Earth Structure — Set 4

Correct Answer: A. 84%

• **~84%** = the Mantle occupies approximately 84% of Earth's total volume, making it the most voluminous layer by far. • **67% of Earth's mass** — the mantle also accounts for about 67% of Earth's total mass, extending from the Moho to the Gutenberg Discontinuity. • 💡 Wrong-option analysis: 50%: an underestimate; the mantle is the dominant layer; 15%: approximately the volume of the outer core; 99%: an extreme overestimate; the crust alone is less than 1%.

Correct Answer: D. Less than 1%

• **Less than 1%** = despite being the layer we live on, the crust occupies less than 1% of Earth's total volume, making it the thinnest shell. • **~5–70 km thickness** — the crust is 5–10 km thick under oceans and up to 70 km thick under mountains, yet still less than 1% of Earth's 6,371 km radius. • 💡 Wrong-option analysis: 10%: vast overestimate; 5%: still a large overestimate; 15%: this is approximately the outer core's volume percentage.

Correct Answer: C. 6,000°C

• **~6,000°C** = the estimated temperature at Earth's center (inner core), comparable to the surface temperature of the Sun. • **As hot as Sun's surface** — Earth's inner core reaches ~6,000°C, similar to the Sun's photosphere (~5,500°C), heated by radioactive decay and primordial energy. • 💡 Wrong-option analysis: 3,000°C: temperature at approximately the lower mantle; 10,000°C: overestimate, higher than Sun's surface; 1,000°C: temperature near the base of Earth's crust/upper mantle.

Correct Answer: D. Outer Core

• **Outer Core** = Earth's magnetic field is generated in the liquid outer core through the Geodynamo Effect, where convection currents of molten iron create electric currents. • **Geodynamo Effect** — convection and rotation of liquid iron in the outer core generate electric currents that produce Earth's protective magnetic field (magnetosphere). • 💡 Wrong-option analysis: Crust: too thin and cool to generate magnetic fields; Mantle: solid/plastic silicate, not electrically conductive enough; Inner Core: solid iron-nickel, does not have the fluid motion needed for the dynamo effect.

Correct Answer: A. Lehmann

• **Lehmann Discontinuity** = separates the liquid outer core from the solid inner core at approximately 5,150 km depth, marking the liquid-to-solid transition. • **5,150 km depth** — discovered by Inge Lehmann in 1936 by analyzing P-wave reflections from the inner core boundary. • 💡 Wrong-option analysis: Conrad: separates upper crust (SIAL) from lower crust (SIMA); Mohorovicic: separates crust from mantle; Gutenberg: separates mantle from outer core at 2,900 km.

Correct Answer: D. 2,900 km

• **2,900 km** = the Mantle extends from the Moho (~30 km) to the Gutenberg Discontinuity at ~2,900 km depth, where it meets the liquid outer core. • **Divided into upper and lower** — the mantle is divided at ~700 km (Repetti Discontinuity): upper mantle (includes asthenosphere) and lower mantle (more rigid). • 💡 Wrong-option analysis: 700 km: depth of the Repetti Discontinuity, the transition within the mantle; 100 km: approximate depth of the lithosphere-asthenosphere boundary; 5,150 km: depth of the Lehmann Discontinuity (outer/inner core boundary).

Correct Answer: B. 2.7 to 3.0 g/cm³

• **2.7 to 3.0 g/cm³** = the average density of Earth's crust; continental crust (~2.7 g/cm³) is lighter than oceanic crust (~3.0 g/cm³). • **Continental vs Oceanic** — because oceanic crust (SIMA, ~3.0) is denser than continental crust (SIAL, ~2.7), oceanic crust subducts under continental at convergent boundaries. • 💡 Wrong-option analysis: 5.5 g/cm³: average density of the whole Earth; 13.0 g/cm³: density of the inner core; 11.0 g/cm³: approximate density of outer core material.

Correct Answer: B. 13.0 g/cm³

• **~13.0 g/cm³** = the approximate density of the inner core (up to ~13 g/cm³); the outer core ranges from ~9.9–12.2 g/cm³ due to heavy iron-nickel under extreme pressure. • **9.9–13.0 g/cm³ range** — the core's density increases with depth; the center is the densest point in Earth at approximately 13 g/cm³. • 💡 Wrong-option analysis: 2.7 g/cm³: density of continental crust (SIAL); 8.0 g/cm³: intermediate value, not representative of the core; 5.5 g/cm³: average density of the whole Earth.

Correct Answer: A. Silica

• **Silica (SiO₂)** = the common constituent in both SIAL (Silica + Aluminum) and SIMA (Silica + Magnesium); silicate minerals dominate Earth's crust. • **~59% of crust** — silicate minerals (silica-based) make up approximately 59% of Earth's crust by volume. • 💡 Wrong-option analysis: Alumina: only in SIAL (upper crust), not SIMA; Magnesia: only in SIMA (lower/oceanic crust), not SIAL; Iron Oxide: found in some rocks but not the common constituent of both SIAL and SIMA.

Correct Answer: A. Volcanic eruptions

• **Volcanic eruptions** = a direct source of Earth's interior information, as magma brings material from deep within the mantle to the surface for direct analysis. • **Kola Superdeep Borehole** — the deepest drilling project reached ~12 km; volcanic eruptions provide deeper material directly from 50–200 km depth. • 💡 Wrong-option analysis: Magnetic field: an indirect source revealing the liquid outer core; Gravity anomalies: an indirect source suggesting density variations; Seismic waves: an indirect source used to infer layer properties.