By April 7 the combined crater was 1,700 feet (520 m) long, 1,200 feet (365 m) wide and 500 feet (150 m) deep.[16] A USGS team determined in the last week of April that a 1.5-mile (2.4 km) diameter section of St. Helens' north face was displaced out at least 270 feet (82 m).[11] For the rest of April and early May this bulge grew 5 to 6 ft (1.5 to 1.8 m) per day, and by mid-May it extended more than 400 feet (120 m) north.[11] As the bulge moved northward, the summit area behind it progressively sank, forming a complex, down-dropped block called a graben. Geologists announced on April 30 that sliding of the bulge area was the greatest immediate danger and that such a landslide might spark an eruption. These changes in the volcano's shape were related to the overall deformation that increased the volume of the volcano by 0.03 cubic miles (0.1 km³) by mid-May.[17] This volume increase presumably corresponded to the volume of magma that pushed into the volcano and deformed its surface. Because the intruded magma remained below ground and was not directly visible, it was called a cryptodome, in contrast to a true lava dome exposed at the surface.



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