This is geology bites with Oliver Strimple scattered around
the globe. Within both oceanic and
continental plates are some massive accumulations of igneous
rocks called large igneous provinces.
What causes such huge volumes of magma to flow onto the Earth's
surface? And what impact does their
emplacement have on the plates where they erupt, the atmosphere
and climate and life? Richard Ernst has been studying
all aspects of large Igneous provinces for nearly three
decades and has written the definitive textbook on the
subject. He is scientists in residence at
Carlton University in Ottawa. Richard Ernst, Welcome to
Geology Bites. Thank you, Oliver.
It's great to be here. There are many places where
volcanism has produced a lot of basalt on the surface of the
Earth, not least from mid ocean spreading ridges and volcanic
arks. Is there an accepted definition
of a large anus province that distinguishes them from such
other forms of volcanism? As originally defined in 1990,
the basic aspect was a large volume intraplate event,
typically with a short duration, and the minimum size was
conceived as around 100,000 cubic kilometers.
The intraplate setting distinguished from ordinary sea
floor spreading and collisional related activities and they can
be very very short duration, often times less than a million
years, or can be multiple pulses over a longer period of time.
But basically it's a bulk of extruded magma is where the
definition lies, right? That's the main thing, that's
how it's it was originally recognized it was these huge
flood basalts observed in many parts of the world that could be
kilometers thick of basaltic lava with very little interflow
sediments. And so it was really recognized
that these were kind of unique sort of thing and then they took
this to the oceanic realm and recognized similar kind of thick
accumulations of extrusive material.
Can you describe the structure of a larger nears province?
At the surface you could see the huge flood basalts that might be
extensive over many many hundreds of kilometers, covering
up to 1,000,000 or more square kilometers.
But as the flood basalts are eroded, you see more of the
plumbing system. You see giant radiating dike
swarms that can have a radius of up to 2000 kilometers or more,
giant circular swarms that could have a radius of almost 1000
kilometers. Also you have massive sills,
dolerite sill provinces associated with sedimentary
basins. You also have the important
group of Mavic ultromathic intrusive complexes that are
also an important part of the plumbing system.
And then as you go deeper, you get down to the magmatic under
plate that's often present where you might have up to 10 million
cubic kilometers or more of material created to the base of
the crust as Mavic to ultromathic material.
So the plumbing system is really quite extensive and this
indicates that these events are really even more of a luminous
than we realise from just looking at the extruded flood
basalt. But if I was standing on top of
a large Igneous province or on a continent to flood basalt, would
I recognise it from the plumbing system or would I just see
endless sheets or thick sheets of successive lava flows?
Yeah, that's the way you would recognize it.
You'd see yourself sitting on top of as far as the eye could
see. Basaltic volcanism, Hawaiian
style volcanism, but on steroids.
If you have windows through it where the basalts were eroded,
then you might see some of the dyke swarm component and the
other plumbing system component. I have been to some of these
large English provinces, for example on the western coast of
Greenland and then also in the Deccan.
And you can, when it's eroded through by a gully or something,
just see layer upon layer upon layer upon layer, endless layers
of successive lava flows. Yeah, dramatically so.
And even in the ones that have been deeply eroded, like the
McKenzie event of Northern Canada, where the dyke swarm
component goes out to 2000 kilometers, in the focal region
of this overall radiating swarm, you have the remnant of what
must have been originally very, very extensive flood basalt
sequence, the copper mine volcanics couple kilometers
thick or so. What are the best known large
igneous provinces? Where are they and roughly how
many have been recognized? One of the most dramatic is the
Siberian trapped large Igneous province, which extends over
about 5,000,000 square kilometers in Siberia.
Other hugely dramatic ones are the Central Atlantic Magmatic
Province at 201 million years ago.
So those are two well known ones.
You mentioned the Deccan as well of India at 66,000,000 years.
It is widespread in India. Those are the youngest ones and
there's lots more young ones. But also as you go back through
time and you look at the plumbing system component, we
can trace them all the way back into the Archaean and up through
the protozoic about 2 1/2 billion years ago.
At the end of the Archaean we have them occurring every 20 or
30 million years somewhere on the planet.
And if you add in the oceanic plateaus which are only visible
in the last 200 million years, while we still have current
oceanic crust, but we know the oceanic plateau component
continues back through time, we can see them as remnants in
collisional zones of oceanic plateaus from closed oceans.
And so we suspect that the rate of large in these provinces back
through time might be as often as one every 15,000,000 years or
so back through about 2, 1/2 billion years ago.
And then in the Archaean, we're doing a new compilation based on
all the new geocrinology etcetera and it's looking like
the rate is going to be fairly similar.
So you're saying really all the way back to the Archaean, We're
talking about one of these being in place roughly every
15,000,000 years? Potentially as often as and
certainly every 20 to 30 million years based on the continental
large Igneous Province record. But to my mind, we talk about
plate tectonics a lot and it's a dominant mechanism on Earth.
But this whole other process, this whole other voluminous
magma production feature of Earth, is also incredibly
important. Could you talk about the
geochemical makeup? So being basaltic, we're talking
fundamentally about Mayfair composition, but there's also,
as I understand it, a variation between ultra mafic at the one
end and also some fairly solicit components of these large
Igneous provinces. That is certainly true for the
continental examples, where it's mainly mafic with some ultra
mafic component, but you can also have some more granitic
compositions. As magma crystallizes, the
residual liquid becomes more granitic through a fractionation
of the magma. You can generate from the mapic
magma some amounts of granitic type magma.
But the other thing that is really fascinating component of
continental large igneous provinces is the potential for
the thermal source causing massive partial melting of the
lower mapic crust. And if you melt a basaltic lower
crust, you produce a granitic melt and so you can get massive
provinces of granitic melt associated with large igneous
provinces from the same heat source.
But these massive provinces are interesting because they can
have remarkable scale, 100,000 square kilometers or more.
It's well known that you produce a lot of Brinitic type magnetism
in collisional environments associated with subduction, and
as you go back in time it can be hard to tell what setting you're
in. And so when you have a large
granitic magmatic province of some age, is it evidence of a
plate tectonics collisional bone, or is it an example of a
large igneous province producing solicit magnetism?
Can you give an example of one of those?
The Karu Large Igneous province, which is a massive event across
southern Africa into Antarctica, and there's a felsic equivalent
of it in southern South America. The Chon Ike event.
How long does it take to form a particular larger province?
Or to put it another way, do they have a typical magma
extrusion rate? The geocronology on the Siberian
Trap event indicate that this 5,000,000 cubic kilometer or
more event, not including any under plate component which
would probably make it much larger, all came out within less
than a million years. The Central Atlantic Magmatic
Province, which covers some 10 million square kilometers,
Uranium lead dating suggests that most of it must have come
out again within a million years or so, an incredibly short
period. Where exactly is the Central
Atlantic Magmatic Province today?
The center of activity from the North American perspective in
Florida, from the African perspective, it would be the
corner of Africa. If you close the Atlantic Ocean,
you tuck it in adjacent to Florida there and that's the
center. But you have magnetism along the
entire coast all the way up to Newfoundland on the Canadian
side. It's also extensive down into
Brazil, into the Amazon basin and then into West Africa, all
the way up into Mauritania, Mali, up into Morocco, Algeria,
up into Iberian Peninsula as well.
So it's a massive event and everywhere we see it and dated
it seems to have a 201 ± 1 or so million year age.
However, there are also events that are coming out in multiple
pulses that might span 10s of millions of years.
One of the ones that's important is the North Atlantic large
Igneous province which is on Greenland, the units in
Greenland you were talking about and also in the UK and offshore
as well. That North Atlantic Igneous
province has a main pulse at 62 to 58,000,000 and then a second
pulse with the onset of rifting in the North Atlantic at 55 And
Iceland right in the middle of the North Atlantic which is
active today, represents the continuation of the North
Atlantic large Igneous province at a much lower volume and flux
rate. What happens many times is you
have these large Igneous provinces with a huge volume of
magma in a short period of time, and then there's protracted
magnetism at a much lower pace for another 100 to 200 million
years. So Iceland has pulses at 60
million years, then a second pulse of 55.
Other events also have multiple pulses, oftentimes with the
second pulse associated with the onset of rifting and continental
breakup are. They unique to our planet.
They are not unique to our planet.
On Venus we see just a wealth of what we think of as large
igneous provinces. On Venus, there's no plate
tectonics. And so one of the things that's
come from the study of huge flood basalt events, lava flows
continuing for 1000 kilometers or more.
We've also recently discovered huge dyke swarms on Venus.
Venus has no erosion, no plate tectonics.
Erosion is important because we see the surface expression of
lava flows. We also see the surface
expression of dykes laterally. We think of dykes moving up.
They do, but they also can move sideways in the crust for long,
long distances. We know that from Earth we see
that demonstrated vividly on Venus, and we can trace them for
long distances as swarms of dykes radiating for thousands of
kilometers. We just recently traced a single
one for almost 4000 kilometers, and because of the absence of
plate tectonics, it really lends support to a mantle plume model.
Let me also mention Mars. Mars has grabbing systems
interpreted to be dyke storms 3 or 4000 kilometers long,
associated with and radiating from this area of protracted
plume related volcanism called the Tharsis region, and also on
another region antipodally called the Elysium region.
So there's boundless radiating grabbing systems on Mars that
are also interpreted to represent huge, laterally in
place, nathic dyke swarms associated with what we would
call large igneous provinces on Earth.
Let's talk about what might generate these prodigious
volumes of lava that form these large igneous provinces.
Then I understand that there are two categories of model, each of
them providing A mechanism that generates anomalously hot mantle
material at shallow levels. The first involves shallow, by
which we mean crustal and upper mantle processes, and the 2nd
invokes the deep mantle with the arrival of a hot mantle plume.
Let's start with the former explanations.
So there are a number of shallow mechanisms.
The lithospheric portion of the surface plate can drop off and
descend into the mantle. That's a process called
delamination. And if so, then hotter material
from the mantle below can rise to a shallower level and produce
large volumes of melt. It's also been well recognized
that rifting extension, as you thin the the plate, then you
also bring hotter mantle material to a shallower level
and it can then also cause partial melting.
There's another class of shallow models that also sort of things
to a supercontinent cycle. We know that the continents come
together and break apart and come together and that's
happened several times in Earth history, the most recent time
being Pangaea that was together over the last few 100 million
years and started to break apart around 200 million years ago.
When you assemble all these cottons together, there's a
region of mantle that is covered by thicker crust, grenaded
crust, and it represents some sort of thermal blanket.
So the temperature can then build up in the mantle
underneath then potentially enough to generate melting.
I'm reminded of the episode of Georgie Bites that we did with
Peter Molnar, where he talked about the foundering of an
anchor below Tibet as an explanation of why it's so high
today. And I think that is what you're
referring to by delamination of the lithosphere in a a heavier
portion below sinking into the sinospheric mantle below.
And similarly, Damien Nance also talked about the thermal
blanketing just at the last episode of ways in which you
could get extra heating that would then lead to the demise of
a supercontinent. So let's talk about the second
category of model, namely the ones that involve the deep
mantle and the mantle plume. You have a plume that rises, and
the top of the plume is growing as it rises because it is being
filled from below faster than it can push its way up through the
mantle. And so the size of the plume
head is predictable by the modeling.
And you get a plume head at the top that's many hundreds of
kilometers across and flattens when it reaches the base of the
plate to something potentially 2000 kilometers across.
And this kind of arrival and flattening can take place within
a very short time, meaning that you suddenly get into a zone
where massive melting happens very, very quickly, leading to
large India's provinces in providing an explanation for
very rapid generation of huge volumes of melt.
So this mantle plume idea has also been useful for us with
respect to radiating dyke swarms we see in the McKenzie swarm,
which is one of the most magnificent in the world.
It extends out over 2000 kilometers in radius.
Where is that located? In northern Canada, so it
continues from northern Canada, the Arctic coast all the way
down, not too far from Lake Superior.
The radiating swarms, the kind of thing you get from domal
uplift above a mantle plume. A plume lifts up the crust a
kilometer or so and produces radio fractures that are filled
by magma from the plume and you get a radio swarm.
The swarm and the McKenzie and the swarm of many others is
radio for a distance and then swings beyond a certain distance
influenced by some sort of regional stress sterile to the
plume. And we take that transition as a
measure of the size of the underlying plumes.
So in the case of the McKenzie, it transitioned from radiating
to a swinging into another regional stress pattern beyond
at about 1000 kilometers consistent with this mantle
plume modeling. Do these various models also
help explain what we see on Mars and Venus then?
Indeed they do. On Venus, I mentioned this
nearly 4000 kilometer long individual dike.
It radiates away from a big volcanic center, radiates out
1200 kilometers, and then swings about 50° into a trend that's
parallel to a rift zone that's 10,000 kilometers long.
So we have a dramatic change in trend right at 1200 kilometers,
consistent with hundreds of other dikes that we are seeing
as part of the radiating system. But this particular individual
dike shows it so dramatically. On Mars, we see beautiful
radiating swarms that seem to go out three or 4000 kilometers,
and the entire Tharsis Rise is larger than 5000 kilometers
across. So it's a big event on its own.
How does the model explain the initial radial pattern?
And then there's a certain radius, the swinging out there
dykes. In order to drive a dyke out
laterally for that part, you have to have a lot of magma.
So you have to have magma continually being pumped into
the starting end of the dyke. The source area for Dike Magnus
is typically near the plume center.
So within a few 100 kilometers of the plume center, we can see
in some cases gravity anomalies marking intrusions from which
the dikes were pumped out. And they're pumped out initially
in the direction given by the Domal uplift, which imposes the
stress on the crust that favors radiating openings.
And so the magma that's being pumped outward from the center
region takes advantage of these preferred opening directions and
happily drives out laterally and keeps driving out sideways.
And then if it gets beyond the plume head, there is still
regional stress. On Earth, it's related to plate
boundary stresses. In continental areas away from
spreading zones or collisional zones, there's intraplate
stresses that have preferred directions.
And so the idea is that the magma keeps being pumped into
the starting end of the dike, and the magma keeps moving at
the far end. And once it gets beyond the
domal uplift, beyond the influence of the stresses
related to the uplifting plume head, the magma still wants to
go. And it's going to happily go in
the direction of whatever the stress it experiences at that
point. And on Earth is presumably
related to some sort of plate boundary or collisional
subduction stresses. On Venus we see this one that
parallels this huge rift zone for 2000 kilometres of it's
10,000 kilometres long, full length of that Rift Zone.
As we look through the geological record, we often see
a temporal coincidence between the emplacement of a large ignis
province and continental rifting.
Can you give some examples of this and do we understand why
this is so? We clearly see many examples
where a rival of a large Igneous province is associated with
attempted breakup and often successful breakup to open a new
ocean. On one hand, you have the
Central Atlantic Magmatic Province at 201 million years,
which precedes the opening of the Atlantic by 5/10/15 million
years. Beautiful correlation there.
On the other hand you have the magnificent Siberian Traps,
Archignis Province, which did try to break up Siberia from
Europe. The West of Siberia has major
rift zones that are part of the Siberian Trap event and
represent serious attempted breakup, but ultimately
unsuccessful. So this points out that there's
two aspects that are relevant. You have to have favorables,
plate boundary stresses in order for there to be successful.
In the case of the Central Atlantic, there was already a
whole series of rift basins, at least on the North American
side, called the Newark series of basins that represented NS
rifting prior to the ultimate breakup.
That happened 30 or 40 million years later after the arrival of
the central magnetic province. So in that case, you'd say there
was favorable plate boundary stresses, they were already
interested in causing a breakup, and then the mantle plume
arrives, causes some uplift, causes radiating dikes from, but
also causes potential triple junction rifting in multiple
directions, going north to separate North America from
Europe and Africa into the South to move South America away.
Ore deposits are linked to large igneous provinces.
What kind of ores are these, and do we understand what the
connection is? Yes, we have some good ideas.
Most ores for copper, nickel and platinum group elements,
platinum and Palladium in particular.
They are genetically associated with large igneous provinces,
and part of the story has to be with the kind of magma you get
both mafic and ultra mafic magnetisms in large igneous
provinces. Ultra mafic magnetism can often
be associated with elevated levels of some of these elements
that then can get concentrated in a layered intrusion.
For instance, like the Bushfeld intrusion in Southern Africa, a
huge reservoir of many rich ore deposits.
It's a single body that extends 400 kilometers across and maybe
7 kilometers thick. A huge body of magma that
differentiates and produces significant ore deposits at
different levels. And it's part of a large Igneous
province that a little over 2 billion years ago in southern
Africa. Then the real magmatic sulphide
deposits in the Siberian trap event.
Those seem to have interacted with silicon rich units at some
point, causing sulphur saturation and production of a
separate magmatic sulphide liquid that can interact with
the huge volumes of magma that might be moving by it in a magma
chamber. The idea is if you have a
separate liquid adjacent to the basaltic liquid, as the basaltic
liquid is moving by it, it's sulfur loving elements like
nickel and copper and platinum group elements jump out of the
basaltic magma and jump into the sulfide magma and progressively
get more and more rich in the magmatic sulfide.
And that's part of the reason then that large in these
provinces are important in magmatic sulfides because they
have a potential for huge volumes of magma to move by
magmatic sulfides and continually offer them the
opportunity to get more and more enriched in the sulfur loving
elements, nickel, copper and PGS that are very important in the
Green Revolution. We also see a correlation
between the emplacement of a large Igneous province and a
major mass extinction. Can you give examples of these
and how do we think this comes about?
Yeah, the best perhaps is the Siberian Trap event that I
mentioned before, which covers Northern Siberia and Central
Siberia. All of it seems to be in place
within less than a million years and precisely within a short
time both preceding and post dating the well dated timing of
the Permo Triassic mass extinction.
So that's a superb example. I mentioned the Central Atlantic
Magmatic Province, two O 1,000,000 years.
It precisely brackets the Triassic, Jurassic mass
extinction. The Deccan event marching this
province in India. Again, most of it is less than a
million years. It does seem to have two or
three distinct pulses, very short duration for the main
pulse, again bracketing in time the mass extinction associated
with the demise of the dinosaurs.
In that particular case, the dating on the chick cha Lube
meteorite impact in the Yucatan peninsula seems to give a
similar age, and so it looks like it's a joint effort between
a large igneous province and a meteorite impact in that case,
and causing that extinction. I presume the extinctions are
thought to be a consequence of the global environmental impacts
of these enormous eruptions that create these larger Guinness
provinces. But what exactly is so inimical
to life? There's a bounty of riches in
that regard. There's rapid global warming,
1520° temperature increases above present and acid rain can
be an important component. The Siberian Traps event
modeling shows that there's CO2 and SO2 sulfur dioxide coming
out, but there's also the thermogenetic gases.
So this huge amount of gas release from sills, the
intrusive component interacting with sediments and in the case
Siberian trap. There's the opportunity to
interact with hydrocarbons with oil deposits.
Also an opportunity to interact with evaporate sedimentary
deposits you get in deserts from evaporation of water.
They heated up some evaporates of sediments to see what would
be released from SIL at 1000° being adjacent to such sediments
and they would release chlorine and bromine compounds.
So those are ozone destroying. So the ozone destroying is also
a possibility. Mercury releases well poisonous
mercury. It seems that mercury levels are
higher from large against province events, so they are a
source of mercury poisoning potentially.
Paradoxically, you can also have remarkable cooling episodes that
are associated with large ING this provinces.
If you erupt a large ING this province at the equator, there's
a lot of flood basalts and then they can be rapidly eroded
within a million years or so, and that sucks a lot of CO2 of
the atmosphere. So if you're in a cooler
transition time, it's enough to potentially pull you into a
snowball Earth where the oceans are frozen to the equator.
And an example of that is the longest snowball Earth in this
Earth's history from about 719,000,000 to 660,000,000 years
ago, so-called sturtine glaciation.
And what occurred right at the absolute beginning of that?
The huge Largigius province in Canada, known as the Franklin
Largigius Province, 2 or 3,000,000 square kilometers in
Canada, whose precise ages 719 like a million years before the
onset of the sturtine glaciation.
And yet there's more. We have the same age event in
formerly attached Siberia called the Irkutsk Largeignius
Province, and there's more. We have at the same time in the
portion of southern Africa and probably into adjacent
Antarctica, another largeignius province called the Mutari
Fingerin that's of the same age. And also in South China there's
a Largeignius province of the same age, and probably in
Northern Europe. In Baltica there's another a
large igneous province of the same age.
Some of these other ones are also at the equator, so the
causing of the Grand 13 Glaciation Snowball Earth event
due to a large Igneous province, almost certainly the Franklin
Large Igneous Province was the cause and in a time where it was
already cooling down. It's interesting that for some
of these you eject a ton of greenhouse gases into the
atmosphere and you Jack up the temperature by 1020°.
Yet for others it's the subsequent effect of the
drawdown of CO2 from all the erosion leading to cooling that
is the major effect. I suppose it's a question of
different time scales. It's a little more complicated
than that because you also have sulfur dioxide release and so
that's initially greenhouse warming, but it can also convert
to a sulfate very rapidly and become solar blocking.
So you have the CO2 pulse initially from the gas, both
from the volcanics and from the heating up by seals of adjacent
sediments. The sulfur dioxide component, if
it's large, takes you from an initial warming and start some
cooling. And then if you rapidly remove
and erode the the salts at an equator, you more than
compensate for whatever CO2 you put into the atmosphere and you
have a net loss. So you would expect a little bit
of a pulse of warming right at the beginning to sulfite
aerosol, slight transition, then to some cooling and then if the
erosions kicks in then it can be even more dramatic cooling.
If three or four coincident Lodgingness provinces triggered
the Sturtian glaciation in the near Protrozoic, what about the
Marin Owens Snowball episode that came soon after the end of
the Sturtian? There's some candidates both for
the interruption between the Sturti and the Marinolan that
you have pump a lot of CO2 in the atmosphere and that may have
temporarily taken us out of the Sturti and and then there's
another candidate at the end of the Marinolan for potentially
ending that period. And then there's also the gas
gear's glaciation at 580 or so, which seems to be very short and
it may have been both started and ended by a large ignis
problem, started by a basaltic large ignis component and ended
by the felsa component. Can I mention one of the things
is we have this wild idea that Venus had a massive climate
change from earlier habitable conditions to its modern
climate, with no plate tectonics and no erosion.
Could multiple large igneous provinces happening at the same
time on Venus, each of them increasing the global
temperature by 10 or 15°? Say have 3 superimposed.
You can increase the global temperature by 45° and that
would lead to runaway ocean evaporation and lead to the end
of plate tectonics. Water is considered important
for subduction to continue, so we propose that this great
climate transition from Earth like conditions on Venus to the
current 450°C self cleaning oven type temperatures was initiated
through multiple large igneous provinces happening at the same
time. And we have cheekily said
perhaps this is an Earth's distant future.
There's nothing to stop a similar mechanism from happening
hundreds of millions of years from now.
Richard Anst, thank you very much.
Thanks so much, Oliver. To see pictures and
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