Also intersecting at Mount St . Helens is a NE ( 045?) trending line ( red ) of Pleistocene ( about 4 Ma ) plug domes and a topographic lineament ( followed in part by Highway 12 ).
12.
Soon before or after the buried ice had melted away, dacite lava quietly erupted from Opal Cone southeast of the Atwell Peak plug dome 10, 700 to 9, 300 years ago One of the lava flows traveled down a 30 % to 35 % grade over the landslide scar on the western flank.
