Mount Etna is among the few volcanoes on Earth that erupt nearly continuously, but its activity (in terms of output rate and flank eruption frequency) undergoes significant fluctuations in time. Such fluctuations do not occur randomly, but represent various stages of cycles on a scale of decades and centuries. Recurrent patterns are particularly evident since 1865, with four complete cycles and a fifth one initiated in 1993. Each cycle consists of three phases, beginning with low-level activity followed by nearly continuous summit activity and culminating with a series of flank eruptions, the last commonly being the most voluminous. A distinct increase in the output rate of Etna, along with more frequent and voluminous summit and flank eruptions since 1950, may be interpreted as part of a longer cycle that began after a large eruption in 1669 and has not yet reached its culminating phase. If that trend continues, the activity of Etna might become similar to that of the 17th century, when flank eruptions were more voluminous than they have been since; however, it is difficult to assess when this will take place.

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