It's classed as a "steam erruption" and more detail can be found here:
Originally posted by cpr12r
I wonder when another significant eruption will happen.
From ASTER Image Gallery Mt. St. Helens
This 3-D anaglyph image of Mt. St. Helens volcano combines the nadir-looking and back-looking band 3 images of ASTER. To view the image in stereo, you will need blue-red glasses. Make sure to look through the red lens with your left eye.
Click Image to Enlarge