Making magma chambers from mush Shallow magma chambers either erupt as volcanoes or solidify as intrusive magma bodies. These magma bodies are traditionally considered to be long-lived and dominated by melt. Cashman et al. review the evidence that shallow magma chambers are actually assembled quickly from much larger, crystal-rich transcrustal magmatic systems. This paradigm helps explain many geophysical and geochemical features of volcanic systems. It also presents challenges for understanding the evolution of magma and provides insight into how and why volcanoes erupt. Science , this issue p. eaag3055