Earth's Internal structure Because Earth became partly fluid, less dense molten materials(silicon, aluminum, sodium, and potassium) were freed to migrate toward the surface a Denser melted materials. such as molten iron, sank toward the center of the planet.◼ Because Earth became partly fluid, less￾dense molten materials (silicon, aluminum, sodium, and potassium) were freed to migrate toward the surface. ◼ Denser melted materials, such as molten iron, sank toward the center of the planet. Earth’s Internal Structure