Final answer:
Earth's outer core is inferred to be liquid because S waves, or shear waves, cannot pass through it, while P waves, or compression waves, can.
Step-by-step explanation:
Geologists have inferred that Earth's outer core is liquid because S waves cannot pass through the outer core. Shear or transverse waves (S waves) do not travel through liquids, which can be observed in the case of seismic activity. When the Earth experiences earthquakes, S waves are not transmitted through the outer core, while compression or longitudinal waves (P waves) can pass through it. This behavior of seismic waves is critical for understanding the internal structure of the Earth, since these waves get reflected and refracted as they encounter materials of different densities and states (solid or liquid).
Geologists deduce the liquid nature of Earth's outer core based on the seismic behavior during earthquakes. S waves, or shear waves, cannot traverse through liquids, a phenomenon observed in seismic activity. When earthquakes occur, S waves are impeded from penetrating the outer core, unlike P waves, or compression waves, which can traverse both solid and liquid mediums. This seismic behavior is pivotal for unraveling Earth's internal structure. As seismic waves encounter materials with distinct densities and states (solid or liquid), they undergo reflection and refraction. The inability of S waves to propagate through the outer core strongly implies its liquid composition. This seismic evidence has been fundamental in shaping our understanding of the Earth's layered structure, elucidating the presence of a liquid outer core beneath the solid mantle.