Final answer:
The Earth's core being composed mainly of iron is evidenced by the behavior of seismic waves and measurements of the Earth's magnetic field, both indicating interior structures containing iron.
Step-by-step explanation:
Physical evidence for the Earth's core being composed mainly of iron is provided by the seismic wave behavior and magnetic field measurements. The movement of seismic waves (waves that spread through the interior of Earth from earthquakes or explosion sites) reveals the interior structure and composition of the Earth. Also, the Earth's magnetic field, which behaves like a giant bar magnet inside the planet, provides the clue. This magnetic field is generated by the circulation of liquid metal inside the Earth's liquid metallic core, which includes iron.
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