Questions: Earth's Interior Structure

Question 1

Seismologists observe that S-waves do not arrive in a large shadow zone on the far side of Earth from an earthquake. What does this most directly imply about Earth's outer core?

• A. The outer core is extremely dense and absorbs S-waves
• B. The outer core is liquid, because S-waves cannot propagate through fluids
• C. The outer core is composed of iron and nickel
• D. The outer core is hotter than the surrounding mantle

Answer: (B)

S-waves (shear waves) require a solid medium — they propagate by displacing material perpendicular to the wave direction, which fluids cannot sustain because fluids have no shear strength. The observed S-wave shadow zone is direct seismic evidence that Earth contains a liquid region at depth. While the outer core is also dense, iron-rich, and hot, the S-wave absence specifically and uniquely indicates a liquid state.

Question 2

The lithosphere and the crust are synonymous terms for the same layer of Earth.

• T. True
• F. False

Answer: False

The crust is defined compositionally — it is the outermost shell of rock with a distinct chemical composition (silica-rich for continental crust, more mafic for oceanic crust). The lithosphere is defined mechanically — it is the rigid outer layer comprising the crust plus the uppermost brittle portion of the mantle. The lithosphere is thicker than the crust alone. Its base, the lithosphere-asthenosphere boundary, is where the mantle transitions from rigid to ductile, and this mechanical boundary is what matters for plate tectonics.

Question 3

If the inner core is deeper than the outer core and therefore at a higher temperature, why is the inner core solid while the outer core is liquid?

Model answer: The inner core is solid because the enormous pressure at that depth raises iron's melting point above the local temperature. Even though the inner core is hotter in absolute terms, the pressure-adjusted melting point of iron is even higher, so it solidifies. The outer core is at lower pressure, so its melting point is lower and iron remains liquid there despite the high temperature.

This illustrates that melting point depends on pressure, not just temperature. In Earth's deep interior, pressure increases faster with depth than temperature does (the geothermal gradient), so beyond a certain point the melting curve of iron lies above the actual temperature. The inner core is a relatively young feature — Earth was fully molten when it formed 4.5 Ga, and the inner core has been slowly solidifying as the planet cools. Its growth releases latent heat and light elements that drive convection in the outer core, sustaining Earth's magnetic field.