A group of calc-alkaline andesitic and lamprophyre cinder cones and lava domes, along with associated lava flows, is located near the town of Mascota about 50 km E of Puerto Vallarta. Mascota is the youngest of a group of potassic volcanic fields in extensional grabens of the Jalisco block south of the Río Ameca. Most volcanism here is confined to two NNW-SSE-trending grabens cutting Cretaceous ash-flow tuffs. The youngest eruption was a basaltic-andesite lava flow at Volcán Malpaís, north of Mascota; the lack of soil development and sparse vegetation suggest that it may be only a few thousand years old (Carmichael et al., 1996). The field is noted for its unusual diversity of volcanic rocks, and contains Earth's youngest known minettes (lamprophyres).