Igneous Sedimentary and Metamorphic Rocks Questions and Answers

Answer: A

The heat of a pluton bakes a narrow aureole around it, producing non-foliated rocks such as hornfels and marble. Regional metamorphism acts over orogenic belts with directed pressure and produces foliated rocks instead.

Answer: B

Correct answer: directed pressure aligning platy minerals

Foliation requires two things: pressure that is greater in one direction than another, and minerals with a platy or elongate habit - the micas and amphiboles - that can rotate and grow perpendicular to the maximum stress. The result is a planar fabric along which the rock splits. Uniform pressure produces no preferred direction, and a rock of one equidimensional mineral cannot develop one at all.

Answer: C

Correct answer: grade of metamorphism

The three are the successive products of increasing temperature and pressure acting on a shale. Slate is low grade, with microscopic micas giving perfect cleavage; schist is medium grade, with visible micas and a wavy foliation; gneiss is high grade, with the minerals segregated into light and dark bands and the foliation coarse. The sequence is the standard illustration of metamorphic grade.

Answer: D

Correct answer: limestone

Marble is recrystallised limestone: the heat and pressure convert the fine calcite of the original into an interlocking mosaic of coarser calcite crystals. It effervesces in dilute acid exactly as its parent does, since the chemistry is unchanged. The corresponding pairs are quartzite from sandstone, slate from shale, and gneiss from granite or from high-grade schist.

Answer: A

In the parent sandstone the cement is the weak link and fracture follows the grain boundaries, whereas in quartzite there is no weaker path. That is why quartzite is both an excellent aggregate and expensive to drill and blast.

Answer: B

Correct answer: essentially of one mineral, with no platy minerals to align

Marble is almost all calcite and quartzite almost all quartz, and both minerals are equidimensional rather than platy, so directed pressure has nothing to align and the rock recrystallises into an interlocking mosaic instead. Foliation requires micas or amphiboles to rotate and grow, which is why it develops in metamorphosed shales and not in metamorphosed limestones or sandstones.

Answer: C

Correct answer: strong, massive and free of bedding planes

Granite has high strength, no bedding, no foliation and no solubility, so it presents a uniform mass with no preferred plane of weakness - the ideal foundation. Its only significant discontinuities are its joints, including the sheet joints produced by unloading near the surface, and those are mapped and allowed for rather than being an inherent defect of the material.

Answer: D

Correct answer: weak, and liable to slake and soften on wetting

Shale is weak to begin with and, being made of clay minerals, it slakes when wetted - breaking down into chips and then into mud, and losing much of what strength it had. Freshly exposed shale in an excavation may deteriorate within days, so it is protected immediately with shotcrete or a blinding layer, and its long-term strength must be assessed in the softened condition.

Answer: A

Circulating groundwater dissolves it along joints and bedding to form karst features that may be unroofed by loading or by drawdown. Probing between boreholes is essential, since a cavity can sit undetected between them.

Answer: B

Cooling contraction produces regular columns, and gas cavities give vesicular horizons of low strength, while some flows carry weathered clay-rich zones between them. Sound basalt is nonetheless a strong and durable rock.