Formation of rocks Revision Notes

The rock cycle is a natural, ongoing process that describes how rocks transform from one type to another over geological time. It involves several key processes: melting, cooling, erosion, sedimentation, compaction, and metamorphism. Rocks do not remain static; instead, they are constantly recycled through these processes, driven by Earth's internal heat and surface forces like weathering and erosion.

For example, an igneous rock exposed at the surface can be broken down by weathering into sediments, which may later form sedimentary rock. That sedimentary rock can then be subjected to heat and pressure, transforming into metamorphic rock. If conditions cause melting, the rock becomes magma, which upon cooling forms igneous rock again, completing the cycle.

• The rock cycle shows continuous transformation of rocks through geological processes.
• Processes include melting, cooling, erosion, sedimentation, compaction, and metamorphism.
• Rocks can change from igneous to sedimentary to metamorphic and back again over time.

Igneous rocks form when molten rock material called magma cools and solidifies. The location and rate of cooling determine the texture and crystal size of the igneous rock.

Granite forms when magma cools slowly beneath the Earth's surface (intrusive igneous rock). Slow cooling allows large crystals of minerals like quartz, feldspar, and mica to develop, giving granite a coarse-grained texture. Granite is often used in construction due to its strength and attractive appearance.

Basalt forms when lava erupts onto the Earth's surface and cools quickly (extrusive igneous rock). Rapid cooling results in very small crystals, producing a fine-grained texture. Basalt is common in oceanic crust and volcanic regions and is often dark-coloured and dense.

• Igneous rocks form from cooled magma or lava.
• Granite cools slowly underground, producing large crystals (coarse-grained).
• Basalt cools quickly on the surface, producing small crystals (fine-grained).
• Texture and crystal size depend on cooling rate and environment.

Sedimentary rocks form from the accumulation, compaction, and cementation of sediments derived from weathered and eroded rocks or biological material.

Limestone mainly forms from the remains of marine organisms such as corals and shellfish, which accumulate on the sea floor. Over time, these calcium carbonate-rich deposits compact and cement to form limestone. It often reacts with dilute acid due to its calcium carbonate content.

Sandstone forms from compacted sand grains, typically quartz, deposited by wind, rivers, or in deserts. The grains are cemented together by minerals like silica or calcium carbonate, creating a rock that is often porous and used as a building material or reservoir rock in geology studies for groundwater or oil storage potential. Sandstone’s grain size and sorting reflect its depositional environment, such as river channels or beaches, providing clues to past conditions. Shale forms from compacted clay and silt particles, which are much finer than sand. It typically forms in calm environments like lake beds or deep ocean floors where fine sediments settle. Shale is characterised by its thin layers and fissility (ability to split into thin sheets). It often contains organic material and can be a source rock for fossil fuels like oil and gas.

• Sedimentary rocks form from compacted and cemented sediments.
• Limestone forms mainly from marine organism remains rich in calcium carbonate.
• Sandstone forms from compacted sand grains cemented by minerals.
• Shale forms from compacted clay and silt particles in calm environments.
• Sedimentary rocks often show layering and may contain fossils.

Metamorphic rocks originate from existing igneous, sedimentary, or other metamorphic rocks that undergo physical and chemical changes due to heat and pressure, without melting. This process is called metamorphism.

Marble forms when limestone is subjected to heat and pressure deep within the Earth. The calcite crystals in limestone recrystallise, producing a denser, crystalline rock with a characteristic interlocking texture. Marble is often used in sculpture and architecture due to its hardness and aesthetic appeal.

Slate forms from shale under relatively low-grade metamorphic conditions. The clay minerals realign under pressure, creating a foliated texture that allows slate to split into thin, flat sheets. This property makes slate useful for roofing tiles and flooring materials. The transformation preserves some original features but changes the rock’s texture and mineral composition.

• Metamorphic rocks form from existing rocks under heat and pressure without melting.
• Marble forms from limestone recrystallisation under metamorphism.
• Slate forms from shale by mineral realignment and foliation.
• Metamorphism changes texture and mineralogy but not the rock’s chemical composition significantly.

The rock cycle links all rock types through continuous transformation. For example, granite (igneous) exposed at the surface can erode into sediments that form sandstone (sedimentary). Sandstone buried deep may become slate (metamorphic) under pressure. If slate melts, it forms magma, which cools to form new igneous rock like basalt.

Understanding these transitions helps explain Earth’s dynamic geology and the distribution of rock types. For instance, limestone’s origin from marine organisms indicates past sea environments, while basalt’s presence signals volcanic activity.

This knowledge is essential for environmental management, mining, construction, and interpreting Earth’s history.

• Rocks continuously transform through the rock cycle.
• Igneous, sedimentary, and metamorphic rocks are interconnected.
• Examples: granite → sandstone → slate → magma → basalt.
• Rock types reveal past environmental and geological conditions.