How do you do that?
Well, there's a classic kind of instrument called a tiltmeter that's basically
a water level that's connected by a pipe to a water level at some distance away.
And so when one part of that system or
one of those detectors moves up relative to the other the water level changes.
And we can observe that using the gauge that's on the instrument.
Of course, these days we do these instruments electronically.
There's a bubble level in the interior of the tiltmeter, and
as the tiltmeter starts to change its orientation, electrodes are able
to measure the motion of that bubble and make a record of the tilt of the volcano.
These instruments are extremely accurate.
For example, a volcanologist can detect the tilt to
the surface of a volcano of only 0.00006 degrees.
So what does that mean?
Well, that's like being able to detect a change in
distance the thickness of a coin, over a distance of about a kilometer.
The changes in shape of a volcano can also be detected using satellite imagery.
What it's measuring, using radar, is the distance between the satellite and
the volcano.
So you can imagine a satellite goes around the earth.
It measures the distance to the ground, continues around its orbit, comes back and
sometime later, makes exactly the same measurement.
Well, it's able to tell by just a few wavelengths of the radar beam,
whether the volcanic surface has gone up or down relative to where it was before.
And it can represent that information as a series of color bands.
When the color bands are closer together, there's been more motion.
When the color bands are further apart, there's been less motion.
So you're able to get an image,
by looking at the volcano, of whether it's surface is going up or down.
Of course, another way of doing this is by simply using GPS
over a longer period of time.
Because GPS is now able to detect displacements on the order of a millimeter
over a period of time, and
that allows you to again, be able to see the motion of the volcanic surface.
We can also use another kind of measurement to detect the injection of
magma, and that's the heat flow that's being admitted by the volcano.
Remember, magma is really hot.
And so when the magma gets into the volcano, it starts conducting heat
into its surroundings, and that heat will radiate off the side of the volcano.
We can measure that heat and measure the rate at which heat is being emitted,
and that's called the heat flow.