How can satellite EO measure terrain deformations around volcanoes?

Only about 10% of the potentially active volcanoes worldwide are monitored on a regular basis. Most monitoring relies on visual observation and terrain inspection and there are less than a hundred observatories worldwide. A key role for EO data is at volcanoes where little or no ground-based monitoring exists. This includes large-scale radar interferometric (InSAR) surveys that look for deformation, especially at volcanoes without any seismic monitoring stations; imagery to detect surface change (visual or radar) to identify, for example, dome growth and pyroclastic flows; thermal studies that look for the first sign of magma close to the surface and active lava flows on the surface; gas emissions before, during, and after an eruption, and tracking of large ash clouds during and following eruptions.