This is Geology Bites with Oliver Strimple.
One of my first geological field trips was to Sicker Point on the
southeast coast of Scotland, the site of James Hutton's famous On
Conformity. The underlying rock there is a
440 million year old marine turbidite and I soon discovered
similar looking turbidites on other trips from Greenland to
South Georgia. I was struck by their ubiquity
in the geological record. So how do they?
Form and why they're so widespread.
In the record. The answer lies on the sea floor
in one of the most powerful and least witnessed phenomena in
geology. Turbidity currents, massive
underwater avalanches of sediment that can travel
hundreds of kilometres, snap cables and reshape the deep
ocean floor in a matter of hours.
In 2006, a single earthquake off Taiwan triggered a cascade of
turbidity currents that sequentially severed more than
20 undersea cables, cutting Internet and phone connections
across Southeast Asia for nearly two months.
And yet, because these events happen in the deep ocean, often
without warning, we've rarely observed them directly.
Today's guest is one of the few researchers who has, sometimes
more directly than she'd have liked.
Esther Sumner is an associate professor of geology and
geophysics at the University of Southampton.
She studies turbidity currents by combining direct sea floor
monitoring with laboratory experiments and field studies of
ancient turbidite outcrops. In our conversation, she
describes what it's like to instrument an active submarine
Canyon, what these flows have revealed about the way sediment
moves across the sea floor, and the day her team accidentally
flew an underwater robot into a live turbidity current.
Esther Sumner, welcome to GEOLOGY BITES.
Thank you very much for inviting me.
So what exactly are turbidites? Turbidites are the deposits of a
type of sea floor avalanche called a turbidity current, and
so turbidity currents are the same family as flows, as things
like snow avalanches and pyroclastic flows.
So that gives you an idea of what they look like.
But the types of materials that are there carrying anything from
clay sized material to sand sized material and even up to
boulders and bigger. And these are really big flows
that carry lots of sediments. They can travel big distances,
so hundreds, sometimes even in excess of 1000 kilometers and
they can have a range of durations that but that can be
hours or in some cases even days.
So when we look at a rock in the field, how can we distinguish A
turbidite from other kinds of marine sedimentary rocks?
Generally, if you're looking at a sort of sequence of
turbidites, so a turbidite succession, you're going to be
looking at sequence of interbedded sandstones and mud
stones. Now if we're thinking about an
individual turbidity current events or an individual
turbidite, then I guess classically we would identify
this as a normally graded sand with a mud cap.
So normal grading being big grains at the bottom, small
grains at the top, and then a characteristic series of
sedimentary structures with structureless sand at the bottom
overlaying by plain laminator sand oberlain by ripple scale
cross laminated sand. I think in reality actually it's
quite rare to see exactly that and deposits are much more
varied. And that really probably
reflects that that deposit would form if we had a very simple
dilute decelerating flow and that's just not always the case.
What triggers A turbidity flow? So my simple answer would be
that we know that things like earthquakes can trigger
turbidity currents, big storms. So if you've got a big storm,
you might have storm waves basically pounding the
continental shelf, re suspending sediment that can form a
turbidity current. River discharge can cause
turbidity currents, especially at for example, deltas.
So there it might be that the sort of sediment laden river
water is actually sort of plunging down into the ocean
forming A turbidity current, or it might be that the delta
front, which is relatively steep area collapses to form a
turbidity current. Pyroclastic flows entering the
sea can generate turbidity currents and so and things like
fishing. So if there's human impacts like
trawling in an area, basically anything where you're
destabilizing some sediment in water can form a turbidity
current. I think I should probably make
the point that just because you have one of these triggers
doesn't mean you'll get a turbidity current.
And actually many times when we're measuring turbidity
currents, we don't really understand what triggered them.
And so I think what we're starting to understand is what
might be equally important or or maybe even more important than
triggers are actually preconditioning.
So preconditioning often just means basically what sediment is
there available to fail in the event of one of these triggers
happening. So you can imagine, let's say
you have an earthquake, but there isn't a lot of sediment
available to be destabilized, then you won't necessarily
generate A turbidity current. So I guess in a simple way is
this combination of preconditioning combined with
some kind of trigger? Is there a typical?
Setting where turbidity currents happen, like on continental
shells. Or on the flanks.
Of seamounts. So in terms of sort of settings,
so anywhere really where you've got a sediment water mixture
going into another body of water, so you can form turbidity
currents in lakes, you can form them in fields, you can form
them in the oceans. And I think most commonly we
think about them occurring in the oceans and there really they
can flow all the way from the coastlines to the continental
shelves right out into the deep sea.
I think one of the interesting things about this is that they
can actually do this on remarkably low gradients.
So when we talk about slopes in the ocean, these are normally
actually really quite pathetic slopes.
So let's say we're in a submarine Canyon.
There's probably a slope of say 2°, maybe a little more, a
little less. If we exit that Canyon in deeper
waters, so we go out onto a basin plain, you're looking at a
slope of 1° or even much less than a degree, maybe half a
degree or less. So I often say to people, well,
as far as I'm aware, something like a Premier League football
pitch has that kind of slope and over say half a degree for
drainage. And we would think of that as
flat. But these are the kind of Flopes
that turbidity currents can travel along, and they can
travel great distances, maybe hundreds of kilometers on those
slopes, going at meters per second.
That's interesting. So I guess this is all because
these flows are happening underwater where because of the
buoyancy effect, you can think of the gravity as being much
lower. And so that can enable them to
move down very gentle gradients and also have these much longer
durations that you mentioned. Yeah.
So certainly that has to play a part.
You've got your sediment particles and they're settling
in water. If we put the same particles in
air, they fall out more quickly. And so the thing that's driving
these flows is the density difference between the flow and
the surrounding ambient fluid, in our case seawater.
And so that's one part of the story, but I think there's more
to the story. So there's some really
interesting recent work showing that some turbidity currents can
reach a steady state, and so really steady state, meaning a
more or less constant velocity. And so when they're doing that,
they must be achieving a more or less constant density because
that's what's driving them. And the way that we image them
doing this is that the flows that do this, at the front of
the flow, there's a dense near bed layer.
And that dense layer seems to be able to erode sediment into
itself. So it's effectively feeding the
flow. And then two things can happen.
If it keeps feeding itself, your flow is going to speed up and
get faster. But what often seems to happen
is it must then Chuck some of that sediment into the rear
parts of the flow where it can get deposited out.
And so the flow as a whole is maintaining this constant
density and therefore constant velocity.
And so the long durations somewhat an effect of that
probably that you reach this constant velocity.
And then the other thing that's happening is that flow head
tends to accelerate away from the body of the flow.
So your flow stretches out and gets longer.
So through time, it's quite weird, but through time, the
length of your turbidity current is getting longer and longer.
I mean, this gets called auto suspension.
It's actually something which was sort of theorised in the
1960s, nineteen 80s, that actually very recently we have
data from direct monitoring of tubidity currents that suggests
it does indeed happen and might be quite an important reason for
how they can travel such long distances.
And when we're talking about distances and lateral spatial
scales, what are we talking about?
In terms of distances travelled, so I mean, it depends on your
turbidity current, right? Not all of them go these great
distances and that's quite interesting in itself.
But the really long duration flows can be travelling hundreds
or in excess of 1000 kilometers of distance.
How far they spread laterally really depends on their setting.
So if they're combined within a submarine Canyon, they can't
spread laterally. Once you get out of the Canyon,
out of the mouth of the Canyon onto a submarine fan where they
can spread out, then it's somewhat going to depend
actually on the sort of characteristics of that
submarine fan. Because even in that setting you
have some confinement from channels, but individual flows
could be many kilometres wide. So these are big, big flows.
And how long can they last? So in terms of durations, when
we're monitoring them, I think we have to say that the typical
duration for a turbidity current is on the scale of hours.
But then in certain settings, so Congo Canyon off West Africa
being one of them, turbidity grants have been monitored for
much, much longer durations, so days or even in excess of weeks
in duration. So really long duration flows
and these are the flows that are going really long distances.
You mentioned the flows off the mouth of the Congo River.
In fact, there's a giant submarine fan in the Bay of
Bengal at the mouth of the Ganges River, which I suppose is
filling up with sediment eroded down from the Himalayas.
Are those kind of huge submarine fans also built up by repeated
turbidity currents? Yes, yes, absolutely.
Basically you've got the aggregation from lots of
turbidity currents over long time periods.
So with something like that system, you've got the Ganges
and other rivers, which are effectively feeding sediment
into your coastline. And then that sediment ends up
in submarine canyons. So submarine canyons are
basically just giant canyons on the sea floor.
So you imagine a Canyon on land, they look somewhat like that,
often on a slightly larger scale.
So your sediment ends up in your Canyon, gets destabilised, forms
the turbidity current. When that turbidity current is
in that Canyon, it's quite an efficient place for it to be.
It's confined, so it keeps going.
Once it reaches that Canyon mouth, suddenly it's unconfined
and it can spread out and so it slows down, it loses energy and
so starts depositing its sediment load.
And so you end up with this basically cone shaped pile of
sediment at your Canyon mouth called the submarine fan.
And then this is forming over really long time scales.
So you're looking at millions, if not 10s of millions of years.
So lots and lots of turbidity currents stacked up.
And these are huge volumes of sediment.
They're some of the largest sediment accumulations on Earth.
So the Bengal fan that you mentioned, I mean, this is in
excess of 3000 kilometers long, 1000 kilometers wide and in
places greater than 15 kilometers thick.
So that's a massive amount of sediment.
And then you're saying, you know, this sediment comes from
weathering of the Himalayas. And that's quite interesting in
itself, that this big pile of sediment on the sea floor is
basically an archive of how the Himalayas have eroded in
response to changing tectonics and climate.
So yeah, they're just really interesting, basically archives
of sort of tectonic climate histories in the past.
But I think if we think about in the present day, one of the
things that turbidity currents might be doing is feeding
pollutants to that submarine fan.
So if we've got pollutants mixed up with our sediments on the
continental shelf, and those might be things like plastics or
pesticides, they also have the possibility of making the way
all the way out to the submarine fan.
Let's talk about how you go about studying turbidity
currents, both with your experimental modelling and with
your work in the field. I guess I'll start with the
field. I may be a little bit weird
because when I talk about the field, sometimes I'm talking
about on land and sometimes I'm talking about field work at sea.
So on land we're talking about sort of hiking up mountains and
looking at outcrops on the sea floor.
We can do something really similar by taking sediment.
Cause so basically we throw a big metal tube with a weight on
it at the sea floor. It sinks into those sort of
layers of sediment beneath the sea floor.
And hopefully we pull out a tube of sediment, split it open and
actually we can look at the layers within.
My reason for starting with field deposits is I think it's
really important point to make that this is the biggest data
set we have about turbidites, which tells us about their
characteristics and their dynamics.
And it's also a data set that spans geological time.
In terms of how we actually look at turbidites in the geological
record. It's actually really simple.
We look at things like grading patterns, we look at things like
sedimentary strictures, and then we try to interpret what does
that mean about how that flow was behaving however many
million years ago. Now, there's a slight issue in
doing this, and this was once illustrated to me quite
amusingly on a sort of conference field trip.
So we were up in the Italian Apennines where there's some
really exquisite sequences of turbidites.
And so all the people stood around this outcrop, their job
is to research turbidites. We're looking at this outcrop,
we're making really similar observations to one another, but
actually some of our interpretations are wildly
different. And so this I think illustrates
the problem of trying to interpret flow dynamics from
deposits is that you're trying to interpret the dynamics of
something that actually we have very few actual observations of
the thing we're trying to interpret.
And so for me, this is where experiments can be really,
really useful. So there's different ways we can
do experiments about turbidity currents.
So one way is to generate sort of small scale turbidity
currents in the laboratory, so actually generating a little
density current. And the other thing we can do,
which is I've what I've focused on more is actually trying to
look at aspects of their behaviour.
So for example, in my own experiments, what I've tended to
do is sort of generate fast moving slurries of sand and clay
and then change how the flow is behaving.
So artificially change the velocity of the flow,
artificially decelerate the flow in different ways, and then
actually look how does that information about the flow get
recorded in the deposit? Because then you can then
hopefully take that information into the field to try and help
you make better interpretations. What I would say is someone
who's done quite a few experiments, they also make me
slightly nervous because you always have this slight worry of
how well does what I'm doing in the lab actually scale with
these full scale flows in the real world.
So I guess where we're at now is in the last couple of decades
we've started being able to actually directly monitor
turbidity currents in the oceans in some situations.
So I I can talk a little bit more about that if you like.
Yeah. That would be great.
I'd love to hear. About how you.
Monitor turbidity currents in real time in the ocean.
The main way we do this is using moorings, so a mooring is
something we put on the sea floor.
It's got a big weight at the bottom, some floatation at the
top, and a basically a vertical wire in between.
And then we can attach instruments to that wire.
So the overall mooring's a couple of 100 meters high and it
sat on the sea floor. Now, one of the main instruments
that we used to do this is a thing called an acoustic Doppler
Current Profiler or ADCP. And so these are basically the
speed traps of the ocean. So they work in quite a similar
way to say, a police speed trap. But whereas the police officer
is using radar, is firing out radio waves that are bouncing
off moving vehicles, we're using sound pulses.
So this instrument's basically looking down towards the sea
floor, sending sound pulses down into the water column.
They're then bouncing off moving particles and we can build up a
really nice picture of the velocity structure of flow.
We do have a major issue with these instruments is that they
actually don't penetrate very well into concentrated layers
within flows, which we keep finding in the oceans.
The other issue we have sometimes have with them is they
actually get transported by the flow because obviously these are
destructive flows that we're trying to measure.
So we have some other quite neat bits of kit to deal with this.
So we basically have smart boulders.
So a smart boulder, if you imagine it's about the size of a
basketball and it's got its own pressure sensors, you know,
water depth it's at, it's got an accelerometer in it.
And that accelerometer is basically just telling us how is
that boulder moving? Is it, say, sliding in the flow?
Is it rolling in the flow? So what's the nature of the flow
that it's moving within? And then other people use all
sorts of instruments, so hydrophones to listen for
turbidity currents, seismometers to sort of feel turbidity
currents. One thing in all of these cases,
though, is we have to go somewhere we know has frequent
flows because we have to leave those instruments on the sea
floor for quite long periods of time and then either go back and
collect our instruments. Or in the case of a mooring, if
we've been unlucky, it's it's been damaged and cut loose and
floated to the surface. Or in the case of some
instruments like those smart boulders, they're actually
designed that with time they'll pop some weights out, float to
the surface. If we're working in a coastal
area, float to the shoreline. They've got GPS on them.
We can go and collect them. In reality, beachcombers usually
collect them for us. So those are kind of approaches
to understanding flow dynamics. So I think it's really this
combination of field work, experiments and then more
recently we've been able to actually start directly
monitoring flows. So as I mentioned in the
introduction, you actually use remotely operated vehicles, sort
of underwater robots to study turbidity currents and even flew
into one once. Can you tell us about that?
So we do use remotely operated vehicles to, for example, take
sediment cores from the sea floors.
There are methods by which we can attach coring devices to
Rovs, collect turbidites from the sea floor.
We did have quite an exciting day in the office, so to speak,
back in 2013 where we did indeed get a remotely operated vehicle
and trained in a turbidity current.
So at the time I was working for an institute in California
called Monterey Bay Chrome Research Institute, and we were
doing some work in Northern California.
We were actually meant to be collecting samples of gas
hydrates, but it was really bad weather.
It's a really awful set of days to be honest it.
And what we realised is there was basically one place we could
go and do some work on this particular day, and that was we
could collect some cores using this remotely operated vehicle
from Mendocino Canyon. And so I should explain probably
what a remotely operated vehicle is.
So this is basically an underwater robot.
You can think of it as being about very roughly the size of a
car. It's an unmanned vehicle, so
there's nobody inside it, which is good news on this day.
It's attached to the ship by a wire and it's controlled from a
control room on board the ship. So you imagine on board the
ship, you've got a room with sort of pilots looking like
they're playing video games, flying the ROV round on the sea
floor and scientists telling them what they'd like them to
do. So we've basically flown this
remotely operated vehicle down into this Canyon.
We're sitting on the sea floor. Now, obviously it's very dark on
the sea floor, but these vehicles have incredible light.
So normally we can see many meters ahead of us.
And that was our situation when we landed.
But then suddenly it all goes dark and we're basically
surrounded by sediment and there's a strong current coming
down the Canyon. And the pilots are complaining
that it's it's really quite difficult to keep the vehicles,
the ROV in position. And so in order to try and
stabilise the vehicle, they lift it off the sea floor.
At the moment they lift it off the sea floor, the vehicle
starts travelling down the Canyon at nearly a metre per
second, despite the fact that they're trying to fly it up the
Canyon at about a metre per second.
So we got some very interesting observations basically from the
perspective of being a particle within the turbidity currents.
They did get the vehicle out of the turbidity currents and I
think they described the situation quite well because
they described it as like flying an ROV in a tornado.
So that's what it feels like to be a particle in a turbidity
current. There were some instruments on
this vehicle. So actually we were able to get
some quite nice data about the stratification in this flow.
There's basically 2 layers to the flow, a layer at the bottom
flow, which was relatively dense and then a really dilute cloud
on top of that. So that near bed layer was a
couple of meters thick, whereas the cloud on top of it was maybe
in excess of 100 meters thick. And that actually ties in kind
of more generally with other studies that we've done with
more conventional methods and other canyons that we do see
these dense layers at the base of flow, usually much denser
than the one we measured with the ROV.
And that as I said earlier, they're potentially really,
really important part of actually how these flows are
driven. When you were flying this ROV,
what was the goal of the project?
What were you trying to discover?
We were meant to be doing something entirely different.
We were meant to be collecting gas hydrates from a different
part of the coastline, but that just was not possible.
And what we'd been doing at that time as part of a larger study,
was actually collecting sediment cores from lots of canyons along
the California coastline in order to look at the nature of
turbidites and therefore turbidity currents within those
different canyons. So we've got some canyons where
the heads are very close to the shoreline, some canyons where
the heads are away from the shoreline and looking at
basically what are the different processes acting in those
canyons. So really what we realised was,
well we don't want to do nothing on this day on this ship, you
know, it's really expensive running ships.
So what we could do, rather than doing nothing, is we could go
and collect some cause from this Canyon.
And it just so happened that we encountered A turbidity current.
So I mean, I think it was quite fortuitous.
You know, the vehicle survived and so did everybody, so.
At those speeds, A metre per second, is the water flowing in
a turbulent way or is it laminar flow?
So the flow is probably going about two metres per second
because we were actually trying to fly up the Canyon at about a
metre per second and we're heading down at a metre per
second. It's quite hard to say what the
nature of that dense layer near the sea floor was doing exactly
because a lot of the time the vehicle was really is actually
thicker than that layer. But certainly on the interface
between that dense near bed layer and the more dilute cloud
on top, there was actually quite a stable waveform.
So between the layers of fluid of different densities.
And then that upper layer was very turbulent, which relates to
that comment about like flying in a tornado.
They were just sort of getting thrown all over the place while
they're trying to keep this this vehicle stable.
And where you experiments in the lab roughly consistent with what
you were finding in the field there?
So I guess the interesting thing there is it seems like if you
compare it to lab experiments, the flow we measured in the
field looks quite like what you get if you in the lab generate A
turbidity current and then intrude another turbidity
current into it. So you've basically got pulses
of turbidity current, which is probably not uncommon within
submarine canyons. So there was a storm going on at
this point. So you may well have been sort
of destabilizing the sediments at the head of the Canyon.
You won't necessarily do that all in one go.
So you may actually generate multiple stability currents that
intrude into one another. And the structure of the flow we
measured in that Canyon looked quite like the structure of flow
seen in labs, although all the lab experiments that show that
that I'm aware of are actually just done with dense fluids, not
with sediment loading water. And I should say that the sort
of near bed dense layer we saw in Mendocino Canyon, I think is
quite different to the near bed dense layers we've seen in
places like Monterey Canyon and Congo Canyon, which are much,
much higher concentrations than what we saw with the ROV
thankfully. Because I think we wouldn't have
had an ROV at the end of it if it they'd been those kind of
concentrations. There's a great deal of human
infrastructure on the sea floor, such as fibre optic cables and
oil and gas installations. Are they affected?
By turbidity currents. Yeah, yeah, absolutely.
This is really important reason actually for understanding the
dynamics of these flows. So anything from say, fiber
optic cables, high voltage cables, oil and gas pipelines,
there's a lot of infrastructure on the sea floor.
If we think about say, fiber optic cables, I mean, they are
basically the network that we use that underpins the Internet.
And certainly those fiber optic cables aren't actually typically
protected on the sea floor in deep water.
They're just lying on the sea floor and they're vulnerable to
lots of things. So they're vulnerable to things
like ships, anchors or trawling from fishing boats, basically
dragging them and breaking them. They're also vulnerable to
turbidity currents. And the issue we have with
turbidity currents is whereas if say an anchor drags and it
breaks a cable, you break 1 cable with a turbidity current,
you can break lots and lots of cables at the same time.
And that causes further problems because what you normally do
when you break a cable is reroute the data traffic
somewhere else around the network.
If you've just broken a lot of the network, you can't do that.
So I guess sort of a classic example of this, it's in Taiwan
in 2006. So Taiwan experiences a lot of
natural hazards from sort of earthquakes to typhoons.
And both of these things can be responsible for generating
turbidity currents or certainly be associated with turbidity
currents. So in 2006, there was an
earthquake, and in association with this earthquake, over 20
cables on the sea floor were sequentially broken.
So you've just broken a massive bit of the network.
So temporarily, communications from Southeast Asia were
basically severed. And then on a longer time scale,
as they're trying to mend these cables, the Internet was slowed
down. And I think that was for nearly
two months. And you see the impacts of that
on things like financial markets.
So, yeah, these cables are definitely vulnerable to
turbidity currents, similar issues with oil and gas
pipelines. So this is one of the really
important reasons we study them. I should say, and I I should say
it quietly probably because there is a positive to these
cables getting broken. It's not positive for many
people, but it is positive if you study turbidity currents
because they do give you quite a nice data set about the speed of
some of these flows in the deeper ocean, say on submarine
fans, which otherwise we aren't really monitoring.
So if you imagine you've got a series of cables, you know when
they got broken, you know the distance between the cables,
then you're getting an idea of the velocity of the flows that
are breaking them. So there are some positives.
Does your work also help these companies figure out where not
to put infrastructure? Yeah, that's a really important
part. So they've always got to decide
where am I going to route this pipeline or route this cable.
And I think it's a really difficult job to do because
you've got this trade off between hazards to your cable
and the length of your cable. So often what happens if if it's
known this, you've got to say an active Canyon fan system with
turbidity currents, then that cable needs to be rooted into
deeper water. That's going to cost a lot more
money. Or for example with some oil and
gas pipelines are sometimes actually directionally drilled
into bedrock beneath sea floor canyons, which is phenomenally
expensive thing to do. What are you working on at the
moment? I guess a couple of different
areas. So I think one of the really
interesting things at the moment as we get more data sets from
direct monitoring from different places is understanding actually
the diversity of turbidity currents that exist.
So if you think about something like a river, we all know what a
river looks like, but you also know that, well, if I look at a
bedrock river in the mountains versus say, a river on a plain
like the Mississippi River, those two rivers look quite
different from one another. And I think this is what we're
starting to see with turbidity currents, that there's some
similarities when we look at them in different places, but
there's also some differences. And so and it would be great if
we could predict their character with less need for monitoring.
And that would really, really help with what you're talking
about a moment ago, which is how we design things like cable
routes. So that's sort of 1 area of
interest. So another is actually
understanding land detached canyons.
So I don't think I've mentioned these yet, but we often divide
sea floor canyons into either being land attached or land
detached. So a land attached Canyon would
be something like Monterey Canyon, where the Canyon head's
really, really close to the shoreline.
It's in really shallow water, it's almost up at the beach, and
there's a really obvious sediment source for the
turbidity currents. So you have longshore drift
that's moving sediment along the coastline.
That sediment effectively falls in the hole in the ground that
is the canyons. Then actually most canyons
around the world are actually land detached canyons.
So their Canyon head is actually detached from the shoreline.
Now for a long time we assumed that these types of canyons were
inactive in sort of present day sea level.
So if they had turbidity currents in them, they'd be
quite small turbidity currents. We didn't need to worry about
them. But more recently we've realised
that this actually isn't the case.
They can actually have quite fast and destructive turbidity
currents in them. So for example, this summer part
of an expedition which is going out into the Atlantic to a
Canyon called Whittard Canyon. So Whittard Canyon is a Canyon
over 300 kilometres southwest of the UK.
So it's very, very land attached.
There's a big distance between it and the shoreline.
So this is an expedition led by my colleague Mike Claire, who
works at National Oceanography Centre in Southampton.
And so as part of his project, we're going to put an array of
moorings into this Canyon. So we already know from previous
work we've got fast and destructive turbidity currents
in there, but it's really work in progress to understand why.
So what's actually triggering them and then kind of how far
down the Kenya system they go, how they actually behave as they
do so? As to Sumner, thank you very
much. Thank you very much for having
me, that was a lot of fun. To see pictures and
illustrations that support this podcast, go to geologybytes.com,
where you'll also find a subject matter index of all the
episodes. There you can also give me
feedback which I welcome, as well as sign up to get my emails
about new episodes.