(TMZ) - "Vampire Diaries" star Ian Somerhalder can make a girl go crazy just by touching her -- at least that's the way it looks in this video, showing a USC student spontaneously bursting into "Natural Woman" last Tuesday ... as soon as she holds his hand, according to TMZ.

TMZ reports Ian had just finished an Earth Day talk at the school -- he's a big conservationist -- when student Shelby Miguel dropped the Aretha assault during a meet-and-greet ... out of nowhere.

It's pretty ballsy -- not to mention awkward -- but check out the video ... she's got a great voice.