Volcanos are often classified by plotting their degree of "explosiveness" (y axis) and height of the erpution column (x axis).... the largest and most dangerous eruptions have high values of both. Check over each type for examples.
Note- this example is here is purely for showing how flickr can be used to create "hotspot" learning objects.... my geology knowledge is extremely rusty!
Flickr has neutered the notes capability, but you can see the notes
they refuse to show via