A red filament of lava stiffens at a volcanic vent as vapor snaps from fractures in the dark crust, offering a deceptive answer to a familiar question. Not the core. What reaches the surface is magma assembled much nearer to it, where partial melting in the mantle produces melt and faults or porous rock provide a path upward. Heat is only part of it. Dissolved water, carbon dioxide, and sulfur spe...More