Igneous Sedimentary and Metamorphic Rocks Questions and Answers

Answer: C

Correct answer: the foliation is a plane along which the rock fails

A strong foliation gives the rock a pronounced anisotropy: it is far weaker across the mica-rich planes than along them, and blocks detach along that fabric. In a tunnel the consequence depends on the orientation - foliation dipping into the crown gives slabbing and overbreak, and foliation parallel to the axis can allow large wedges to fall. The rock may be strong when tested on a core and still behave badly in the mass.

Answer: D

Correct answer: hard, tough and chemically inert

Quartzite is almost pure quartz recrystallised into an interlocking mosaic, so it is very hard, tough and chemically unreactive - excellent aggregate that resists abrasion and does not react with cement. Those same properties make it expensive to drill, blast and crush, so it is difficult to excavate for the same reason it is valuable once excavated.

Answer: A

Correct answer: acid

The acid class begins at 66 per cent silica, so 70 per cent falls within it - typical of a granite or rhyolite, rich in quartz and feldspar, pale in colour and of comparatively low density. Intermediate runs from 52 to 66 per cent, basic from 45 to 52, and ultrabasic below 45.

Answer: B

Correct answer: 45 to 52 per cent

Basic rocks contain 45 to 52 per cent silica - basalt and gabbro are the type examples. Below 45 the rock is ultrabasic, such as peridotite; above 52 it is intermediate. The silica-poor melts crystallise the ferromagnesian minerals in abundance, which is why basic rocks are dark and dense.

Answer: C

Correct answer: sand

The standard grade boundaries are 2 mm between gravel and sand, 0.06 mm between sand and silt, and 0.002 mm between silt and clay, so 1 mm falls in the middle of the sand range. The scale is logarithmic, each boundary being roughly a factor of thirty from the next, which reflects how differently the transporting fluid behaves across those sizes.

Answer: D

Correct answer: 0.002 mm

Two micrometres separates silt from clay. The distinction matters far beyond grain size, because particles below it are generally clay minerals, plate-like and electrically active, and it is those properties - plasticity, cohesion, swelling, low permeability - that give clay its engineering behaviour. Silt of the same size range but different mineralogy behaves quite unlike it.

Answer: A

Correct answer: 2.65 g/cm\( ^3 \)

Density is mass over volume: 265 g in 100 cm cubed gives 2.65 g/cm cubed. The figure is worth recognising, since 2.65 is the density of quartz and hence of most sandstones and granites - it is the standard specific gravity of soil solids used throughout soil mechanics.

Answer: B

Correct answer: 13 per cent

The difference relative to the granite is (3.00 - 2.65)/2.65 = 0.132, about 13 per cent. It reflects the ferromagnesian minerals of the gabbro against the quartz and feldspar of the granite, and it is why basic and ultrabasic rocks stand out on a gravity survey and why the oceanic crust, being basaltic, is denser than the continental.

Answer: C

Correct answer: granite

Quartz with alkali feldspar and a little biotite, in a coarse interlocking texture, is granite. The quartz places it in the acid class, the coarse grain places it as plutonic, and the small proportion of ferromagnesian mineral accounts for its pale colour. Limestone and marble are carbonates, and basalt is fine-grained and basic.

Answer: D

Correct answer: marble

Effervescence in dilute acid identifies calcite, interlocking coarse crystals indicate recrystallisation, and the absence of foliation rules out the mica-bearing metamorphic rocks. That combination is marble - metamorphosed limestone, unfoliated because calcite is equidimensional and has nothing to align. Quartzite would not effervesce, and slate and gneiss are both foliated.