- Now this,
is perhaps one of the most beautiful
kind of features that volcanoes can make.
A very big high eruption column.
And in the last 10 years or so
we've made really a lot of progress
in understanding what these explosions do,
and the kind of things that can go on.
And for most of the rest of this lecture,
I'm going to be talking to you about
the kind of things that go on in these big explosions.
Why are there problem?
Because this stuff coming out of volcano
is coming out as a very fast stream of hot gas,
and dust and magma and bits of liquid
and all sorts of stuff.
It's coming out at a few 100 meters per second,
that's about the speed of a jet airliner.
That's how fast it's coming out.
But if I were just to throw something into the air,
the speed of a jet airliner,
it would go about one or two kilometers into the air,
and then it would fall down.
Now these things go 20 or 30 kilometers into the air,
they go much, much higher than they should.
We're going to do an experiment now,
which is going to help us to understand
what actually goes on.
This is a tank of water,
but rather special water.
At the top of the tank here is freshwater.
And at the bottom of the tank is salt water,
which is much heavier,
is denser than the freshwater,
and there's a gradient between the two.
This is to match the atmosphere.
And the atmosphere of the air, is much lighter at the top,
then down the bottom,
where we are here where it's denser.
In the bottom of my tank, I have a volcano,
and I'm going to feed a stream of liquid into that volcano,
which is colored red.
And the liquid which is coming into the volcano
is freshwater.
It's the same density as the top of the tank.
And it's a lot lighter than everything else in the tank.
So you'd think that yes,
of course, the stream will rise up to the surface,
where it's got the same density as the surface water.
Well, let's see what actually happens when we do it.
So I'm going to open the tap here.
See what happens.
Here it comes.
And it's coming up,
and it's not getting up to the top.
It's going up about halfway.
And then it's spreading out sideways.
It's making a mushroom cloud.
And this mushroom cloud is growing
more and more sideways, it's not going up.
Now that's surprising.
How can that be if the liquid which is coming out
is the same density as the surface of the tank.
And it's much lighter
than the density in the middle of the tank.
What's actually happening in this demonstration
is that as this stream of liquid is coming out
from the volcano,
it's sucking in water from roundabout.
And of course, the water roundabout is denser
than the stream of liquid coming out
of the bottom of the volcano.
And so that makes the stream as a whole denser,
and it can't rise up as high as we expected at first,
it can only get halfway
because it's got denser
by sucking in all this liquid around the side.
And this process of fast jets,
sucking in whatever is surrounding them
is actually very common,
it happens all over the place.
And this rather lovely demo here,
demonstration, which produces these big mushroom clouds
is gonna help us understand
what happens at Mount St. Helens.
Thank you very much.
(audience clapping)
Now this mush,
this big cloud here,
this big eruption column
at the top also made a mushroom cloud.
And the next picture will show
what that mushroom cloud looks like from a distance.
Mount St. Helens is buried
in the middle of the foot of that cloud.
And this thing is so big
that it couldn't actually fit in a camera frame.
And so this is a picture made up of
lots of pictures which have been glued together
to make this one.
And this mushroom cloud rose up
to a height of about 20 kilometers
before it spread out sideways.
Now what's happening in this,
is that the stream, the hot stream of gas and rocks
and magma coming out of that volcano
is again sucking in the surroundings,
the surrounding air,
but the air of course, is light.
And the air is then heated
by that hot stuff coming out of the volcano.
So the air which is sucked in expands.
And this is like tying lots of hot air balloons
on to what's coming out to the volcano.
So it rises much higher than would otherwise rise.
So whereas that tank,
the mushroom cloud,
the stream went lower than we expected
here it went higher than we expected.
The process is exactly the same.
Both of these streams coming out of the volcano
sucked in the surroundings
and that's what makes the effect.
It's so happens here
that because the air which is sucked in
is heated and expands, this goes up.
In the tank, the liquid which was sucked in
was actually heavy which prevented it going higher,
but the process is the same.