The summit cone of Mt Etna is cut by an approximately NNE-SSW trending, narrow fissure zone that controls much of the terminal activity of this volcano. Within the central crater, which lies at the top of the summit cone, there are two main vents alined on this fissure zone: the chasm, a deep pit; and the 1964 crater. Both of these vents were blocked before 1971. The NE crater first opened in 1911 on the lower slopes of the summit cone on the same fissure zone, and has had an open conduit from which lava has been erupted at a constant rate from 1966 to the beginning of the 1971 eruption. In 1968 a small gas bocca known as the Bocca Nuova opened near a fissure that had previously erupted lavas in 1956. The Bocca Nuova collapsed in early 1970 to give a 100 m wide crater. It is suggested that collapse resulted from movement of magma at depth into the 1956 fissure causing sudden lowering in the magma column below the Bocca, and that continued migration of magma into the fissure eventually lead to the eruption of April 1971, the site of this eruption being in line with the fissure. A rise in temperature of fumaroles at the edge of the central crater after the Bocca Nuova collapse is attributed to this incursion of magma across the crater.