How do we know that earths crust is different from what’s inside?

The Earth's interior is composed of four layers, three solid and one liquid—not magma but molten metal, nearly as hot as the surface of the sun. The deepest layer is a solid iron ball, about 1,500 miles (2,400 kilometers) in diameter. Although this inner core is white hot, the pressure is so high the iron cannot melt\

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