Scientists came to know that the Earth's outer core is liquid primarily through the study of seismic waves generated by earthquakes. The key observations include:
- S-waves (shear waves), which can only travel through solid materials, do not pass through the outer core. This creates an "S-wave shadow zone," indicating that the outer core is not solid but liquid because liquids do not transmit S-waves.
- P-waves (primary waves), which can travel through both solids and liquids, do pass through the outer core but with different speeds, consistent with it being a liquid layer.
- The disappearance and reappearance of S-waves at certain depths suggest a liquid outer core layer between the solid mantle and solid inner core.
- Additionally, the Earth's magnetic field supports the idea of a liquid outer core because the movement of molten, electrically conductive iron in the liquid outer core is responsible for generating the geomagnetic field (dynamo theory).
- Laboratory experiments simulating high pressures and temperatures inside the Earth also show that iron under those conditions would be liquid in the outer core region.
These lines of evidence—seismic wave behavior, magnetic field generation, and high-pressure experiments—together confirm that Earth's outer core is liquid.
