Final answer:
Seismologists have deduced the composition of Earth's interior by studying the behavior of seismic waves, with discoveries such as the liquid outer core and solid inner core made by Inge Lehmann based on the propagation of these waves through different materials.
Step-by-step explanation:
Seismologists have inferred what Earth's interior is made up of by analyzing seismic waves generated by earthquakes. In the early 1900s, it was discovered that these waves, specifically primary and secondary waves (or P and S waves), change velocity as they travel through materials of varying densities. By observing the way these waves are reflected and refracted, seismologists concluded that the Earth is composed of distinct layers with varying properties. The renowned geophysicist Inge Lehmann used such data to determine that Earth has a solid inner core surrounded by a liquid outer core, as shear waves cannot travel through the liquid but compression waves can. Further details about the interior are provided by seismic imaging which resembles techniques like ultrasound used in medicine. Thus, seismological studies have shown that Earth's interior consists of the crust, mantle, and inner and outer cores, with variations in solid and liquid states attributed to material composition and ongoing heat from radioactive decay.