). For every kilometre the crust descends, the more rock is pressing down on it. The temperature increases too, but to a lesser extent – the cold crust takes a long time to heat up.

As conditions change, two things start to happen, metamorphism and metasomatism. Firstly the minerals forming the basaltic rock (such as plagioclase, pyroxene, olivine) become unstable and new minerals are formed –metamorphism. This process is fairly continuous as the conditions change, but the most dramatic (and attractive) transformation occurs about 2 million years after subduction starts, at around 50 kilometers depth. Here the basaltic rock turns into eclogite.

Many of the metamorphic reactions affecting the crust release water which flows from the subducting crust up into the wedge of mantle sitting above it. Water is a fantastic solvent, so it takes other elements up dissolved with it – this flow of material is called metasomatism. These elements tend to be ones that make large ions, like Potassium and Boron- they are harder to fit into the increasingly tightly-packed mineral structures that form at depth. It’s as if they are being squeezed out of the rock.

How on earth does a slab of rock force its way deep into the earth? The driving force is density. Old cold subducting crust starts off denser than the surrounding rock and the process of turning it into eclogite makes it 10% denser still. Over geological timescales the mantle behaves like a stiff fluid and a cold and rigid dense plate is able to force its way into it. The force generated by the sinking plate is called slab pull and is one of the major drivers of plate tectonics. Eclogites make the world’s plates go round.

After burial, rebirth

A lot of what we know about subducting crust comes from pieces of it that have somehow got back to the surface. We don’t know of any eclogite that has been deeper that 150km, so as our plate sinks further down we have to infer what is going on using indirect methods.

One such method is to study the pattern of earthquakes associated with modern subduction zones. Over time, plotting their distribution picks out the subducting plate and shows that typically it carries moving down to at least 650km depth.

One, rather important, consequence of subduction is the creation of volcanic arcs. These are chains of volcanoes, parallel with the subduction zone, typically about 100km along the surface from the trench. Many major modern day volcanoes, such as Mount St. Helens and Krakatoa are found in volcanic arcs.

Working out what is going on below volcanic arcs relies on the indirect tools provided by geochemistry – studying the composition of earth materials. We know the composition of what we start with (subducting crust and mantle materials) and of what we end up with (volcanic rocks) and comparing the two gives insights into the process.  It’s complex.

At depths of 100-250 km the oceanic crust begins to melt. The resulting magma, along with the water mentioned earlier, rises up into the wedge of mantle above. This rising material then lowers the melting point of the hot mantle wedge, so in turn parts of that melt. This new mantle-wedge melt is what rises to the surface and forms the volcanic arc. On the way it may be further modified by melting and then mixing with the crust it is intruded into. The end result is that mantle melting ends up  producing rock with a very different composition, called andesite.

This process is worth studying in detail as it is one of the main engines of continental crust formation, producing the stuff that most of you are currently sitting on. Over time, volcanic arcs have been the major mechanism for turning mantle rocks into continental crust. [If you are not sitting on continent, how’s the pineapple/cod cheeks tasting tonight?  I’ll get to the creation of your oceanic island in the next post]

Subduction is involved in not one but two interlocking cycles of creation and destruction. Oceanic crust is created, but it is destined soon to return to the mantle at subduction zones, to make space for newer crust.  Squeezing out of the water the crust gained from the oceans helps create new continental crust. Eclogite plays an important role in both cycles – it helps pull the mid-ocean ridges apart by slab pull and it sweats out the fluids that kick-off the creation of volcanic arcs.

I talk of destruction, but our crust still lives on, transformed. Maybe 40% of it has melted and flowed upwards, but it it is still a distinct slab, different from the surrounding mantle. By now it  has travelled 250km down, after about 10 million years of subduction. It’s journey is far from over with 10 times as far to sink still. We’ll continue the incredible journey in another post.

Further reading

I found the paper Metamorphic chemical geodynamics of subduction zones an invaluable recent summary of the details of metamorphism within subduction zones and how it drives melting. The link is to a freely available copy.

Dana Hunter’s recent post over at Rosetta Stones gives a perspective on what its like to live above a subduction zone. She’s also writing extensively about Mount St. Helens and the drama caused (indirectly) by devolatilisation reactions in eclogites.