Igneous rocks were once molten. This is a simple statement, but it’s exactly what sets them apart from the other two great divisions of rocks: sedimentary and metamorphic. So, deriving from the Latin word for fire–ignis, the same word that gives us ignition–igneous rocks are associated with heat. Some are simply solidified lava, but most originated by slowly cooling below the Earth’s surface. Thanks to erosion through time of the overlying material, such rocks are now widespread at the Earth’s surface and consequently underlie many of the world’s vineyard regions, from Washington State to the mountains of Hungary, from Lodi, California, to the Cape Peninsula of South Africa. Although it gets warmer with depth everywhere across the Earth, generally the weight of the overlying rocks makes the pressure too great to allow melting, so as a rule the rocks below our feet are solid. In some places, however, the heat increases so rapidly that temperatures can reach over 600°C at just a few kilometers below the ground surface, a temperature at which some rocks are molten, even under pressure. The initial melting usually takes place in and below the lower part of the Earth’s crust, but the molten rock then rises, typically to reside tens of kilometers or so below the surface, though less under volcanically active areas. Such depths may seem large to us, but seeing as its well over 6000 kilometers to the center of the Earth, geologically they are pretty close to the surface. In other words, the igneous rocks we now see at the surface did not form incredibly deep in the Earth’s interior; they were nowhere near Earth’s core, as some writings claim. We call this underground molten material magma. People seem to like the word. Not only does it appear on wine labels, but it is also the name of a number of wine shops, bistros, and various drinks. It exists in the Earth in magma chambers. It would be simplistic to picture these as some sort of enormous underground caves filled with liquid rock: there may be patches that are wholly liquid, but almost certainly there will be plenty of solid matter, minerals that are below their melting point.
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