 The volcano erupted on December 14 , 1989 , and continued to erupt for over six months . Sudden melting of snow and ice at the summit caused by pyroclastic flows and dome collapses caused lahars , or mudflows , which flowed down the north flank of the mountain . A majority of the mudflows coursed to Cook Inlet , about 22 miles ( 35 km ) from the volcano . The lahars entered a nearby river , worrying officials that they might destroy an oil storage facility located along it . 
