Igneous Sedimentary and Metamorphic Rocks Questions and Answers

Answer: C

Correct answer: crystallise from a cooling magma

Bowen showed that minerals crystallise from a cooling magma in a definite order, in two branches: a discontinuous series from olivine through pyroxene and amphibole to biotite, and a continuous series in which plagioclase becomes progressively more sodic, the two converging on potash feldspar, muscovite and finally quartz. The order explains why certain minerals occur together and why fractional crystallisation changes a magma’s composition as it cools.

Answer: D

Correct answer: stratification into beds

Sediment is deposited layer upon layer, so bedding is the defining field characteristic of a sedimentary rock and the one on which structural geology depends - without beds there would be no dip, no strike, no folds and no way to read the sequence. Interlocking crystals point to an igneous origin, foliation to a metamorphic one, and glass to rapid chilling.

Answer: A

Correct answer: fragments of pre-existing rock

Clastic rocks are built from clasts - fragments produced by the weathering and erosion of older rock, transported and then deposited and cemented. Conglomerate, sandstone and shale are the familiar members, distinguished by grain size. The non-clastic sediments form differently: by chemical precipitation, as with rock salt, or by the accumulation of organic remains, as with many limestones and coal.

Answer: B

Correct answer: grain size

Sandstone is made of grains between 0.06 and 2 mm; shale of particles finer than 0.002 mm, laid down in still water and splitting along the lamination. Composition overlaps considerably - both may be quartz-rich - and both may be cemented by the same materials. It is the grain size, and the depositional energy it records, that separates them.

Answer: C

Correct answer: rounded

Both are coarse clastic rocks with fragments above 2 mm; the difference is in the shape of those fragments. Rounded clasts have been transported far enough for their corners to be abraded away, so a conglomerate records distance or vigorous water. Angular clasts have travelled little, so a breccia records a scree, a fault zone or a collapse near its source.

Answer: D

Correct answer: precipitation or the accumulation of shells

Limestone is calcium carbonate, formed either by direct chemical precipitation from sea water or, far more commonly, by the accumulation of the shells and skeletons of marine organisms. Either way, no transported rock fragments are involved, which is what places it outside the clastic group - and its solubility in slightly acid groundwater, the source of all its engineering difficulties, follows from that chemistry.

Answer: A

Correct answer: deposition from a decelerating current

As a current slows it can no longer carry its coarsest load, which settles first, and progressively finer material follows as the velocity falls further. The bed therefore grades from coarse at the base to fine at the top. Turbidity currents on a continental slope produce exactly this, and graded bedding is also a way-up indicator: the coarse end is the bottom, whatever the rock has done since.

Answer: B

Correct answer: exposed and dried after deposition

Mud cracks form when a wet fine sediment is exposed to the air and shrinks as it dries, and their preservation means the cracked surface was then buried by the next deposit. They therefore record a depositional environment that was periodically dry - a tidal flat, a river floodplain, a playa lake - and, like graded bedding, they indicate which way was up.

Answer: C

Correct answer: the migration of ripples or dunes

Cross-bedding is produced when sediment avalanches down the lee face of a ripple or dune that is migrating in the current, so the internal laminae are inclined to the main bedding. It is common in river, beach and desert sandstones, and the direction of inclination records the direction of the current or wind that formed it - which makes it a palaeocurrent indicator as well as a way-up one.

Answer: D

Correct answer: chemically active fluids

The three agents of metamorphism are heat, pressure and chemically active fluids, the last carrying material into and out of the rock and greatly accelerating recrystallisation. Weathering acts at the surface and is not metamorphism, and melting would take the rock out of the metamorphic realm altogether - metamorphism proceeds in the solid state, which is its defining constraint.