Earth Structure — Set 3

Correct Answer: C. Plate Tectonics

• **Plate Tectonics** = explains the movement of lithospheric plates over the semi-fluid asthenosphere, building on Wegener's Continental Drift concept. • **~7 major plates** — Earth's lithosphere is divided into ~7 major and several minor tectonic plates that interact at convergent, divergent, and transform boundaries. • 💡 Wrong-option analysis: Big Bang Theory: explains the origin of the Universe; Nebular Hypothesis: explains the formation of the solar system from a gas cloud; Isostasy: describes how the crust floats in equilibrium on the mantle, not plate movement.

Correct Answer: C. Crust

• **Crust** = the thinnest layer of Earth, ranging from ~5 km under oceans to ~70 km under continental mountains. • **Less than 1% of volume** — despite being the layer we live on, the crust occupies less than 1% of Earth's total volume. • 💡 Wrong-option analysis: Mantle: the thickest layer, ~2,900 km; Outer Core: ~2,250 km thick; Inner Core: ~1,220 km radius — all much thicker than the crust.

Correct Answer: B. 6,371 km

• **6,371 km** = the approximate mean radius of the Earth, measured from the surface to the center of the core. • **Equatorial vs Polar** — Earth's equatorial radius is 6,378 km (slightly larger) and polar radius is 6,357 km (slightly smaller) due to its oblate spheroid shape. • 💡 Wrong-option analysis: 12,800 km: this is approximately Earth's diameter, not radius; 3,200 km: roughly the radius of Mars; 4,500 km: approximately the radius of Uranus.

Correct Answer: A. 5.5 g/cm³

• **5.5 g/cm³** = the average density of Earth as a whole, making it the densest planet in the Solar System. • **Densest planet** — Earth's average density (5.5 g/cm³) is much higher than the crust (2.7–3.0 g/cm³) because the dense core (~13 g/cm³) pulls the average up. • 💡 Wrong-option analysis: 13.0 g/cm³: density of the inner core only, not the whole Earth; 3.0 g/cm³: density of oceanic crust; 2.7 g/cm³: density of continental crust (SIAL).

Correct Answer: C. A liquid Outer Core

• **Liquid Outer Core** = the S-wave shadow zone (103°–180° from epicenter) proves the outer core is liquid, because S-waves cannot travel through liquids. • **103°–180° shadow zone** — S-waves are absent in this zone on Earth's opposite side, providing direct evidence of the liquid outer core. • 💡 Wrong-option analysis: A plastic Mantle: the mantle is solid/plastic but S-waves do travel through it; A broken Crust: crustal cracks do not create a global shadow zone; A solid Inner Core: the inner core's solidity is proven by P-wave reflections, not S-wave shadow zone.

Correct Answer: A. Asthenosphere

• **Asthenosphere** = the primary source of magma, where partial melting of rocks in the upper mantle creates molten material that rises through the lithosphere. • **~1%–5% partial melting** — only 1%–5% of asthenosphere rock partially melts to produce magma; subduction zones and mid-ocean ridges trigger more melting. • 💡 Wrong-option analysis: Lower Mantle: very high pressure keeps it solid, rarely a direct magma source; Inner Core: solid iron-nickel, no magma; Outer Core: liquid iron-nickel, does not produce silicate magma.

Correct Answer: A. Oxygen

• **Oxygen** = the most abundant element in Earth's crust by weight (~46%), primarily bonded with silicon to form silicate minerals. • **46% of crust** — oxygen makes up about 46% of the crust's mass; silicon is 2nd at ~28%; together they form silicates, the most common mineral group. • 💡 Wrong-option analysis: Silicon: 2nd most abundant in the crust (~28%); Iron: most abundant in the whole Earth but only ~5% of the crust; Aluminum: 3rd most abundant in the crust (~8%).

Correct Answer: D. Iron

• **Iron** = the most abundant element in the entire Earth by mass (~35%), mostly concentrated in the core. • **~35% of Earth's mass** — while oxygen dominates the crust, iron dominates the whole-planet composition because the enormous iron-nickel core makes up ~32% of Earth's mass. • 💡 Wrong-option analysis: Nickel: about 2.4% of Earth's mass; Silicon: about 15% of Earth's mass (2nd most in crust, 5th overall); Oxygen: most abundant in the crust (~46% by weight) but ~30% overall.

Correct Answer: C. Extreme Pressure

• **Extreme Pressure** = despite temperatures of 5,000–6,000°C in the inner core, immense pressure (~3.6 million atm) raises iron's melting point above the actual temperature. • **3.6 million atmospheres** — the pressure in the inner core is about 3.6 million times Earth's surface atmospheric pressure, keeping iron solid. • 💡 Wrong-option analysis: Rapid cooling: the inner core is actually heating the surrounding outer core; Low Density: the inner core is the densest part of Earth; High Silica content: silica is found in the crust, not the core.

Correct Answer: C. Asthenosphere

• **Asthenosphere** = the lithospheric tectonic plates rest and move upon the asthenosphere, which acts as a lubricant due to its plasticity and partial melting. • **~80–200 km deep** — the asthenosphere's semi-fluid nature (due to partial melting) enables the convection-driven movement of overlying tectonic plates. • 💡 Wrong-option analysis: Lower Mantle: below the asthenosphere, too rigid for plate movement; Inner Core: the solid center of Earth, far below the lithosphere; Outer Core: liquid iron-nickel, not in contact with the lithosphere.