What is a caldera?

A caldera is defined as a large depression that forms at the top of a volcano following a catastrophic eruption, where the underlying magma chamber has emptied and can no longer support the weight of the land above it. When the magma drains out, the ground collapses, creating a significant circular or oval-shaped hole. This process can result in a caldera that is several kilometers in diameter.

The formation of a caldera involves substantial geological activity, often associated with explosive eruptions. While erosion can shape various geological structures, it does not account for the specific formation mechanism of a caldera. The other options describe different geological features or phenomena but do not accurately capture the specific characteristics and formation process of a caldera in the context of volcanic activity.