Earth Structure — Set 1

Correct Answer: C. Radioactive decay and primordial heat

• **Radioactive decay and primordial heat** = the two main sources of Earth's internal heat, which drive geological processes like plate tectonics and volcanism. • **~47 terawatts** — Earth radiates approximately 47 terawatts of heat from its interior, about half from radioactive decay and half from primordial heat. • 💡 Wrong-option analysis: Friction from ocean tides: produces negligible heat compared to internal sources; Chemical reactions in the atmosphere: atmospheric chemistry produces no significant internal heat; Solar radiation absorbed by the surface: heats only the surface, not the deep interior.

Correct Answer: B. Mantle

• **Mantle** = the layer immediately beneath the crust, composed of silicate rocks rich in iron and magnesium, extending to ~2,900 km depth. • **~2,900 km thick** — the Mantle is Earth's thickest layer, constituting about 84% of Earth's total volume. • 💡 Wrong-option analysis: Inner Core: located at the very center (~5,150–6,371 km depth); Outer Core: located below the mantle (~2,900–5,150 km depth); Barysphere: another name for the Core, not directly beneath the crust.

Correct Answer: B. A semi-fluid weak zone in the upper mantle

• **Asthenosphere** = a mechanically weak, semi-fluid zone in the upper mantle, located just below the lithosphere, enabling tectonic plate movement. • **80–200 km depth** — the Asthenosphere lies approximately 80–200 km below the surface and acts as a lubricant for tectonic plates. • 💡 Wrong-option analysis: The solid inner core: located at Earth's center under extreme pressure; The hard rock of the lower crust: the lower crust is solid, not the asthenosphere; The gaseous atmosphere: the atmosphere is above Earth's surface, not within it.

Correct Answer: C. Nickel and Iron

• **Nickel and Iron (NIFE)** = the primary composition of Earth's core; heavy metals that sank to the center during early planetary differentiation. • **~3,485 km radius** — Earth's core has a radius of about 3,485 km, divided into liquid outer core and solid inner core, both made of iron-nickel. • 💡 Wrong-option analysis: Carbon and Iron: carbon is not a primary core element; Magnesium and Silica: these describe SIMA (lower crust/oceanic crust); Silicon and Aluminum: these describe SIAL (upper continental crust).

Correct Answer: B. The Crust and the uppermost solid Mantle

• **Crust + uppermost solid Mantle** = together they form the Lithosphere, a rigid, brittle layer broken into tectonic plates that float on the asthenosphere. • **100 km average thickness** — the Lithosphere is approximately 100 km thick (thinner under oceans, thicker under continents) and moves over the asthenosphere. • 💡 Wrong-option analysis: The Crust and the Asthenosphere: the asthenosphere is below the lithosphere, not part of it; The Upper and Lower Mantle: mantle alone does not constitute the lithosphere; The Outer and Inner Core: the core is at Earth's center, unrelated to the lithosphere.

Correct Answer: D. Basalt

• **Basalt** = the primary rock of oceanic crust, a dark, fine-grained igneous rock rich in iron and magnesium (SIMA composition). • **5–10 km thick** — oceanic crust is only 5–10 km thick but denser (~3.0 g/cm³) than continental crust, causing it to subduct under continents. • 💡 Wrong-option analysis: Granite: makes up continental crust (SIAL), not oceanic; Limestone: a sedimentary rock, not the primary oceanic crust rock; Sandstone: a sedimentary rock, not the primary oceanic crust rock.

Correct Answer: B. Granite

• **Granite** = the primary rock of continental crust, lighter in color and less dense than oceanic basalt, allowing continents to 'float' higher on the mantle. • **30–70 km thick** — continental crust is 30–70 km thick (thicker under mountains) and has density ~2.7 g/cm³ (SIAL composition). • 💡 Wrong-option analysis: Gabbro: a dark igneous rock similar to basalt, found in oceanic crust; Peridotite: the primary rock of the upper mantle, not the crust; Basalt: makes up oceanic crust, denser than granite.

Correct Answer: B. Mohorovicic Discontinuity

• **Mohorovicic Discontinuity (Moho)** = the boundary between the crust and mantle, marked by a sharp increase in seismic wave velocity, discovered in 1909. • **~5–70 km depth** — the Moho is ~5–10 km deep under oceans and ~30–70 km deep under continents, varying with crustal thickness. • 💡 Wrong-option analysis: Gutenberg Discontinuity: separates the mantle and outer core at ~2,900 km depth; Lehmann Discontinuity: separates the outer and inner core at ~5,150 km depth; Conrad Discontinuity: separates upper crust (SIAL) from lower crust (SIMA).

Correct Answer: C. S-waves (Secondary waves)

• **S-waves (Secondary/Shear waves)** = cannot travel through liquids or gases; they require a rigid medium, which proves the outer core is liquid. • **S-wave shadow zone** — the absence of S-waves on the far side of Earth (103°–180° from earthquake epicenter) reveals the liquid outer core. • 💡 Wrong-option analysis: L-waves (Love waves): surface waves that travel through solid ground; P-waves (Primary waves): can travel through solids, liquids, and gases; R-waves (Rayleigh waves): surface waves, not affected by liquid/solid distinction in the same way.

Correct Answer: C. P-waves (Primary waves)

• **P-waves (Primary waves)** = the fastest seismic waves, compressional waves that travel through solids, liquids, and gases, arriving first at seismographs. • **6–8 km/s speed** — P-waves travel at approximately 6–8 km/s in the crust, compared to S-waves at 3–4 km/s. • 💡 Wrong-option analysis: Surface waves: the slowest seismic waves but cause the most damage; Tidal waves: not seismic waves; S-waves (Secondary waves): arrive after P-waves because they travel more slowly.