6.2 They may be lowering of the floor after an eruption : formation of caldera

Before an eruption, the magma chamber size increase and decrease a lot during an eruption creating a big hole inside the Earth. The material on the top, lots of time it is the top of the volcano, are not stable an fall. This event creates a crater after the formation of the ring fault.

“A caldera is a large, usually circular volcanic depression formed when magma is withdrawn or erupted from a shallow underground magma reservoir.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.”

The Dolomieu crater on 31 October 2006 (a), on April 6 in the afternoon (b) and on April 10 (c). The Bory fracture network studied by Carter et al. [2007] in indicated for relocation.
source: Laurent Michon et al., 2007

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