A book I'm reading about Mars mentioned caldera in relation to volcanoes, so I looked up the term. ------------------ cal·de·ra Pronunciation (kl-dâr, -dîr, käl-) n. A large crater formed by volcanic explosion or by collapse of a volcanic cone. caldera n : a large crater caused by the violent explosion of a volcano that collapses into a depression [Spanish, cauldron, caldera, from Late Latin caldria. See cauldron.] Brian Kelsay