 Since about 6.6 million years ago the volcanic activity produced rocks of both mafic and silicic compositions.The increase of volcanic activity has been attributed to the steepening of the Nazca Plate slab which allowed mantle material to penetrate into the space between the lower crust and the slab.North of 21° degrees southern latitude ignimbritic volcanism started earlier, generating the Altos de Pica and Oxaya formations.