List the layers of the Earth in order from least dense to most

The layers of the Earth, ordered from least dense to most dense, are the crust, mantle, outer core, and inner core. The crust, the outermost layer, is the least dense and composed primarily of silicate rocks. Below it lies the mantle, which is denser and made of semi-solid silicate minerals rich in magnesium and iron. The outer core, consisting of liquid iron and nickel, is denser still. Finally, the inner core is the densest layer, composed of solid iron and nickel, compressed under immense pressure. This density gradient arises from variations in material composition and the effects of pressure and temperature.