Igneous Sedimentary and Metamorphic Rocks Questions and Answers

Answer: A

Correct answer: 1-Q, 2-P, 3-R

Each metamorphic rock inherits the chemistry of its parent. Marble comes from limestone, its calcite recrystallised coarser but unchanged in composition. Quartzite comes from sandstone, its quartz grains fused into an interlocking mosaic. Slate comes from shale, its clay minerals recrystallised into microscopic micas whose alignment gives the cleavage.

Answer: B

Correct answer: 1-Q, 2-P, 3-R

Texture records cooling history directly. Phaneritic means coarse crystals throughout, so cooling was slow at depth. Porphyritic means large crystals set in a fine groundmass, so the magma cooled slowly at first and then rapidly - typically it began crystallising at depth and was then erupted. Glassy means no crystals at all, so the chill was too fast for any lattice to form.

Answer: C

Correct answer: 3, 1, 2

The sequence from a shale parent runs slate, then phyllite, then schist, then gneiss, as temperature and pressure rise. Slate is fine and cleaves perfectly; schist has visible micas and a wavy foliation; gneiss has its light and dark minerals segregated into bands and is close to the point at which melting would begin and the rock would leave the metamorphic realm.

Answer: D

Correct answer: 1 and 3 only are correct

Statements 1 and 3 are correct: granite and rhyolite differ only in grain size, and bedding is the characteristic feature of sedimentary rock. Statement 2 reverses the definitions - a dyke is discordant, cutting across the bedding, while a sill is concordant and follows it. The distinction matters in the field, since it tells you at once how the body was emplaced.

Answer: A

Correct answer: shale

Shale is a fine-grained clastic sedimentary rock, formed from clay deposited in still water and compacted. Granite, basalt and gabbro are all igneous - granite acid and plutonic, gabbro basic and plutonic, basalt basic and volcanic - and all three show interlocking crystals with no bedding at all.

Answer: B

Correct answer: marble

Marble lacks foliation because calcite is equidimensional and directed pressure has no platy mineral to align; it recrystallises into an interlocking mosaic instead. Slate, schist and gneiss are all foliated, being derived from shales rich in clay minerals that become micas, and they differ from one another only in the grade of metamorphism.

Answer: C

Correct answer: slabs detaching along the bedding planes

Bedding dipping steeply into the face gives planes on which slabs can detach from the crown and the walls, since the rock is far weaker across the bedding than along it. The orientation of the discontinuities relative to the tunnel axis therefore matters as much as the strength of the rock itself, and support is designed to pin the beds back rather than to carry the intact material.

Answer: D

Correct answer: locating solution cavities and leakage paths

Limestone dissolves in slightly acid groundwater, so a thick limestone may be riddled with enlarged joints, sinkholes and caverns that no strength test would reveal. A reservoir on such ground can lose its whole storage through them, and there are well-known cases where it did. The investigation must therefore be aimed at the karst features and the groundwater regime, not at the material properties.

Answer: A

Correct answer: both the assertion and the reason are true and the reason explains the assertion

Both are true and the reason supplies the mechanism. Large crystals require a long period at slowly falling temperature to grow, and a fine or glassy groundmass requires the remaining melt to be chilled quickly - two conditions that cannot occur at once. The rock therefore records a magma that began crystallising at depth and was then erupted or intruded to a shallow level, which is exactly what a porphyry is.

Answer: B

Correct answer: the assertion is false although the reason offered is a true statement in itself

The reason is true - metamorphism does require elevated temperature and pressure - but the assertion is false and states the boundary of the process. Metamorphism proceeds in the solid state: the minerals recrystallise and new ones grow without the rock ever melting. Once melting begins the material becomes a magma and the product is igneous, which is why migmatites, caught between the two, are so interesting.