volcanic rock dust soil will only improve drainage as by nature it has know goodness in it . its what you mite use in hydroponics or aquaponics in its more solid form because of this . its airy and light .

Living on a volcanic island, guess what I use. Drainage is super fast, drying out in summer is a problem. But the air holes in the rock retain water, something you would loose if it was in a dust form. You would get the same results by adding an extra handful of perlite to your mix.

hear hear wylie .

Volcanic larva is an alkaline substance so the rock dust has similar properties to garden lime. One advantage is that it would also have extra trace minerals but a garden soil in good heart would have plenty of these trace minerals in it anyway. I guess that the rock dust is more expensive than the lime so, personally, I would stick with lime.

As I inderstand it, VRD is a waste product from quarrying and if it wasn't sold as a soil improver it would go to landfill. So it provides some trace elements and is used to improve water retention in dry soils but you're doing more for the environment by saving it from landfill.
I found this which might interest you http://youtu.be/uOJ9vBdHZGM