Calderas are huge bowl-shaped craters, usually formed by volcanic activity. Some of the earliest geologists thought the calderas are formed when violent volcanic eruptions blew the tops off the volcano. However, few calderas are formed this way.
Calderas are formed because eruptions of huge volumes of pyroclastic materials had left the roof of the magma chamber unsupported, causing it to
fracture and fall downwards into the chambers. Magma is also being drained from the chamber through fissures at depth. Collapse of the cone occurs, as it becomes a jumble of enormous blocks, some of which sink through the magma. This process is termed cauldron subsidence. This process may take a long time to complete and often happens in an extinct volcano. An example is the caldera of Tengger in Java, Indonesia.
Crater lakes are formed when a caldera becomes filled with water sometime after it is formed. An example is the Crater Lake in Oregon, America. It is nine kilometers in diameter whose floor is 600 meters in depth, while the surrounding 6800-year-old caldera walls rise steeply 600 meters above it.