Two things separate the two really, both have been mentioned already. Recently though, kids have been asking me for a third thing so there's a new name being banded about.
So.
A traditional spine must have coping, and those two copings must touch.
A volcano is a spine with a flat top on it, usually 8" - 12" ish wide. It may or may not have coping.
Recently, people have been talking about a 'street spine'. This is simply a volcano without coping.
The name comes from the shape, stand side on and look - the profile of the ramp looks like a volcano...
The technique to get over either is similar, with smaller volcanos being the easier to get over, you can pretty much just monster truck over em if you're determined enough.
Basically though, approach medium speed (not as fast as going at a jump box) and aim for the left or right of centre as is your preference. Work this out beforehand as you want to ride up the ramp and carve slightly your opposite way. As you pop over the top, this slightly sideways orientation will put you in a nice familiar dropping in position to come down t'other side. You need to be in the 'attack position', head and shoulders over the bars, as Scott has said, you must get that front wheel in and you won't do that with your arse hanging over the back wheel. Don't get left behind..
Got to stay loose, and be brave. If you get into the habit of dabbing a foot on the top, it can be a hard habit to break to habit in the future.
Like lots and lots of riders, I just launched to flat the first time just to break the spell and get rid of the fear. But going over these things straight up and down is just bloody hard work. Got to get your carve on
