At erupting volcanoes, just before magma, or molten rock, arrives at the surface to produce lava and ash, it can become much more viscous and reluctant to flow. This change in character of the magma in turn affects a number of other processes - high pressures build, gas flows change and the rate of flow of the magma itself becomes variable. Sometimes these changes vary systematically every few hours to produce a periodic behaviour. Being able to measure such periodic behaviour is very useful to scientists in volcano observatories for two reasons. Firstly, certain times in the period are much more prone to explosions and hazardous flows, and so being able to forecast their occurrences is useful. Secondly, by observing how a variety of phenomena change during each cycle allows the conditions that give rise to the periodic flow to be understood. This in turn allows the longer-term behaviour of the volcano to be better anticipated, with benefits to people affected. In this project we will improve our understanding such behaviour at Soufriere Hills Volcano, Montserrat. This type of periodic behaviour is probably common at the more dangerous type of volcano with magma rich in silica. However, it is very difficult to observe and as a consequence not well understood. This is because some of the signals associated with it are restricted to near the vent of volcano and are difficult to measure. One place where such periodic signals were measured is Soufriere Hills. Over an interval of a few months in early 1997, tiltmeters that measure the inclination of the ground surface, recorded a remarkable series of cycles of ground motion up and down with a period of about 9 hours. Unfortunately, the tiltmeters were destroyed by the volcano and the location was subsequently too dangerous to re-install new ones. We plan to bring a new technology to bear on this problem in a 2-year project based at the University of Reading and applied at the Montserrat Volcano Observatory. Rather than measure the ground movement using an instrument buried in the ground we will do so from a safe distance using radar interferometry. From a few kilometres away we will measure the outward and inward movement of the ground around the lava dome growing within the crater at Soufriere Hill. We expect the cycle to be measured over a few hours and to an accuracy of a few millimetres for a signal ten times as large. A portable, ground-based radar interferometer has been developed for this type of task, and we will be the first to use it on a volcano like this. Because the instrument gives an image of the ground displacement rather than a point reading it will be able to measure the spatial pattern of motion, by making measurements from different viewpoints. This will enable the new measurements to test a hypothesis that the conduit feeding the magma to the surface below Soufriere Hills Volcano has a shape like a vertical cylinder joined onto a fissure below depths of about one kilometre. The technology of the measurements of earthquakes, gas and wider deformation of the whole island routinely made by the Montserrat Volcano Observatory has advanced greatly since 1997, particularly the frequency of measurements. We will use these frequent (very hour and less) measurements of the cycle to compare with a computer simulation of the magma-filled conduit. This will help us to understand better how the conduit behaves and how it might behave in the future.