So much physics. How could I resist?
There's the ability to use the sights and sounds to determine distance from the volcano to the camera.
There's the projectile motion of the ejected boulders. Launch speeds, maximum ranges, maximum heights—start with ideal estimates.
Then use PhET's Projectile Motion sim to bring air resistance into the analysis. Huge differences!
Are the spectators at a safe distance? No need to wonder: find out how far those boulders could fly for maximum range. And maximum range with air resistance doesn't happen with a launch angle of 45°.
And don't forget about the adiabatic expansion cloud that appears and disappears at the outset.
Here's the student worksheet that I prepared. The lesson can be done in or out of class.