Since 1948, it has been been operated by the USGS. During the past 75 years of research, HVO scientists have developed and refined most of the surveillance techniques now commonly employed by volcano observatories worldwide. Volcano monitoring The term volcano monitoring refers to the observations and measurements scientists make to document changes in the state of the volcano during and between eruptions. Such changes are now well known for Kilauea, and a pattern of similar changes is becoming apparent for the less studied Mauna Loa. As magma enters the shallow summit reservoir, the volcano undergoes swelling or inflation (a process similar to the stretching of a balloon being filled with air). This swelling in turn causes changes in the shape of the volcano's surface. During inflation, the slope or tilt of the volcano increases, and reference points (benchmarks) on the volcano are uplifted relative to a stable point and move farther apart from one another. For Hawaiian volcanoes, pre-eruption inflation generally is slow and gradual, lasting for weeks to years. However, once eruption begins, the shrinking or deflation of the volcano typically occurs rapidly as pressure on the magma reservoir is relieved -- a process not unlike deflating a balloon. During deflation, changes in tilt and in vertical horizontal distances between benchmarks are opposite to those during inflation.

Hypothetical cross section of Kilauea Volcano. Magma entering the shallow reservoir exerts pressure on the volcano, causing earthquakes and distorting its shape from the dotted-line profile to the solid-line profile. During inflation, reference points (benchmarks) on the volcano's surface are pushed upward and outward relative to points assumed to be stable. The changes in the volcano's shape and the occurrence of the earthquakes can be tracked precisely by volcano- monitoring techniques.

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