The cinder cones of Michoacán—Guanajuato, central Mexico: their age, volume and distribution, and magma discharge rate

Abstract

The Michoacán—Guanajuato Volcanic Field (MGVF) in central Mexico contains over 1000 late Quaternary volcanic centers, of which approximately 90% are cinder cones. This area is distinct from other parts of the Mexican Volcanic Belt (MVB), where composite volcanoes predominate. Other volcanic forms in this field include lava cones, lava domes, maars, tuff rings, small shield volcanoes, and coneless lava flows. Most of the shield volcanoes are eroded and predate the currently observable cinder cones. Within the MGVF, cinder cones are situated between 200 km and 440 km from the Middle America Trench. Nearly 75% of the volcanoes are distributed between 200 km and 300 km from the trench, and cone density is highest at 250 km. Overall cone density is 2.5 cones/100 km 2, and median separation distance is 2 km. The median cinder cone has a height of 90 m, a basal diameter of 800 m, a crater diameter of 230 m, and a volume of 0.021 km 3. The cinder cones typically erupted olivine-basalt or basaltic andesite; these rock types are less silicic than those of composite volcanoes in the MVB. In general, samples from the MGVF show higher MgO, Cr, and Ni and lower K 2O, P 2O 5, and Zr than those farther from the trench. Cinder cones show various stages of degradation, from which relative ages can be estimated; radiocarbon dates of seven cinder cones were obtained for calibration. Of several morphological indices of age, gully density and surface morphology of associated lava flows are the most sensitive. The morphological classification, based on gully density and lava flow surface features, revealed that 78 volcanoes are younger than 40,000 years. All of them are situated in the south, and some have a rough NE alignment, parallel to the relative motion vector between the Cocos and North America plates. Such NE alignments are also found locally for older cones, although in general cones are randomly spaced. Local cinder cone alignments are E—W in the northern part of the volcanic field, where E—W normal faults also occur. Despite the large number of scattered cinder cones and other small volcanoes in the MGVF, total erupted volume suggests a low magma supply rate. The estimated total volume of lava flows, ash, and cones erupted during the last 40,000 years for an area of 15,000 km 2 is 31 km 3. The calculated magma eruption rate of 0.8 km 3/1000 years is small in comparison to a single composite volcano in the MVB.