Volcanic material erupting on the earth surface provides the one direct means to study the physical conditions, chemical composition, and mineralogy of magmas in magma chambers. Most magmas from the Galápagos spreading center have not cooled or crystallized perceptibly on the way to the surface, and these magmas can be used to determine the composition, temperature, density, viscosity, and crystal content of magmas in magma chambers below the Galápagos spreading center. The constraints placed on these ridge crest magmas indicate that they will not erupt if they cool to less than 1150 °C, if their viscosity is more than 820 P, or if their density is greater than 2.8 g/cm3. The magma chambers are zoned: high-temperature (∼1200 °C), low-density, crystal-free magma is at the top, and low-temperature (∼1170 °C), high-density, crystal-rich magma is at the bottom. These conclusions can be extended to all ocean ridges that erupt basalts with low crystal content and unzoned olivine phenocrysts.