What is a caldera?
A large (>1 km) circular-elliptical depression formed by loss of support and roof subsidence after magma eruption or drainage.
What is a cauldron?
A deeply eroded caldera where the collapsed roof is exposed; may include cauldron blocks.
What are the two main caldera styles?
Mazama-type (volcano blown apart) and Valles-type (large magma chamber collapse).
What is a ring-fault?
A margin fault where caldera subsidence takes place.
What is a ring-fracture?
An arcuate fracture parallel to ring faults, often associated with postcaldera vents.
What is resurgence?
Uplift of caldera floor due to rising magma after collapse.
What is downsagging?
Downwarping of surface strata forming a caldera without ring faults.
What is a syncaldera deposit?
Deposits formed during caldera collapse, often pyroclastic.
What is a postcaldera deposit?
Materials deposited after collapse, draping margins or erupting along faults.
What is a nested caldera?
Overlapping calderas formed by multiple collapse events at the same volcano.
What is a pit crater?
A mini-caldera (<1 km) formed by drainage of magma; floor subsidence must be evident.
Are calderas common?
Yes, at large-volume volcanoes of mafic to felsic composition.
How do mafic calderas usually form?
By drainage of magma at depth or phreatomagmatic explosions.
How do felsic calderas usually form?
By large-volume pyroclastic eruptions (ignimbrites).
What controls caldera diameter?
Eruption volume.
What is trapdoor subsidence?
Subsidence on one side via faulting while the opposite side sags.
What is Stage 1 of caldera formation?
Chamber intrusion and tumescence via emplacement of shallow sill-like magma.
What is Stage 2 of caldera formation?
Plinian overpressure phase—eruption, depressurization, vesiculation.
What is Stage 3 of caldera formation?
Onset of subsidence with pyroclastic flows and development of ring faults.
What happens during main subsidence?
Formation of ring faults and block collapse (piston-like models).
What is the funnel-collapse model?
A mechanism where the roof collapses downward into a narrowing shape.
What is the piston-collapse model?
An oversimplified model where a block drops vertically during collapse.
What are caldera margins like?
Ring-faulted, warped, or complex; may include polygonal shapes in tectonically influenced areas.
What is a caldera complex?
Multiple overlapping calderas within a volcanic field.
