What was different about st. helens eruption from other volcanoes?
The Mt St Helens eruption was the first recorded example of a lateral blast. The northern slope of the volcano collapsed which relieved pressure from the north side of the volcano. This was sufficient to enable the rising magma to explosively decompress. Instead of the normal situation when a volcano erupts from a summit crater the eruption blasted out sideways exploiting the weakness of the northern face. Previously this behaviour was unknown to volcanologists and had not been considered. I recommend you go away and read through the relevant wikipedia articles, the USGS will have further information. As ever, be careful about un-cited information from wikipedia. It suggests that David A. Johnston (the volcanologist manning the observation post when it was wiped out) had suggested the danger of the lateral blast. I have not found anything which corroborates that however.
