 The most recent volcanic phase began about 4 mya , producing largely andesitic eruptions . A series of eruptions built up a broad plateau of lava , and extended over the area where Calabozos now lies . Locally , this plateau was composed of mafic andesite with olivine , which over time gathered to form 300 @-@ metre ( 1 @,@ 000 ft ) to 500 @-@ metre ( 1 @,@ 600 ft ) thick layers . Nearby volcanoes sit on top of two mya lavas that formed during this period , while the Loma Seca Tuff lies atop andesitic deposits from Descabezado Grande . 
