Rock Permeability (Cambridge International IGCSE Environmental Management)

What is rock & soil permeability?

• Permeability is the ability of water to pass through the pore spaces of rock and soil

  • Pore spaces are the tiny gaps or holes between rock or soil particles that allow water and air to move through

• Rocks and soils with many connected pore spaces allow water to flow through easily

  • They are described as permeable

• Rocks and soils with few or no connected pore spaces do not let water pass through easily

  • They are described as impermeable

• The size and connection of the pores determine how quickly water can move through the material

Rock permeability

Exam Hint Students sometimes mix up 'porous' and 'permeable'. 'Porous' means the rock has spaces; 'permeable' means water can actually flow through. If the pores aren’t connected, the rock might be porous but still impermeable.

Permeable rocks

• Permeable rocks allow water to pass through them because they contain many connected pores or cracks

• Most sedimentary rocks are permeable because they form from layers of particles that leave spaces between grains

• Examples of permeable rocks include:

  • Limestone has small cracks and spaces that let water flow through
  • Sandstone is made of sand-sized grains with spaces between them that allow water to move easily

• Permeable rocks often store groundwater

  • This forms aquifers, which are underground layers of rock that hold water

Impermeable rocks

• Impermeable rocks do not allow water to pass through easily because their pores are very small or not connected

• Igneous and metamorphic rocks are usually impermeable

  • This is because they have a dense, compact structure with few spaces

• Some sedimentary rocks, such as shale, are also impermeable

  • This is because the fine clay particles are packed tightly together

• Water cannot soak into impermeable rocks

  • This means that water often flows over their surface or collects above them to form springs or surface water

Exam Hint When classifying rock types, avoid vague answers like 'igneous rocks are hard.' Instead, state why they’re impermeable—because the crystals fit tightly together and there are no spaces for water to pass through.