 In comparison to the well @-@ preserved rocks of the dry , central part of the Andes , the record that defines the southern sector is poorly preserved . Remnants of Miocene and Quaternary eruptions within the central part are clearly preserved in the rock record . Ash flow sheets constitute as much as 40 percent of the area 's total erupted material , suggesting that pyroclastic eruptions were rather important during this time . By examining the eruption rate of individual volcanoes , Hildreth et al. concluded that similar amounts of ash @-@ flow volcanism took place in the southern sector as well . Their study established that in particular silicic ash was missing from the record , and concluded that erosion had probably disrupted deposition of volcaniclastic rock . 
