In the first video about Flood Basalts we
have learnt what Flood Basalt Eruptions are and what scope they had. We explored using
the example of the Siberian Traps, which are the remnants of one of the largest of these
eruptions in history, what impact this kind of volcanism can have on our planet and its
inhabitants and we have learned where and when else these so called large igneous provinces
arose. One question, however, remains to be answered.
Where did they come from? Or more specifically, what causes these cataclysmic eruptions. That’s
what we will explore today. The exact processes are of course still subjecte
to heated debate – but the principal model for their origin which is widely accepted
today, is the so called Mantle Plume Model. Before we can discuss what exactly this is,
however, we first have to understand how volcanoes are typically formed. Almost all volcanoes
on Earth are the result of tectonic processes and are formed either by two colliding plates
or two plates drifting apart. Subduction zones are convergent boundaries
where the edge of one plate (usually a denser oceanic one) is forced under the edge of a
less dense plate and then pushed into the mantle. During the subduction the down-going
plate experiences increasing pressure and temperature. At depths of around 100 km or
60 mi the pressure is great enough that water inclusions trapped in the rock are freed and
released into the overlying mantel which lowers the melting point of the already hot mantle
rocks resulting in partial melting. This process is called Flux melting and the magma it produces
then slowly rises into the crust above and finally to the surface forming a chain of
volcanoes alongside to the Subduction zone In contrast divergent boundaries are zones
where two plates move apart. Here a spreading ridge – either in form of a rift valley
or a mid ocean ridge is created – through which hot mantle material can rise to the
surface. This also reduces the melting point of the rocks, this time as a result of decompression
melting caused by the reduction in pressure during the ascent. The resulting magma then
leaks onto the surface, cools and creates new ocean floor in form of giant undersea
mountain ranges. But how do mantle plumes fit into this picture?
– Surprisingly little. The volcanic processes we just covered are all the result of mechanisms
inside the uppermost layers of the earth – roughly the first 100-200 kilometres. Mantles Plumes
on the other hand have a much deeper origin. They are columns of enormous quantities of
hot rock, upwellings that rise to the surface from the depth of earth’s mantle – 2900
kilometres or 1800 mi below our feet. As such they are part of the on-going convection processes
that take place in the mantle in which hot material from inside the earth continuously
rises up below the tectonic plates, cools and moves back down again: Similar to what
happens inside a lava lamp, just way more complex. Because the mantle consists for the
most part out of solid rock that only behaves like a fluid over a geologic timeframe these
processes are of course very slow and take millions and hundreds of millions of years.
The formation of a mantle plume starts at the core-mantle-boundary. Here, in the thermal
and chemical boundary layer at the base of the mantle which separates the liquid outer
core from the solid lower mantle temperatures rise rapidly, faster than in any other layer.
The temperature of the outer core is already approximately 1,000 degrees Celsius higher
than that of the overlying mantle just a few kilometres above. This causes large amounts
of heat to be transferred into the mantle through conduction where it heats up the rock
causing it to start rising: A Mantle Plume forms.
As it ascends through the mantle it slowly start to take on a mushroom shape because
the hot material rises faster through the plume than the plume itself rises through
its surroundings – not unlike during the explosion of a nuclear bomb. Most plumes never
make it to the surface before they cool down again and lose their momentum but some of
the largest can rise all the way through the mantle and below the lithosphere. When such
a plume hits the tectonic plates, which act like a barrier it flattens out and deforms
into a thinner and wider disk. As we have discussed before the reducing pressure will
eventually reduce the melting point of the hot solid rock so much that it begins to melt.
This produces enormous quantities of liquid basaltic rock, basically a giant bubble of
magma with a diameter of multiple hundred kilometres directly beneath earth’s plates.
From here it will start to rise into the crust, build up in countless magma chambers and ultimately
produce large scale flood basalt volcanism on the surface.
This can go on for a few hundred thousand to a couple million years but eventually the
plume head will cool down so much that the large scale volcanisms stops. What remains
is the much more narrow tail of the plume which will periodically continue to transport
magma to surface for 100 million years or more until it dries up too.
This model is of course a very simplified view of the process. The reality is much more
complex and chaotic. But it gives us at least a basic understanding for why and how these
massive eruptions have occurred, why they are so rare and why their chemical composition
is so different from regular volcanoes. Beyond the formation of large igneous provinces this
models also allows us to explain the dozen or so volcanic hotspots that you can find
all across the planet. These regions of continuous volcanic activity are unusual because they
are often far away from plate boundaries – in some cases thousands of kilometres – and thus
can’t be explained through tectonic processes. Their chemical composition is also notably
different from other volcanoes and more in line with that of flood basalt eruptions.
The most well-known of these hotspots are probably Hawaii, Yellowstone and Iceland.
Because Mantle Plumes aren’t the result of plate movements they aren’t tied to them.
Quite the contrary: Because each plume is anchored at the core-mantle boundary and is
therefore relatively stationary in relation to the core the hotspot is constantly changing
is position on the plate– not because the hotspot is moving but because the plate is
moving. As the plate moves across the hotspot over the course millions of years it creates
a chain of volcanic structures. This explains for instance the formation of the Hawaii-Emperor
Chain, a chain of around 130 dormant sea-mounts stretching over 5800 kilometres across the
pacific plate like a string of pearls with the Hawaiian Islands at the end.
Other hotspots follow similar patterns. Yellowstone for instance has over the last 15 million
years slowly moved east-ward as the north American plate moved westward over the hotspot.
Follow this trail of breadcrumbs and you can see an image of the volcanic activity through
time. When you reach the point in time 16 million years ago the position of the Yellowstone
hotspot on the North American plate overlaps suspiciously well with the Columbia River
Basalt Group. This suggests that the hotspot was responsible for the enormous flood basalt
province that formed between 16-9 mya when the Plume head hit the lithosphere and that
since then the tail of the plume was responsible for the regular supervolcano eruptions
In Hawaii’s case however you can’t trace the breadcrumbs back a large igneous province
as the subduction zone off the coast of Russia has already destroyed all evidence of its
existence. On first glance you might think: Doesn’t the trail point perfectly to the
Siberian traps? This is true, of course, but only a coincidence. Once you cross plate boundaries
you also have to consider the position of the plates relative to one another over time
and Eurasian Plate was of course at no point in time over todays Hawaii, at least not in
the last 500 million years or so – In fact no continental plate was – The Hawaiian
Hotspot was always surrounded by oceanic plate. When exactly it happened and how destructive
its first eruption was, we will therefore probably never find out.
For other hotspots this is however still possible. The Reuinon Hotspot off the coast of Madagascar
for instance can be linked to the Deccan Traps. When 66 million years ago the Indian Plate
on its way to its current position moved across the hotspot apparently right as the plume
head hit the lithosphere the resulting eruption then formed the massive lava province that
even today covers nearly a third of India. The some 4,300 km long chain of seamounts
produced by the Louisville Hotspot may point towards the Ontong Java Plateau while the
Iceland Hotpot was likely responsible for the formation of the North Atlantic Igneous
Province. But connections like these aren’t always possible – The Siberian Traps for
instance can’t definitely be linked to a present day hotspot. Iceland as well as the
Ural mountains are proposed locations of the plume today but it’s also possible that
its corresponding hotspot already went dormant in the past 250 million years.
But not every large ignous province can be explained through the mantle plume model – there
are exceptions. One such exception is the Central Atlantic
magmatic province whose remnants today stretch across 4 continents. This flood basalt province
lacks some of the key features usually attributed to mantle plumes for instance the characteristic
chemical composition of the rock. Evidence suggests that its formation instead was the
result of the rifting and breakup of the supercontinent Pangaea. This rift which later formed the
Atlantic Ocean could’ve freed enormous quantities of magma trapped inside and beneath the continental
crust produced by the nearly the 360° subduction zone around the supercontinent. Despite this
many researchers also believe that the arrival of a plume must have at least played some
role in the initial breakup of the Continent – which goes to show that there is still a
lot to learn about the geology of our planet. One tool that we are now using more and more
is so called Seismic tomography. This method – which is basically a CT scan of the interior
of our planet – makes use of seismic waves caused by earthquakes. Because these waves
travel at different speeds in various types of rocks and rocks of varying temperatures
we can measure the velocity of the waves at various points on the planet to make conclusions
about the physical properties of the subsurface. While the models these scans produce have
at the moment still a very poor resolution they have at least finally confirmed what
for a long time was only conjecture – namely deep-mantle plume-like structures under most
major hotspots like Yellowstone, Hawaii and Iceland. But these are obviously only still
images of convection processes that take millions of years. To fully understand the formation
of mantle plumes and flood basalt provinces, it therefore still requires a lot more research.
But large ignous provinces are of course not only of geological importance but also of
biological interest. What catastrophic impact the formation of such provinces can have on
the planet we have already discussed in the last part using the example of the Siberian
Traps – But this was by no means an isolated case – quite the opposite.
If you illustrate the formation of the largest flood basalt provinces of the last 500 million
years graphically, you can see a striking temporal correlation with the boundaries of
geologic time periods. These boundaries are defined by abrupt and significant changes
of earth’s biosphere and climate and the planet as a whole and often mark the point
of catastrophic mass extinctions. The formation of the Deccan Traps for instance
falls almost perfectly on the Cretaceous-Paleogene or KT boundary that marks the extinction of
the non-avian dinosaurs, the formation of the central Atlantic magmatic province overlaps
with the Triassic Jurassic boundary and the formation of the Siberian Traps coincides
as we have discussed with the Permian-Triassic Boundary and the End-Permian Extinction.
At the end of the Devonian Period the Earth also experienced this kind of volcanism – in
fact an exceptional amount of it. It is speculated that this was the result of the arrival of
a super plume under the east European platform resulting in the formation of at least 4 flood
basalt provinces over a relatively short 20 million year timeframe. Although the timing
and importance of the individual events is in this case still poorly understand it might
be an explanation for the series of smaller extinction events typically combined into
the Late Devonian extinction. Particularly the formation of the Viluy Traps seems to
correspond well with the so called Kellwasser Event – the first and most severe of the
Late Devonian extinction Events. Together these four boundaries describe 4
of the 5 largest mass extinctions in history, each responsible for the loss of 70% or more
of all species and probably 99% or more of all individuals. Even the fifth and earliest
of these so called “Big 5” can be linked to large-scale volcanism due to elevated mercury
concentrations in the Ordovician rock layers although a corresponding lava province seems
to no longer exist. After the discovery of the Chicxulub crater
in the mid-20th century and the realisation that an asteroid likely was what had wiped
out the dinosaurs the consensus in the scientific community subsequently became that asteroid
impacts must the main driving force behind mass extinctions in general.
Today however, thanks to intensive research on the formation and role of large igneous
provinces over the past few decades we now know that this is likely not the case. Asteroids
certainly had an important impact on this planet but It simply can’t be a coincidence
that all 5 of the largest mass extinctions in history as well as many more – more or
less – dramatic changes of our planet all happened at the same time as some of the most
catastrophic lava eruptions the world has ever seen.
Advancements in high precision dating both of the mass extinction events and the formation
of the corresponding flood basalt provinces during the last 20 years have in almost all
cases only strengthened this temporal link and the examination of the events individually
has in most cases confirmed volcanism as the main cause for the extinctions and not as
previously thought, asteroid impacts. Even for the case example – the extinction
of the non-avian dinosaurs – we now know that large scale volcanism has at least played
a role: Through the formation of the Deccan traps that started erupting about 400,000
years before the Chicxulub impact and continued for another 600,000 years after expelling
a total of over half a million km³ of lava. Although this eruption didn’t nearly have
an as catastrophic effect on the climate as previous ones, probably due to the lack of
large scale sill intrusion, it still caused alternating episodes of warming and cooling
for multiple hundred thousand years. This must have put significant stress on the biosphere
and likely weakened many animal populations as a result which is why many researchers
believe the later impact might have not been enough to drive the dinosaurs to extinction
if it hadn’t been for the contemporary volcanism. If you take all this into consideration it
can no longer be denied what impact flood basalts had and likely will continue to have
on the development and evolution of life on earth. It seems no other natural disaster
has on such a scale decided who lives and who dies.
We owe these catastrophes our existence. Humans, just like every other animal alive today are
the ancestors of the few survivors of these mega eruptions. The ancestors of the species
that managed to survive literal hell on earth for sometimes hundreds of thousand years only
to then rise of the ashes to bring new life to the planet that we call home.