DEM

Before                                                                      After

This is a DEM or Digital Elevation Model of Mount St Helens before and after her eruption on May 18, 1980.  The digital elevation model give a better visualization of the land elevations.  Information can be gathered from land surveys, Lidar, or Radar.
The photo above shows that the eruption had a clear effect on the surface of the Mountain.

This map is found at:
http://ned.usgs.gov/Ned/historic.asp