Final answer:
Geologists determine a model of the Earth's interior by using seismic waves to infer the composition and structure of its layers.
Step-by-step explanation:
Geologists determine a model of the Earth's interior by using seismic waves. Seismic waves are generated by earthquakes and they travel through the Earth's interior. These waves have different velocities depending on the materials they pass through, allowing geologists to infer the composition and structure of the Earth's layers. By monitoring seismic waves in different locations and using seismographs, geologists can construct a model of the Earth's interior, showing liquid and solid layers.