This is geology B with all of us trampled.
Much of the Earth's land surface is flat.
There are huge steps in Eurasia tundra, in the Arctic,
grasslands, and farmland in North America.
We can easily imagine a simple flat planet with nothing but
planes, but in fact, dramatic mountain ranges rise up on every
continent. The distinguished geologist Mike
cell has been studying mountain ranges for 40 years Mike cell.
Welcome to geology B mountain ranges such as the Himalaya the
Andes or the Rockies are some of the most prominent features on
the surface of our planet. Anyone trying to understand how
the others evolved over geological time has to explain
how he get these huge, topographic landmarks.
So I'm going to jump right in and ask you how do mountains
form Well, most mountain ranges are formed along plate
boundaries. So everyone's heard of plate
tectonics. This is the theory that divides,
the Earth's crust up insist seven or eight fairly large
plate and the plates move around driven by convection in the
mantle and those convection currents come up and generate
new crust along mid-ocean ridges.
And they consume ocean crust as the crust plunges.
Down subduction zones, when we see convection say in the
atmosphere, when air rises and makes clouds, it tends to be a
fairly chaotic Affair. So I suppose, the convection in
the Earth's, mantle that drives these tectonic plates is also
periodic orbit at much larger scales.
This means that the plates must all be moving in different
directions with plates, moving away.
A from each other along some of their margins such as at the
mid-ocean ridges you mentioned but also bumping into each other
on the other side. Well that's the interesting part
when mountains get made. So the Alpine Himalayan, Shane
runs pretty much all around the world and it's a result of the
Collision of a series of southern continents that were
all part of this great. Super continent called gondwana
colliding. With the Some consonants which
are part of Asia that collision between the two continental
plates varies along the strike, but it has very similarity in
processes that are happening at the same time, all the way for
example, from the Alps to the Himalayas, and even down towards
Indonesia. Well, that makes sense
intuitively that mountain should form where plates are colliding
with each other but that is a truly gigantic scale.
You're talking about From the Himalaya to the Alps.
There must be a lot of diversity in.
What's happened along this Collision belt, though, because
the Himalaya quite different from the apps.
Yes, that's true. The Alps in the Himalayas were
both formed by two, continental Collision but they are very
different indeed. As you say, the Himalayas is
probably one of the simplest of the Collision belts that we see,
which is why it's such a beautiful place for a geologist
to work because we have along the Himalayas a mountain chain.
In about two and a half thousand kilometers long.
Probably 100 to 200 kilometers wide.
And the processes of collision are very, very similar all the
way along that mountain chain from Pakistan to India, to Nepal
to Southern Tibet. And we can determine them using
the geology and stratigraphy and geochronology how you date rocks
using fossils or using radioactive minerals like
Zircon, we can determine the processes.
That formed that major Collision belt.
So, for example, in the Himalayas we know that the
Indian plate and Asian plate. Collided about 50 million years
ago, roughly down at equatorial latitudes since when India has
ploughed on North Woods, indenting into Asia as it goes,
and creating the Himalayas in the last 50 million years.
Okay, let's talk then about what's gone on and still going
on during this drawn out, 50 million year collision.
And I want to throw in an amazing fact, namely that we
find fossils of marine creatures, right at the very top
of Everest. So, how did rocks from the
bottom of an ocean manage to find their way to the summit of
the highest mountain in the world?
That's absolutely correct. The summit rocks of Mount
Everest, the highest rocks on the planet. 650 meters above
sea. Level are actually Marine
limestone's. They were formed during the
ordovician about 450 million years ago and they still contain
relics fossils in them. They had little tiny crinoids
ossicles. Ossicles of the stems of
crinoids which has see fans or sea lilies, but are still in the
oceans today and they are perfectly preserved.
In these limestone's taken from the summit of Mount Everest.
Well, how did sedimentary rocks formed at sea level end up at
nearly nine kilometers above sea level.
And that is because as India is colliding with Asia, the Indian
place is diving down beneath the Asian plate, and as it Dives
down, it is stacking units up. So all the rocks that form the
northern boundary of India, the sedimentary rocks that form the
northern boundary of India are all folded.
And thrust and that folding and thrusting shortens the rocks and
as India is diving down, it is completely jacking up the crust
of Tibet. As it goes, it's a bit like
putting a jack underneath the car to fix your puncture and as
the jack is continually wound up, the car goes up and up and
up. So was that devastating
earthquake in Nepal in 2015 part of that jacking up process and
where the mountains suddenly a bit higher after that
earthquake. Yes, exactly.
The Gawker earthquake of 2015 was just one of a whole series
of hundreds and thousands of earthquakes that have similarly
affected the Himalayas through time and that's how you build up
mountain ranges. Like the Himalayas, each one of
those earthquakes results in the Rocks.
Moving up to 10 metres, in some cases in some of the largest
earthquakes. With regard to The Gawker
earthquake in Nepal. That was a thrust fault so it
records shortening on the basil thrust as India is diving down
beneath the Himalayas and the Rocks beneath Katmandu shortened
by anything up to a meter and a half instantly and the mountains
immediately to the north of the Kathmandu Valley, Rose by just
over one meter. In a few seconds, it may seem
ironic global positioning We have scattered around the pool
showed that the highest peaks around Mount Everest actually
sank a little bit. But in fact, they sank because
the whole of Nepal was tilted down to the north and up to the
South. So the most devastating parts of
that earthquake were all around the parts along the northern
side of the Kathmandu Valley. For example, where the Rocks
raised by over a metre and horizontally shortened okay, I
can see how a plate can jack up. Another plate when it's squeezed
underneath it, but that still doesn't really explain how we
get these spectacular Peaks and valleys as opposed to say a flat
Plateau. Well, it does make a flat
plateau in Tibet to the north of the Himalayas.
So Tibet is by far the largest area of highest elevation and
thick crust anywhere on the planet.
The average elevation of the Tibetan Plateau is 5 kilometers
above sea level. And the crust is double the
thickness of anywhere else around it in India, or on the
continent, it's up to 70 or 80 km today.
So, the Himalayas of the collisional mountain range that
borders, the southern edge of this plateau.
And the only reason we know what the structure of the crust is
like, underneath Tibet is from geophysics, you can't see it in
the The layers, you can see it. You can walk up any of the
valleys which drain north-south on the most incredible thing
about the Himalayas is the early geologists discovered a hundred
years ago was the further north, you tracked, along those valleys
towards Tibet, the deeper the Rocks exposed, so you're putting
deeper hired pressure higher temperature, rocks on top of
shallower sedimentary rocks, and that is pretty unusual.
Emily on the planet, the deeper you go.
You increase pressure and you increase temperature.
So you would convert a sedimentary rock like a
limestone or a Shale to a metamorphic, rock like a marble
or AP like sappy light is just a metamorphose Shale and it has
these distinctive minerals in metamorphic minerals that we can
derive, an idea of the depth of burial of those rocks and as you
walk, Woods across the Himalayas, you're going
progressively deeper, as you go to the north, and this is what
we call an inverted metamorphism.
So the whole of the sudden, slopes of the Himalayas, all the
way along the mountain range are actually upside down.
I don't to keep harping on this question, but it's still not
clear to me. Just how any of these geological
processes actually makes the mountains into the shapes that
we see well that is largely to do with the geomorphology.
What we See today, along the Himalayas is the results of
dominantly glacial erosion mountains are above 5 km high
are going to be glaciated. So when you track up to Everest,
for example, the Western Combe which is the classic climbing
route up from the Nepal side up through, Everest is a classic
U-shaped valley. And those glaciers that khumbu
Glacier has eroded maybe three or four kilometers deep into the
crust of Everest. so what we're seeing today on the surface, all
those beautiful geomorphological mountains are actually a result
of dominantly glacial erosion but also flew the alliteration
further to the South, the rivers cut these v-shaped valleys in
and as you go further north towards the highest peaks those
v-shaped valleys become glacial u-shaped valleys with the
classic geomorphology of glacial erosion, really the Glacial
erosion is only the upper few kilometers of the crust, and
that's what makes the mountains that we see today.
Okay, so we've talked about making mountain ranges like the
Alps and Himalaya. When two, continental plates,
collide, what happens when we get an ocean plate colliding?
With a continental plate? Well, the classic example of
that is the Andes in South America, which is a long linear
mountain range, thousands of kilometers long and formed in a
completely different process. It's the Collision of two
plates, but as you say, the Pacific Ocean plate is
subducting eastwards, beneath the South American crust.
And as that plate subducts, it is releasing fluids, which melt
the base of the crust to form. Very distinctive rocks that we
call Granite. Granite is a quartz and
feldspar, Rich rock which intrudes up forming these.
Great Granite batholiths. That we see a batholith is
simply a long linear, granitic structure that it actually forms
the substrate of some of the most explosive volcanoes on the
planet. Things like Mount Saint Helens.
The great eruptions that we see in South America and Central
America. Roll the most siliceous volcanic
rocks, that are the Upper Crust, all parts of the deeper Granite
backless. And the granite batholiths are
exposed in some of the most classic climbing, areas of the
world Yosemite Valley. For example, the El Capitan
Granite is an example of one of these or the Patagonian Granite
spires down in the southern parts of South America and the
Upper Crust above. Those granite's.
Are these explosive Volcanoes, which are made dominantly of
rocks we call and decides named after the Andes and they are
much richer in silica than the rocks formed in the oceans and
hence, they're much more explosive and much more
dangerous. So there's one more type of
collision between two plates. And that's when no continental
plate is involved. And we have one oceanic plate
colliding, with another. If those also produce mountains,
they certainly can't be built. On top of a continent.
Yes. No.
Those are examples of island arcs and Island arcs are very
important mountain ranges because they actually form some
of the highest topography that we see on the planet, the
classic examples of island arcs are in the Western Pacific.
And most notably the Mariana Islands Arc which is an island
arc that stretches from south of Japan.
All the way down through the Philippines to Indonesia.
And the trench where the Pacific Ocean is subducting to the West
is actually the deepest part of the Earth today, 12 km below.
Sea level, remember Mount Everest is just under 9
kilometers above sea level, but the deepest parts of the oceans
are even more. So, you could actually say that
the highest mountains on Earth, are the volcanoes that make up
parts of the Mariana Islands, dark.
If you go from the bottom of the trench, to the top of the
volcano, It's 12 km the Rocks produced at Island arcs, and
much more basaltic that there are sort of intermediary between
the explosive andesitic. Volcanoes, we see in the Andes
for example, so there's other incredibly explosive volcanoes
along Island Arts that we see in Kamchatka.
Mount Fuji in Japan, Mount Pinatubo in the Philippines.
All of these examples forming Island aren't Mountain chains in
the All of the oceans. Okay, so you've told us about
three very different kinds of mountain ranges that form near
the edges of tectonic plates when they Collide.
So do we only see mountains on plate margins dominantly yes.
The longest mountain range is probably in the oceans along the
Mid-Atlantic Ridge which runs all the way across the globe and
fortunately in the north the middle and sick Ridge is
actually Exposed on the island of Iceland.
Where we can see volcanoes forming along a rifting margin
underneath the ocean that is a very very prominent mountain
range that stretches right the way down the Atlantic and the
Atlantic is a growing ocean so it's expanding with time.
So mid-ocean ridges are important mountain ranges.
There are not that high compared to the Andes, but they're very
very prominent, you drain all the water out of the Atlantic
Ocean. This would be a very Melons
range of mountains stretching the entire globe and they're all
made of battled. So they're much much simpler and
anything on the continent and wondering whether these kinds of
mountains, perhaps with the exception of Iceland happen only
under the ocean. No, they also occur under the
continents and the classic example of this is the East
African Rift. Where the very old continent of
Africa is splitting apart all the way down, its eastern
margins. And in the far north of this
East African Rift in Ethiopia is a place where you can see this
ocean, propagating from the Red Sea down into the continent.
So the afar triangle region, the danakil depression in Ethiopia
is an absolutely incredible place.
It's below. Sea level is highly volcanic.
And you can see faults and basaltic.
Eruptions forming new ocean crust.
And that ocean crust in the North.
And the Red Sea has been there for about 10 million years.
And as it's propagating down towards the African continent is
forming, this Great Rift Valley where the two sides of Africa
are being pulled apart dominantly by mantle convection
in the Deep crust and the mantle below southern Africa, and they
are typified by huge volcanoes. Like Mount Kilimanjaro.
And these are Again, very distinct volcanic rocks.
They are not the same as the Rocks, we see along the
mid-ocean ridges. And there's certainly not the
same as the Rocks. We see in the Andes or an island
arcs. There are much more Alkali Rich
battled because they are more rich in elements like sodium and
potassium. But the whole of the African
place is actually rifting apart East West and the island of
Madagascar is an older part So, back Rica that Rift it off 20 or
30 million years ago. And there's now oceanic crust in
between the two and predicting ahead, maybe 20 or 30 million
years time, the Red Sea will be expanding pushing Arabia and
Africa apart. And the African continent will
be split into East Africa to Western and central Africa.
So we've seen that mountain, ranges form, not only at exist.
Ting plate, boundaries where a collision is going on, but also
within the plate where a plate is splitting apart and you
plate, boundaries are forming. Do these multiple types of
mountain building on the edges of plates account for all of the
mountains on the planet? But not all of them.
There are other distinct mountains in the middle of the
ocean islands, like Hawaii, or the Galapagos Islands are very
unique because they are huge. Volcanic edifices sitting Top of
what we call hot spots. These are anomalous thermal hot
spots that go deep into the mantle and they've been
generating basaltic magma for a long, long time.
So Hawaii is the classic example of a hotspot Island and we can
see if we look at a map of the Pacific, but there is a train of
islands, going off to the Northwest and eventually all the
way across the Pacific to the Kamchatka trench.
This is the Hawaii. Emperor seamount chain.
Say, and those islands are getting progressively older.
As you go Northwest words away from the big island of Hawaii,
which is the site of the presently, active top spot,
you've talked about several different kinds of mountain
ranges that we can't really see because they're largely
submerged the island arcs, the mid-ocean ridges and the hotspot
chains, if we emptied out the oceans, would they look
different from each other? They would if you drain the
oceans, you would be surprised because you would end up looking
at thousands and thousands of isolated volcanic seamounts.
The Pacific has literally thousands of these islands and
90% of them are under sea level. We don't see them, The big ones
that we do see like Hawaii and Galapagos are major hot spots
that have been around for 40 50, 60, million years, or more other
ones that you see in the Indian Ocean.
For example, the island of reunion is a very interesting
volcano because that is another one of these Hawaiian type
Alkali, Basalt ocean Islands, a fixed hotspot.
And there is a chain of islands. Going to the north of rainy on
which goes along the Ow, gasps Mall died lacquered, I've chain,
and it eventually ends up at the Deccan battles in Western India.
Mount Everest has nearly nine kilometers high, and you've said
that the Mariana Trench has nearly 12 kilometers deep.
What determines this? What determines?
How high mountains can grow various points.
Mostly the tectonics defines, whether you have a steady
eruption of larvas like Hawaii, which is gradually built up over
a long period of time. Whether you have sudden, huge
eruptions, like the, some of the island arc volcanoes along the
Mariana Trench. But with the case of Mount
Everest, the reason the Himalayas are so high and
Everest in particular, being the highest point of the planet,
it's a trade-off between tectonics that creates mountains
uplifts them and erosion forces that break them down by the
weathering or chemical erosion. And that forces mountains to
come back down to sea level, erosion, always wants to
maintain the status quo. So the quicker, the place is
moving the higher. The mountains of the Himalayas
of forming and erosion cannot keep Pace with tectonics along
the Himalayas as soon as the Indian plate starts to slow down
and tectonics will give way through erosion, erosion, will
eventually Wipe Out the Himalayas and It's a sea level.
This is what's happened in the past a lot of Precambrian
mountain ranges. We see, for example, in Northern
Canada were probably Himalayan type Mountain belts in their day
and then 500 600 million years ago, they were eroded back down,
towards sea level, and planed off, and to an extent.
That's what you see today in the Great Northern Prairie lands of
America and in the Tundra's of the Arctic Canada, Those were
big mountain. Ranges 500, 600 million years
ago and they've been flattened by erosion.
So, does that mean that when we see flat planes on Earth that
they've all been mountainous and then flattened by erosion or
have some of them actually, escaped mountain building events
for all of geological time. No, no, a lot of the great flat
places of the Earth today have escaped mountain ranges.
They would they never were. So if you look at the Great
Plains of central Asia, for example, the oldest mountain
ranges in Europe, go up towards Northern to that and along the
Carpe. The ins that sort of area,
everything to the north of that has never really been in a
mountain building. Phase ever.
Other places, for example, in North America, most of Northern
Canada has been involved in big mountain range mountain,
building processes right up until 500 600 million years ago.
And before that, they were very active mountain ranges.
Some of them like we see, for example, in the Himalayas today,
but they've been eroded down and being pretty close to sea level
for the last four or five hundred million years.
So, no, not all Flatlands of the Earth, been through mountain
building processes. As plates continue to drift
around the surface of the Earth and mountain building on their
margins. And within them above hotspots,
continues is the earth going to get more and more mountainous
over time. Probably not the Earth during
Earth history has gone through cycles of big mountain building
processes. So the Alpine Himalayan cycle
that we see at the moment for the last 50 or 60 million years
has created many of the mountain belts that we see stretching all
the way from the Pyrenees and the Alps down through Central
Ron through Oman cross the Himalayas down towards
Indonesia. That's the very young.
Very active collisional processes.
Process. That's forming mountains.
In the geological past, we've seen other very large mountain,
ranges across huge tracts of the Earth.
There was one in the Caledonian and the Appalachian timing which
was back in the ordovician 450 million years ago.
But that was another huge collisional mountain range that
stretched all along the Eastern Seaboard of south of North
America, right? The way from the Amazon all the
way across up to Scandinavia. There was another one across
Central Europe in the pacinian about 250 to 300 million years
ago. That formed the mountain range
that we see across most of central and Northern Europe
today. So these things do go in Cycles.
But the average over the huge amounts of geological time that
we're looking at 500 million years in the case of the
phanerozoic, four and a half billion years in terms of total
Earth, history probably maintains a continuance Now for
my last question, I'm going to ask you to speculate if we could
be Time Travelers and we sped about 100 million years into the
future. What would we see?
Well, you're asking me to play God.
Now mean what we would see in my purely speculative mode is a
very different planet from what we're looking at today.
I would expect that the Pacific and the Indian Ocean Ian's would
be getting smaller and closing as those subduction zones are
consuming crust, I would expect the Atlantic Ocean to be
expanding so that could become the Great Pacific of its time in
a hundred million years time. Everything depends on the
mountain ranges. For example for Himalayas are
all formed because India has been pushed northwards by the
spreading Oceanic spreading centers in the central, and
southern Indian Ocean. And it says stop.
Then the Indian plate would no longer indents and collide with
Asia and erosion would take over in the Himalayas would be
flattened down to sea level. God, forbid.
Now, the most beautiful mountains on the planet, and we
rely on the Indian plate. Still to actively collide with
Asia to make these beautiful high mountains.
We see today Mike cell. Thank you very much, my
pleasure.