Rocks & Minerals — Set 2

Correct Answer: B. Diamond

• **Diamond (Hardness 10)** = the hardest known natural mineral on the Mohs scale, owing its extreme hardness to strong covalent bonds between carbon atoms. • **Only diamond can scratch diamond** — this unique property makes diamonds valuable for industrial cutting, drilling, and grinding tools. • 💡 Wrong-option analysis: Quartz: hardness 7; Topaz: hardness 8; Corundum: hardness 9 (includes rubies and sapphires), just one step below diamond.

Correct Answer: D. Feldspars

• **Feldspars** = the most abundant mineral group, comprising about 60% of the Earth's crust, containing silica and aluminum, found in all three rock types. • **60% of Earth's crust** — feldspars dominate all three major rock types (igneous, sedimentary, metamorphic) and include orthoclase and plagioclase varieties. • 💡 Wrong-option analysis: Amphiboles: dark silicate minerals, common but less abundant than feldspars; Micas: sheet silicates like biotite/muscovite, abundant but less than feldspars; Quartz: very common (SiO2) but second to feldspars in total crustal abundance.

Correct Answer: D. Silicon Dioxide (SiO2)

• **Quartz = SiO2 (Silicon Dioxide / Silica)** = one of Earth's most common and chemically resistant minerals, a major component of sand, granite, and sandstone. • **SiO2** — silicon dioxide; quartz resists weathering, which is why it accumulates in sand; hardness 7 on Mohs scale. • 💡 Wrong-option analysis: Iron Oxide (Fe2O3): composition of hematite (iron ore), not quartz; Sodium Chloride (NaCl): composition of halite (rock salt); Calcium Carbonate (CaCO3): composition of calcite/limestone.

Correct Answer: A. Obsidian

• **Obsidian ('Volcanic Glass')** = formed when lava cools so extremely rapidly that no crystals develop, producing a smooth, glassy, amorphous rock with sharp edges. • **Ancient tool use** — obsidian's extremely sharp edges made it valuable to ancient cultures for knives, arrowheads, and surgical-quality cutting tools. • 💡 Wrong-option analysis: Basalt: fine-grained extrusive rock with tiny crystals (not glassy); Pumice: porous volcanic rock with gas bubbles, used for skin exfoliation; Rhyolite: felsic extrusive rock with small crystals, not truly glassy.

Correct Answer: A. Pumice

• **Pumice** = a highly porous, frothy volcanic rock filled with gas bubbles, so low in density that it can float on water. • **Gas bubble porosity** — during explosive volcanic eruptions, trapped gases create countless tiny holes (vesicles) in solidifying lava, making pumice lighter than water. • 💡 Wrong-option analysis: Basalt: dense, fine-grained volcanic rock, sinks in water; Granite: dense intrusive rock, much heavier than water; Obsidian: dense volcanic glass, sinks in water.

Correct Answer: B. Conglomerate

• **Conglomerate** = a clastic sedimentary rock composed of rounded gravel and pebbles cemented together, indicating long-distance water transport of the fragments. • **Rounded = water-transported** — the rounded shape of fragments indicates abrasion during long transport; angular fragments indicate breccia, not conglomerate. • 💡 Wrong-option analysis: Sandstone: composed of sand-sized grains; Breccia: similar to conglomerate but with angular (not rounded) fragments; Shale: composed of very fine clay-sized particles.

Correct Answer: B. Magma

• **Magma** = molten rock stored beneath the Earth's surface, containing dissolved gases, minerals, and crystals suspended in the melt. • **Name change at surface** — magma becomes 'lava' the moment it erupts onto the surface; the distinction is purely based on location. • 💡 Wrong-option analysis: Lava: same material as magma but on the surface after eruption; Tephra: fragmented volcanic material ejected into the air; Tuff: rock formed from compacted volcanic ash.

Correct Answer: C. Lava

• **Lava** = molten rock (magma) that has erupted through a volcano or vent onto Earth's surface, losing dissolved gases as it flows and cools. • **Extrusive igneous rocks** — solidified lava forms extrusive igneous rocks; fast cooling produces fine-grained basalt, very fast produces glassy obsidian. • 💡 Wrong-option analysis: Granite: solid intrusive igneous rock formed underground from cooled magma; Magma: same material but underground; Pluton: a body of intrusive igneous rock formed underground.

Correct Answer: B. Batholith

• **Batholith** = a massive body of intrusive igneous rock (usually granite) covering over 100 km², formed deep underground and exposed by millions of years of erosion. • **Mountain cores** — batholiths form the cores of many mountain ranges (e.g., Sierra Nevada, Patagonian Batholith); exposed by uplift and erosion over geological time. • 💡 Wrong-option analysis: Sill: horizontal sheet-like intrusion running parallel to rock layers; Dyke: vertical or steep sheet-like intrusion cutting across rock layers; Laccolith: dome-shaped intrusion that arches overlying strata.

Correct Answer: A. Dyke

• **Dyke** = a sheet-like igneous intrusion that cuts vertically or steeply across existing rock layers, often harder than surrounding rock and appearing as walls after erosion. • **Vertical vs horizontal** — Dyke cuts across layers (discordant); Sill intrudes parallel between layers (concordant); the distinction is key. • 💡 Wrong-option analysis: Sill: horizontal intrusion parallel to rock layers, not cutting across; Batholith: massive deep igneous body, not a sheet-like intrusion; Laccolith: dome-shaped intrusion between layers, not cutting across.