Remote Sensing Can Measure Volcanoes Grow and Shrink

The image shows subtle expansion of Kilauea, which implies that magma is applying pressure from underneath. The image has concentric rings of color to show whether the volcano's radial movement is toward or away from the sensor and by how much, with three distinct rings equating to three half-wavelengths of 1.65 cm for a movement of 4.95 cm over nearly two months. This kind of information is especially important in places like Hawaii and places on the Pacific ring of fire, like Japan and Indonesia, where people live by volcanoes. 

https://www.hawaiitribune-herald.com/2019/06/23/features/volcano-watch-reading-the-rainbow-how-to-interpret-an-interferogram/

As for the interferometry, I was displeased with the article's explanation for it so I like the way the following site describes it. Basically, two beams of light traveling the same distance through the same medium should be moving at the same speed. Therefore, the return signals, or lack thereof when a single beam is split and sent in two different directions before coming back and effectively neutralizing each other's waves, should arrive to a sensor at the same time. However, when something causes a beam to slow down, then the difference can be recorded.

https://www.ligo.caltech.edu/page/what-is-interferometer