Parícutin Volcano (Mexico) is situated on a northeast-southwest zone of weakness, which is reflected in the location of the lava vents and a satellite cone. That the lava rises from considerable depths is shown by inclusions of several rock types not known to crop out anywhere in the vicinity.

A detailed 4 month record of activity at the crater and in the area of lava outflow at the side of the cone shows little correlation between them. Only once, when the eruption became unusually vigorous, was there a direct response in the lava movement. No cyclic change was observed in the eruptive behavior of the crater except a vaguely defined alternation of moderate activity and quiet in 3 week periods; no periodicity whatever appeared in the lava movement, and the amount of flowing lava remained roughly constant during the 4 months of observation.

A mechanism of eruption is suggested in which lava stands continuously at a high level in the conduit. Dissolved gas bubbles out through the crater, while the liquid spills over into a network of fissures which connect with lava vents at the side of the cone. This mechanism accounts for the simultaneous emission of huge amounts of gas from the crater and emission of lava with little gas from fissures near the base of the cone. It explains also why SO2 is prominent in gas from the crater, while the small amount of gas accompanying the lava contains practically no sulfur compounds but is rich in HCl. Because the lava vents connect with the main conduit by narrow, winding channels, the lava flows are responsive only to major changes in the activity of the crater.

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