The earth's major layers formed because ________.
◦ radioactive decay caused materials to change over time and resulted in the layering
◦ the earth's mantle grew like a crystal forming around the earth's core but the crust was exposed to space and stayed more like the original material
◦ the earth was heated after its formation resulting in melting and redistribution of major chemical compositions
◦ the chemical composition of the solar nebula changed over time