Original subtitles

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this

is mount peck 2 also known as chiang mai

mountain by the chinese

it lays here in northeast asia on the

border between china and north korea

although few outside the region even

though it exists the enormous strata

volcano is one of the largest and most

dangerous volcanoes on earth

its massive caldera which today is

partially filled by an equally massive

crater lake

has an area of almost 20 square

kilometers

that's more than five times larger than

the caldera of mount st helens

the eruption that shaped the caldera

around 1 000 years ago is considered to

be the second largest volcanic eruption

in the last 5

000 years classified as a 7 or

super colossal eruption on the volcanic

explosivity index

it ejected more than 100 cubic

kilometers of material enough material

to bury even villages more than 1200

kilometers away in northern japan

under a thick layer of ash and yet

despite its size and its destructive

pest mountain peck 2 was for the longest

time

and to some degree still is today a

mystery

even for the scientists that have

studied it the volcano remained a

conundrum only until very recently

nobody could explain why exactly here in

a tectonic sense

in the middle of nowhere one of the

largest volcanoes on earth exists

to understand why mount peck 2 is so

unique

we first have to understand what makes

ordinary volcanoes

well ordinary take a look at this map

it is a map of all the active volcanoes

in the pacific most of them run along

clearly visible lines

almost like a string of pearls why this

is becomes clear when you lay a map of

the tectonic plates over it

almost all volcanoes are found along

plate boundaries more specifically along

subduction zones

so zones where two plates collide and

the denser oceanic plates are pushed

under the continental plates

here in japan for instance where the

pacific plate is being subducted beneath

the okosuke and the philippine seaplate

and where the philippine sea plate is in

turn being subducted beneath the

eurasian plate

as a consequence of these complex

subduction zones japan

is one of the most volcanically active

regions in the world

in total the small island nation has

over 100 active volcanoes

the fuel of this subduction zone

volcanism is slightly ironically

water

oceanic crust holds huge amounts of

water stored in hydrous minerals which

form when crust interacts with seawater

as the subducting slab descends into the

mantle it encounters greater and greater

temperatures and pressures

this ultimately at depths of around 120

kilometers or 75

miles causes the rock to change its

chemical composition

and release the water that was trapped

in the crust into the mantle above

water added to the hot solid mantle

lowers its melting point

because the water molecules weaken the

crystalline structure of the rock

much in the same way that salt weakens

the crystalline structure of ice

the magma produced this way then rises

up into the overriding plate

collects in magma chambers and

eventually causes volcanic eruptions on

the surface

and so a volcanic arc that runs parallel

to the subduction zones is created along

the plate boundaries

a similar pattern can be found all

around the pacific ocean

like here in indonesia or in chile

in fact almost 80 percent of all active

volcanoes on earth run along the

numerous subduction zones in the pacific

which is why the region is commonly

referred to as the ring of fire

we already know this since the 1960s

when the model of continental drift

first proposed by alfred wegener in 1912

was finally proven

and together with the model of seafloor

spreading combined into the theory of

plate tectonics

there was only one problem there were

still a few volcanoes that didn't

conform to this theory

volcanoes that were seemingly sprinkled

across the globe at random

like here in hawaii some 3 800

kilometers or 2 400 miles away from the

next plate boundary

or in yellowstone chiang bai mountain

which sits more than a thousand

kilometers northwest of the japan trench

is another one of these weird

interpolated volcanoes

to explain this kind of volcanism a new

model was needed

an important clue to solve this mystery

could be found on the ocean floor in the

north pacific

if you follow the line of islands that

make up the hawaiian archipelago

northwest

you could find a series of volcanic

structures that once form very similar

islands before millions of years of

erosion reduce them to a series of

atolls and sea mounts

this so-called hawaiian emperor chain

contains more than 80 individual

structures like this

stretching nearly 6 000 kilometers or 3

900 miles across the ocean floor

on first glance you might come to the

conclusion that the source of hawaii's

volcanism must be moving

from northwest to southeast like a giant

marker drawing a line across the pacific

plate

however you can also explain this line

in a different way

the same pattern would be produced if

instead the oceanic floor is moving

to this realization came the canadian

geologist john wilson 2 in the 60s

the movement of the pacific plate from

southeast to northwest above a

stationary source of magma would also

produce this image

and sure enough this is exactly the

current path of the plate

such volcanism that isn't influenced by

plate tectonics

would need to have a much deeper origin

than other volcanoes

this idea was then developed into the

mantle plume model it explained these

volcanoes as hot spots that are fed by

enormous upwellings of hot mantle rock

that rise to the surface from the depths

of earth's mantle

driven by heat exchange across the core

mantle boundary 2 900 kilometers or 1800

miles below the surface

and while it would take a few more

decades before technology had advanced

to the point that it could actually

prove what wilson had only hypothesized

based on a few pieces of evidence

there was finally an explanation for

intra-plate volcanism

with this the puzzle surrounding the

source of the mysterious volcanoes in

places such as hawaii yellowstone and

northeast asia seemed to be solved

there was only one slight issue mount

peck 2

is not such a hot spot but scientists

wouldn't realize this

until the end of the 20th century

one reason the mountain has remained

elusive for such a long time certainly

has to do with its location

it not only lays in a very remote region

of asia

it also sits at the border of two

countries that lived in almost complete

isolation for much of the 20th century

it was only once china started to open

up more and more to the outside world in

the 80s and 90s

and began to promote scientific research

that gathering information about the

solitary giant became less of a

political impossibility

still it wasn't until the end of the 90s

that the first permanent seismographs

were installed on the chinese side of

the mountain and this was hugely

important because seismographs had

become perhaps

the most essential tool for studying the

geology of earth's interior

that's because of one clever invention

seismic tomography

in the 1970s computed tomography or ct

scans

revolutionized medicine by giving

doctors the ability to look into their

patients bodies without having to make

any cuts

and that way look for tumors fractures

infections

and a variety of other problems and

diseases safely and effectively

geologists realize that they can adopt

this technology for themselves to

finally reveal

what's going on hundreds or even

thousands of kilometers below our feet

the seismic waves produced by

earthquakes move at different speeds

depending on the temperature

density water content and state of the

rocks they travel through

you can measure the arrival times of

these waves at various points on the

surface

to map out the interior of our planet

this allowed scientists to quite

literally cut open the earth and see

what's inside

like medicine before this technology

revolutionized earth science

finally it was possible to see processes

that were long hypothesized

like the subduction of plates into the

mantle the formation of new ocean floor

through upwellings of hot mantle

material below mid ocean ridges and of

course plumes that rise up from the core

mantle boundary and feed hot spots such

as hawaii and yellowstone

and so for a brief moment in time we had

an explanation

and explanation for every form of

volcanism on the planet

all three forms subduction-induced

volcanism

mid-ocean ridge volcanism and mantle

plumes were supported by evidence

and seemed to explain every volcano or

volcanic structure on the planet

that was until the first tomographic

models of mount changbai appeared in the

late 90s and early 2000s what they

showed was significantly different from

what images of hawaii or yellowstone

showed

there wasn't the prominent low velocity

anomaly extending all the way down to

the core mantle boundary

instead the anomaly seemed to end

roughly 400 kilometers or 250 miles

below the surface

which is deep much deeper than the

source of any volcano near subduction

zones

but not nearly as deep as the 2900

kilometers or 1800 miles geologists had

expected

not only that but below the low velocity

anomaly there was a distinct high

velocity anomaly that showed up in the

images at depths of about 600 kilometers

or 380 miles

right here it showed what appeared to be

a large barrier of unusually

cold material this puzzled the

geological community

once again there was a form of

intra-plate volcanism that couldn't be

explained

that wasn't a typical hot spot like

previously thought

one of the largest volcanoes on earth

suddenly didn't seem to fit into

any existing category it was a

completely new

kind of volcano to understand what's

going on here

we first have to understand this zone

curiously the cold anomaly observed here

sits right inside the so-called mantle

transition zone

this is the known transition zone

between the lower and upper mantle

here the pressure is great enough that

it changes the internal arrangement of

the atoms by forcing the rock into a

much

denser more compact crystalline

structure

this results in a sudden quite

significant jump in viscosity in the

zone

because of this the mantle transition

zone acts as a sort of barrier in the

mantle

which at least partially separates the

processes in the upper and lower mantle

from one another

geologists quickly realized that the

distinct anomaly of cold material in

this zone can only be a piece of ancient

crust

expanding the image further to the east

quickly reveals which plate it is

it appears that the pacific plate after

subducting into the mantle

bends horizontally and becomes stagnant

in the mantle transition zone

right below mount pek 2. this wasn't

even that surprising

ever since we began to understand more

about the interior of our planet and its

layers

it has been speculated that slabs after

their subduction would become stagnant

here

in fact there was a fierce debate going

on in the geological community with one

side arguing that the convection

processes in the upper and lower mantle

are separated from one another and that

slabs after their subduction becomes

stagnant in the boundary layer

while the other side argued for whole

mantle convection and subducting slabs

sinking all the way to the bottom of the

mantle

forming a graveyard of slabs on top of

earth's core

but before you think clearly the first

group had it right here's an image of

the subduction zone just a few hundred

kilometers to the south

here you can clearly see the pacific

slab penetrating the mantle transition

zone and sinking into the lower mantle

likewise global seismic tomography

models have revealed an entire catalog

of roughly 100 old slabs that have

penetrated

deep into the mantle so clearly it's not

as simple as picking one model over the

other

that aside there was also the question

how a stagnant slab could even produce

volcanism so far away from the

subduction zone

earthquakes provided an important clue

most earthquakes on earth occur just

like volcanoes

near plate boundaries where the crust is

bent and deformed by the collision of

the plates

but seismographs in asia also frequently

pick up strong earthquakes deep below

mount chiang bai

600 kilometers or 380 miles below the

surface

remember this chart it'll be important

later

here our story takes a bit of a turn

because at the start of the 20th century

earthquakes became the center of

discussion for a different reason

in 2002 the newly established tiansher

volcano observatory

started to pick up a worrying increase

in seismic activity

5 kilometers or 3 miles below the

mountain

over the next year the number of

earthquakes increased from an average of

10 per month

to over a hundred in some months even

over 200

signaling that an outbreak might be

imminent

if we look at mount chiang bai's

eruptive history we can see that it can

be broadly divided into three

evolutionary stages

the volcanic activity started around 20

to 30 million years ago

as part of the large-scale interplayed

volcanism that took place in northeast

asia since the late jurassic period

around 2.5 million years ago

partial melting in the mantle below the

volcano increased significantly

and resulted in the eruption of

extensive amounts of basaltic lavas

which even today cover an area of around

ten thousand square kilometers

or four thousand square miles with an

enormous shield like plateau

the largest in the region due to the

enormous weight of this plateau

the rise of the magma below was

eventually slowed down

so much that it began to collect in a

shallower magma chamber before erupting

to the surface

this was actually very important as the

extra time changed the chemical

composition of the melt due to

contamination from the crust

and the deposition or removal of certain

minerals in the magma

as a result the lava that erupted onto

the surface became a lot more viscous

which led to the formation of a volcanic

cone on top of the basalt plateau

this second phase lasted from around 1

million to 4

000 years ago and by the end of it had

produced a cone roughly 4 kilometers or

13 000

feet high the last stage the explosive

stage

then resulted in the almost complete

destruction of this cone

which brings us back to the millennium's

eruption of 946

this enormous eruption blew off the

entire top half of the volcano

creating the caldera that we see today

if you imagine the amount of material

that must have existed

between the two kilometer high surface

of heaven lake that partially fills the

caldera today

and the once four kilometer high summit

of the volcanic cone

it's not difficult to see that the

millennium eruption must have been one

of the most catastrophic eruptions in

human history

it produced a plume of smoke and ash

that shot an estimated

25 kilometers or 15 miles into the

stratosphere

and ultimately covered more than 350 000

square kilometers

between northern korea and the kuril

trench in 5 to 10 centimeters or 2 to 4

inches of ash

that's an area larger than the entire

korean peninsula

the total amount of ash dispersed by the

eruption would have been enough to bury

the whole of new york city

under almost 150 meters or 500 feet of

ash

additionally pyroclastic flows

avalanches of superheated gas and debris

filled valleys and canyons as far as 50

kilometers or 30 miles in every

direction

burning an area the size of tokyo in the

process

at the time the area around the mountain

and northeast asia in general were not

particularly densely populated

but today more than 1.5 million people

live within the immediate

danger zone of the volcano and almost a

billion people live within the potential

fallout zone

within this 1500 kilometer radius you

can find many of the largest cities in

the world

such as seoul pyongyang tokyo osaka

beijing and shanghai if only one or two

of these mega cities would be buried in

volcanic ash from a similarly strong

eruption

the damages caused by that could easily

make it one of the most costly natural

disasters in history

but in mid-2005 the mountain went silent

again

the seismic activity went back to its

pre-2002 levels

and has remained that low ever since

making an eruption within the next few

years

unlikely at least at the present day but

what these years have shown us is that

the volcano is very much

active and will likely erupt again at

some point in the future

this ignited the scientific interest in

the volcano and resulted in a flood of

new research

primarily from china mountain pectus

sudden unrest

even got the normally very reclusive

north korean government worried

to the point that they started to reach

out to the west and eventually even

invited a team of scientists to the

country to

study the volcano together with their

north korean colleagues

the group collected data took rock

samples and installed a set of

seismographs to monitor the mountain

the first results of this unique

collaboration were published in 2016.

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with all the new research that has been

done over the last 10 to 20 years

we now have a pretty good understanding

of what causes this unusual form of

volcanism

using the latest seismic tomography

models created taking nearly 80

000 earthquakes recorded by over 3 000

seismic stations

we can now map the mantle below

northeast asia with never seen before

clarity

from here we can create a schematic 3d

model of the main features that drive

the volcanism

under chiang mai mountain let's start

with the most obvious one

the pacific plate as you can see a huge

portion of the slab has become

horizontal and stagnant under northeast

asia

after subducting at a roughly 30 degree

angle below the island of japan

in the process it has trapped much of

the upper mantle in the region between

the crust and itself

creating what geologists have termed a

big mantle wedge

under the korean peninsula and east

china

also of note is this gap roughly at 35

degrees northern latitude

here the pacific plate is visibly being

torn apart at depths of around 300

kilometers or 190 miles

with the southern part descending at a

much steeper 70 degree angle

and subsequently penetrating the lower

mantle

the length of the western edge of the

stagnant slab which is much longer than

the corresponding part of the subduction

zone

indicates that these two pieces were

once joined together

before they were ripped apart likely as

a result of the huge forces generated by

the differences in the subduction rate

direction and angle focusing on the

mantle below mount peck 2

reveals a prominent low velocity anomaly

of partially molten rock

that sits on top of the stagnant slab

and has a diameter of roughly 100

kilometers or 60 miles

it can be described as a plume that

rises from the mantle transition zone

this is an important distinction because

unlike normal mantle plumes that are

relatively unaffected by the tectonic

processes near the surface

this plume seems to be the direct result

of these processes

namely the subduction and stagnation of

the pacific plate

furthermore the source of its buoyancy

also seems to be different

while normal mantle plumes rise as a

result of heat exchange between the

lower mantle and the much hotter core

this plume is not significantly hotter

than the surrounding mantle

instead its buoyancy seems to mainly be

caused by water

electrical conductivity studies have

shown that the mantle transition between

northeast asia

holds huge amounts of water 5 to 10

times

more than you would normally find here

this water was likely trapped here by

more than 100 million years of

continuous subduction

and dehydration of the ocean floor along

the eastern seaboard of asia

deep earthquakes in the stagnant slab

seem to indicate that these dehydration

processes are still going on today

here's where the chart from earlier

comes in as you can see

earthquakes occur pretty much everywhere

in the downgoing slab

but they seem to pick up again in

magnitude and quantity once we get to

this point

the mantle transition zone below chiang

mai mountain where we can see a visible

cluster of deep earthquakes

these earthquakes may reflect the

reactivation of faults deep in the slab

caused by the horizontal bending of the

plate

near the japan trench many normal

faulting earthquakes occur in the outer

rise portion

because of the upward bending of the

oceanic lithosphere before the plate

subduction

a large amount of seawater may enter

into the deep portion of the pacific

plate through these normal faults

as the stress regime changes from

extension to compression during the

subduction

the normal faults are closed and thus a

large amount of water could be preserved

deep within the slab

while dehydration processes of the

overlying crust cause the commonly

observed arc volcanism

the water stored deep within the slab is

brought down to the mantle transition

zone

here the stress regime once again

changes because of the huge resistance

at the bottom of this zone

as the slab bends and deforms the

preserved faults are reactivated which

allows for dehydration processes deep

within the slab

how these dehydration processes look

like in detail is

however still subject to debate the

extreme temperatures and pressures

as well as the unique minerals found in

this zone make it difficult to predict

how exactly the water interacts with the

surrounding mantle

one thing however has become clear over

the last couple of years

the mantle transition zone below

northeast asia and likely elsewhere in

the world

is an enormous water reservoir and

likely plays a critical role in global

water recycling processes

thanks to the minerals in this zone that

are particularly good at incorporating

water into their structure

the mantle transition zone may hold more

water than all the oceans combined

below chiang bhai mountain in particular

it seems the water content has reached a

critical amount

enough that the hydrous mantle becomes

buoyant and starts to rise into the

upper mantle

here it accumulates on top of the mantle

transition zone and forms a hydrous

plume

convection processes in the big mantle

wedge caused by the fast subduction of

the pacific slab

likely further aid the plume's rise and

the reduction in pressure ultimately

causes dehydration melting of the wet

mantle rocks

the result is a continuous wet and hot

upwelling

right below northeast asia so to

summarize

mountain peck 2 is not a traditional hot

spot but a sort of back arc volcano

caused by a hydras plume

that rises from the mantle transition

zone and that is fueled by the

subduction and stagnation of the pacific

slab

under northeast asia detailed

tomographic images of the other volcanic

fields in the region

show similar structures below them

indicating that they were the result of

similar subduction related processes

in fact it seems the entirety of east

china and the korean peninsula were

heavily shaped by these mechanisms if

you map the western edge of the pacific

slab

you can find it lines up perfectly with

the so-called north-south

gravity linument an important

geophysical boundary in east china where

the surface topography visibly changes

you can see it quite clearly on this

height map when you cross this boundary

from west to east

the thickness of the continental plate

dramatically decreases

from around 100 kilometers or 60 miles

to about 50 kilometers or 30 miles

the result is a significant reduction in

surface elevation in northeast asia

that this boundary lines up so well with

the pacific plate below is no

coincidence

evidence suggests that the entire region

was the result of millions of years of

lithospheric destruction from below

caused by the continuous subduction in

the subduction related melting and

upwelling of mantle material

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the result of these processes was

extensive large-scale volcanism

and the formation of interplayed

volcanoes in northeast asia

of which chiang mai mountain is

currently the largest and most active of

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while this might seem like a story about

this one very unique volcano in a remote

corner of asia

it's actually much more than that what

the geodynamics of northeast asia have

shown us over the last 20 years

is that the mantle transition zone plays

a key role in continental magnetism

and that the importance of water in

these processes has been significantly

underestimated in the past

and this has historically not just

happened here in asia

wet upwellings related to the subduction

and stagnation of oceanic crust in the

mantle transition zone

may in fact provide a possible

explanation for much of the terrestrial

interplay volcanism

that has happened around the globe

geochemical analyses even suggest

similar processes are likely linked to

the formation of continental flood

basalts

which represent the largest volcanic

events in earth's history

these cataclysmic eruptions that covered

areas of hundreds of thousands to more

than a million square kilometers in lava

are important points in our geological

history

points that mark the rifting and breakup

of continents or catastrophic mass

extinctions

conventionally the formation of these

provinces was like their oceanic

counterparts

attributed to the initial arrival of

plumes from the core mantle boundary

but the chemical composition of

continental flood basalts is often

distinct

hydrous melt that seems to have

originated from recycled oceanic crust

could for instance be identified in the

siberian traps and the central atlantic

magmatic province

which instead implies a subduction

related origin

this means chiang mai mountain is not an

isolated case

instead it has become an important

window into processes that likely have

in the past

albeit on a much larger scale not just

played a key role in the formation and

breakup of continents

but also severely impacted the course of

evolution

on a global scale

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you