What tells us that the inner core is made up of iron?

This has to do with abundance of Iron (in the Universe, Milky Way & Solar system), the density of iron, magnetism of Iron (and Nickel), the presence of a magnetic field around the Earth, melting point of Iron (under pressure), and the heat trapped/generated within the Earth. read more

Iron represents the material that is the best fit with the velocity of the seismic waves moving through the earth's core. Finally, iron is one the most plentiful elements on earth, and many asteroids are also composed of iron. read more

Since both iron and nickel and attracted to magnets earth's magnetic field drew the to metals into the inner core mixing them into the alloy. C. Earth started as a giant asteroid composed of an iron-nickel alloy (which many are), and other materials from space built upon it causing earth to have an iron-nickel core (I told you they were crazy!). read more