Earth Structure — Set 2

Correct Answer: C. Liquid

• **Liquid** = the Earth's Outer Core is in a liquid state, proven by S-waves' inability to pass through it, creating a shadow zone. • **2,900–5,150 km depth** — the liquid Outer Core spans from ~2,900 to ~5,150 km depth; convection of molten iron generates Earth's magnetic field. • 💡 Wrong-option analysis: Plasma: an ionized gas state, not the outer core's state; Gaseous: extreme pressure prevents gaseous state; Solid: the Inner Core is solid, not the Outer Core.

Correct Answer: B. Immense pressure

• **Immense pressure** = despite temperatures around 5,000–6,000°C, the Inner Core remains solid because extreme pressure raises iron's melting point above the ambient temperature. • **3.6 million atm pressure** — the Inner Core is under approximately 3.6 million atmospheres of pressure, keeping iron-nickel in solid form. • 💡 Wrong-option analysis: Cooling by the mantle: the mantle is hot; it does not cool the core; Presence of silica: silica is found in the crust/mantle, not the core; Low presence of iron: incorrect — the inner core is primarily iron-nickel.

Correct Answer: C. Mantle

• **Mantle** = constitutes approximately 84% of Earth's total volume, making it the most voluminous layer, extending from the Moho to ~2,900 km depth. • **~84% volume, ~67% mass** — the mantle occupies 84% of Earth's volume and about 67% of its mass, primarily composed of silicate rocks. • 💡 Wrong-option analysis: Inner Core: constitutes only about 1% of Earth's volume; Outer Core: constitutes about 15% of Earth's volume; Crust: constitutes less than 1% of Earth's volume.

Correct Answer: D. Increases constantly

• **Increases constantly** = Earth's density increases from the surface (~2.7 g/cm³ crust) to the center (~13 g/cm³ inner core) due to gravitational stratification. • **2.7 to 13 g/cm³** — density increases from crust (2.7 g/cm³) through mantle (3.3–5.6 g/cm³) to outer core (~9.9–12.2 g/cm³) to inner core (~13 g/cm³). • 💡 Wrong-option analysis: Decreases constantly: opposite of what occurs; Remains the same: each layer has distinctly different density; Increases then decreases: no layer reversal in density trend.

Correct Answer: B. Upper Crust and Lower Crust

• **Upper Crust (SIAL) and Lower Crust (SIMA)** = the Conrad Discontinuity marks the boundary within the crust, separating granitic upper crust from denser lower crust. • **~15–20 km depth** — the Conrad Discontinuity is found at approximately 15–20 km depth in continental regions. • 💡 Wrong-option analysis: Upper Mantle and Lower Mantle: separated by the Repetti Discontinuity at ~700 km; Mantle and Core: separated by the Gutenberg Discontinuity at ~2,900 km; Outer Core and Inner Core: separated by the Lehmann Discontinuity at ~5,150 km.

Correct Answer: A. Upper Mantle and Lower Mantle

• **Upper Mantle and Lower Mantle** = the Repetti Discontinuity separates the upper and lower mantle at approximately 700 km depth, marked by changes in seismic wave velocities. • **~700 km depth** — below this depth, minerals undergo phase transitions to denser forms, causing a seismic velocity jump. • 💡 Wrong-option analysis: Crust and Mantle: separated by the Mohorovicic Discontinuity; Outer Core and Inner Core: separated by the Lehmann Discontinuity; Outer Mantle and Core: separated by the Gutenberg Discontinuity.

Correct Answer: B. Mantle and Core

• **Mantle and Core** = the Gutenberg Discontinuity at ~2,900 km depth separates the solid silicate mantle from the liquid iron-nickel outer core. • **2,900 km depth** — at this boundary, P-wave velocity drops sharply and S-waves disappear, proving the core below is liquid. • 💡 Wrong-option analysis: Upper and Lower Crust: separated by the Conrad Discontinuity; Outer and Inner Core: separated by the Lehmann Discontinuity at ~5,150 km; Crust and Mantle: separated by the Mohorovicic (Moho) Discontinuity.

Correct Answer: C. Silica and Aluminum

• **SIAL = Silica + Aluminum** = represents the granitic upper continental crust, which is less dense than the underlying SIMA layer. • **~2.7 g/cm³ density** — SIAL rocks (granite-type) have density ~2.7 g/cm³, lower than SIMA (~3.0 g/cm³), allowing continents to stand higher. • 💡 Wrong-option analysis: Silicon and Iron: Iron is in the core (NIFE), not the upper crust; Silver and Aluminum: Silver is not a significant crustal element; Silica and Magnesium: that is SIMA (lower/oceanic crust).

Correct Answer: D. Silica and Magnesium

• **SIMA = Silica + Magnesium** = represents the basaltic lower continental crust and oceanic crust, denser than the SIAL layer above. • **~3.0 g/cm³ density** — SIMA (basalt-type rocks) are denser (~3.0 g/cm³) than SIAL, which is why oceanic crust subducts under continental crust. • 💡 Wrong-option analysis: Silicon and Manganese: Manganese is not the key element in oceanic crust; Sulfur and Iron: these are not the primary elements of oceanic crust; Silver and Magnesium: Silver is not a significant crustal component.

Correct Answer: D. Core

• **Core (NIFE = Nickel + Iron)** = the Earth's Core is primarily composed of Nickel (Ni) and Iron (Fe), giving it high density and magnetic properties. • **~35% of Earth's mass** — iron is the most abundant element in the whole Earth by mass, mostly concentrated in the core. • 💡 Wrong-option analysis: Crust: composed of SIAL (silica + aluminum); Asthenosphere: part of the upper mantle, composed of silicate minerals; Upper Mantle: composed of silicate minerals like peridotite.