How melted rock migrates.

For most nongeologists, the idea of liquids moving through solid rock is a strange one. But liquids of one sort or another are thought to be ubiquitous in the Earth. There are the familiar hydrothermal fluids, dominated by water, which occur in the very shallow crust (the Old Faithful geyser in Yellowstone National Park in the United States is a dramatic example). But in the deeper parts of the Earth there are hydrous and carbon dioxide (CO2) liquids formed by the heating of rocks as the minerals containing these molecules break down