The earth's mantle is different from the outer core because the mantle is

Mantle is composed of rocks where as core is composed of metals.

Explanation:

Mantle and core both are part of the earth's crust. Though there have a difference between them - The mantle is consists of semi solidified rocks and core is consists of hot metals like iron and nickel. Core is the inner most layer surrounding the earth's center. The portion of the interior beyond the crust is called mantle. The core is the densest layer of the earth with its density ranges from 9.5-14.5 g/cm^3 where the density of mantle varies between 2.9-3.3 g/cm^3.