I've posted about the Ben Nevis Caldera before but this sign outside the Ben Nevis Inn gives the best explanation I've seen. Granite is formed when magma solidifies underground over up to a million years. This gives the crystals time to grow. What seems to have happened here is that granite was forming in the underground magma chamber when the top of the volcano cone collapsed in. I've seen a caldera on Tenerife. It was huge and spectacular. Trying to imagine it in this ice ravaged landscape is difficult.
