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