- Now it wasn't just in the oceans
that people were looking at magnetism.
Magnetism was a popular thing to look at in the 1950s.
And people were also doing it on land.
And in particular, they were looking at magnetism in rocks.
Now, volcanic rocks in particular form,
like this one here, this is a volcanic rock.
Volcanic rocks form from molten lava,
which erupts at the surface, and then cools.
And the rock contains little tiny grains of iron oxide.
And when they cool, they become magnetized
and they become magnetized in the direction
of the Earth's magnetic field.
And then the rock itself can become a little magnet.
Let me show you that this rock here is a little magnet.
Here's a compass needle pointing North at the moment.
I can actually make it move
by pushing this magnet around.
So rocks can become magnetized.
Now, organic crops that form today
become magnetized in the direction
of the Earth's magnetic field today.
That's what you would expect,
but some magnetic rocks or some rocks
were magnetized apparently backwards.
Let me explain what that means.
Here is a bar magnets
and the Earth's magnetic field looks
like that from a bar magnet.
Now, if I produce this.
This is a magic wand which has another magnet in here
and this magnet can move around here.
If I bring this magnet close to this magnet here,
move it around.
You'll see that the yellow end of my wand
is pointing to the red end of this magnet.
Now, if I reverse the magnetic field
in this sample here,
so supposing this was my rock with a magnet inside it.
When I say it's reversed, magnetized backwards,
what I mean is it's like that.
Now, instead of the yellow end
of my magnet pointing to that,
the red end will.
So that's what I mean by being magnetized backwards.
It looks as though the magnet in the rock
was pointing in the different direction
from the Earth's magnetic field,
it was pointing the other way.
Now, We don't measure the direction
of the magnetization in rocks by this kind of magic wand.
We use a much more sensitive instrument
which is like this thing here.
And if I show you this,
how this works,
I'm going to bring up this rock,
which is like a tiny little magnet
towards the end of this probe.
And we should see that needle on the screen, move.
There it is, it moves to the right.
And if I turn this up the other way,
it'll move the other way.
So that this instrument can tell
what direction the magnetization is in my rock.
What way up the magnetization is.
Now what we have here is six rocks
which are all from the volcano in Britain.
These are from a volcano in Scotland,
off the West coast of Scotland, the Island of Skye.
And they're from that volcano was active
about 50 million years ago.
It's not doing anything now.
These are old lavas.
Now the extraordinary thing about these lavas here
is when we measure the direction
of the magnetic field in them, they are backwards.
The magnetic field is in the opposite sense
from the magnetic field of the earth.
And these are much less magnetic
than this rock over here.
They're very weakly magnetic,
but this can still pick it up.
Let me show you,
you'll have to watch the screen very carefully now
because the movement will only be a small amount.
So if I bring this one close to the probe,
so the move, just move very slightly.
If I turn it over, it moves very slightly the other way.
It's a small effect
but these instruments are very, very sensitive
and we can actually measure
the direction of magnetization in these rocks.
So what are we to make of all this?
Some rocks are magnetized
in the direction of the Earth's magnetic field
and some are magnetized backwards.
Well, the next thing,
next advance in this whole business
was to actually be able to date these rocks.
To find out how old they were.
And in the late 1950s,
some instruments were developed
which could actually work out how old those rocks are
by looking at the natural radioactivity in them.
These rocks, as I say are 50 million years old
and their magnetization is backwards.
If we take other rocks from some other parts
of the world, Japan may be,
or America or India, it doesn't matter.
If they are the same age as these rocks
you find that they are also magnetized backwards.
And if we find rocks which are magnetized
in the normal sense of the Earth's magnetic field,
all other rocks in the world
of the same age are also magnetized in that same direction.
So it really does look as though
the magnetic field of the earth itself flipped
that the North Pole became the South Pole magnetically
and that the South magnetic pole became the North Pole.
And by dating these rocks,
you could find out when that happened.
And by about 1966,
people had made a calendar of what had happened.
And this is the calendar.
What we have here is time.
This is today, naught.
1 million years, two, three, 4 million years.
And the black means that the magnetic field
is in the same direction as it is today.
But here, which is about 700,000 years ago
the magnetic field flipped.
The magnetic field turned round.
And here just before 1 million years ago,
it flipped back again.
And here it was reversed and here it was normal, normal.
I mean, reversed, normal, reversed, normal, and so on.
It's a change with time.
And what this thing tells us is when it happened.
This is a calendar of when the magnetic field reversed.