 In the months after the eruption , the crater floor of St. Helens remained hot and unstable , with five minor volcanic eruptions , and lava dome construction between May and October 1980 . After the eruptions ceased in the winter of 1980 , the crater floor cooled down enough for snow and ice accumulation . Beginning with snowfall in the winter of 1980 @-@ 1981 , the glacier began to grow very rapidly in the shadow of the crater . The glacier thickened at a rate of as high as 50 ft ( 15 m ) per year and advanced northward as much as 135 ft ( 41 m ) a year . This glacier growth was discovered by scientists working in the crater about seven to nine years later . However , the existence of the glacier was not publicized until 1999 . By 2004 , Crater Glacier covered about 0 @.@ 36 square mile ( 0 @.@ 93 km2 ) , about 20 % of the glacier area in the pre @-@ 1980 glaciers , and there was a western and eastern lobe flowing around the 1980s dome . Due to the gas emissions on the crater floor , there were glacier caves ( ice caves ) in the once smooth glacial ice , and several of them had been explored by the late 1990s . 
