It might not seem odd until you think about it: why the heck is the Earth full of molten rock, just waiting to burst all over us as red-hot lava? The origin of lava is an ancient scientific puzzle, and the theories about it have gone from fanciful (burning of subterranean sulfur) to sophisticated. With plate tectonics, we finally have a decent theoretical framework for the origin and behavior of magma, and I'm happy to treat you to the basics in this new article, About Magma.
An intriguing bit of history is that the word "magma" originally meant a paste, such as a druggist might prepare. In 1794 when Dolomieu first used the word in a geological way, he was talking about what we would call diagenetic or hydrothermal processes involving water. It took more than 40 years before geologists started using the word the way we do today. Yet magma really is a kind of pastea red-hot mush of mineral crystals in a thick fluid, only sometimes like the splashy stuff at Kilauea.
Plutonic emulsion, Donner Pass Geology Guide photo