Question

S waves temporarily change the shape of the material that transmits them. Can you identify a place in Earth's interior that would not transmit S waves? Why?

Short answer

S waves cannot be transmitted through the Earth's outer core because it is liquid.

Step-by-step solution

Understanding S Waves

S waves, also known as secondary waves or shear waves, move through the Earth's interior, causing particles to oscillate perpendicular to the direction of wave propagation. They can only travel through solid materials and are unable to pass through fluids, which do not support shear stresses.

Identifying Fluid Layers

Inside the Earth, there are two major fluid layers: the outer core and the partially molten asthenosphere, which lies beneath the lithosphere. The outer core is completely liquid, primarily composed of iron and nickel.

S Waves and the Outer Core

Since S waves cannot travel through liquids, they are unable to pass through the Earth's outer core. The outer core's fluid nature absorbs and completely attenuates S waves, preventing them from re-emerging elsewhere.

Conclusion

The outer core of the Earth cannot transmit S waves due to its liquid state, which offers no resistance to shear stresses required for the propagation of S waves.

Key concepts

Earth's Interior

The Earth's interior is a fascinating and complex structure comprising several layers. Each of these layers has distinct physical and chemical properties. The primary layers of the Earth include the crust, mantle, and core. Each layer plays a critical role in the geological and seismic phenomena observed on our planet.

• Crust: The outermost solid layer, which is relatively thin compared to other layers. It's where we live and contains the continental and oceanic crust.
• Mantle: Beneath the crust, this layer makes up most of Earth's volume. It's semi-solid and convects, driving plate tectonics.
• Core: The innermost layer divided into the solid inner core and the fluid outer core.

Understanding these layers is crucial in learning how seismic waves, like S waves, interact differently with each.

S Waves

S waves, or secondary waves, are a type of seismic wave crucial for understanding seismic activity and the Earth's inner structure. These waves are called shear waves because they move the ground perpendicular to their propagation direction. Here's how they work:

• Propagation: They travel through solid materials and transfer energy through them.
• Shear Stress: They cause the material to change shape momentarily as they pass, demonstrated by twisting or shearing motions.
• Speed: S waves travel slower than P waves, which are the first to be detected by seismographs during an earthquake.

Importantly, S waves cannot propagate through liquids or gases, as these phases do not have the shear strength to support their motion.

Outer Core

The outer core is an essential component of Earth's inner structure. It is the layer above the solid inner core and below the mantle. This region is primarily composed of molten iron and nickel, creating unique properties affecting seismic wave propagation.

• Liquid State: Unlike the inner core, the outer core is liquid, which is why S waves cannot pass through it.
• Magnetic Field: The movement of the liquid iron generates Earth's magnetic field via convection currents and the Coriolis effect.
• Seismic Shadow Zone: The inability of S waves to pass through it leads to a shadow zone, helping geophysicists determine its fluid nature.

This liquid state is a significant reason the outer core does not support the transmission of S waves.

Shear Waves

Shear waves are integral in studying seismic activities and Earth's structural analysis. Their ability to change the shape of the Earth material through which they travel makes them instrumental in seismic interpretations. Here's what makes shear waves distinctive:

• Rock Movement: They induce horizontal displacement, making the rock layer temporarily deform.
• Material Type: Only solids allow shear waves to travel through them, as they possess the rigidity necessary to resist shear forces.
• Seismic Observation: The behavior of shear waves, including their absorption by Earth's outer core, helps scientists understand both earth's composition and dynamic phenomena like earthquakes.

The properties of shear waves make them an essential tool for seismologists probing the Earth’s many mysteries.