What does a Volcano burp up?

With the recent activity of Mt. Redoubt going off, it got me thinking. Hmmmm.....What exactly is that volcano burping into the air over 12 miles high? Just think, everything that comes up, once was miles beneath the earth. So.....hopefully someone somewhere can take some of the ash and whatever it is that's spread all over for miles, and will learn from this and share it with the generations to come.
With all the eruptions lately comes air traffic cancellations, people stranded all over the country not being able to get to their work, or loved ones, vacations or their final destinations. It's quit an inconvenience to lots of people and yet we do NOT have any control over it. It's just one of God's ways to show us who really IS in control.