This is Geology bites with Oliver Strimple.
Almost all erupted magma is initially generated in the
Earth's mantle as the melt forces its way up through the
many kilometres of lithosphere to the surface.
It usually pauses in one or more magma chambers or partially
melted mosh zones for periods of up to a few thousand years
before erupting. During these pauses, the magma
can separate into fractions, having different compositions
and also interact with the surrounding crust.
But while we have seismic evidence and models that support
this picture, we've not hitherto been able to watch how magma
actually moves in the upper mantle and crust.
Bob White has set out to change that.
Using a dense array of seismometers, he has been able
to pinpoint thousands of tiny earthquakes that reveal the
detailed movement of melt through the thick crust of
Iceland just before it erupted. He combines this seismic data
with geochemical analysis of the lava that can tell us about the
depths at which the melt is formed.
He is Emeritus Professor of Geophysics in the Department of
Earth Sciences at the University of Cambridge.
Bob White, welcome to Geology Bites.
Well, it's a pleasure to be with you.
And of course, this work was done not just by myself, but by
a whole team of graduate students and colleagues in
Iceland. Why did you pick Iceland?
For your detailed study of magma movement.
The Mid Ocean Ridge The Mid-Atlantic Ridge cuts right
through Iceland. So Iceland is a brilliant place
to study the processes where the plates are spreading apart and
generating a lot of volcanic rock.
And then how did you decide exactly where in Iceland to
focus on? Well, we started working in the
centre of Iceland. It's a place where absolutely
nobody lives. You can only get there in the
summer and that has a huge advantage that it's seismically
very quiet. There's no cars and lorries and
all the other noise that comes about where people live, so it's
very seismically quiet. It's also the most active
volcanic area in Iceland because it happens to lie over a mantle
hotspot underneath it. So it was a great place to watch
volcanoes doing what volcanoes do.
So as I mentioned, you exploited two types of data in your study,
seismic and geochemical. Let's talk about how these work
to give us detailed information on the movement of melt.
Starting with the seismic. By definition, melt is liquid,
so we can't be seeing earthquakes directly from the
melt. Indeed you can't.
That's correct. But the melt has formed very
deep in the upper part of the mantle.
The melt under Iceland probably starts forming at depths of
about 100 kilometres. And then it migrates up very
quickly until it hits the brittle part at the top or the
lithosphere. And once it gets there, it has
to get its way through to the surface.
It's buoyant, so it wants to go upwards, but it has to create
cracks to do that, to make a pathway for it.
So a crack is actually an earthquake, or rather, an
earthquake is a very big crack. And the molten rock, as it
pushes its way through the crust, creates many, many, many
tiny earthquakes, tiny cracks. So by mapping where those tiny
earthquakes are occurring, you're actually mapping where
the melt is moving. If it stalls, it tends not to
generate any earthquakes, but when it's on the move, it
generates lots to open a pathway for itself.
So to be able to watch in detail where the melt is going then you
need to track these micro quakes and you also need to know
exactly where they are. So how do you pinpoint the
location of these very faint earthquakes?
Yes, it's a technique called triangulation.
We helped a little bit in seismology about the fact there
are two different types of seismic wave.
One is called P waves or compression of waves.
The other is called shear waves or S waves.
And the S waves travel at about half the speed of the P waves,
but they come from the same spot, of course.
So actually, if you've only got 1 seismometer and you can
measure the P wave and the S wave, you at least know how far
away it is because you know the speed at which they travel, but
you don't know where it is. So you still need a number of
seismometers. And these are very, very
sensitive instruments recording the motion of the earth in 3
dimensions vertically and two horizontal directions.
So you can use that to work out how long it took for the
earthquake wave to reach you, and also actually the direction
of travel, because the wavefront hitting the seismometer can tell
you where it's coming from. So you have quite a lot of
information. But the more the merrier, the
more seismometers you can get, the better the location.
And then at the end of the day, roughly how precise a location
can you get? Well, the thing you don't know
is exactly how fast the seismic waves travel through the Earth.
That gives some uncertainty. And also if there's any noise on
the arrival that can perturb it. So in absolute terms, we
probably know the location to 100 meters or so.
But it turns out that if you've got a lot of earthquakes in
essentially the same place, which is what you get at the tip
of a crack as it's propagating forward, as the melt moves
forward, you can do rather better at calculating the
relative locations of those different earthquakes, because
if you imagine it, the energy from each earthquake is
travelling through the same heterogeneous crust to your
receiver every time. So if you've got several
earthquakes essentially in the same place, then you can cancel
out all that heterogeneity and just look at the relative
locations between them. And we can get that down to
about 10 metres, which is remarkable really because we can
look at these tiny earthquakes down at six, 810 kilometres
depth and know where they are to within 10 metres or so.
I imagine you have to get relatively close to these
earthquakes in order to be able to get that level of accuracy.
So when I asked you why you picked Iceland and where exactly
in Iceland, you said, well, the centre of Iceland is free of
people and it's more secure and so on.
But presumably you had to narrow it down way more than that in
order to get close enough. So were you able to predict
where you think the melt was going to move before a
particular eruption, or did you just get lucky?
We got lucky in one respect, but we also had an array out over
quite a big area, so it was over 100 kilometres square, something
like that. If the earthquake's big enough
and you can record it across the whole array, which might span
100 kilometres, then you can still locate it very accurately.
If it's a tiny earthquake, you need to be a bit closer to it
and ideally you want run receiver more or less directly
above it because that's what controls the depth, which is
probably the biggest uncertainty.
But we had a regional array out in the middle of Iceland.
We've been operating one there for the last 15 years or so
because it is very active. And we've actually caught a
couple of other small injections of melt into the crust before.
The one I'm going to talk about, the big one in 2014, and on this
occasion in 2014, we'd actually put our extra seismometers.
We had more than we'd ever had before.
We had more than 70 of these seismometers arrayed across an
area because what we're planning to do was to find out where the
melt was residing under another active volcano called ASCIA.
Now that's not the one that erupted, but it was adjacent to
it, and we knew it had molten rock underneath it, and we were
going to use tomography. And the way that worked in
medical systems is that you have a very high frequency sound wave
and you bounce it off your internal organs to look for
tumors or babies as they develop in pregnant women.
And you can reconstruct an image of what's going on inside your
body with tomography. Well, we just pinched the same
idea. We have a much longer
wavelength. The wavelength of the seismic
waves is about one kilometre or at the very best several 100
metres. So our resolution is much poorer
and we can't generate sound at the surface unless we let off
explosives, which occasionally we do, but people are not very
keen on you doing that these days.
So mostly we use the earthquake source itself, the energy from
the earthquake to make the energy source, and then we
record how it travels through the earth and we can invert all
that information to do tomography.
And particularly we can see where molten rock is because one
of those two types of waves, I explained the shear waves don't
go through molten material, the compression of waves do.
So I'm talking to you now by compression of waves, which is
going through the air. But shear waves would not go
through the air. So it's rather a good technique
for seeing where molten rock is. And we were hoping to image
molten rock underneath this volcano, Askia, which we have
since done. It's about 1 kilometre depth,
some of it, but it meant we had a big array out when this big
eruption in 2014 happened. So we were very fortunate and we
actually had about a dozen spare seismometers that we just
brought off the glacier where we're doing another experiment.
And we quickly deployed them where we thought the eruption
would occur. And we thought the eruption
would occur in the place where the overweight of the rocks
above it was leased now at any one place that's in a valley,
not under a mountain, because the weight of the mountain
creates greater pressure at a given depth.
So we quickly put these out in the valley where we thought it
would erupt. And indeed it did.
So close, in fact, that we had to rescue two of the
seismometers shortly after the eruption started because it
arrived in the path of the lava. So we had amazing data.
Yeah, we're in the right place at the right time, but it was
for another reason originally. Was this eruption associated in
any way with what's going on in the Rakhines peninsula at the
moment near Reykjavik? The 2014 eruption wasn't no,
it's completely separate. There's so much going on in
Iceland at the moment. As you rightly say, Reykjavik is
erupting as we speak. Last time it did it about 800
years ago. The eruptions went on
intermittently for about 30 years, so I think it's going to
carry on for a long time. But the ones in the middle of
Iceland erupt less often, but probably more voluminous.
They're bigger eruptions. When you say you measure
hundreds or thousands of micro quakes, how micro are they and
how do they compare to the earthquakes that make the news
that create damage? They're tiny, really tiny, and
we record 50 to 100,000 micro earthquakes a year in the centre
of Iceland. But if you take a magnitude 6
earthquake is one that makes the news, generally 6 or bigger.
These are magnitude one. Now the magnitude scale.
It's not linear, it's logarithmic.
And in energy terms, every time you go down one number, that's a
decrease in energy of about a factor of 30.
So a magnitude one is about 24,000,000 times less energy
than a magnitude 6. And the magnitude 6 that you
would be worried about, houses falling down and people getting
hurt and so on, breaks the surface of the Earth for
distances of 10 or 10s of kilometres typically, and moves
a few metres. That's the sort of scale of it.
The ones of magnitude one that we're talking about are perhaps
a metre long and they move perhaps a centimetre or
something like that. So much, much smaller scale, but
exactly the same process. It's just the ground cracking,
so very small. In fact we get a lot smaller
because it's a logarithmic scale.
You go to negative numbers, oddly, so we get down to
magnitude -1. We can measure down at depths of
10 kilometres. So they are very sensitive
instruments and it's a very quiet location.
Let's talk a little about the geochemistry of the lava.
What exactly can we measure, and what can that tell us about the
depths at which the melt formed and fractionated on the way out?
Yes, well, there's two different things where it forms, We can do
quite well at because we can look at the major element
compositions. The deeper it forms in the
earth, the more magnesium there is in it, for example.
And also there are things called rare earth elements, which don't
react with anything, but they come out of the solid into the
molten rock with different affinities.
So at different depths, different rare earths come out
into the melt. And they have names like
neodymium samarium. But the beauty of them is that
they don't react with anything. And so once they're in the melt,
they just get carried along with it.
And so once it's erupted at the surface, you can look at the
rare earth element composition, work out how deep it was
generated in the mantle, which is a really powerful tool.
Whereas the other elements, the more common ones that we're used
to like magnesium and iron and calcium, all these so-called
major elements, as they come up, they react with the country rock
which pollutes them. They fractionate out and
different things fractionate out with different affinities.
And so the difficulty with geochemistry is that you can
only ever measure it on the rock that's come out the surface.
And as you alluded to, it may have stalled on the way at
several different depths. And that can be a problem
because it usually gets over printed at the last depth it
stopped at. So you can usually tell quite
well the depth where it ponded just before it erupted.
But even that can be difficult because it reacts as it erupts.
And if you haven't got a very fresh sample, it's reacted with
the air when it comes out. Now the rare earths, because
they just fractionate along with everything else, they're OK.
They don't change their relative concentration.
So we can tell how deep it formed.
A bit harder to tell how deep it stalled on the way up.
If you're lucky, you can do it. And what you have to do is find
some elements which are in equilibrium with each other at
some given depth and hope that they've come out at the surface
quite quickly without reacting with other things.
So then you can work out at what depth those three and things
would be in equilibrium. And so I can give you some idea
of the depth, but not nearly so precise.
If you can do it with a few kilometres, you're doing well
for the depth, whereas we can tell much more accurately than
that with the seismology. So can you then describe what
your study actually showed about the path taken by the melt in
the instance that you just described in 2014?
Yes, 2014 was a fascinating eruption because it's the
biggest historic eruption in ice and since 1783 I think it was,
it was 1.8 cubic kilometres of rock, which is a huge amount of
rock. It would cover most of our
city's many hundreds of meters deep and it lasted 6 months.
But what was interesting about this one was that it came from a
volcano which was underneath the big ice cap in Iceland called
Vatniokol, which is the biggest ice cap in the Northern
hemisphere, I think biggest temperate ice cap I should say.
And it is precisely there because it's over this mantle
hotspot underneath. So it's actually the most
elevated region of Iceland, which is why it's got ice on top
of it, but it's also the hottest piece of mantle underneath, so
it's where more melt is generated than anywhere else.
So it's a funny Juxta vision. It's the hottest mantle bit and
the coldest at the surface. So the volcano is called Bada
Bunga and it's been active for thousands of years.
And what would normally happen is that the melt will come up
straight through the glacier, through the cold area.
It's buried by about 800 meters of ice, and you'd expect the
melt to come straight out the top of the volcano, as most
volcanoes do. Now.
In this case, it didn't do that at about 6 kilometres depth.
It started moving laterally sideways about 6 kilometres
under the surface of the Earth, and it did so for nearly 50
kilometres before it erupted. And the way it moved was it was
moving away from this high ice cap and it erupted just at a low
point before the topography started increasing up towards
the next volcano along which was called Ascia.
So it was a fascinating thing to happen to go sideways 50
kilometres and a bit scary because, you know, if you live
50 kilometres from an active volcano, you might think, well,
that's fine, but maybe lava can pop up underneath your feet.
Now actually, it's not completely unusual because in
Hawaii that is exactly what happens.
And there was a case a few years ago where the lava was flowing
laterally from the high point, the Kilauea Caldera on Big
Island of Hawaii, and then went laterally 10s of kilometers
before it erupted and it erupted under a Township on the coast.
Literally a crack opened in the streets and molten rock started
coming up and actually they had to evacuate.
And I don't think they can go back now.
Even so, lava does flow sideways, and I think it would
do it if the pressure gradient is easier for it to go sideways
than to keep going up another 6 kilometers of overburden of
rock. And then it erupts, usually in a
low point where the pressure above it is the least.
And so that's exactly what happened in our case.
But interestingly, some of my students were out there when the
first eruption started in 2014 and they've been putting out
these seismometers and they actually saw the eruption start
and they said it came up through the same craters as have been
used in the previous big eruption there in the mid 19th
century, actually reoccupied them.
So clearly there was a pathway down at six kilometers depth
which the lava was reusing, flowing sideways.
So in that respect it wasn't surprising because that's
probably an easier way for it to go.
There was lava route which would have been blocked up with lava,
but lava contracts a bit when it cools, so there's there's
probably some void space and it's easier to go there than to
breakthrough virgin rock above it.
So yeah, it was a fascinating story, and it took about two
weeks for the lava to propagate that distance of 50 kilometers.
About what speed does that correspond to?
We found it went in burst, and this was one of the things we've
discovered by seeing it actually in the act of propagating,
because of course, usually you just get geological exposures,
you just get the rock exposed that service and you can't tell
the history of it. This would propagate forward in
bursts that would propagate a few kilometers forward at a
speed of between 1:00 and 4:00 kilometers per hour.
So about the speed, you can walk comfortably if it had been on
the surface and that's very typical, but we found it would
propagate sideways and then it would stall for a few days up to
a week and then it would have a burst of propagating forward
again. And this is happening and we
think because it has a pressure head behind it of the molten
rock in the reservoir and the the volcano where it's coming
from. And as it propagates forward, it
uses a lot of energy of course to crack its way forward and it
uses up that pressure head. And then you have to stop and
wait for more melt to flow in and build up the pressure again
before it could break its way forward the next bit.
But once it's broken its way forward, it can flow quite
freely. And interestingly, you could see
that the seismicity, the earthquakes, were concentrated
in a few kilometers just to the front of the propagation.
Once it opened up a passageway, no more earthquakes.
And you can estimate how big the passage was, and we estimate it
was roughly 10 meters diameter, which is 30 foot.
You know, it's pretty reasonably large for molten rock to be
flowing along. And when this finally broke
surface started erupting out the surface, you could see the
reservoir dropping down in Bardabunga, the parent volcano,
and it dropped something like 65 meters over that six months that
it was erupting. You know the size of the
caldera, so you know how far it dropped.
So you can estimate the volume of how much was evacuated.
And that all pretty well matched, one for one, how much
we know was erupted at the other end out of the final eruption
site. So it's a very simple system
actually, you know, you sort of evacuate one end and it spurts
out the other, just flows laterally in between.
But watching how it flowed was fascinating.
And to be able to document that. Just coming back to the question
of what depth it either originated or where it last
paused on its way up. So if it evacuated A caldera,
was that just coming from 1 shallow location to another or
were you able to see that it had actually pondered at some
greater depth? We don't know for sure what
depth it's ponding at. We think it's probably ponding
at about 6 to 8 kilometers under that volcano.
But what we need to do is to do tomography on that volcano to
see where the molten rock is. And it's obviously a huge, great
reservoir, so we should be able to see it quite easily.
With the exception it's quite hard to work on a glacier.
It's a temperate glacier that accumulates several meters of
snow every winter and melts back several meters every summer, so
it's not an easy place to work. But that's precisely what one of
my ex students is doing. He's going off to put an array
of seismometers right over the top of this Badamunga volcano to
actually map what's happening directly underneath and see if
he can image this storage area. Eruptions in Iceland come in
various styles. They're explosive eruptions,
most famously in recent times being the one that blasted out
enough ash to bring most of European aviation to a
standstill. You're going to tell me the name
of the volcano? I'm waiting for you to tell me
that. AF Yachtly Yes.
It was a source of great amusement in Britain with the
news readers trying to read it. But other eruptions, such as the
one going on at the moment in the Rakhiness Peninsula, emerge
as fountains of lava along fissures.
What is it that causes the eruption style to vary so much
from one place to another in Iceland over just a few 10s of
kilometres? It's whether it has a lot of ice
above it, because that eruption in 2010 from Ayef Yerkla Yoko
was under an ice cap of several 100 meters at the top of the
volcano. And so you can imagine that
molten rock, which is at a temperature of 1100°C.
It hits the ice as it comes out of the rock and melts.
It makes a big pond of water under the ice, and then
subsequent melt coming in at the same temperature will promptly
freeze very quickly in this water.
And what happens when you freeze molten rock is you make glass.
Actually, the way you make glass is to make molten silica and
freeze it. So you make this massive glass,
but there's a lot of gas in the molten rock as well, and so it
actually makes lots of tiny bubbles of glass.
And then the glass is hot, so the glass expands and these
little bubbles expand and they burst.
So you get millions and millions of tiny shards of glass, which
then can erupt out the top because it's trying to expand.
So it just blows its way out the top.
And that's basically what the ash was of the explosive
eruptions. It was tiny, tiny shards of
glass coming up to, in that case, went up to 35,000 feet and
then drifted down over Europe. So it's very nasty, carcinogenic
stuff. You don't really want to breathe
it, but it's not very good for jet engines either.
I've seen a picture of a plane that flew through an ash cloud
and it just stripped off all the paint.
So that's the difference really. Most of the volcanoes start in a
fissure, a crack which opens. They often then just concentrate
into a crater, but if it's under a glacier, it can be explosive.
The other thing that can make it explosive is that if there's
some melt hanging around under that glacier that's been there
for a few 100 years and it's fractionated, changed its
composition, and then some fresh melt comes in which is rich in
volatiles, it can hit the old melt and that can cause a very
explosive eruption. And that happened under a FIACO
in 2010 as well. So yeah, there's a couple of
reasons they can be explosive, but in general they're much more
calm eruptions than the ones around subduction zones where
plates are pushing together, which really are dangerous
because you don't get much warning, and there's a lot of
volatiles from the melt, from the rocks that have been carried
down in the subduction zone. So is volatiles expanding that
makes it so explosive. In those cases, there are
volatiles in the Icelandic ones, but not so much, so they're
generally not so explosive. Let's step back and talk a bit
about what causes all the volcanism that we see in
Iceland. And as you mentioned, it's
generally thought that there's a hot spot or a plume of
anomalously hot mantle below Iceland.
And that in addition, you mentioned there's the
Mid-Atlantic spreading Ridge, which runs right through it.
So where does the location you studied in 2014 lie with respect
to these structures? Well, it's pretty well over the
top of the mantle plume underneath, which probably has a
diameter of about 100 kilometres.
They're not particularly big things as they rise up from
deep. They get deflected into a big
mushroom head when they hit the bottom of the lithosphere, the
rigid part, But we were right over the central part of it, so
the hottest part of it if you like.
And because the melts are generated so deep, they are, as
you said, relatively low viscosity, so they can flow
easily. They're high magnesium, high
iron, and it flows compared to the subduction zone ones, which
have much more silica in them, which makes them much more
viscous, much more sticky. So they don't flow very far, but
they instead they explode. So as I mentioned in the
introduction, almost all the erupted melt originates in the
mantle at least so we think. So what is it about this
particular location that causes the mantle to melt there and not
in other places? If you go back to what causes
melting in the 1st place, The mantle of the Earth under the
rigid outer layer, which is a lithosphere.
Typically that's 100 kilometres thick on an old stable
continent. The mantle is actually solid,
despite some people think it's molten, but it's basically solid
and it has a very high viscosity.
It does flow, but pretty slowly on a human time scale.
But if you take a piece of that mantle and bring it to the
surface very quickly, it melts as it decompresses.
Now the reason for that is it's sitting quite close to the
melting point of mantle rocks when it's down at 100 kilometres
depth. It's called the solidus.
It's where you change from solid to liquid, but as the pressure
decreases as you get nearer the surface, because there's
tremendous pressure. Of course at 100 kilometres
depth, as you come nearer the surface the pressure decreases
until your atmospheric pressure at the surface and at the same
time the melting point decreases by quite a significant amount.
So if you take a parcel of mantle and move it up very
quickly, it will cross this solidus because at its
temperature it won't have time to lose heat by conduction
because that's quite slow. So it retains its heat, but it
comes to an area where the melting point is much lower and
so it melts instantaneously. Actually, you don't have to pump
heat in, it just melts because the melting point is lower, so
it's crossed that phase change from solid to liquid.
So that's exactly what happens under normal mid ocean ridges.
You're pulling the plates apart and so the thickness is
essentially going to zero at the rift in the middle.
So the mantle can move very close to the surface and it
comes up very quickly because the plates are spreading quite
fast, between 20 and 150 millimetres a year.
And on a geological time scale that's very fast.
So the mantle doesn't have time to cool down as it rises, so it
retains its temperature, but it's crossed the solid US.
It's now hotter than the melting point at that low pressure, so
it melts and mid ocean ridges. You generate about 7 kilometres
of melt, which is very buoyant forms of crust, and that's the
crust of the oceans, typically 7 kilometres thick.
Now under a mantle plume you get mantle plumes essentially
because the center of the Earth is very hot, still trying to
lose heat, can't lose it fast enough by conduction.
So you get blobs of material coming up and then in training
material behind them, which is very hot.
Bit like those old lava lamps that were so popular in the 60s,
and mantle plumes just like that.
You get a big BLOB which rises up and it pulls the tail up
behind it of material which is hotter than the surrounding
mantle by not a huge amount, 150° or 200°, something like
that. So when you stretch that and
allow it to decompress then it will generate a lot more melt
because you're sitting so close to the melting point at depth.
So instead of it melting when it gets up to about 30 or 40
kilometres depth as you do under mid ocean ridges, it'll start
melting at 80 or 100 kilometres instead.
And so that all accumulates and under Iceland you generate about
40 kilometres thick crust above the spreading centre, even
though it's just a rift, just like the bit further South in
the oceanic crust. So instead of 7 kilometres you
get 40 kilometres and that's why you get lots of volcanoes there.
Now most of the melt doesn't come out as a lava flow, most of
it is just frozen in the crust. We only see a small percentage
of it. So there is a great deal
generated directly above the top of the mantle plume and then
that tails off. So as you move down towards the
oceanic crust from the centre of Iceland, it decreases from 40
kilometres to about 14 kilometres just on the
Raikkonen's peninsula. And then you keep going offshore
and it gets thinner and thinner till you get back to the normal
7 kilometres. But it takes about 1000
kilometres before it gets back to that normal temperature.
Why is there such a huge variation in the amount of melt
that you get in a place like Iceland, say, as opposed to say,
Hawaii, the mid ocean hotspot volcano where you're not getting
that kind of volume at all? That's a very good question
because the difference is that Hawaii is in the middle of the
Pacific plate, which is about 70 kilometers thick there.
And so this rising material from deep in the Earth, which is very
hot, actually probably hotter than Iceland.
It gets blocked by the rigid lithosphere above it at about 70
kilometers depth. So you can generate melt from
when it crosses A solidus at maybe 100 kilometers depth, up
to that depth of about 70 kilometers.
But after that it gets deflected sideways, and once it's going
sideways, then there's no more decompression.
Of course, it's at the same pressure, and so you don't
generate any more melt, so you only generate it directly above
the rising plume because there's no rift across the middle of
Hawaii. And so the rate of melt
generation is much decreased because of that thick
lithosphere plate it's sitting on.
And there are other volcanoes under continental crust as well.
In North Africa there's quite a number of hotspots which run the
continental crust where the lithosphere is even thicker and
so you get less melting still. So it all depends on how thick
the lithosphere is and if there's a mid ocean Ridge
spreading above it, then it wants to stretch to 0 because
it's spreading apart about 20 millimetres a year in Iceland
and that's why so much melt is generated there.
So it's much more to do with the thickness of the lithosphere
that happens to be encountered where the plume reaches the
lithosphere, rather than variations in temperature of the
plume from one plume to another. Well, it's both things.
Of course. I think the temperatures
probably do vary a bit, but they probably don't vary hugely,
because in order to go unstable, the core of the earth is very
hot, the mantle's much cooler, and you get what's called a
boundary layer between it, where you go from the hot to the
cooler. And to make that boundary layer
go unstable, you need a certain temperature difference, and so
probably most of the plumes are pretty similar temperature.
So the main control, yes, is the thickness of the lithosphere
when the mantle plume impinges on the lithosphere.
And it's not quite happenstance that it's impinging under the
spreading centre under Iceland, because the spreading centre
where it's pulling apart likes to be in the lowest energy
state. And if it's above an elevated
mantle plume, because it's hot, it elevates.
It's more buoyant, so it elevates the Earth's surface a
bit there, and so it's easier for the rifting to occur above
that point than it is off the side of the plume.
And so actually, we can see that the spreading axis has actually
been tracking the plume eastwards across Iceland.
And there's a big dog leg as you come from the Atlantic in the
South. There's a big dog leg to the
east across the top of the plume, and then a dog leg to the
West again to the spreading as you go up towards the Arctic.
And so the spreading centre is trying to keep itself above the
plume which is drifting eastwards with respect to the
spreading centre. Eventually it will give up and
it will jump back to just go straight through Iceland, and we
can see that's happened in places like Tristandacuna in the
South Atlantic. Tristandacuna is off on the east
side of the Mid-Atlantic Ridge, but originally the hotspot was
sitting right underneath it and it made a big underwater Ridge
which you can still see underwater, called the Walvis
Ridge on one side and the Rio Grande Rise on the other.
So you can actually see when the spreading centre stopped
tracking the mantle plume and then gave up, and then Tristan
de Kuna above the mantle plume is still carrying on moving
eastward. We know that continental rifting
often accompanies the arrival of a mantle plume and the
emplacement of a large igneous province.
As Richard Ernst talked about in a recent Geology Bias Podcast,
Is it the thickness of the crust and the consequent volume of
milk produced that determines whether rifting will occur at a
given plume? It's a really good chicken and
egg question, isn't it? Did the plume cause the
continental breakup, or did the continental breakup allow the
plume to rise? Personally, I think the answer
is that the plume, which originates deep in the Earth, at
least 650 kilometers but probably much deeper than that
at the core mantle boundary, doesn't know what's going on
with the plates sipping around on the surface of the Earth.
It doesn't really have any knowledge of that.
There's no direct connection between them, and so where it
comes up is not controlled really by what's above it.
But you can imagine that if a plume does come up underneath a
continent which is predisposed to be stretched of it, it's
intention for some reason, then the extra elevation of the plume
material underneath the lithosphere, which will give it
an elevation of two or three kilometres.
Quite a lot of potential energy in that elevation might give it
the impetus to finally break. And then when it does finally
break, then of course you've thinned the lithosphere and so
you allowed that mantle plume material to rise right up near
the surface and create lots of melt, as we were just
discussing. So it is slightly chicken and
egg, but I think it's likely that if a plume hits underneath
a continental block which is predisposed to break up, it will
give it the finest push and then it will generate as you said
huge volumes of melt, maybe 10 million cubic kilometers in the
space of a million years or so. So it's a huge rate of
production. But clearly we know sometimes
plumes do arrive under continental crust and don't
cause break up. So as I said in North Africa,
there's several plumes which we can identify which have not
caused breakup in North Africa. But the one that hits on what is
now the West Coast of Africa caused breakup between South
America and Africa. But again that was predisposed
to break. There was there was some rifting
in the north from the North Atlantic and from the South.
So the plume gave it the final push and then it generated loads
of melt. What are you working on at the
moment? Well, you could guess, couldn't
you, which on Reykjiani's peninsula.
There's 20 or 30 seismometers deployed on Reykjiani's
peninsula now. And we're tracking all the
multiple dikes and melt intrusions that are happening
there with unprecedented detail, actually, because it's so easy
to get to. You know, it's only an hour's
drive from Reykjavik. And so something changes.
You can nip out and fix it. The instruments in the interior
of Iceland I was talking about before, you can only really get
there in the summer for two months because it's -20° in the
winter covered with ice. You literally can't get your
instruments out the ground because they're frozen in so
hard. You'd have to use pickaxes.
So Ray Connis is going to produce all sorts of new
information, but we are still working in the interior.
We've still got an array there because there's a lot of benefit
to having a long term array and we're still doing that
tomography we set off to do in 2014.
In fact, just this week actually as I as I speak to you, a paper
has been published on tomography under Askia where we can see
melt at one kilometre depth and also at six kilometres depth in
at least two storage areas. And it is inflating.
We know from GPS measurements it's gone up getting on for a
metre now over the past couple of years.
There's always something going on in Iceland.
Bob White, thank you very much. It's a pleasure.
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