Geological and environmental controls on the change of eruptive style (phreatomagmatic to Strombolian-effusive) of Late Pleistocene El Caracol tuff cone and its comparison with adjacent volcanoes around the Zacapu basin (Michoacan, Mexico)

摘要

The 28,300 year BP (cal 32,300 BP) El Caracol tuff cone complex is one of the few phreatomagmatic volcanoes in the scoria-cone dominated No-Quaternary Michoacan-Guanajuato Volcanic Field (MGVF). It displays a shallow circular crater of similar to 1 km in diameter that is filled with several meter-thick lava flows and is located between two NE-SW trending normal faults dipping NW. It lies directly on top of Pliocene lavas of the San Lorenzo shield volcano that forms part of a tectonic horst (topographic high) separating the Zacapu lake basin (1980 m) in the south from the Rio Angulo river valley (1760 m) to the north. Detailed study of the tephra sequence indicates that the eruption occurred in two stages: 1) Weak phreatomagmatic, in which about 0.1-0.5 km(3) dense rock equivalent (DRE) of magma was issued within similar to 1 to 3 months at the rate of 4-40 m(3)/s, and 2) purely magmatic (Strombolian-effusive) during which the vent shifted slightly its position toward the NW, forming a small scoria cone (similar to 100 m high) on the crater rim of the tuff cone. From this scoria cone lava flows were issued, first into the tuff cone crater occupying its bottom, and subsequently toward the NW, down the outer flank of the tuff cone and into the plain, where they reached a distance of similar to 3.5 km. During this stage similar to 0.6 km(3) DRE of magma was produced at the rate of similar to 4 m(3)/s in a period of similar to 5 months.