Convection homogenizes magma intrusions. Plutons are mountain-sized formations of igneous rock that poke through Earth’s surface. Their origin as solidified eruptions of magma is straightforward to explain. What’s puzzling is why plutons are so homogeneous on large scales, despite their immense size and despite being inhomogeneous in their mineral composition on small scales. Alain Burgisser of the Institute of Earth Sciences in Orléans, France, and George Bergantz of the University of Washington in Seattle have proposed an answer. In their model, a mass of viscous, semisolid magma—”mush” is the technical term—lies beneath the surface, hemmed in by walls of more solid rock. Pluton formation begins when the slow churning of the mantle below happens to bring a body of hot magma into contact with the bottom surface of the cooler mush. Over the ensuing decades, the heat rises slowly via conduction, making the mush less viscous and—crucially—less dense. In a...

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