How are Rocks Formed? (Cambridge International IGCSE Environmental Management)

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

The difference between magma and lava

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

Types of igneous rock

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

Structure of the Earth

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

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

Exam Hint Students often think that all rocks with crystals are igneous, but that’s not true. Metamorphic rocks like marble can also have crystals—the difference is that these crystals form under heat and pressure, not from cooling magma or lava.