Which layer of the Earth is composed mostly of iron and nickel?

The inner core is primarily composed of iron and nickel, making it a dense and solid layer located at the very center of the Earth. This layer extends from about 5,100 kilometers (3,200 miles) beneath the Earth's surface to the Earth's center at about 6,371 kilometers (3,959 miles). The immense pressures found at such depths keep the iron and nickel solid, even though the temperatures are extremely high, exceeding those on the surface of the sun.

In contrast, the outer core, which is often mentioned in discussions about Earth's composition, is liquid and composed of the same materials, but it is not solid like the inner core. The crust is primarily made up of silicate rocks and does not contain significant amounts of iron and nickel. The upper mantle, while it does have some metal content, consists mainly of silicate minerals and is not predominantly iron and nickel. Therefore, when identifying the layer of the Earth that is primarily made of iron and nickel, the inner core is the correct choice.