He structure of Earth's deep interior cannot be studied directly. But geologists use seismic (earthquake) waves to determine the depths of layers of molten and
semi-molten material within Earth. Because different types of earthquake waves behave differently when they encounter material in different states (for example,
molten, semi-molten, solid), seismic stations established around Earth detect and record the strengths of the different types of waves and the directions from which
they came. Geologists use these records to establish the structure of Earth's interior.