The late 1950's and 1960's have seen the advent of the concept of Plate Tectonics which is now generally accepted and appears to explain neatly a vast range of geological phenomena of the past and the present. In this framework, volcanism is basically associated with three tectonic processes: Subduction.
Where the margins of two lithospheric plates - one oceanic, the other continental - collide, the denser, oceanic plate is thrust (=subducted) under the lighter continental one. Magmas are generated by the partial melting of subducted oceanic lithosphere (that is, the rigid outer stratum of the globe), which consists of basalt covered by mainly silicic sediments (which contain large quantities of water), and the resulting magmas have a high silica content because much of the melt is constituted by the sediments.
Almost all volcanoes around the Pacific Ocean (the so-called "Ring of Fire") are the result of subduction processes, and their activity is highly explosive. Rifting. This occurs in areas where the Earth's crust is torn apart, allowing magma to rise to the surface.
Most of this activity occurs in the ocean basins, the most famous example being the Middle Atlantic Ridge where two lithospheric plates are "drifting" apart, and new crust is formed by the emission of basaltic magmas. Only in a few places, mainly Iceland and the Afar region in northeastern Africa, rifting, which generates oceanic lithosphere, is occurring on land.
Hot Spots. In various places on Earth magma is rising from the Upper Mantle to the surface where there are no plate boundaries (like in the previous two cases), and magmatism of this type is called "intraplate magmatism". The places where this occurs are known as "hot spots" which appear to occupy relatively fixed positions in space while lithospheric plates move across them. Magma is being fed by so-called mantle plumes.
The result is a long-lived volcanism which often builds a chain of volcanoes which become progressively older with distance from the active hot spot. Volcanoes related to this type of process occur both on oceanic and continental lithosphere. The most famous examples are the Hawaiian islands, with the currently active volcanoes of Mauna Loa, Kilauea, and Loihi. Etna apparently does not fit into any of these tectonic settings.
Subduction is believed to have contributed to the volcanic activity in the Aeolian Islands, off the northern coast of Sicily, but recently proposed hypotheses envisage a peculiar type of rifting as another factor acting simultaneously in the same area. Etna is not directly related to the Aeolian volcanism. To understand what may be the reason for the long-lived and voluminous volcanic activity at Etna, one has to get familar with the complicated structural situation of the area, and in the following I will try to render an idea as comprehensive as possible of that situation.
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