 True to its name , the glacier lies inside the north @-@ facing crater left by the 1980 eruption of Mount St. Helens and the glacier 's elevation is about 6 @,@ 794 ft ( 2 @,@ 071 m ) . A massive central lava dome emplaced from 1980 to 1987 occupies the center of the crater , and the glacier formed in the shape of a horseshoe around the dome , with two terminal moraines on the eastern and western sides . Heavy winter snowfall , repeated snow avalanches , rockfalls , and sun @-@ shading by the surrounding cliffs to the south , led to the exceptionally rapid growth of this glacier . Thus , the glacier composition is estimated to be six @-@ tenths ice and four @-@ tenths rock , with an average thickness of 328 feet ( 100 m ) and a maximum thickness of around 656 feet ( 200 m ) ; nearly as deep as Mount Rainier 's Carbon Glacier . None of the ice is older than the year 1980 , however , the volume of the new glacier is about the same as all the pre – 1980 glaciers combined . The surface of the glacier looks dark and dirty in the summer due to the numerous rockfalls from the steep , unstable crater walls along with ash from eruptions , all of which help to insulate and protect the growing glacier . The 2004 @-@ 2008 volcanic activity created a series of domes that nearly split the glacier into two lobes at the south end of the crater . In spite of the four @-@ year lava dome building period , the glacier remains North America 's youngest and fastest growing glacier . With the joining of the termini on the north end of Crater Glacier in May 2008 , the body of ice completely encircles the lava domes . Meltwater from the glacier gives rise to Loowit Creek . 
