Questions: Earth's Interior: Density and Composition

Question 1

At approximately 660 km depth, seismic wave velocities change abruptly. What best explains this discontinuity?

• A. The chemical composition changes from silicate rock to iron-rich material
• B. Minerals undergo a phase transition to denser crystal structures without changing chemical composition
• C. Temperature drops sharply, causing rock to become more rigid
• D. This is the core-mantle boundary where rock transitions to liquid iron

Answer: (B)

The 660 km discontinuity is a phase transition: olivine-family minerals transform into denser bridgmanite under pressure. The chemical composition remains silicate throughout — only the crystal structure changes. The iron-rich core boundary is much deeper (~2,900 km). Temperature generally increases, not decreases, with depth. Distinguishing phase-change layering from compositional layering is a key insight in this topic.

Question 2

What is the primary seismological evidence that Earth's outer core is liquid?

• A. Drilling samples from the outer core contain liquid iron
• B. S-waves do not pass through the outer core, while P-waves do
• C. Temperature measurements show material exceeds the melting point throughout the outer core
• D. The density of the outer core exceeds that of any known solid material

Answer: (B)

S-waves (shear waves) cannot propagate through liquids because liquids do not resist shear stress. When S-waves disappear at the core-mantle boundary (~2,900 km) but P-waves continue, geophysicists conclude the outer core is liquid. Direct sampling is impossible — the deepest drilling projects have only reached ~12 km. Temperature measurements at those depths are extrapolated, not directly observed.

Question 3

Oceanic crust is denser than continental crust because it contains more iron- and magnesium-rich basaltic rock.

• T. True
• F. False

Answer: True

Oceanic crust (~3.0 g/cm³) is basaltic — relatively rich in iron and magnesium. Continental crust (~2.7 g/cm³) is more granitic, dominated by lighter silicon- and aluminum-rich minerals. This density difference is tectonically important: when oceanic and continental plates collide, the denser oceanic plate subducts. The compositional difference, not just density, drives this behavior.

Question 4

Density increases smoothly and continuously from Earth's surface to its center.

• T. True
• F. False

Answer: False

Density increases in steps, not smoothly. Compositional boundaries — the Moho (crust-mantle) at ~35 km average depth and the core-mantle boundary at ~2,900 km — produce abrupt density jumps. Phase transitions within the mantle at ~410 km and ~660 km add additional steps. Within each layer, density does increase gradually with pressure, but the overall depth profile is a staircase, not a ramp.

Question 5

How do seismic waves allow geophysicists to determine the composition and physical state of Earth's interior layers without directly sampling them?

Model answer: Seismic waves change speed when they cross boundaries between materials with different density, rigidity, or physical state. P-waves (compressional) travel through both solids and liquids; S-waves (shear) travel only through solids. Abrupt velocity changes reveal compositional or phase boundaries — for instance, the Moho marks the sharp transition from lighter crustal rock to denser mantle rock. The disappearance of S-waves at ~2,900 km reveals the outer core is liquid. Speed profiles within layers, compared against laboratory experiments on minerals at high pressure and temperature, allow identification of specific mineral compositions at each depth.

This indirect method is analogous to medical imaging — you send waves through an opaque body and infer internal structure from how the waves are altered. The combination of P-wave and S-wave behavior is especially powerful: P-waves alone can't distinguish solid from liquid, but the S-wave shadow zone directly reveals where liquid exists.