Fig. 44.1 shows a diagram of the three main types of rock and how they are formed. Rocks are classified into three main types based on how they are formed: ...

(a) How igneous rocks are formed [2]

1. Igneous rocks are formed when magma (molten rock from deep within the Earth) rises toward the surface and cools, or when lava (magma that has erupted onto the surface) cools and solidifies. The cooling causes the liquid rock to crystallise into a solid mass of interlocking mineral crystals. [1]
2. The rate of cooling determines the texture: fast cooling at the Earth's surface produces fine-grained (extrusive) igneous rocks such as basalt, because crystals have very little time to grow. Slow cooling deep underground produces coarse-grained (intrusive) igneous rocks such as granite, because crystals have thousands of years to grow large. [1]

(b) Processes involved in the formation of sedimentary rocks [3]

1. Weathering: Existing rocks at the Earth's surface are broken down into smaller fragments by physical weathering (e.g. freeze-thaw, where water expands as it freezes in cracks), chemical weathering (e.g. acid rain dissolving limestone), and biological weathering (e.g. plant roots widening cracks). [1]
2. Erosion and deposition: These rock fragments (sediments) are transported by agents of erosion such as rivers, wind, glaciers, and waves. When the transporting agent loses energy, the sediments are deposited in layers, typically in low-energy environments such as river deltas, lake beds, and the sea floor. [1]
3. Compaction and cementation: Over millions of years, successive layers of sediment accumulate and the lower layers are compressed by the weight of overlying material (compaction). Minerals dissolved in groundwater precipitate in the spaces between sediment grains, binding them together (cementation) to form solid sedimentary rocks such as sandstone, limestone, and shale. [1]

(c) How metamorphic rocks are formed from sedimentary rocks [2]

1. Sedimentary rocks that are buried deep underground, typically at convergent plate boundaries or near magma intrusions, are subjected to intense heat and immense pressure from overlying rocks and the Earth's internal thermal energy. [1]
2. These extreme conditions cause the minerals within the rock to recrystallise and the rock's internal structure to change, forming metamorphic rocks with new textures and properties. For example, limestone (a soft sedimentary rock) becomes marble (a harder, crystalline metamorphic rock), and mudstone becomes slate. Crucially, this transformation occurs in the solid state; the rock does not melt. [1]

(d) Why the rock cycle is described as a continuous process [3]

1. The rock cycle has no definitive beginning or end. Any type of rock (igneous, sedimentary, or metamorphic) can be transformed into any other type through different geological processes. The starting point is arbitrary. [1]
2. The cycle operates through multiple interconnected pathways: igneous rocks can be weathered and eroded into sediments that form sedimentary rocks; sedimentary rocks can be changed by heat and pressure into metamorphic rocks; metamorphic rocks can melt to form magma that solidifies into new igneous rocks. [1]
3. These transformations have been occurring continuously throughout Earth's 4.6-billion-year history and will continue as long as the Earth has internal heat (driving melting, metamorphism, and volcanism) and surface processes (driving weathering, erosion, and deposition). [1]