 The first eruption , which took place 0 @.@ 8 mya , was distinct from later eruptions in that its product lacks flattened <unk> ( lens @-@ shaped layers of mineral or rock embedded in rock ) . The tuff is dotted with predominantly plagioclase phenocrysts , which make up between less than 5 to approximately 15 percent of each particle 's mass . After being erupted , the material settled in canyons , where it underwent glaciation . This carved cliffs which drop as much as 100 metres ( 328 ft ) . The tuff is limited to just a few kilometers around the complex . 
