How are Rocks Formed? (Cambridge (CIE) IGCSE Environmental Management): Revision Note

Formation of igneous rocks: granite & basalt

What are igneous rocks?

• Igneous rocks form when molten rock (magma or lava) cools and solidifies

  • Magma: molten rock beneath the Earth’s surface

  • Lava: molten rock that reaches the surface through volcanic eruptions

• Magma is found in the mantle

Types of igneous rocks

• Intrusive (plutonic) igneous rocks

  • Form below the surface when magma cools slowly

  • Crystals have time to grow, so they are large and visible

  • Example: Granite

• Extrusive (volcanic) igneous rocks

  • Form on the surface when lava cools quickly

  • Crystals are small because there isn’t enough time for them to grow

  • Example: Basalt

• Igneous rocks are hard and more resistant to erosion

Formation of granite (intrusive)

• Magma rises and gets trapped below the Earth’s crust

• It cools slowly over thousands of years

• Large crystals of minerals (e.g. quartz) form

• Granite is:

  • Coarse-grained (large crystals)

  • Hard and durable

  • Commonly found in continental crust

Formation of basalt (extrusive)

• Lava erupts from a volcano or fissure onto the Earth’s surface

• It cools rapidly in contact with air or water

• Small crystals or no visible crystals form

• Basalt is:

  • Fine-grained (small crystals)

  • Dark in colour

  • Found mainly in oceanic crust

Formation of sedimentary rocks: limestone, sandstone & shale

What are sedimentary rocks?

• Sedimentary rocks form from sediments (small rock particles, minerals, or organic remains) that are:

  • Eroded, transported, deposited, and then

  • Compressed and cemented over time

• They usually form in layers (strata) at the Earth’s surface or in water bodies

Main processes forming sedimentary rocks

• Weathering—breakdown of existing rocks into smaller particles

• Erosion and transportation—movement of these particles by wind, water, or ice

• Deposition—settling of materials in lakes, seas, or riverbeds

• Compaction—pressure from layers above squeezes sediments together

• Cementation—particles bind together, forming solid rock

Formation of limestone

• Formed mainly from calcium carbonate

• Originates from:

  • Shells and skeletons of marine organisms

• Over time:

  • Sediments build up on the seabed

  • Layers are compressed and cemented to form limestone

• Limestone is:

  • White, grey, or yellow in colour

  • Can contain fossils

Formation of sandstone

• Formed from sand-sized mineral grains (mainly quartz)

• Deposited by rivers, wind, or waves in deserts, beaches, or riverbeds

• Layers of sand are buried, then compacted and cemented together

Formation of shale

• Formed from clay and silt particles that settle in calm water (like lakes or deep seas)

• Layers of mud build up and are compacted under pressure

• Shale is:

  • Often dark grey due to organic material

Insert image: Formation of sedimentary rock
Formation of sedimentary rock

Formation of metamorphic rocks: marble & slate

What are metamorphic rocks?

• Metamorphic rocks form when existing rocks (igneous or sedimentary) are changed by heat and/or pressure inside the Earth

• The process is called metamorphism

  • Metamorphism: the alteration of rocks due to heat, pressure, or chemical processes, without melting

Formation of marble

• Limestone is the parent rock

• When exposed to high temperature and pressure, the calcium carbonate crystals in limestone recrystallise

• This forms marble, a hard, crystalline, and non-layered rock

• Marble is:

  • Usually white (can have coloured streaks from impurities)

  • Used in building and sculpture

Formation of slate

• Shale is the parent rock

• Under moderate heat and pressure, the minerals align, creating thin layers

• This forms slate, a fine-grained, layered metamorphic rock

• Slate is

  • Hard, dense, and splits easily into thin sheets

  • Commonly used for roofing tiles and flooring