More generally, perhaps, such migration results in concealed intrusions, and only when these are large are they reflected by engulfment at central craters.
Measurements of tilt have demonstrated that volcanoes rise and fall in sympathy with movement of buried magma. The entire edifice of Kilauea seems almost to breathe in response to underground displacements of magma. Jaggar’s painstaking researches at this volcano have shown that “sudden downward phases are just as common and rhythmic as gradualrisings, and this introduces a whole world of magmatic force, magmatic earthquakes and magmatic intrusion in pulsating stresses through time, where down-go and opening of voids must be granted.”2 Slow accumulation of magmatic pressures followed by quick release is the natural rhythm of volcanoes. The opening of radial fissures on the slopes of large volcanic cones is much less likely to be caused by hydrostatic pressure of magma in the narrow central conduits than by a general doming of the roof of the magma chamber following increased pressure from below.
At times of vigorous magmatic movement, volcanic cones are often subject to stresses adequate to open far-stretching rifts. Tension fractures produced by upsurging magma and openings caused by withdrawal of liquid provide pathways for migration. The central pipe of a volcano is only one of countless channels. Since it leads to the summit vent, what happens there may be plainly observed; nevertheless the events witnessed in the crater may be only incidental to more fundamental movements far below, along channels far removed from the central pipe. Already we have seen that during the later stages of the history of Mount Mazama, a semicircular arc of vents opened on the northern flank, probably along a ring fracture. Presumably this fracture resulted from withdrawal of magma at depth and consequent breakage of the chamber roof. Again, we have noted that during its late history, Mount Mazama developed linear groups of parasitic cones and domes on its flanks. Some of these are disposed on radial fractures, developed as tension fissures during times of uprising magma. It seems reasonable to suppose, therefore, that during the long interval of quiet preceding the climactic eruptions of Mazama, internal pressures may have accumulated until they were able to rupture the walls of the magma chamber in many directions. The time of maximum magmatic pressure is not during eruption, but precisely the moment preceding the fist explosion. It is then that fracturing of the walls and injection of dikes are most likely to occur.
Recent studies by Jaggar, Stehn, van Bemmelen, and Reck, to mention only a few, all point to the importance of engulfment. Jaggar, whose lifelong experience at Kilauea gives unusual weight to his opinion, strongly advocates the idea of engulfment as the prime cause of calderas. During the explosive activity of Kilauea in 1924, the pit was enormously enlarged. Yet the volume of old rock thrown out was only 1/253 of the volume which disappeared by engulfment. “The great steam-blast eruptions of ‘the world,” he writes,3 “have been accompanied by what Mercalli called ‘sprofondamenti,‘ deepenings by collapse.” Referring to the destruction of the ancient summit of Vesuvius and the consequent formation of Monte Somma, unreasonably described by most writers as the result of decapitation by the great Plinian eruptions of A.D. 79, he adds:
The tremendous event of the Vesuvian crisis, when a steam-blast eruption happens, is a “sprofondamento,” a deepening. The amount of surface lava, dust and rocks is trivial compared to crateral evacuation.
