Quick questions on Igneous, sedimentary and metamorphic rocks - WJEC A-Level Geology

An igneous rock is made of interlocking crystals that grew as the melt cooled, fitting together with no pore space and no cement. There is no bedding, no rounding and no fossils. Crystal size records cooling rate: slow cooling at depth gives large, visible crystals (coarse-grained, as in granite), while fast cooling at the surface gives small crystals (fine-grained, as in basalt) or, with quenching, a glassy texture (as in obsidian).

A clastic sedimentary rock is made of separate grains (fragments of older rock) that were transported, deposited and then bound by a cement, so individual grains sit in a matrix. Non-clastic sedimentary rocks form by chemical precipitation (such as rock salt) or from organic remains (such as fossil-rich limestone or coal). Diagnostic features are bedding (layering), rounding and sorting of grains, fossils, and structures such as ripple marks. Many limestones fizz with dilute acid.

A metamorphic rock forms in the solid state and shows either a foliated texture (parallel alignment of platy minerals from directed pressure, seen as slaty cleavage in slate, schistosity in schist or banding in gneiss) or a non-foliated crystalline texture from contact metamorphism (such as marble or quartzite, which have interlocking recrystallised grains but no alignment). New minerals such as garnet may grow.

Quartz sand cemented into sandstone, then metamorphosed to quartzite, then melted into a granite shows how one mineral threads through all three classes while the texture records the process. Building stone choice. Slate roofs exploit the foliation of a metamorphic rock that splits into thin sheets, while granite kerbstones exploit the interlocking, poreless igneous texture that resists weathering. Reading Earth history. A sequence of bedded fossil-rich limestone, cut by a coarse igneous intrusion and folded into a foliated schist, lets a geologist reconstruct deposition, intrusion and mountain building in order from the textures alone.

State the process that forms each rock class. [3 marks]

A specimen is made of large, interlocking, randomly oriented crystals. Which class is it, and was it intrusive or extrusive? [2 marks]

Give one feature that distinguishes a non-foliated metamorphic rock from an igneous rock. [1 mark]