The irony of magma’s temperature

What is the earth made off? The inner core is iron under high pressure. Then we have the outer core made of heavy metals, the mantle made out of hot liquid rock, and finally the crust on which we all live. Alright, this is a globally accepted scheme. Although, why after all those billions of years that the earth exists is that rock, the magma, still hot and liquid? Didn't it have more than enough time to cool down to a solid state?  View More The irony of magma’s temperature