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Our transcription: Belts of volcanoes have grown, shifted, and died with changes in plate boundaries. The history of individual volcanoes themselves can be quite complex. A volcano can look very different at different stages in its development. Here at Panam Crater, for example, the initial eruptions were gas charged explosions. They produced this shallow crater surrounded by the rim of pumice that I'm standing on. Having lost most of its gases, the remaining magma grows slowly out of the vent in the next eruption. This magma plugged the vent by forming this dome composed of volcanic glass. This glass known as "obsidian" is the product of an extremely viscous magma. It is composed almost entirely of silica.
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