Scientists infer that Earth's outer core is liquid based on their studies of a) earthquake waves b) magnetic polarity reversals c) lithospheric heat flow- d) magma

Scientists infer that Earth's outer core is liquid based on their studies of a) earthquake waves. They use the information about two main types of seismic waves generated by earthquakes: P-waves (primary waves) and S-waves (secondary waves). P-waves are compressional waves that can travel through both liquids and solids, whereas S-waves are shear waves that can only travel through solid materials and cannot travel through liquids.

When an earthquake occurs, these earthquake waves travel through the Earth and are recorded by seismometers at various locations around the globe. Scientists have observed that P-waves travel through the Earth and can be detected on the other side, but S-waves stop at a certain point and don't appear beyond that. The point where S-waves disappear corresponds to the depth where the solid mantle transitions into the liquid outer core. This behavior of S-waves provides direct evidence that the outer core of the Earth is liquid.

This inference is also supported by a phenomenon called the P-wave shadow zone. P-waves are refracted (bent) when they enter a liquid from a solid due to the change in density between the two materials. This refraction creates a zone on the Earth's surface where no P-waves are detected because they are bent around the outer core. The existence of this P-wave shadow zone further supports the conclusion that the outer core is in a liquid state.