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Abstract
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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