We know Earth has a mantle primarily through the study of seismic waves.
How Seismic Waves Reveal Earth's Mantle
Scientists use seismic waves generated by earthquakes to understand the Earth's internal structure. These waves travel through the planet and change in speed and direction depending on the materials they encounter. Different types of waves provide different kinds of information:
- P-waves (Primary Waves): These waves are compressional, meaning they cause the rock to expand and contract. They can travel through solids, liquids, and gases.
- S-waves (Secondary Waves): These waves are shear waves, moving the rock from side to side. They can only travel through solids.
Key Findings:
| Wave Type | Behavior at Mantle-Core Boundary | Implication |
|---|---|---|
| P-waves | Slow down at the mantle-core boundary | The outer core is less rigid than the mantle |
| S-waves | Disappear at the mantle-core boundary | The outer core is a liquid, not a solid. |
The Mantle's Properties
- Composition: The mantle is a mostly solid layer beneath the crust, composed of silicate rocks rich in iron and magnesium.
- Depth and Thickness: The mantle extends from the base of the crust down to the Earth's core, making it the thickest layer of the Earth.
- State: While primarily solid, the mantle behaves like a very viscous (thick) liquid over long periods. This allows the convection currents to move the plates.
- Seismic Wave Analysis:
- The analysis of seismic waves has been crucial to determining the mantle's position, depth, and density.
- Changes in P-wave and S-wave velocities as they travel through the mantle, and their behavior at the mantle-core boundary, provide evidence for the existence of a distinct mantle layer with unique physical properties.
Practical Insights
- The ability of S-waves to travel through the mantle confirms it is solid.
- The slowing down of P-waves at the mantle-core boundary indicates a shift in material density and rigidity.
- The disappearance of S-waves at the same boundary shows that the outer core must be liquid because liquids cannot transmit S-waves.
Through the study of seismic wave behavior, specifically how P-waves slow down at the mantle-core boundary and how S-waves disappear, scientists can definitively conclude that Earth has a mantle distinct from both the crust and the outer core.
