 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.
