Earth Structure — Set 5

Correct Answer: B. Analysis of Meteors

• **Analysis of Meteors** = an indirect source; meteors are assumed to share composition with the early Earth, providing clues about core and mantle materials. • **Primitive meteorites** — chondritic meteorites (unchanged since solar system formation) match Earth's estimated bulk composition, suggesting similar formation processes. • 💡 Wrong-option analysis: Volcanic lava: a direct source as it brings interior material to the surface; Deep ocean drilling: a direct source, though limited to ~12 km depth; Surface rock mining: a direct source of crustal information.

Correct Answer: A. Alfred Wegener

• **Alfred Wegener** = proposed Continental Drift theory in 1912, suggesting all continents were once a single supercontinent called Pangaea. • **1912 Pangaea theory** — Wegener's evidence included matching coastlines, fossil records, and rock formations across continents; his theory later evolved into Plate Tectonics. • 💡 Wrong-option analysis: Arthur Holmes: proposed mantle convection as the mechanism for plate movement; Harry Hess: proposed seafloor spreading; Charles Darwin: proposed the theory of evolution by natural selection.

Correct Answer: D. Convection currents in the Mantle

• **Convection currents in the Mantle** = thermal convection driven by heat from the core causes mantle rock to rise, cool, and sink, dragging tectonic plates with it. • **Slab pull + ridge push** — besides mantle convection, slab pull (dense oceanic crust sinking) and ridge push (magma pushing plates at mid-ocean ridges) also drive plate movement. • 💡 Wrong-option analysis: Earth's rotation: has minor influence but is not the primary driver; Solar wind: influences Earth's magnetosphere, not plate tectonics; Lunar gravity: creates tidal forces but does not move tectonic plates.

Correct Answer: A. Rock/Stone sphere

• **Rock/Stone sphere** = from Greek 'lithos' (stone/rock), the lithosphere is the rigid outer layer of Earth comprising the crust and uppermost solid mantle. • **~100 km thick** — the lithosphere is broken into tectonic plates that float on the asthenosphere; 'lithos' means stone and 'sphaira' means sphere. • 💡 Wrong-option analysis: Water sphere: that is the 'Hydrosphere' (hydro = water); Air sphere: that is the 'Atmosphere' (atmos = vapor/air); Weak sphere: that is the 'Asthenosphere' (asthenes = weak).

Correct Answer: A. The Core

• **The Core** = Barysphere, from Greek 'barys' (heavy), refers to Earth's core containing heavy metals (iron and nickel), lying beneath the lithosphere and mantle. • **Greek 'barys' = heavy** — the term reflects the heavy metallic composition of the core (NIFE), which gives Earth its high average density of 5.5 g/cm³. • 💡 Wrong-option analysis: The Mantle: sometimes called the 'Pyrosphere' (pyro = fire/heat); The Atmosphere: the gaseous envelope above Earth's surface; The Crust: the thin outer rocky shell, part of the lithosphere.

Correct Answer: B. 2,900 km

• **2,900 km** = the Gutenberg Discontinuity lies at ~2,900 km depth, marking the boundary between solid silicate mantle and liquid iron-nickel outer core. • **Named after Beno Gutenberg** — discovered by seismologist Beno Gutenberg; at this boundary, P-wave velocity drops from ~13.7 km/s to ~8 km/s and S-waves stop. • 💡 Wrong-option analysis: 5,100 km: depth of the Lehmann Discontinuity (outer/inner core boundary); 30 km: approximate depth of the Moho under continents (crust-mantle boundary); 6,371 km: Earth's total radius, not a discontinuity.

Correct Answer: B. Mantle

• **Mantle** = in the boiled egg analogy, the 'White' (egg albumen) represents the Mantle — the thickest layer surrounding the core. • **Egg analogy** — the shell = Crust (thin outer layer), the white = Mantle (thick middle layer), the yolk = Core (central dense part). • 💡 Wrong-option analysis: Outer Core: part of the yolk region in this analogy; Crust: represented by the shell (thin outer layer); Inner Core: the innermost part of the yolk in this analogy.

Correct Answer: A. Core

• **Core** = in the boiled egg analogy, the 'Yolk' represents Earth's Core — the dense central region of iron-nickel composition. • **Yolk = Core (NIFE)** — just as the yolk is centrally located and different in composition, Earth's core is at the center composed of iron-nickel (NIFE) at high density. • 💡 Wrong-option analysis: Crust: represented by the egg shell; Lithosphere: the rigid outer layer (crust + upper mantle), represented by the shell and outer white; Mantle: represented by the egg white.

Correct Answer: D. Seismology

• **Seismology** = the scientific study of earthquakes and seismic waves, used to map Earth's internal structure by analyzing how waves refract and reflect. • **Primary mapping tool** — seismology has revealed all of Earth's internal discontinuities (Moho, Gutenberg, Lehmann) without any direct access to those depths. • 💡 Wrong-option analysis: Cosmology: study of the origin and evolution of the Universe; Meteorology: study of weather and atmosphere; Petrology: study of rocks and their origin, not earthquake waves.

Correct Answer: A. It increases the melting point

• **Increases the melting point** = extreme pressure in the inner core raises iron's melting point above the actual temperature (~6,000°C), keeping it solid despite the heat. • **3.6 million atm pressure** — at this immense pressure, iron's melting point exceeds 6,000°C, which is why the inner core remains solid while the outer core remains liquid. • 💡 Wrong-option analysis: It decreases the melting point: opposite of what happens under extreme pressure for iron; It turns iron into gas: impossible under such extreme pressure; It has no effect: incorrect — pressure is precisely what keeps the inner core solid.