Someone help me (alternator problem)

I was heading to work today and my battery light came on so I turned around and went home. When I looked under my hood I saw that the cable coming off the back of my alternator had actually melted. What in the world would cause this? I've noticed a burning smell for the past week but was unable to find where it was coming from. I guess now I know but I have no clue what caused it. I'll post pics but I'm waiting on my husband to take them since I had to grab his truck and run to work before I was late. Has anyone else had this problem???
http://img133.imageshack.us/img133/823/z7gg.png