Stephen Wessells Dina Venezky 2010 A caldera is a large, usually circular volcanic depression formed when magma is withdrawn or erupted from a shallow underground magma reservoir. It is often difficult to visualize how calderas form. This simple experiment using flour, a balloon, tubing, and a bicycle pump, provides a helpful visualization for caldera formation. application/pdf 10.3133/ofr20101173 en U.S. Geological Survey Caldera demonstration model reports