Rocks & Minerals — Set 5

Correct Answer: B. Lava

• **Lava** = molten rock expelled from a volcano onto Earth's surface, losing dissolved gases as it flows; solidifies into extrusive igneous rocks. • **Extrusive igneous** — solidified lava forms rocks like basalt (slow lava) and obsidian (very fast cooling); the type depends on cooling rate and composition. • 💡 Wrong-option analysis: Granite: solid intrusive igneous rock that cools underground, not lava; Magma: same material underground before eruption; Ash: fine volcanic particles ejected into the air, not flowing rock.

Correct Answer: D. Plutonic rocks

• **Plutonic Rocks** = intrusive igneous rocks that cool slowly at great depths, named after Pluto (Roman god of underworld); Granite is the most common example. • **Slow deep cooling** — plutonic rocks develop large, visible crystals (coarse-grained) due to slow underground cooling; exposed at surface by uplift and erosion. • 💡 Wrong-option analysis: Volcanic rocks: another name for extrusive igneous rocks formed at the surface; Foliated rocks: metamorphic rocks with parallel mineral alignment; Sedimentary rocks: formed from compressed sediment layers.

Correct Answer: C. Sedimentary

• **Sedimentary Rocks (Stratification)** = deposited in horizontal layers (strata) as new sediment accumulates on top of older layers; this bedding is the defining feature. • **Strata reading** — geologists read sedimentary strata like pages of Earth's history; older layers are deeper, newer layers on top (principle of superposition). • 💡 Wrong-option analysis: Igneous: form as massive crystalline bodies without layering; Volcanic: extrusive igneous rocks, also typically lack stratification; Plutonic: intrusive igneous rocks forming large undifferentiated masses.

Correct Answer: D. Sedimentary

• **Sedimentary Rocks (Petroleum/Gas reservoir)** = porous sedimentary rocks like sandstone and limestone store hydrocarbons formed from ancient organic matter trapped in sediment. • **Porous reservoir rocks** — petroleum and gas migrate through porous sedimentary layers until trapped by impermeable rocks or structural traps (anticlines). • 💡 Wrong-option analysis: Igneous: dense crystalline rocks cannot store hydrocarbons; Volcanic: too dense and impermeable; Metamorphic: high heat and pressure destroy organic hydrocarbons.

Correct Answer: D. Shale

• **Slate = low-grade metamorphosed Shale** = pressure aligns clay minerals in shale into flat parallel planes (cleavage), allowing slate to split into thin durable sheets. • **Cleavage planes** — aligned microscopic clay minerals create distinct breakage planes; this makes slate ideal for roofing tiles and flooring. • 💡 Wrong-option analysis: Coal: metamorphoses into graphite under extreme conditions; Sandstone: metamorphoses into quartzite; Limestone: metamorphoses into marble.

Correct Answer: B. Igneous

• **Granite (Intrusive Igneous)** = the quintessential plutonic rock composed mainly of quartz, feldspar, and mica, formed by slow underground cooling of magma. • **Quartz + Feldspar + Mica** — the three primary minerals of granite; its coarse texture (visible crystals) results from slow cooling deep in the Earth's crust. • 💡 Wrong-option analysis: Sedimentary: Granite is igneous, not formed from sediment; Metamorphic: Granite is not transformed from another rock, but metamorphoses into gneiss; Chemical: a sub-category of sedimentary rocks.

Correct Answer: D. Igneous

• **Basalt (Extrusive Igneous)** = the most common extrusive igneous rock, forming most of the ocean floor and large volcanic provinces on continents. • **Ocean floor dominance** — approximately 70% of Earth's surface is covered by basalt-based oceanic crust, formed at mid-ocean ridges by rapid lava cooling. • 💡 Wrong-option analysis: Biochemical: a sub-category of sedimentary rocks (e.g., coal, limestone); Sedimentary: basalt is formed from cooling lava, not sediment accumulation; Metamorphic: basalt is not formed from transformation of another rock.

Correct Answer: C. Limestone

• **Marble = metamorphosed Limestone (or Dolostone)** = heat and pressure recrystallize calcium carbonate in limestone, forming interlocking calcite crystals of marble. • **Fossil destruction** — the recrystallization process destroys any fossils present in the original limestone, which is why marble contains no fossils. • 💡 Wrong-option analysis: Granite: metamorphoses into gneiss, not marble; Shale: metamorphoses into slate (low grade) or schist (high grade); Sandstone: metamorphoses into quartzite.

Correct Answer: D. Limestone

• **Limestone** = a sedimentary rock composed primarily of calcium carbonate (CaCO3), the essential raw material for cement manufacture when heated with clay. • **Cement production** — limestone heated to ~1450°C with clay produces clinker; grinding clinker with gypsum makes Portland cement used in construction worldwide. • 💡 Wrong-option analysis: Slate: metamorphic rock used for roofing/flooring, not cement; Granite: igneous rock used in construction but not for making cement; Basalt: igneous rock used for road aggregate, not cement raw material.

Correct Answer: D. Heat and Pressure

• **Heat and Pressure** = the two primary agents driving metamorphism; heat softens rocks enabling mineral recrystallization, while pressure realigns or compresses minerals. • **Chemically active fluids** — a third factor; hot mineral-rich fluids can also drive chemical changes in rocks, but heat and pressure are the primary drivers. • 💡 Wrong-option analysis: Wind and Water: agents of erosion and transportation, not metamorphism; Erosion and Deposition: agents forming sedimentary rocks; Cooling and Melting: agents forming igneous rocks, not metamorphic.