Mantle:
The layer of Earth that is formed of iron and nickel is the mantle. The mantle is located between the outer core and the Earth's crust. It is the thickest layer of the Earth, accounting for about 84% of its volume.

Composition:
The mantle is primarily composed of silicate minerals rich in magnesium and iron. These minerals include olivine, pyroxene, and garnet. However, the mantle also contains significant amounts of iron and nickel, which give it its dense and metallic properties.

Structure:
The mantle is not a solid layer but rather a partially molten region. It is divided into two parts: the upper mantle and the lower mantle. The upper mantle is solid, while the lower mantle is in a semi-solid state due to the high temperatures and pressures found at that depth.

Convection Currents:
The mantle plays a crucial role in the movement of tectonic plates on the Earth's surface. Heat generated from the core causes convection currents to form within the mantle. These currents transfer heat from the core towards the surface, driving the movement of the lithospheric plates.

Role in Plate Tectonics:
The movement of the tectonic plates results in various geological phenomena such as earthquakes, volcanic activity, and the formation of mountain ranges. The mantle's high iron and nickel content, along with its semi-solid state, contribute to its ability to transmit these convection currents and facilitate plate movement.

Implications:
The presence of iron and nickel in the mantle also has important implications for the Earth's magnetic field. The liquid outer core, composed primarily of iron and nickel, generates a magnetic field through a process called the dynamo effect. This magnetic field extends into the mantle and influences the movement of the convection currents.

In conclusion, the layer of the Earth formed of iron and nickel is the mantle. This layer plays a significant role in plate tectonics, heat transfer, and the generation of the Earth's magnetic field. Its composition and semi-solid state allow for the movement of convection currents, which drive the motion of tectonic plates on the Earth's surface.