WEBVTT

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We know that an
element is defined

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by the number of protons it has.

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For example, potassium.

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We look at the periodic
table of elements.

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And I have a snapshot of
it, of not the entire table

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but part of it here.

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Potassium has 19 protons.

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And we could write it like this.

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And this is a little
bit redundant.

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We know that if it's potassium
that atom has 19 protons.

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And we know if an
atom has 19 protons

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it is going to be potassium.

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Now, we also know that not all
of the atoms of a given element

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have the same
number of neutrons.

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And when we talk
about a given element,

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but we have different
numbers of neutrons

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we call them isotopes
of that element.

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So for example,
potassium can come

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in a form that has
exactly 20 neutrons.

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And we call that potassium-39.

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And 39, this mass
number, it's a count

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of the 19 protons
plus 20 neutrons.

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And this is actually the most
common isotope of potassium.

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It accounts for, I'm
just rounding off,

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93.3% of the potassium that
you would find on Earth.

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Now, some of the other
isotopes of potassium.

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You also have potassium--
and once again writing

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the K and the 19 are a
little bit redundant--

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you also have potassium-41.

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So this would have 22 neutrons.

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22 plus 19 is 41.

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This accounts for about 6.7%
of the potassium on the planet.

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And then you have a
very scarce isotope

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of potassium called
potassium-40.

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Potassium-40 clearly
has 21 neutrons.

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And it's very, very,
very, very scarce.

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It accounts for only 0.0117%
of all the potassium.

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But this is also the
isotope of potassium

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that's interesting to us
from the point of view

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of dating old, old rock, and
especially old volcanic rock.

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And as we'll see, when you
can date old volcanic rock

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it allows you to date
other types of rock

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or other types of fossils
that might be sandwiched

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in between old volcanic rock.

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And so what's really interesting
about potassium-40 here

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is that it has a half-life
of 1.25 billion years.

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So the good thing about
that, as opposed to something

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like carbon-14, it can
be used to date really,

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really, really old things.

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And every 1.25
billion years-- let

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me write it like this,
that's its half-life--

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so 50% of any given
sample will have decayed.

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And 11% will have
decayed into argon-40.

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So argon is right over here.

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It has 18 protons.

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So when you think about
it decaying into argon-40,

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what you see is that
it lost a proton,

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but it has the same mass number.

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So one of the protons must of
somehow turned into a neutron.

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And it actually captures
one of the inner electrons,

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and then it emits
other things, and I

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won't go into all the
quantum physics of it,

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but it turns into argon-40.

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And 89% turn into calcium-40.

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And you see calcium on the
periodic table right over here

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has 20 protons.

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So this is a situation
where one of the neutrons

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turns into a proton.

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This is a situation
where one of the protons

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turns into a neutron.

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And what's really
interesting to us

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is this part right over here.

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Because what's cool about argon,
and we study this a little bit

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in the chemistry playlist, it is
a noble gas, it is unreactive.

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And so when it is embedded
in something that's

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in a liquid state it'll
kind of just bubble out.

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It's not bonded to
anything, and so it'll just

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bubble out and just go
out into the atmosphere.

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So what's interesting
about this whole situation

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is you can imagine what happens
during a volcanic eruption.

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Let me draw a volcano here.

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So let's say that
this is our volcano.

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And it erupts at some
time in the past.

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So it erupts, and you have
all of this lava flowing.

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That lava will contain some
amount of potassium-40.

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And actually, it'll
already contain

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some amount of argon-40.

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But what's neat
about argon-40 is

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that while it's lava, while it's
in this liquid state-- so let's

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imagine this lava
right over here.

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It's a bunch of stuff
right over here.

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I'll do the potassium-40.

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And let me do it in a color
that I haven't used yet.

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I'll do the
potassium-40 in magenta.

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It'll have some
potassium-40 in it.

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I'm maybe over doing it.

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It's a very scarce isotope.

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But it'll have some
potassium-40 in it.

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And it might already have some
argon-40 in it just like that.

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But argon-40 is a noble gas.

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It's not going to bond anything.

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And while this lava
is in a liquid state

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it's going to be
able to bubble out.

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It'll just float to the top.

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It has no bonds.

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And it'll just evaporate.

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I shouldn't say evaporate.

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It'll just bubble
out essentially,

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because it's not
bonded to anything,

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and it'll sort of just seep out
while we are in a liquid state.

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And what's really
interesting about that

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is that when you have
these volcanic eruptions,

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and because this argon-40
is seeping out, by the time

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this lava has hardened
into volcanic rock--

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and I'll do that volcanic
rock in a different color.

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By the time it has
hardened into volcanic rock

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all of the argon-40
will be gone.

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It won't be there anymore.

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And so what's neat is, this
volcanic event, the fact

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that this rock
has become liquid,

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it kind of resets the
amount of argon-40 there.

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So then you're only going to
be left with potassium-40 here.

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And that's why the argon-40
is more interesting,

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because the calcium-40 won't
necessarily have seeped out.

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And there might have already
been calcium-40 here.

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So it won't
necessarily seep out.

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But the argon-40 will seep out.

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So it kind of resets it.

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The volcanic event resets
the amount of argon-40.

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So right when the
event happened,

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you shouldn't have any argon-40
right when that lava actually

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becomes solid.

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And so if you fast forward
to some future date,

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and if you look at the sample--
let me copy and paste it.

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So if you fast forward to
some future date, and you

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see that there is some
argon-40 there, in that sample,

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you know this is
a volcanic rock.

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You know that it was due to
some previous volcanic event.

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You know that this argon-40 is
from the decayed potassium-40.

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And you know that it has decayed
since that volcanic event,

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because if it was there before
it would have seeped out.

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So the only way that this would
have been able to get trapped

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is, while it was liquid
it would seep out,

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but once it's solid it can
get trapped inside the rock.

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And so you know the only
way this argon-40 can

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exist there is by decay
from that potassium-40.

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So you can look at the ratio.

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So you know for every
one of these argon-40's,

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because only 11% of the decay
products are argon-40's,

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for every one of
those you must have

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on the order of about nine
calcium-40's that also decayed.

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And so for every one of these
argon-40's you know that there

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must have been 10
original potassium-40's.

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And so what you
can do is you can

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look at the ratio of the
number of potassium-40's there

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are today to the number
that there must have been,

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based on this evidence right
over here, to actually date it.

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And in the next
video I'll actually

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go through the
mathematical calculation

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to show you that you
can actually date it.

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And the reason this
is really useful

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is, you can look
at those ratios.

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And volcanic eruptions
aren't happening every day,

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but if you start looking over
millions and millions of years,

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on that time scale,
they're actually

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happening reasonably frequent.

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And so let's dig in the ground.

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So let's say this is the
ground right over here.

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And you dig enough and you
see a volcanic eruption,

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you see some volcanic
rock right over there,

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and then you dig even more.

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There's another layer of
volcanic rock right over there.

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So this is another
layer of volcanic rock.

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So they're all going to have a
certain amount of potassium-40

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in it.

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This is going to have some
amount of potassium-40 in it.

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And then let's say this one
over here has more argon-40.

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This one has a little bit less.

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And using the math that we're
going to do in the next video,

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let's say you're
able to say that this

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is, using the half-life, and
using the ratio of argon-40

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that's left, or using the
ratio of the potassium-40 left

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to what you know was there
before, you say that this must

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have solidified 100
million years ago, 100

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million years
before the present.

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And you know that this layer
right over here solidified.

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Let's say, you know it
solidified about 150

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million years
before the present.

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And let's say you feel pretty
good that this soil hasn't been

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dug up and mixed or
anything like that.

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It looks like it's been
pretty untouched when

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you look at these soil
samples right over here.

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And let's say you see
some fossils in here.

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Then, even though carbon-14
dating is kind of useless,

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really, when you get
beyond 50,000 years,

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you see these fossils in
between these two periods.

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It's a pretty good
indicator, if you

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can assume that this soil hasn't
been dug around and mixed,

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that this fossil is
between 100 million and 150

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million years old.

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This event happened.

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Then you have these
fossils got deposited.

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These animals died, or
they lived and they died.

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And then you had this
other volcanic event.

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So it allows you, even though
you're only directly dating

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the volcanic rock,
it allows you,

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when you look at the layers,
to relatively date things

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in between those layer.

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So it isn't just about
dating volcanic rock.

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It allows us to date things
that are very, very, very old

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and go way further back in time
than just carbon-14 dating.