Supporting the teaching of Earth sciences at all levels
VIRTUAL EXPERIMENTS
EARTH SCIENCE TEACHERS’ ASSOCIATION
Experiment: Mike Tuke              Realised by: Julian & Phillipa Priddle                  Sponsor: PESGB

POROSITY

A rock is said to be porous if it contains many small holes.  In rocks these holes are usually too small to be easily seen.  Bread, a sponge and Aero chocolate are all porous and in those you can see the holes.   Porosity is the degree to which a rock or other substance is full of holes.  Porosity is always given as a percentage. Porosity = volume of holes x 100/volume of whole rock Porous rocks are able to hold fluids.  Permeability is the ability of a substance to allow fluids to pass through it. A sponge is porous and so is Aero chocolate, but in a sponge the holes are connected so fluids such as water or gas can pass through the sponge; it is both porous and permeable.  In Aero chocolate the pores are not connected so it is porous but not permeable.   Loose sediment is porous because there are gaps between the grains. Sedimentary rocks are generally porous because the cement does not fill up all the gaps between the grains.   Most igneous and metamorphic rocks are made up of interlocking crystals, have no gaps between the grains and are therefore not porous. The only exceptions are vesicular lava and pumice.

Importance of porosity

Here is a list of some of the reasons why porosity of rocks is of great economic importance; Oil and natural gas are stored in the pores within sedimentary rocks Water is stored underground within the pores in sedimentary rocks Some mineral deposits occur within the pores in rocks Roadstone must not be porous because water in the pores would freeze and shatter the rock Physical weathering may occur because of expansion of the water in the pore spaces when it freezes. Rainwater in the pore spaces may cause chemical weathering.

Some terms

Porosity It is volume of the holes in a rock divided by the rock’s bulk volume.  It is always given as a percentage of the bulk volume of the rock.  Bulk volume is the volume of the rock including the volume of all the pores. Yield Yield is the volume of water (or oil) which will drain out of a rock.  It is given as a percentage of the bulk volume of the rock. Retention Retention is the volume of the water that will not drain out. It is held in the rock by capillary attraction. It is given as a percentage of the bulk volume of the rock.

Activities

Porosity, yield & retention -  20 min. Porosity and grain size in well sorted sediment- 2 min. Grain size, yield and retention in well sorted sands - 40 min. Porosity and sorting - 15 min. Porosity of  poorly sorted rocks - 15 min. Porosity of rounded and angular grains - 15 min. Porosity and sphericity - 15min. Porosity and packing - 2 min. Porosity of cubic packing - 5 min. Porosity of a rectangular piece of rock - 20 min. Porosity of irregular shaped rocks - 20 min. Porosity of clay 3 min. Dry porosity - 15 min. Porosity of vesicular basalt - 10 min.