Prior to a volcanic eruption there are generally some warning signs, one of which is bulging near vents as
magma moves upwards towards the surface. Throughout history attempts to measure
these movements have been made to aide with the prediction of an eruption.
Romans noticed ground movement
and swelling preceding a volcanic eruption. They tried to monitor the degree of
tilt by filling clay vessels to the top with water. As the vessel began to tilt,
water would flow out of it and they could measure the loss in water, which
would help them determine how much tilting had occurred. In theory, this simple
method sounds very useful, however, it is too susceptible to external forces. Changing
weather conditions meant much of the water could be lost due to evaporation, or
more added through rainfall. Wild animals roaming the mountains may also have
found it very considerate that humans had left some water for them to drink.
A basic clinometer
can be made at home to measure changes in inclination.
A more accurate method of
measuring the changes in tilt is to use a clinometer. A basic clinometer can be
made by suspending a plumb bob above a protractor on a base so it can stand
upright. As the volcano bulges, the protractor will tilt more, and the angle of
tilt can be read off the protractor. A joiner’s spirit level can also be used
to measure the angle. These days most spirit levels give the exact angle of
inclination, rather than just showing a bubble in liquid. Although using a
clinometer or spirit level will give much more useful data than the amount of
water displaced from a vase, a volcano can tilt and grow in many directions, so
many careful measurements would have to be taken to give a clear picture of the
amount of bulging.
This tiltmeter (or tilt-o-meter) works similarly to a joiner’s spirit level. (Wikipedia,
accessed 3/7/2020)
These days, most volcanoes are
closely and accurately monitored using high precision equipment, such as
tiltmeters and GPS readings from satellites, capable of measuring changes in
altitude to within a millimetre. However, these devices only show changes in
altitude or inclination at the exact location they are placed, so numerous
devices are needed to give a clearer overall picture.
InSAR image
created from radar data collected by satellites. (USGS,
accessed 3/7/2020)
The most modern and particularly
useful method of monitoring deformation is InSAR (Interferometric Synthetic
Aperture Radar). This works a bit like echolocation (or seismic), where a
signal is sent out from a satellite overhead which then reflects off the ground
surface back to the satellite. The time taken for the two-way journey is
recorded and this enables the distance to be calculated. A large area can be
covered quickly by this means and the data can be collated to create a picture
of the whole area. Regular measurements of the same area can be taken and
compared to show if ground deformation is occurring. This technique is
especially useful for difficult-to-access volcanoes.
- Why not try making your own basic tiltmeter and clinometer using the instructions from this WASP activity or watch the correlating video
- For more information on volcano monitoring have a look at the USGS Volcanic Hazards Website