The layer of the Earth mostly made up of melted iron is the outer core.

Here's why:

* Outer Core: This layer is about 1,400 miles (2,200 kilometers) thick and lies beneath the mantle. It's primarily composed of liquid iron and nickel, with trace amounts of other elements like sulfur. The intense pressure and heat within the Earth keep this iron molten.

* Inner Core: While also composed primarily of iron, the inner core is solid due to the immense pressure. This pressure is so great it overcomes the melting point of iron, forcing it into a solid state despite the extremely high temperatures.