Section view of the different layers of Earth’s interior

Earth’s mantle is the mostly solid, rocky interior of our planet. At mid-ocean ridges, the tectonic plates that form the sea floor gradually spread apart. Rock from the upper mantle slowly rises to fill that void, melting as the pressure decreases, then cooling and re-solidifying as new crust along the seafloor. To be able to model this process, scientists needed to know the temperature at which rising mantle rock starts to melt. (Illustration by Jack Cook, Woods Hole Oceanographic Institution)