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.