This is geology, B. I'm all of us.
Trampled, a former astrophysicists and Museum
director with an endless curiosity about the geology of
our extraordinary Planet, most rocks, have a history stretching
back, tens, hundreds, or even thousands of millions of years,
During that time, some rocks have just sat there and been
largely ignored. While others have been subjected
to enormous pressure and or temperature changes, knowing
what has happened to Iraq. During its lifetime gives us,
crucial insight into how geological processes work such
as how mountains form or how economically valuable mineral
resources originated. But how can we find out what
sort of History? A rock has been subjected to
Claire Warren is a senior lecturer in the school of
environment as an ecosystem Sciences at the open University,
how workers helped us, understand how and when deeply
buried rocks, preserve a record of that burial and subsequent
Journey up to the surface. Armed with This Record.
She determines the processes and mechanisms.
That Barry transform and bring rocks up to the surface.
Claire Warren. Welcome to geology, B.
Thanks very much for inviting me Oliver.
It's great to be here today. The planet is covered with
rocks. So how do you decide which
rocks? Might tell you something
interesting about the geological questions.
You're addressing There are three broad rock types that we
find on Earth. Igneous ones that have
crystallized from molten rock, sedimentary, rocks and
metamorphic rocks. And it's these latter ones that
have been subjected to high pressures and high temperatures
that tell us about how Mountain belts formed in the past, and
it's that process of mountain building that I am particularly
interested in, and particularly ones that form, where two
continents Collide, so I presume to get the Rocks, you're
interested in, you have to go to Mountain, Yes.
But they can be vast. Yes they are huge and it's
difficult. Sometimes to figure out which
parts of the mountain belt to go to to find rocks of interest and
different parts of mountain belts that we find can tell us
different parts of the history. So some parts tell us about the
early history of formation. Some parts tell us about the
later parts of formation so we have a rough idea of where to go
to but then once we're there sometimes you've just got to
start exploring and see what you find.
And when you're up there, are they Rocks.
Just lying around and say that looks interesting.
I'll grab that one. Or How does it go?
It really depends on where you're going.
So, some places now are protected in law their
geological Heritage sites. Of course, you can't sample from
there and we always operate on a leave, no Trace kind of basis.
So, when you take the geological Hammer, you don't want to leave
evidence where you've collected samples from.
But otherwise pretty much apart from local laws about collecting
things, you can go and take samples where you want to.
I mean, obviously, there's practical, Cooties when you're
working in modern Mountain belts because sometimes the altitudes
really high. So there's not much oxygen in
the air. Sometimes there isn't much
exposure so you can't see many rocks for the taking and
sometimes where there's been glacial action.
For example, it's actually really hard to sample at all
because there's no nice sharp edges that you can hit off with
your Hammer. But generally you can go and
take what you want. So when you find a rock, you'd
like to focus on do you start by just having a good look at it?
Where you Actually find it in the field.
Yeah, we start from a really broad scale, so sometimes you
might take a pair of binoculars out and have a scan of what's in
the distance and you look for different colors and you look
for different textures. And you might look for the way
that rocks have folded or sit next to each other.
And then once you've identified an area of Interest, that's
where you start getting up closer and having a look at how
the Rocks fit together on the outcrop scale.
So on the sort of couple of meters scale and then once
you've done that, do find that it's pretty clear.
Which rocks you really want to hone in.
On our they ones that flash in sparkling covers or does it take
a bit of Hit and Miss to get rocks?
That are really going to tell you the story or after.
Well that's a really interesting question because obviously our
eyes are nearly always drawn to the most beautiful rocks and
those are the ones which will end up on our mantelpiece or on
the office equivalent of a mantelpiece but actually it's
quite often. It's the dull-looking works next
to the pretty rocks that actually yield the best
scientific information. So there's one part for example,
in Norway one Particular place in Western Norway, where
everybody gets taken to on these geotourism holidays.
And a lot of people have got a lump of this beautiful green,
and red rock sitting on their desks, but it's actually the
sort of rather Dulla finer grain to the black looking Rock.
Next to it, that yields the minerals that tell us about the
pressure temperature and time history and give us much more
information about how that mountain belt formed.
So in often, you don't actually know which are the best rocks
until you get back and start analyzing them in the lab.
But I presume you, whip out your hand lens.
All the We'll just say I've seen have Pam lens hanging around
their necks. Yeah, so we can tell quite a lot
by just looking at a hand sample.
We can for example identify the different minerals that are in
the Rock and how they fit together.
So it might tell us something about roughly whether this rock
formed, 500 Degrees, which is fairly cool for a metamorphic
rock or 800 Degrees which is quite hot and whether they've
been faulted or whether they've been sheared out like toffee but
until you actually take the rock back to the lab and start
looking. At the chemistry of the
different minerals. There's only so much that you
can tell with the hand lens, okay?
So you've whacked off your samples with a hammer, and
you've put them in your bag, you've got home.
And now, you really want to try and uncover this history, that
tells us what The Rock is been through.
So what's the next step? So I guess the next step then is
to make a thin section. So these are thin slices of rock
that are about 30 microns thick about stickers, your hair and
when you cut rocks that thinly, you can actually start to see
Through them. So first thing we do is look at
the Rocks, under a microscope, which often confirms what we've
seen in hand lens. But it's much clearer to see
which different types of minerals.
There are, we might Identify some that are too fine grain to
be seen with a hand lens, and then once we've identified what
types of minerals are in the Rock, we then take those thin
sections to higher powered equipment.
So there's for example, an electron microprobe where we can
measure chemistry of the minerals specifically the the
major element things like Silicon and iron and aluminium
things that make up quite a lot of the composition of the Rock.
And it also helps us to identify which elements are in the
different minerals. We can then also take that
specific section of the rock to a mass spectrometer and that'll
tell us much finer detail about the elements that are in really
tiny concentrations in the rock. Or for example, if we want to
know the age of the rock, we need to start looking at
Isotopes so ways and different elements behave, but have
different masses and you need really high resolution.
And if the mass spectrometer to be able to tell those apart, and
once you've got all that information, we should be able
to, with a following wind, tell the pressure of the rock the
temperature of the Rock and hopefully the age of the rock as
well. So the complete history of Iraq,
if only you can get it, then would be the history of the
temperature and the pressure, the pressure telling us the rock
depth over all the time since the rock was first conceived was
a twinkle in In the eye of the magma, it came out of something
like that. Yeah.
So if you think about a rock a bit like a tree, the tree rings
on a tree can tell you time from when the tree was just a sapling
all the way through until it was a stately, huge tree, 400 years
later and a rock behaves, a little bit like that as well.
So if we imagine, you know, our nice beach sand, for example, on
the edge of a nice coast and then the ocean disappears and
that bit of Coast collides with the next bit of continent, Bit
of sand on our beach will eventually get buried, and will
turn into a new different rock type and growing different
minerals. But all of that takes millions
of years and so we need to look to see when the tape recorder
starts to see when that rock starts being buried, how deep it
gets at what time it gets to its highest debts in its highest
temperature and then its Journey back towards the surface.
So the best rocks in my eyes are the ones that record that
longest period of time and tell us what the pressure and
temperature. ER, and time was doing over
millions of years. Well, let's start with how you
can figure out pressures and temperatures that The Rock has
been subjected to. So these are recorded mainly by
the minerals that make up the bulk of The Rock, and there are
different ways that you can record, pressure, and
temperature sometimes. It's by the way, that magnesium
and iron, for example, swap between two different minerals.
So if you know, the magnesium and iron concentration of one
mineral and another mineral and you That they grew together at
the same time, you can calculate the pressure and temperature
that those correct there are other newer techniques and more
high-powered techniques where you need to.
Look at Trace element concentration of titanium in a
particular mineral that might also tell you pressure and
temperature. So it's basically by chemistry
and by assuming the magical status of equilibrium and that's
of course always the difficult bit.
But if we make a bunch of assumptions and do some
calculations and we know the chemistry then pressures and
temperatures usually fall out of that.
That appear to make sense. Are there any particular
minerals that are particularly useful for determining pressure
and temperature? So the best ones for the rocks
that I work in our minerals called Garnet, which is a nice,
big bright, red ones, and minerals, like biotite.
That's the shiny Brown Mica that you quite often.
See twinkling in rocks. So, those are two really good
ones, because they record pressure and temperature really
well. Other minerals.
There's a mineral called rutile, which is most commonly used for
making the white color, Your toothpaste.
That's a really really good pressure.
Temperature indicator minerals as well.
So the detailed compositions of Telltale minerals such as garnet
and biotite occurring in a particular, Rock depend in ways
unique to each mineral on what the temperature and pressure was
while they were crystallizing from the Melt.
For example, it turns out that one, especially sensitive
indicator of the temperature, which these minerals
crystallized is the relationship between the iron to magnesium
ratio in the garnet. And the iron to magnesium ratio
in the biotite. So, if you use these very
sensitive instruments, you have to measure the concentrations of
iron and magnesium in these two minerals.
You can get at the temperature. That's right.
And Garnet, grows over a long period of time as well.
So, it's a, particularly good mineral for recording pressures
and temperatures, because it will called different values in
its core compared to an its Rim. So, comparing back.
Of a tree rings. For example, Ghana is a
particular good recorder of pressures and temperatures
because it does grow over such a long period of time.
Let's talk about the time Dimension.
Then how do you figure out the time at, which various things
happen to a rock? So we record or measure
geological time using radioactive decay.
And there are some elements which Decay for example, uranium
or Decatur lead. And there are some minerals in a
rock, which will concentrate uranium and exclude lead when
they start crystallizing. So there's a couple of good
minerals. Zircon is one another's
monocyte. They will have these fantastic
names. Minerals.
And these are particularly good geological clocks and because
again, they grow over long periods of time and they form
these different shells of different composition at
different times. We can actually start using
those as the tape recorder. The problem with those minerals
is that they don't record pressure and temperature.
So we then try to link our pressure temperature, minerals
like, Garnet, with the age information that are
chronometer, our clock minerals record.
How do you get this? This radiometric information out
of a rock. So, that's where we have to go
to our mass spectrometers. And what we do is we have these
thin sections, over these rocks. And we find the minerals, the
clock, minerals in our rock using, for example, the electron
microprobe. And then we use a laser with a
spot size of about 10 microns. So that's about a third of the
size of your hair just to laser tiny little spots all the way
across these minerals. And then we collect the
chemistry information from those minerals.
So how much Rhenium. It's got in it.
How much lead it's got in it and the Isotopes of those two
elements as well as some of the other elements as well like to
taenia more The Rare Earth elements.
They've also got some weird and wonderful names and we collect
all that information spot by spot.
And these grains have probably about 100 to 200 microns big.
So so tiny tiny. We put a whole row of spots on
these grains and then we collect all that age information.
So it is chemistry magic. Really much better than I ever
did at school. Cool.
So if I can spell this out a little more, you start off with
a mineral such as Zircon, which has a certain amount of uranium
trapped inside it and it decays to a particular isotope of lead.
And then when you put the Zircon into the mass, spectrometer you
measure how much of the lead Decay product, there is compared
to the uranium and since you know, the half-life of the Decay
process You can tell how long ago, the Zircon crystallized and
trapped the uranium inside it, starting the radiometric clock.
Yeah, that's right. And the first people to carry
out age dating of rocks like this would measure all the
uranium and all the lead in the whole Rock and of course, they
didn't have mass spectrometers. Then that's about 150 years ago.
So it's very complicated and very time-consuming to get one
age. And now we can measure these
tiny portions of these tiny minerals and measure hundreds of
these minerals in a day and get much more accurate and precise
information out. So, the idea that we can use
radioactive decay, has been around for a long time, but the
techniques that we now use to do that, give an amount of
information that the scientists hundred years ago, they would
have blown their heads, Okay. So now using these radioactive
methods, you can get the crystallization age of a
particular Crystal, when it forms.
So does that then tell us the age of the rock that it's
sitting in? It can do but metamorphic rock
to a little bit complicated because they have changed their
form from one type of rock to another type of rock and it's
the change that I'm particularly interested in, but some of the
minerals in The Rock will still record the age of the original
Rock. For example.
It came out of a volcano originally or crystallized.
There's a large Granite body underground, and then it got
metamorphose later. So what we then need to do is to
try to unravel, which of the ages relate to different parts
of the process that we're interested in and that's where
all these tools of linking pressure and temperature and age
using these geochemical fingerprinting tools comes into
play. So it really is like a detective
story like a forensic science story trying to link little
tent. It has bits of evidence from one
mineral and try to patch that in to when that other mineral was
growing. So nature doesn't make it easy
for us. There isn't anyone mineral whose
tree rings will record both the age from the amount of uranium
lead and the composition saved? Like Darla to tell us the
temperature and pressure at the time, we've got to
cross-correlate different kinds of minerals.
That's right. And we can also use the way that
different minerals sit alongside.
Solid each other. So for example, when Garnet
grows it will engulf other minerals that are in the Rock at
the time that it's growing because if it comes across
something that it doesn't want to make part of its crystal
structure, it will just grow around it.
So Garnet might contain different inclusions of
monocytes in the core and different conclusions of
monocyte in the rim. And so those different
inclusions can then tell us how long that Garnet took to grow.
So there's these different Ways and mechanisms of linking, the
garnet, and the monocyte or the garnet, and the Zircon together,
just by the way that they're growing, so let's say, by
applying this whole battery of techniques that you've managed
to glean enough information. To tell you what The Rock has
been through. Since it was formed, can you
give us an example of a rock? You've done this for.
So I think some of the most interesting examples of worked
on recently. Come from the Himalayas, I
actually went to Bhutan and the Eastern side of the Himalayas
and went all the way up to the northwest corner of And so,
we're camping at altitudes of about 4,000 meters or so.
So this is a proper hardcore, mountainous field, geology.
But we're trying to unravel how the Himalayas formed so which
rocks were colliding between India and Asia and when and what
then happened to them as they got buried and brought back up
to the surface because obviously the rocks that were surfaced
today were buried at depth 10, 20, 30 million years ago.
So we found this one place with two specific samples that I was
looking at one, was a beautiful green and red rocks at high
pressure and metamorphic rock that told us something about the
very depth of the Himalayas and then other rocks, which are
nearby, which were metamorphose that much shallower levels and
we were trying to figure out how these two different rock types
fit together so where they just recording different parts of the
story but had had the same history or did, they have a
different history and then Place together somehow in the vast
washing machine that was underground.
So one of the Rocks told us a lot about the early parts of the
Himalayan history. So India, and Asia collided
about 50 million years ago and the metamorphic processes that
heat up rocks and squash rocks. Start getting recorded in the
Himalayas about 35 million years ago.
And these older, these 35 million are rocks, are quite
rare in the Himalayas, because most of the time, those haven't
come up to the surface yet, actually, it's the younger rocks
which were metamorphose that A lower level which are now at the
surface. So we found this one, rock that
told us a lot about this early history and then this deep
high-pressure Rock was much younger than that.
And so that was also then a bit of a puzzle, how you get a young
deep rock suddenly, a kilometer away from a shallower called
Iraq, but by pulling all these different pieces of the
geochemistry and the field relationships and the mineral
relationships and all of that together, it tells us a lot
about that washing machine. Process of how India and Asia
finally collided. So yeah, it's a fascinating
story which we only managed to unravel back in the lab from two
rocks. That we happened to manage to
get to 4,000 meters altitude on a day when we actually had
enough strength and oxygen to wield a hammer to take these two
bits, a home with us. So this washing machine process,
you're uncovering is the somewhat chaotic burial, partial
melting and uplift of the Indian Rocks, caught up in the
collision with Asia, is this process specific to them earlier
details of how the rocks are now sitting together in the
Himalayas is quite specific to the Himalayas but what we're
finding is that the processes that operate as two continents
Collide, and which bits of rock, tend to ride over each other,
and start melting and start getting mangled around.
That is actually been happening for billions of years.
And some of the oldest mountain belts that we have evidence for
on Earth today, so about 2 billion years old actually
formed in much the same way as the Himalayas, the youngest
Mountain belt of these Continental Collision types
forming today. But what's unusual is that
Himalayas tend to only expose the rocks at the very surface
today, so the higher levels of the mountain belt and over time,
those will erode and we'll start then looking at the deepest
levels and so the older the mountain belt is the deeper you
Look down into the structure. And so, what's great about these
rocks that we've been finding in the northwest of the Himalayas,
is that they're showing us evidence from some of the deep
roots of the Himalayas at the surface today and that gives us
a tie point that we can link the really old mountain belts with
this young Mountain belt and say hey look the processes and the
roots of these Mountain belts were actually very very similar
in the past to the present day. So there's been no major
differences in the way that plate tectonics operates for 2
billion years. In this particular circumstance
and that's quite interesting. Is there a particular ancient
mountain range that you compare them allows with to come to this
conclusion? So I haven't particularly been
to this mountain range of the called the trance Hudson origin
and it's in Canada. And that's about eighteen
hundred million years old, but other workers have worked there.
And the kinds of metamorphic map that they are producing a very,
very similar to the Himalayan ones.
When you look at them in a broad scale.
So the types of rocks that we see, The rates and time scales
of the processes that operate in those Mountain belts, and this
General pattern are very, very similar plate tectonics.
Has been around for most of the history of the earth.
It would seem. Yeah.
Some of the bits of plate. Tectonics appear to be much
younger, but the way that continents Collide appears to
have gone back at least half of the age of the Earth.
Are there any new tools? Emerging now that might help us
elucidate even more about the Rocks history than we can today.
Well, what we're finding is that the more and more we look and
the finer and finer detail that we look at the more and more we
find. So there's a particularly
interesting instrument called the atom probe which allows you
to take tiny tiny bits of minerals.
So 50 nanometers. And we were talking about
microns before so much, much smaller and peel off atom by
atom, the structure of that mineral and all Those atoms then
fly off into a detector and how they hit the detector.
You can then use to unravel where that Adam was in the
structure originally. So it's a very, very high
powered Mass spectrometer. And so, you can actually see,
for example, where the lead is sitting in a Zircon, and we're
probably the original uranium was sitting as well and whether
that led has moved. So you can actually match the
original element that decayed into the new element that
decayed atom by atom by atom. And that is just phenomenal.
Claire Warren, thank you very much.
It's been an absolute pleasure, thank you for more about geology
b, as well as pictures and diagrams that illustrate.
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