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In the first video about Flood Basalts we
have learnt what Flood Basalt Eruptions are
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and what scope they had. We explored using
the example of the Siberian Traps, which are
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the remnants of one of the largest of these
eruptions in history, what impact this kind
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of volcanism can have on our planet and its
inhabitants and we have learned where and
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when else these so called large igneous provinces
arose.
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One question, however, remains to be answered.
Where did they come from? Or more specifically,
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what causes these cataclysmic eruptions. That's
what we will explore today.
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The exact processes are of course still subjecte
to heated debate – but the principal model
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for their origin which is widely accepted
today, is the so called Mantle Plume Model.
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Before we can discuss what exactly this is,
however, we first have to understand how volcanoes
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are typically formed. Almost all volcanoes
on Earth are the result of tectonic processes
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and are formed either by two colliding plates
or two plates drifting apart.
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Subduction zones are convergent boundaries
where the edge of one plate (usually a denser
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oceanic one) is forced under the edge of a
less dense plate and then pushed into the
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mantle. During the subduction the down-going
plate experiences increasing pressure and
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temperature. At depths of around 100 km or
60 mi the pressure is great enough that water
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inclusions trapped in the rock are freed and
released into the overlying mantel which lowers
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the melting point of the already hot mantle
rocks resulting in partial melting. This process
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is called Flux melting and the magma it produces
then slowly rises into the crust above and
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finally to the surface forming a chain of
volcanoes alongside to the Subduction zone
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In contrast divergent boundaries are zones
where two plates move apart. Here a spreading
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ridge – either in form of a rift valley
or a mid ocean ridge is created – through
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which hot mantle material can rise to the
surface. This also reduces the melting point
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of the rocks, this time as a result of decompression
melting caused by the reduction in pressure
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during the ascent. The resulting magma then
leaks onto the surface, cools and creates
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new ocean floor in form of giant undersea
mountain ranges.
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But how do mantle plumes fit into this picture?
- Surprisingly little. The volcanic processes
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we just covered are all the result of mechanisms
inside the uppermost layers of the earth – roughly
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the first 100-200 kilometres. Mantles Plumes
on the other hand have a much deeper origin.
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They are columns of enormous quantities of
hot rock, upwellings that rise to the surface
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from the depth of earth’s mantle – 2900
kilometres or 1800 mi below our feet. As such
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they are part of the on-going convection processes
that take place in the mantle in which hot
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material from inside the earth continuously
rises up below the tectonic plates, cools
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and moves back down again: Similar to what
happens inside a lava lamp, just way more
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complex. Because the mantle consists for the
most part out of solid rock that only behaves
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like a fluid over a geologic timeframe these
processes are of course very slow and take
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millions and hundreds of millions of years.
The formation of a mantle plume starts at
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the core-mantle-boundary. Here, in the thermal
and chemical boundary layer at the base of
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the mantle which separates the liquid outer
core from the solid lower mantle temperatures
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rise rapidly, faster than in any other layer.
The temperature of the outer core is already
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approximately 1,000 degrees Celsius higher
than that of the overlying mantle just a few
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kilometres above. This causes large amounts
of heat to be transferred into the mantle
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through conduction where it heats up the rock
causing it to start rising: A Mantle Plume
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forms.
As it ascends through the mantle it slowly
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start to take on a mushroom shape because
the hot material rises faster through the
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plume than the plume itself rises through
its surroundings – not unlike during the
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explosion of a nuclear bomb. Most plumes never
make it to the surface before they cool down
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again and lose their momentum but some of
the largest can rise all the way through the
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mantle and below the lithosphere. When such
a plume hits the tectonic plates, which act
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like a barrier it flattens out and deforms
into a thinner and wider disk. As we have
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discussed before the reducing pressure will
eventually reduce the melting point of the
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hot solid rock so much that it begins to melt.
This produces enormous quantities of liquid
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basaltic rock, basically a giant bubble of
magma with a diameter of multiple hundred
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kilometres directly beneath earth’s plates.
From here it will start to rise into the crust,
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build up in countless magma chambers and ultimately
produce large scale flood basalt volcanism
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on the surface.
This can go on for a few hundred thousand
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to a couple million years but eventually the
plume head will cool down so much that the
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large scale volcanisms stops. What remains
is the much more narrow tail of the plume
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which will periodically continue to transport
magma to surface for 100 million years or
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more until it dries up too.
This model is of course a very simplified
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view of the process. The reality is much more
complex and chaotic. But it gives us at least
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a basic understanding for why and how these
massive eruptions have occurred, why they
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are so rare and why their chemical composition
is so different from regular volcanoes. Beyond
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the formation of large igneous provinces this
models also allows us to explain the dozen
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or so volcanic hotspots that you can find
all across the planet. These regions of continuous
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volcanic activity are unusual because they
are often far away from plate boundaries - in
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some cases thousands of kilometres - and thus
can’t be explained through tectonic processes.
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Their chemical composition is also notably
different from other volcanoes and more in
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line with that of flood basalt eruptions.
The most well-known of these hotspots are
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probably Hawaii, Yellowstone and Iceland.
Because Mantle Plumes aren’t the result
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of plate movements they aren’t tied to them.
Quite the contrary: Because each plume is
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anchored at the core-mantle boundary and is
therefore relatively stationary in relation
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to the core the hotspot is constantly changing
is position on the plate– not because the
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hotspot is moving but because the plate is
moving. As the plate moves across the hotspot
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over the course millions of years it creates
a chain of volcanic structures. This explains
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for instance the formation of the Hawaii-Emperor
Chain, a chain of around 130 dormant sea-mounts
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stretching over 5800 kilometres across the
pacific plate like a string of pearls with
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the Hawaiian Islands at the end.
Other hotspots follow similar patterns. Yellowstone
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for instance has over the last 15 million
years slowly moved east-ward as the north
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American plate moved westward over the hotspot.
Follow this trail of breadcrumbs and you can
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see an image of the volcanic activity through
time. When you reach the point in time 16
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million years ago the position of the Yellowstone
hotspot on the North American plate overlaps
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suspiciously well with the Columbia River
Basalt Group. This suggests that the hotspot
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was responsible for the enormous flood basalt
province that formed between 16-9 mya when
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the Plume head hit the lithosphere and that
since then the tail of the plume was responsible
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for the regular supervolcano eruptions
In Hawaii’s case however you can’t trace
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the breadcrumbs back a large igneous province
as the subduction zone off the coast of Russia
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has already destroyed all evidence of its
existence. On first glance you might think:
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Doesn’t the trail point perfectly to the
Siberian traps? This is true, of course, but
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only a coincidence. Once you cross plate boundaries
you also have to consider the position of
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the plates relative to one another over time
and Eurasian Plate was of course at no point
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in time over todays Hawaii, at least not in
the last 500 million years or so – In fact
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no continental plate was – The Hawaiian
Hotspot was always surrounded by oceanic plate.
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When exactly it happened and how destructive
its first eruption was, we will therefore
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probably never find out.
For other hotspots this is however still possible.
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The Reuinon Hotspot off the coast of Madagascar
for instance can be linked to the Deccan Traps.
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When 66 million years ago the Indian Plate
on its way to its current position moved across
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the hotspot apparently right as the plume
head hit the lithosphere the resulting eruption
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then formed the massive lava province that
even today covers nearly a third of India.
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The some 4,300 km long chain of seamounts
produced by the Louisville Hotspot may point
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towards the Ontong Java Plateau while the
Iceland Hotpot was likely responsible for
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the formation of the North Atlantic Igneous
Province. But connections like these aren’t
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always possible – The Siberian Traps for
instance can’t definitely be linked to a
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present day hotspot. Iceland as well as the
Ural mountains are proposed locations of the
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plume today but it’s also possible that
its corresponding hotspot already went dormant
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in the past 250 million years.
But not every large ignous province can be
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explained through the mantle plume model - there
are exceptions.
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One such exception is the Central Atlantic
magmatic province whose remnants today stretch
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across 4 continents. This flood basalt province
lacks some of the key features usually attributed
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to mantle plumes for instance the characteristic
chemical composition of the rock. Evidence
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suggests that its formation instead was the
result of the rifting and breakup of the supercontinent
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Pangaea. This rift which later formed the
Atlantic Ocean could’ve freed enormous quantities
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of magma trapped inside and beneath the continental
crust produced by the nearly the 360° subduction
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zone around the supercontinent. Despite this
many researchers also believe that the arrival
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of a plume must have at least played some
role in the initial breakup of the Continent
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- which goes to show that there is still a
lot to learn about the geology of our planet.
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One tool that we are now using more and more
is so called Seismic tomography. This method
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– which is basically a CT scan of the interior
of our planet - makes use of seismic waves
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caused by earthquakes. Because these waves
travel at different speeds in various types
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of rocks and rocks of varying temperatures
we can measure the velocity of the waves at
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various points on the planet to make conclusions
about the physical properties of the subsurface.
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While the models these scans produce have
at the moment still a very poor resolution
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they have at least finally confirmed what
for a long time was only conjecture – namely
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deep-mantle plume-like structures under most
major hotspots like Yellowstone, Hawaii and
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Iceland. But these are obviously only still
images of convection processes that take millions
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of years. To fully understand the formation
of mantle plumes and flood basalt provinces,
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it therefore still requires a lot more research.
But large ignous provinces are of course not
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only of geological importance but also of
biological interest. What catastrophic impact
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the formation of such provinces can have on
the planet we have already discussed in the
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last part using the example of the Siberian
Traps – But this was by no means an isolated
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case – quite the opposite.
If you illustrate the formation of the largest
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flood basalt provinces of the last 500 million
years graphically, you can see a striking
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temporal correlation with the boundaries of
geologic time periods. These boundaries are
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defined by abrupt and significant changes
of earth’s biosphere and climate and the
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planet as a whole and often mark the point
of catastrophic mass extinctions.
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The formation of the Deccan Traps for instance
falls almost perfectly on the Cretaceous-Paleogene
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or KT boundary that marks the extinction of
the non-avian dinosaurs, the formation of
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the central Atlantic magmatic province overlaps
with the Triassic Jurassic boundary and the
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formation of the Siberian Traps coincides
as we have discussed with the Permian-Triassic
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Boundary and the End-Permian Extinction.
At the end of the Devonian Period the Earth
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also experienced this kind of volcanism - in
fact an exceptional amount of it. It is speculated
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that this was the result of the arrival of
a super plume under the east European platform
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resulting in the formation of at least 4 flood
basalt provinces over a relatively short 20
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million year timeframe. Although the timing
and importance of the individual events is
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in this case still poorly understand it might
be an explanation for the series of smaller
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extinction events typically combined into
the Late Devonian extinction. Particularly
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the formation of the Viluy Traps seems to
correspond well with the so called Kellwasser
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Event – the first and most severe of the
Late Devonian extinction Events.
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Together these four boundaries describe 4
of the 5 largest mass extinctions in history,
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each responsible for the loss of 70% or more
of all species and probably 99% or more of
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all individuals. Even the fifth and earliest
of these so called “Big 5” can be linked
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to large-scale volcanism due to elevated mercury
concentrations in the Ordovician rock layers
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although a corresponding lava province seems
to no longer exist.
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After the discovery of the Chicxulub crater
in the mid-20th century and the realisation
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that an asteroid likely was what had wiped
out the dinosaurs the consensus in the scientific
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community subsequently became that asteroid
impacts must the main driving force behind
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mass extinctions in general.
Today however, thanks to intensive research
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on the formation and role of large igneous
provinces over the past few decades we now
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know that this is likely not the case. Asteroids
certainly had an important impact on this
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planet but It simply can’t be a coincidence
that all 5 of the largest mass extinctions
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in history as well as many more - more or
less - dramatic changes of our planet all
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happened at the same time as some of the most
catastrophic lava eruptions the world has
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ever seen.
Advancements in high precision dating both
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of the mass extinction events and the formation
of the corresponding flood basalt provinces
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during the last 20 years have in almost all
cases only strengthened this temporal link
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and the examination of the events individually
has in most cases confirmed volcanism as the
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main cause for the extinctions and not as
previously thought, asteroid impacts.
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Even for the case example - the extinction
of the non-avian dinosaurs – we now know
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that large scale volcanism has at least played
a role: Through the formation of the Deccan
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traps that started erupting about 400,000
years before the Chicxulub impact and continued
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for another 600,000 years after expelling
a total of over half a million km³ of lava.
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Although this eruption didn’t nearly have
an as catastrophic effect on the climate as
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previous ones, probably due to the lack of
large scale sill intrusion, it still caused
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alternating episodes of warming and cooling
for multiple hundred thousand years. This
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must have put significant stress on the biosphere
and likely weakened many animal populations
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as a result which is why many researchers
believe the later impact might have not been
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enough to drive the dinosaurs to extinction
if it hadn’t been for the contemporary volcanism.
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If you take all this into consideration it
can no longer be denied what impact flood
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basalts had and likely will continue to have
on the development and evolution of life on
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earth. It seems no other natural disaster
has on such a scale decided who lives and
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who dies.
We owe these catastrophes our existence. Humans,
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just like every other animal alive today are
the ancestors of the few survivors of these
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mega eruptions. The ancestors of the species
that managed to survive literal hell on earth
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for sometimes hundreds of thousand years only
to then rise of the ashes to bring new life
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to the planet that we call home.22475
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