This is geology B with all of us trampled.
Subduction zones are places where a slab of oceanic
lithosphere plunges down into the mantle below.
The slab, consists of sediments on top crustal, rocks, in the
middle, and the lithospheric mantle on the bottom, all
plunging down together as a kind of sandwich.
In each of these layers is an ingredient that plays a key role
in shaping the evolution of the earth over geological time and
that is water. Jeff Abbas has conducted
extensive research on water and subduction zones.
He uses seismic observations to map, the distribution of water
in subducting plates and in the overriding mantle.
He couples these observations with computer-based models of
the physics and chemistry of the subducting plates.
He is Professor of geological scientists at Cornell
University. Jeff abers.
Welcome to geology B. Thank you.
Happy to be here. I just gave a very high-level
summary of what's inside of subducting.
Oceanic slab before we talk about water in the slab.
Can you flesh out the basic structure and composition of
subducting oceanic slabs a bit more for us?
Sure, almost everything that's adducts, as Oceanic lithosphere
formed at a mid-ocean ridge, so that's where the story starts is
two plates. Pull apart mantle Rises up.
Rises above the melting temperature and some of it
melts, those melts are basaltic in composition and they form the
oceanic crust. It's about five six, seven
kilometers, thick the top of those basalts reps on the
seafloor makes these pillow basalts.
Maybe you've seen movies of these things bubbling around
their full of cracks and pores and they interact at very high
temperatures with seawater. Let's see what it gets in these
cracks and pores and react. To form alteration minerals
minerals that have a lot of water in their structure deeper
in the oceanic crust, the same basaltic material cools, much
more slowly without a lot of water present as equality,
gabbro and below. That is the mantle.
What's left over from the melting?
It's a peridotite rock, that's dominantly Olivine.
So, that's the package that forms at the mid-ocean ridge as
it moves away from the ridge, it cools and the sediments get
deposited. They have a lot of minerals.
I have a lot of water in there. Mineral structure things like
Clays and so forth. How do we know what the
structure of that oceanic plate is?
Well, as a few different Clues, we go anywhere in the world in
the, any ocean Basin, the depth to the seafloor and the heat
flow, the right. The heat comes out like it's
about the same everywhere and that's telling us that this
basic process is very similar globally.
The same package is getting delivered.
The same five, six, seven kilometers of crust, the shallow
Parts. We've drilled lots of places.
The When drilling program samples, the sediments and the
tops of the basalts one or two places we've drilled down to the
top of the gabbro layers. We have not drilled through the
Whole Crust anywhere. So mostly we know about the
deeper parts of the section comes from exposed rocks.
We call ophea lights, these are slivers of oceanic crust that's
thrust on. Two continents through various
geologic. Accidents.
Among all of this is tied together by a tremendous amount
of work done over the last several decades measuring the
Dave speeds of seismic waves usually through some controlled
Source, back in the 50s and 60s people throw, Dynamite off of
boats, and recorded on hydrophones or seismometers at
various distances more recently, a more controlled ways of
creating the Sound Source. But those signals propagate for
tens or hundreds of kilometers show, very distinct changes in
wave speed as you go down through the sediments and the
top of the basaltic layer in through the gabbros and then a
big step in velocities to the faster mantle rocks.
We can also tie these ways speeds, very carefully to
experiments and labs done on minerals and on Rock samples to
see what kinds of compositions correspond to what kind of way
is speeds. And that gives us also some of
the first hints of the effects of water water, when it reacts
with most rocks will form other minerals.
That have sort of the same Bowl composition, plus some water
added, those minerals typically of slower.
Is speeds. This gives us another tool for
calibrating the extent of say, the alteration in the basalt.
Or if any water gets down deeper, we can fingerprint that
as well. Okay, so that's how we can tell
something about what's in this lab as it forms.
The ocean floor and moved away from the mid-ocean ridge towards
the subduction zone. But after it, enters a
subduction zone, and it starts to go deeper.
Can we still get any picture of what the structure is?
How do we look at it? Once it, It actually starts to
dive down to any depth. Well a lot of the same tools
exist for seeing deeper and again some of the main tools
come from looking at seismic wave propagation.
The speeds that waves travel at Oceana crustal section and see
if they change say as the play heats up and minerals would hold
water in their structure break down.
Those are usually unstable at high temperatures and as you get
deeper, people, sample water coming out of seeps in the
ground, or old. Ali.
Get to the volcanic Arc, the law visitor coming out of that to
provide hints of what's gone down.
Let's come back up again and this is very clear at
Fingerprints of subduction zones of sediment.
For example, let's talk about our theme today which is the
geological water cycle. What do you mean when you talk
about the geological water cycle, as distinct from the
water cycle of the atmosphere, and oceans that most people are
much more familiar with? Yeah, that's a great question.
What's the water that we know about sits at the earth's
surface in the oceans and rivers in the atmosphere in ice.
And it moves around, you know, fairly quickly between these
different reservoirs. But there's this other cycle
where water is exchanged between the Surface Reservoir and the
mantle, the Deep interior water comes out in tiny tiny
quantities, in measured in your tens, hundreds of parts per
million through mid ocean, ridges and hotspots, and other
sources of volcanoes. That tap The Earth and then
returns to the mantle primarily in subduction zones and so is
deep water cycle and it's regulated at plate tectonics
speed. So you know, CM tens of
centimeters a year is a cycle between the surface water that
we see and the water that's in the mantle.
Okay? So can you step through what
happens to the water in the subducting oceanic slab after it
enters the trench? Some of the water comes out
pretty quickly. What it's sitting in crack.
Sand, pours. In the shallow part of the
crust, gets squeezed in the upper few kilometers as it's
going down and you can see that coming out and seeps and
trenches. But then a lot of the water is
bound in minerals that are attached to the downgoing plate
and stays in them until these minerals reach conditions where
they're unstable and this is true of nearly all hydrous,
minerals, beat them up enough and you squeeze them enough
water is released the big Flex of that water coming out that we
know about is To the mantle sitting underneath volcanic
arcs. And so that a lot of them will
come out in volcanoes Arc, volcanoes have a few weight
percent water in the Magma's that are coming out of the
mantle that feed them. Those two things would get
squeezed out of the trench comes on, the are probably cancer
about two-thirds of the water coming into the trench.
The rest of it seems to be able to stay in the downgoing slab to
go into the deeper parts of the mantle where it can feed much
longer scale, geologic, processes long term.
Water cycle. Can you connect the dots a bit
between the release of water from this downgoing?
Subducting slab and the water that spewing out into the
atmosphere from a volcanic eruption?
Yeah. So plate.
That's the sending the top of it is the stuff that has the
sediments and that Oceanic basalts that have a lot of water
down in it. That top surface at some point
is going to encounter the hot flowing part of the mantle, he
called the mantle wedge where Hot material is coming in and
getting dragged down by the plate, as it starts to heat up
the minerals that hold the water in.
It will start to break down. That water is released that
water then because it's much more buoyant Rises.
And we call flexes the mantle up above at adding, a little bit of
water, to a hot, rock will lower its melting temperature.
And so the idea is that if it's hot enough, you get enough water
and they'll actually trigger melting in the mantle above the
Uh down going plate, then those melts then rise more or less
vertically to form the volcanic Arc so we see all around the
world and this is a key piece that ties subduction of oceanic
plates going into the trench with this parallel chain of
volcanoes that we see everywhere around the world in my
introduction. I said that you couple your
seismic observations with computer-based models of the
physics and chemistry of subducting plates.
What exactly do the models? Try to replicate my mean,
colleague in this. Peter Van Kirk, and the Carnegie
Institution is doing all the computer modeling.
What we've been working on for the last 10 or 20 years has been
doing largely two dimensional models of the thermal structure
and flow fields of different sections of subduction zones
around the world where we try to pattern inputs and geometries as
closely as we can to real places.
And then the few different things we can do one is is to
make them predictions of the thermal structure of the Arctic
temperatures. But then we put in a model of
the rock composition of the mineralogy and petrology.
Look at what minerals are stable, where given those
temperatures and pressures. How much water they hold?
And then make predictions from that of where we would expect
water to be released at different depths both.
Going down the surface of the slab.
And then within the plate. So the model is then give us
predictions of water, storage and water releases well and
different subduction zones. I'm what do the results predict.
There's a few different things at shallow depths, because
you're being very cold, Oceanic lithosphere fairly quickly
underneath some overriding upper plate, you're effectively
cooling, it sort of refrigerating upper plate, and
so temperatures, inside the down, going play, can stay
pretty cold for several. Tens of kilometers depth at
least probably around 70 or 80 kilometers this plate, starts to
encounter the hot flowing part of the wedge as it goes down.
The deeper Parts, drag down through this kissflow.
The mantle and then hot mantle has to come in to replace it.
So the first thing that we see is that at this depth of around
70, 80 km, the top surface of the plate, undergoes, a fairly
rapid heating pulse that Eating pulses, big enough that we think
in most cases the water that's Bound in minerals in subducting
sediment and probably the top of the oceanic crust is released
there. And that's really the water
that's available to feed. The oceanic volcanic Arc, the
volcanic Arc is just you know the slabs a little bit deeper,
maybe 100 110 120 kilometers deep when the volcanoes appear.
So we think there's is there tight connection between this
thermal structure and where the volcanic Arc is the other thing
that we see Is that if water were to get deeper into the
downgoing plate, and there is some hints to seaward of
trenches at this happens. Water gets through the oceanic
crust into the oceanic mantle that water can stay Bound in
minerals like serpentines for quite a ways deeper than the
plate. At those depths doesn't
necessarily heat up to the point where serpentines breakdown
that's temperatures like 600 and 650 degrees C.
And so that's a pathway for water to bypass the volcanic Arc
and go into the deeper part of the Earth where it can be stored
for much longer, okay? So the models help you predict
how much of it gets recycled out of fatty shot of depth and then
goes to feed the volcanoes and spews out in the atmosphere
again. And how much of it might
actually just stick around in the slab for long enough in the
form of minerals like 17 and then actually wind up going into
the mantle. Yeah, one thing that we see
though is that given the range of Ages Incoming plate which
have to do with sort of how thick the cold part is and how
long it takes to heat up how fast they're descending.
The geometries is actually an incredible variability.
In some subduction zones, the water boils off very quickly is
really not much room for water to stay in the subducting plate.
Past the volcanic Arc. This would include places like
say where the Juan de Fuca plate off of Oregon.
Washington. British, Columbia's subducting
underneath the Cascades there. The Late is maybe only 8 million
years old when it hits the trench and it's descending, very
slowly. So we think they're a water goes
off very quickly. Other places like Tonga, like
Northern Japan, like the Marianas.
He's very old plates subducting very quickly.
The Interiors of those places to can stay cold, for hundreds of
kilometers and so those becomes a highway is for water to go to
the deep Earth. When we add this up globally,
right now, at present rates, probably about a third of the
water that's hitting trenches, is making it past the volcanic
Arc into the deeper through these kinds of processes.
And to get that average, two thuds boils off.
If you like button goes back up and one-third goes deeply, you
actually put real-world parameters for the temperatures
and the thicknesses. How many subduction zones did
you put in parameters for all? We've gone around the world to
serve 55. 60 segments of subduction zones trying to look
at every 500 ish kilometers of Arc.
Segment around the world where we think we know the geometry
and the subduction rate and something about how much
sediments actually going in. I should say, 1/3 2/3 number.
There's a lot of uncertainty were assuming a certain amount
of water is coming in. The mantle of the incoming
plate. We've seen in a few places.
First place was off of Nicaragua and we've seen since that awful
Parts of Alaska De and some other subduction zones where it
looks like as the plate starts to bend for it.
Reaches the trench sort of 50, 100 km seaward, of the trench
that bending makes faults, that seem to act as conduits for
water to get into the mantle. And so we see seismic waves
speeds of the very top of the mantle, I should start to slow
down before the plate. Hits the trench.
We think that's a fingerprint of water, getting into the mantle
reacting to form serpentines. Probably but we don't really
know how deep that goes. How many places that goes is
clearly places that does not happen and because the mantle
part of the system is very big also because those minerals hold
a lot of water, a serpentine old something like fourteen, weight
percent water by mass, not really knowing how much water is
in that. Subducting mantle gives a really
big uncertainty into the system. Now that you've got this model,
how can you? So cool ground truth.
It two main ways we've been doing.
This one observation that we see is a layer that looks like
oceanic crust that has waves speeds.
They look similar to those that we see on the seafloor.
But underneath our 600 kilometers down.
And then this goes away. When we think we're is going
away is where these dehydration reactions occur, where the crust
reacts to form a much denser higher speed, minerals, and we
made these observations using a few different kinds of seismic
waves in thin layers. So we I think we can see the
depth distribution of water release, for instance, in
Cascadia, which is a very hot place.
We think we see this happening. Much shallower depths than we do
say under Alaska, where the incoming plate is colder,
subducting faster, and able to keep the water in the plate Down
Deeper. The second test.
We've been able to do is looking not at the speed of the ways but
sort of the amplitudes of the waves are more precisely how the
energy in the waves being absorbed as it travels through
the Rock the absorption because seismic attenuation is fairly
good function of temperature. As the temperature is especially
approached the melting temperature of the rocks and
what we observed is waves, propagate very efficiently, very
little absorption in the downgoing plate and in the upper
plate, when the diamond plate is less than about 70 or 80 km deep
and very quickly, underneath the volcanic Arc centers is very
high attenuation. Zone where we treat that too
much higher temperatures. So this gives us a little bit
more of a direct handle of testing, the thermal models
themselves that are underlying the mineralogy, then the water
release. We also, look at the depth of
earthquakes, turns out a lot of earthquakes inside these down
going plates. In fact, this is how they were
first recognized. There's a lot of arguments to
suggest the earthquakes are probably in some way due to the
water. Release, either directly or
maybe indirectly through increase in Pore pressure
because the Water is suddenly released in the system.
We've also seen good correlations between the depths
and the geometry of where earthquakes are and where the
thermal models predict the water really should happen.
So when you make this comparison, then for your
population of 50, odd, subduction zones.
Generally speaking, you're able to reproduce the seismic wave
speeds and the attenuation, that's observed by using
reasonable parameters. The reason I'm pushing on this
is there are those people who very skeptical of models that
you have enough. So whatever you see, you can put
together parameter set that will predict that.
And then the question is, have you learned anything?
Yes, as a great Point, our philosophy is been to try to
keep the modeling. No more complicated than it.
Absolutely has to be to explain these observations.
So keeping this in two Dimensions were tying, the
geometries things we know about, we can see from the seismicity
where the plates go. We know how fast the plates go.
We know a lot about the upper plates.
There are these complicated transitions.
The shallow part of the system is essentially a fault with one
rigid plate, going underneath, another one, as you get deeper
as things, warm up that fall transitions into probably a
ductile Shear Zone. We see these things Exposed on
Earth everywhere. We see deep rocks exposed and
then the transitions to this hot flowing viscous mantle wedge.
These kinds of transitions are very hard to replicate without
at least a little bit. Of numerical equation solving.
You could make different decisions but I don't think you
could replicate this whole Suite of observations were talking
about. But purely, simple analytical
models to get there. You can certainly make model
more complicated and there are people who do their people that
make up all sorts of other processes of blobs that fly
around and those, I suppose, help you imagine what might be
down there. But I feel like we are at a
point where these models are just complicated enough, can
capture Sure. The essence of the first order
things that are happening here. Let's come back to the
geological water cycle. What are we talking about?
In terms of quantities of water that we can subtract over
geological time? Yeah, well of course this is a
slow process. This isn't something that
happens overnight. So our numbers suggest that at
the trench we're seeing about the Notions worth of water
subducting in 1 billion, maybe 1.3 billion years assuming
everything stays steady. With today's rates, which is a
big. If if two-thirds of this is
coming out in arcs and for arcs, that means over the age of the
Earth. We've probably selected one
oceans worth of water into the mantle.
Wow. So is that more water in the
mantle normal water? In the oceans, is probably more
water in the mantle than in the oceans right now.
There are things that we know from the very small amounts of
water that do come out at mid-ocean ridge.
Judges tell us about the water and the uppermost mantle is more
water that comes out and hot spots like Hawaii.
These give us some Clues and there's a bunch of other
geochemical arguments but numbers that Jake has to look at
these director, indirect proxies for melting are at somewhere
between one and five or ten. Oceans worth of water are
sitting somewhere in the mantle, where exactly.
That's it seems good question. A lot of water.
Probably sits not in these hydrous minerals things like
Mike has and Clay's and serpentines.
But as hydrogen as defects and nominally anhydrous minerals,
you can hold it. High pressure, is a fair bit of
hydrogen, say in and Olivine, crystalline structure, the
transition zone between about 410 and 670 km depth.
As minerals, that seem to be very good at holding a lot of
water in their structure. So a lot of people think that's
probably the biggest sponge and Mantle to hold a lot of this
water just to clarify what you said earlier about observational
sampling of the matter, you said, at mid-ocean ridges.
We see the water that might be coming from relatively shallow
in the mantle, because that's where we think the material
comes from. But it hot spots, we're sampling
much deeper mantle potentially According to some people all the
way down to the core-mantle boundary.
So, it was that the point, you are making that we can actually
look at these two different geological structures and we can
therefore get a handle on. What water might be in there.
Hello, and the Deep mantle, that's right in mid-ocean ridge,
basalts around the global Ridge system, somewhere between 50 and
200 parts per million by weight of those rocks, are probably
water in the law, visit come out.
And that tells us about the water in the source region.
There seems to be more in hot spots and this is one of the
lines of evidence that people have used to suggest that the
hotspots source region may be over geologic time.
Fed by subduction because we're bringing all this water down and
subduction zones seems to be one of the main conveyors is, of
course debate about this. Because these are very deep
processes that are very hard to fingerprint, but that's at least
a self-consistent model for this very long-term geological water
cycle and the return flow. I'm intrigued by the fact that I
would you logical time, you're talking about subducting and
essentially locking up in the mantle of reservoir of the order
of the same amount of water that we have in the oceans today.
Would it be the case that if plate tectonics was just a bit
more? Vigorous we might have subducted
away the whole ocean. We might have a totally dry
Planet. That's an interesting question.
What happens as you go back in geologic, time or even slightly
different scenarios in terms of what plates are subducting.
When right now, we're in a setting where plates are on
average older than they were 100 million years ago when they were
subjecting, just because of how countenance of moved around,
Round and how ocean basins have formed.
So, even over a couple hundred million years, there may be some
pretty big variations in. How much water is getting into
the mantle. There are a bunch of competing
effects. The so if you subtract things
faster, your of course, bringing more water into the system, but
you're also then spreading at ridges faster and the plates
that hit, the trenches are younger.
So they're going to be hotter and if they're hotter, there's
less room in them to hold the Water.
So they may start with less water going in, so it's not
really clear. If just speeding up plate,
tectonics is going to increase or decrease this.
Net water flux secular cooling of the earth.
Over geologic. Time is also another important
factor, that's coupled to how fast plate tectonics Works.
Some people have done this modelling suggests that the
ability of the subducting mantle lithosphere to hold water and
serpentines, again, has to be colder than about 600 degrees C.
Before it's abducts, this is a relatively recent phenomenon,
over the four and a half billion years of Earth history and so
wasn't really, until that started.
You start to drag down a lot of water.
The key observation here though, is from what we can tell sea.
Level hasn't really vary a lot, maybe tens, maybe a couple
hundred meters over as much of geologic time, as we can tell
anything ocean basins are about four kilometers deep.
So, on average, this Hasn't radically changed.
Interesting question is whether we actually have a stable
equilibrium that? Yeah, I mean, it's hard to think
of what a proper feedback mechanism would be.
There's a lot of feedback mechanisms in the surface water
cycle, that help regulate various parts of it.
The very slow rates of things going into the mantle and what
happens at subduction zones versus what's happening at the
outgassing at ridges and plumes. It's Not so obvious.
Why? Or how these things could talk
to each other? And there could well, be an
imbalance over time. That is changing.
We just don't know this. So I think I'd be a little bit
cautious thinking, there's actually a feedback mechanism in
place, although maybe it's there and we just haven't figured out
how that could work. What are you working on?
At the moment lately, I've been very interested in taking the
same kinds of approaches. We've been using to look at this
sort of larger scale water cycle to look more closely at the
volcanoes themselves. And thinking about, can we see
better the Magma's where they actually form in the mantle that
flux through the crust, to create the volcanic?
Eruptions does this process happen?
Steadily or episodically are their observations that you can
make in the deeper parts of the crust or of the mantle that more
directly tied to eruptive Cycles in volcanoes.
We use these kinds of methods and tools deeper seismic Imaging
to better predict volcano Behavior but Could be extremely
useful if it gets as far as even Hazard prediction.
Yeah, that's a lot of the motivation.
As we understanding how these bigger systems work, then can we
use that understanding and some more useful ways to worry about
volcanic hazards? Understand also better things
like or affirmation other, aspects of things that volcanoes
do that are maybe a little bit more directly applicable to
human existence. Jeff Abus.
Thank you very much. Thank you.
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