Flexi Says: Scientists know that the outer core is liquid and the inner core is solid because: S-waves stop at the inner core. The core remains solid even at temperatures over 5000 degrees Celsius owing to the immense pressure of well above 3 million atmosphere(atm). The strong magnetic field is caused by convection in the liquid outer core. Convection currents in the outer core are due to heat from the even hotter inner core. The heat that keeps the outer core from solidifying is produced by the breakdown of radioactive elements in the inner core.