On a 01 Dodge stratus coupe 3.0 I need help with crank shaft pulley and belt issue?

After my battery light came on I checked the alternator belt and it had come off. Bought a new one only to learn that the crank shaft pulley (harmonic balancer?) is broke. The metal part with grooves isn't there and it's just black rubber. So 2 questions. What would have caused it to break like that in the first place? And once we replace it and get new belt back on, where is the tensioner located (not on the alternator which is what we thought)