The Earth's mantle, contrary to what one might read in some second-level geography text books, is not molten but solid. We know this because seismic S-waves pass through it. From where, then, comes the molten rock that erupts through volcanoes? This lecture will attempt to provide an answer, and will go on to address other issues related to volcanoes and how they work. For example: How does basaltic magma, which has a high density, manage to make its way upwards through the continental crust which has a lower density? What determines the kind of lava - rhyolite, andesite or basalt - that erupts? What determines whether ash or lava is erupted? Why are there no volcanoes in Ireland today? Where are Ireland's ancient volcanoes located, and why? What does lunar basalt look like? What is the oldest known sample of basalt?
