Quick questions on The rock cycle: HSC Earth and Environmental Science Module 5

Igneous rocks form when molten rock cools and crystallises. If magma cools slowly deep underground, large crystals grow and the rock is coarse-grained, like the granite of the Snowy Mountains and much of the eastern Australian highlands. If lava cools quickly at the surface, crystals are tiny and the rock is fine-grained, like basalt; the basalt flows of the Atherton Tableland and the Newer Volcanics Province of western Victoria are Australian examples. Crystal size therefore records cooling rate.

Sedimentary rocks form at the surface in three steps. First, weathering and erosion break existing rock into sediment. Second, water, wind or ice transport and deposit that sediment in layers. Third, burial compacts the grains and dissolved minerals cement them together, a process called lithification.

Metamorphic rocks form when existing rock is altered in the solid state by heat, pressure or chemically active fluids, without fully melting. The minerals recrystallise and often line up to give a banded or foliated texture. Shale can become slate then schist then gneiss as conditions intensify. The high-grade gneisses of Broken Hill in New South Wales are metamorphic rocks formed deep in the crust; the marble quarried at places such as Wombeyan formed by the metamorphism of limestone.

Describe the three processes by which loose sediment becomes a sedimentary rock. [3 marks]

Explain how a sample of granite could eventually become a metamorphic rock and then an igneous rock again. [4 marks]