This is Geology Bites with Oliver Strimpel.
Most of Earth's salt is dissolved in the oceans, but
there is also a significant amount of solid salt among
continental rocks. And because of their mechanical
properties, salt formations have an outsize effect on the
structure and evolution of the rocks that surround them.
This gives rise to what we call salt tectonics, at first sight,
a rather surprising juxtaposition of a soft, powdery
substance with a word that connotes the larger scale
structure of the crust. Michael Hudeck has been working
on salt tectonics since 1989, when he joined the Production
Research Group at Exxon. He is currently investigating
salt tectonics in regions in which rifting has created salt
basins, such as in Brazil, West Africa and the Gulf of Mexico,
and has Co written a definitive textbook on salt tectonics.
He is a research professor at the Bureau of Economic Geology
at the University of Texas at Austin.
Michael Hudeck, welcome to Geology Bytes.
It's a pleasure to be here. What exactly do we mean by salt
tectonics? Well, I think you have to start
by understanding what we mean by salt, because it's a somewhat
slippery term. What most people mean when they
say the word salt is table salt, which is sodium chloride.
The mineral name for it is halite in the Earth's surface or
in the earth generally. A lot of the world salt is
actually halite, but they're actually a whole bunch of other
minerals that are typically deposited with salt and they
have complicated names of carnalite, tachyhydrides,
silvite and hydride, gypsum and so forth.
So salt is very often in geology used as kind of a catch all term
meaning all of those minerals, but the most important one is
the mineral halite. Now, salt tectonics refers to
the structures that form when this salt starts to flow,
because in geology, salt is a fairly unique mineral.
We think of rock as being a word meaning something hard and
impossible to break. Salt actually is a rock that can
flow at geologic strain rates and under geologic conditions,
and so it's actually one of the weakest rocks on planet Earth.
And salt tectonics refers to the whole set of processes and
structures that are formed when the salt starts to flow.
What sort of size structures are we talking about?
Well, single salt domes are often say 5 to 8 kilometers tall
and can be 10 or 15 kilometers across.
When you start getting a larger composite salt structures that
are made of many salt structures that have sort of flowed
together. The largest ones on earth are
the Angola Salt Nap, which runs for several 100 kilometers in
the deep water offshore Gabon and Angola, and then the Sigsbee
Escarpment in the deep water Gulf of Mexico, which likely
covers several 10s of thousands of square kilometers.
So it's simply enormous structure, bigger than most U.S.
states. Wow, that's absolutely enormous.
I had no idea that we had so much salt.
And where does it all come from? Most salt in the earth comes
from evaporation of seawater. If you think about how much
seawater has to evaporate to make salt, I mean, everybody
knows seawater is salty if you taste it.
If you take 1 kilometer of seawater and evaporate it, you
end up with something like 12 meters of salt.
And so to get salt deposits of one or two or three kilometers
thick, which many of them were, you start having to refill the
basin with water many of times and evaporate all the water and
then refill it again and evaporate all the water.
So you take really just enormous quantities of water.
If you had it all in one place, it would be a column of water
more than 100 kilometers tall to make a lot of the salt basins on
earth. So it takes a while to deposit
it, but it reflects the evaporation of just massive
amounts of seawater. There are some minor salt
deposits that come from evaporation of saline lakes, but
by far the most important one on earth is seawater.
Was the earth endowed with its salt when it was formed?
Well the earth wasn't even endowed with water when it was
formed, so it took a while for the rock cycle to get going and
most of the salt was originally bound up in minerals inside the
earth. And there was a process that
went on for a long time called out gassing, where the early
earth had volcanoes and things spewing various gases into the
air. Which then formed water and
oceans, and the salt was one of the substances that came from
both erosion and out gassing of the original planet.
So it took a while for sort of a stable water cycle and a stable
salt cycle to get going. Do we see evidence of such vast
amounts of salt anywhere else in the solar system?
Well, there has been salt tectonics reported on a whole
variety of different planets. On Mars, on Europa, which is a
moon of Jupiter, on Triton, which is a moon of Neptune.
And it's all done from satellite imagery and saying, oh, that
looks like that could be a salt structure.
So we don't really have direct evidence as in a sample that's
salt, but there are weak things flowing around on other planets,
and they are sometimes inferred to be salt, but we actually
don't know the mineralogy. They could be made of borates or
sulfates. They don't necessarily have to
be sodium chloride, the one we're most most used to.
And in some cases, people think that you have salt like
tectonics going on as a result of ice, frozen water in the
subsurface. What you really need is there's
anything that's weaker than the rocks around it and you can get
salt, like tectonics all over the solar system.
In my introduction I said that salt formations have an outsize
effect on the local structures. Why is that?
And what kind of structures form when salt is involved?
Well, if you think about salt flowing in the subsurface and it
moves from point A to point B, and if there are rocks or
sediments over the top of the salt, those rocks above the salt
have to deform and move and go up or down or sideways when the
salt is flowing underneath them. And so a salt basin looks very
different from just a regular basin that does not have salt
because the flow of the salt underground and sometimes up at
the surface has a huge effect on the rocks both above the salt
and to the sides of the salt. So you get styles of structures
that you just don't see anywhere else.
So do we generally see significant salt structures in
any particular plate tectonic setting, for example?
Yeah, plate boundaries. Well, to get salt deposited in
the 1st place, you need to take vast amounts of seawater and you
need to evaporate the seawater. So, for example, there are no
salt basins forming today in the Pacific Ocean because even
though you're evaporating lots and lots of water, that water
gets replaced by flow of water coming in from the Indian Ocean
and from the Atlantic Ocean and from rivers coming in.
So what you need is a lot of water evaporation and coupled
with an isolation of the basin so that you cannot replace the
water that's getting evaporated. And there are some plate
tectonic settings that favor that more than others, in
particular rifting when a continent is splitting apart for
the first time. So you've got Europe and Africa
separating from South America and North America at the birth
of the Atlantic Ocean. Well, initially there's a really
long skinny basin separating the 2 mega continents from each
other as they pull apart. And so that's an ideal situation
for salt to be deposited because there can be a lot of
evaporation in that long skinny basin and it's hard to replace
the water for the water to get in down the axis of that long
basin. So in that case, you had salt
basins forming both in offshore Brazil and offshore West Africa,
and you also had the salt forming in the North Atlantic as
well. Nova Scotia's got salt basins
and so does Morocco, which are a conjugate to each other.
They were originally attached to each other.
So very early in rifting is where you get salt deposition.
Now salt structures, when the salt starts to flow, they
actually sort of have the reverse provenance because it's
when continents collide that this salt, which may have been
perfectly happy to just sit there in the subsurface for long
periods of time, all of a sudden you've got 2 continents ramming
into each other, and any salt that's there wants to get out
any way it can. And so it starts moving all over
the place. So many mountain belts have
large contributions from salt structures in them because the
salt just wasn't happy when the continents started slamming into
each other and it started moving.
Do those structures sometimes make it to the surface?
They do. And in fact, there are some
today in Iran, Iran's undergoing a plate tectonic collision with
Arabia, the Arabian plate, and you've got salt basically
popping out onto the ground surface and flowing sideways as
what are sometimes called salt glaciers.
You actually get salt flowing down the sides of mountains.
And if you actually go on Google Earth, you can find them.
They're very spectacular. Now, Iran's a nice place to see
those because it's a desert, because as you can imagine, or
large parts of Iran are desert anyway, as you can imagine, the
more it's raining, the more the salt tends to get dissolved just
the minute it comes up to the surface.
But to get those things preserved, you need a way not to
have them all dissolved. And a desert's a great place to
do that. So where do we see evidence of
salt tectonics today? Well, salt is active in a whole
bunch of basins, many of them offshore.
So unless you have some sort of echo sounding of the sea floor
or seismic data to look underneath.
But the Gulf of Mexico, both the northern and southern parts of
the Gulf of Mexico, offshore US and offshore Mexico are just
riddled with active salt tectonics.
Salt is flowing all over the place in those basins.
The same is true in offshore West Africa and Gabon and
Angola. The same is true in offshore
Brazil. You've got active salt
structures happening in the North Sea.
So most active salt provinces are actually marine and so the
main people that know about them are people who look in marine
areas, which could be researchers doing hydrography,
but mostly oil and gas companies who are looking for the oil and
gas that is very often associated with salt structures.
So you mentioned the compressive environment.
What happens to the salt then if it doesn't come to the surface?
Do you get domes, diapers forming?
And is that a collapse of the material above it?
Well when you squeeze salt it a really wants to get to the
surface of any kind of way it can.
But if it can't get up to the surface normally it will
accumulate into large bulges which form anticlines and Dome
the rocks over the top and between the domes.
They're what are called synclines, the low area where
the salt flowed out of. But if there are any
pre-existing salt domes that were there before the shortening
started, it's like kind of holding a banana in your hand
and squeezing the banana and the just liquid banana comes
shooting out the top because you're squeezing it shut.
And so that's what happens to any pre-existing domes is they
typically pinch shut or close to shut and the salt goes squirting
out the top. So it can form a variety of
different structures. It's also a very good lubricant
because it's so weak and so if there is a salt layer someplace,
rocks like to slide along salt layers.
So that's another big factor in both compressional environments
actually and extensional ones is salt as a lubricant.
And how does that manifest in the local structures when you
have that sliding movement? Well, when you have that, rocks
can move 10s of kilometers sideways, and so there are
extensional structures in the area where the rocks are pulling
away. It's sort of like the top end of
a landslide and down at the bottom that the down dip end of
the landslide. There are shortening structures,
thrust faults, folds, and so forth.
So if you take 2 margins, 2 continental margins, one of
which has salt and one doesn't, and you crunch them both in the
same plate collision, the one without salt is certainly going
to be deformed. But the style is not going to be
nearly as complicated as what you see when there is salt
present. Salt just makes everything
pretty much go nuts. Has the presence of large salt
structures had a significant effect on the way Earth's global
plate tectonics takes place? The presence or absence of large
salt structures really has no influence at all on the overall
pattern of global plate tectonics.
The plates that we're going to collide, collide anyway.
The rates don't particularly change.
However, what you do get is a very significant modification in
the structural styles that you see now.
The structural styles of a plate collision zone in the absence of
salt structures have been extensively studied and are
really very well known. Classic examples include the
Canadian Rockies and many other fold and thrust belts around the
world. Now when you have salt
structures that are present, the whole structural pattern of the
collision zone changes because the first thing that happens in
a collision zone that involves pre-existing salt structures is
that the salt structures are the first things to activate.
And so the pattern that results depends on the pattern of the
salt structures. And you get patterns that are
completely unlike the normal fold and thrust belts that form
in margins without salt. So in the 1950s, sixties, 70s,
even into the 80s, you had geologists who are unknowingly
mapping collision zones that involved salt and trying to
interpret them with standard fold and thrust belt ideas.
And they would be tying themselves into all kinds of
knots because they're actually were salt structures there that
were confusing everything and it just wasn't a normal fold and
thrust belt. And the geologists would have
all kinds of problems trying to make a salt involved margin that
was involved in a collision as if the salt was not present.
So starting in the late 1980s and into the 1990s, geologists
began to understand the huge influence that salt tectonics
could have on continental collision zone.
There's been a pattern ever since then, one after another,
of looking at collisional belts and identifying where the salt
had actually been present and actually reinterpreting all of
these margins in a new way with the recognition that they
involved salt tectonics. So is there an overall control
of salt tectonics on global plate tectonics?
No, there's not. But does it change the style of
what happens in plate tectonics? Yes, it does, very much.
And one of the big advances in structural geology in the last
30 or 40 years has been the recognition that you really have
to know if salt structures are present or not in order to
correctly interpret the margin. Some of the places that you
mentioned are also centers of oil production, that's true.
Is there a connection between salt tectonics and the formation
of hydrocarbon reserves? There are for a couple of
reasons. One is to get hydrocarbons, oil
and gas, you need what's called a source rock.
And a source rock is generally a very organic rich shale in most
cases doesn't have to be a shale.
So you need a very organic rich shale.
Well, so how do you get an organic rich shale?
So most basins have little microscopic creatures or bigger
creatures dying all the time and they accumulate at the bottom.
But there are all sorts of organisms that eat organic
matter, little bugs both at the surface and in the subsurface.
And so you need an environment in which those bugs are not
active for some reason. You preserve the organic matter
better. And the kind of hyper saline or
very salt rich environments in which salt is deposited are also
very hostile to the organisms that eat organic matter.
They can't survive in hyper saline environments.
So if you've got an environment that is either making salt or
has recently made salt and is still very hyper saline, the
organic matter that dies very often is preserved.
There's nothing at the bottom to eat it because the water is too
salty. So one factor is that salt
basins tend to have very good source rocks associated with the
salt. And then there's the factor that
all this movement of salt that I've been talking about, the
flow of salt in salt tectonics, tends to make a lot of big
structures, big anticlines. And many of those structures
trap hydrocarbons, you know, an anticline, which is a big Dome
of sediments coming up. Well, hydrocarbons tend to be
lighter than water, so they migrate up to the top of the
Dome and they're trapped there. And so oil companies come and
they drill the top of the anticline and produce the oil
and gas. So it's an environment very rich
in traps for oil and gas. And then the last thing is that
salt is itself a very good seal for hydrocarbons.
And by a seal we mean that if the oil or gas is trapped under
the salt, it's very difficult for them to leak through the
salt. It makes a good sort of trap
over the top and if for some reason you do manage to
fracture, the salt itself heals because it re crystallizes very
easily. So if you somehow managed to
disrupt the seal, it heals itself.
So because there's good source rocks, because it makes lots of
traps and because it's a very good top seal.
For example, most of the salt on the Arabian platform or at least
a large part of the salt on the Arabian platform, which is why
Saudis Dr. Mercedes-Benz around is sealed underneath salt.
And in this case, it's not halite, the mineral, it's
actually anhydride is the principal top seal over a lot of
the Arabian platform, another salt, but still illustrates the
prospect that salt makes a very good top seal for oil and gas.
In Saudi Arabia in particular, and the Middle East perhaps more
generally, the oil is very close to the surface, isn't it?
Is that because you're looking at the top of one of these
domes? No.
The oil reservoirs in Saudi Arabia are actually much more
widely distributed than in any single salt structure.
And in fact, in most cases, the oil is actually not in one of
those anticlines. It's in another type of trap
entirely. And the main function of the
salt on the Arabian platform is actually as a top seal, not in
making the structures themselves.
To stop it sort of leaking out into the surrounding rocks.
Yes, to stop it from leaking away.
Exactly. Salt has been a valuable
commodity for much of human history, and it has many uses
even today. What are those uses, and do we
get our salt by mining the salt structures we've been
discussing? Well, for many years we did.
We don't generally anymore get our salt from mining.
So going back into the Neolithic, the principal uses of
salt were for food preservation. It was used in tanning hides,
and it also had some medicinal uses as well.
And so for that reason, a lot of early Neolithic settlements were
located around sources of salt, and they're big archaeological
sites going back many thousands of years around some of these.
Some of these were salt springs where the groundwater just had
salt dissolved in it, and the people took the salt and heated
it up in a pan and got the salt out of that.
In other cases, the salt was exposed at the surface and you
had Neolithic miners going in and mining it.
You can go look up the Salt Men of Iran online if you want to.
A series of miners got trapped in cave insurance in some of
these salt mines. And because salt is very good at
preserving things, they basically got pickled inside the
salt. They died and were buried by
salt and were more recently extracted.
And there's some museums in Iran that have these, you know, heads
of Neolithic miners preserved in them.
So it's sort of a ghoulish, but it does illustrate very well
exactly why salt was so valuable because things are preserved
very well when they're packed in salt.
So these days, up until I'd say 50 years ago, the most common
way to get salt was by mining it.
And so there were many salt mines around the world.
These days, at least in the United States and Europe, most
salt is made in factories by chemical reactions.
It's not naturally occurring salt, although there are still
parts of the world in which salt is mined.
But in most of the industrialized world, it's
actually made in factories. What about evaporation pans in
shallow seas or even in the Dead Sea?
Yes, those still exist, and in many places where there were no
naturally occurring salt basins, again, Neolithic or Middle Ages,
miners evaporated seawater one way or another, even in climates
where you wouldn't think that was possible.
I mean England for example, which isn't a tropical
environment, and you wouldn't think you could do that.
In England a witch was a salt working from usually water
evaporation. So any English city with the
word witch in it, Norwich and so forth was originally a salt
working. Salt was also used as a form of
currency in antiquity, wasn't it?
It was the rumor that Roman soldiers were sometimes paid in
salt is correct. And in fact the Roman word Sal,
which meant salt, was the root of our word salary.
Was somebody being worth their salt?
I mean it actually. They were on occasion paid
directly in salt. It was that valuable.
And in other times wars were fought over access to salt
because it was so critical for civilization, especially food
preservation. In lots of Europe cod there were
wars over COD and salt to pickle the COD or preserve the COD
fought in especially northern Europe.
What are you working on at the moment?
I've been working on a couple of things.
One of them is some of the salt tectonics going on in the very
deep water part of the Nile Delta and offshore Egypt.
It's actually far enough out in the Mediterranean that it's
offshore Cyprus. You've passed from Egyptian
waters to Cypriot waters. And in that area the salt has
been flowing downhill under the weight of the Nile Delta.
And it's actually ramming up against a big basement high
that's out in the middle of the Mediterranean called the
Eratosthenes Seamount. And it making all kinds of
really interesting structures that are visible on seismic
data. And so we're working on a big 3D
seismic cube to try to understand what's going on.
And there have been some recent oil and gas discoveries in that
area, which is why oil companies are interested in the research
and are helping to support us financially to do it.
So the salt structures might be indicative of the presence of
oil. Not directly, but the oil and
gas is certainly associated with salt structures and you need to
be able to correctly interpret the salt structures in order to
find the oil and gas because salt structures, because they
are so complicated when you look at them for the first time on
seismic data, many interpreters, the first response is go, what's
that? You know, it looks terrible.
And so we sort of try to provide techniques for understanding
what's going on and being able to interpret it.
So you can say, oh, I get it, that's what this is, which means
the oil and gas would be over here.
And so it's not so much that the oil and gas is directly
reservoir inside the salt or caused by the salt, but you need
to be able to interpret the salt structures to find it.
Michael Hudeck, thank you very much.
It's been a pleasure, Oliver. To see pictures and
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