Igneous Sedimentary and Metamorphic Rocks Questions and Answers

Answer: A

Correct answer: solidification of molten material

Igneous means born of fire: the rock forms when magma or lava cools and crystallises. That single origin explains its interlocking crystal texture, its lack of bedding, and the fact that grain size records the cooling rate. Cementing of grains gives sedimentary rock, recrystallisation gives metamorphic, and precipitation from solution gives the chemical sediments.

Answer: B

Correct answer: at considerable depth, cooling slowly

A plutonic or intrusive rock crystallises at depth, where the surrounding rock insulates it and cooling takes thousands of years, so the crystals grow large and the texture is coarse. Granite and gabbro are the familiar examples. A volcanic or extrusive rock, cooled at the surface in hours or days, has the same chemistry but a fine or glassy texture - rhyolite and basalt respectively.

Answer: C

Correct answer: slowly, at depth

Crystal size is set by the time available for growth. Slow cooling at depth lets a few nuclei grow into large interlocking crystals visible to the eye, giving the phaneritic texture. Rapid cooling produces many nuclei and no time for any to grow, giving a fine aphanitic texture; and quenching produces no crystals at all. Grain size is therefore read directly as a cooling history.

Answer: D

The magma began to crystallise at depth, growing the large phenocrysts, and was then erupted or intruded to a shallower level where the rest froze quickly. The texture is therefore a history rather than a single condition.

Answer: A

Correct answer: chilled too rapidly for crystals to grow

Crystallisation requires atoms to migrate into an ordered lattice, which takes time. Lava chilled almost instantly - by contact with water or air at the surface - freezes with its atoms still disordered, giving a natural glass. Obsidian is the result, and being a glass rather than a crystalline aggregate it fractures conchoidally, which is why it was so widely used for blades.

Answer: B

Correct answer: 66 per cent

The conventional divisions by silica content are: acid above 66 per cent, intermediate 52 to 66, basic 45 to 52, and ultrabasic below 45. Silica content correlates with almost everything else - acid rocks are rich in quartz and feldspar, pale and light; basic rocks are rich in ferromagnesian minerals, dark and dense - which is why it is the primary classification.

Answer: C

Correct answer: ferromagnesian minerals

The dark colour of a basic rock comes from olivine, pyroxene, amphibole and biotite, the iron and magnesium bearing silicates, which crystallise early from a silica-poor melt. They are also denser than quartz and feldspar, which is why basic rocks are heavier as well as darker - basalt at about 3.0 against granite at 2.65.

Answer: D

Correct answer: grain size, reflecting their cooling rates

The two are chemically identical: a granite is what a rhyolite magma becomes if it cools slowly at depth, and a rhyolite is what the same magma becomes if it erupts and chills at the surface. The difference is entirely one of crystal size, and the same pairing runs through the classification - diorite with andesite, gabbro with basalt.

Answer: A

A dyke fills a fracture and so crosses the strata, while a sill is injected along them. The distinction matters to a tunnel or a foundation because a dyke may be met unexpectedly and is often weathered along its margins.

Answer: B

Correct answer: much greater exposed area

Batholith and stock are both discordant plutonic bodies of the same kind, distinguished only by size: a batholith exposes more than about 100 square kilometres and a stock less. Both cut across the surrounding structure rather than following it, so neither is concordant, and both are coarse-grained rather than glassy.