POROSITY

Water that falls as precipitation can be incorporated into glaciers, it can run off in streams or it can infiltrate into the ground.  (Review the hydrologic cycle from the river page if you don't remember this).  We are now concerned with the part that infiltrates, which is called GROUNDWATER.

If water can in fact infiltrate into the ground, there must be open spaces in the bedrock to accept it.  Such open space is called POROSITY or PORE SPACE.  There are three main types.

INTERGRANULAR POROSITY is that that occurs between the grains of soil, sediment, or incompletely cemented sedimentary rock.  Probably most of the world's groundwater supply occurs in this type of pore space, including most of that in southern Georgia.  The diagram below gives a simple example of how this might look under magnification.  The circles represent rounded sediment grains, perhaps sand.  The cross-hatching represents the pore space.  In general, coarser, better sorted, and more rounded grains provide the maximum amount of porosity.

Some rocks, such as igneous and metamorphic ones, are made of crystals that are very tightly intergrown and which consequently have no intergranular porosity.  The only type of pore space possible in most of these sorts of rocks is FRACTURE POROSITY.  Any rocks can be fractured, but this type of porosity is most important in crystalline rocks.  In the diagram below the crosses represent an igneous rock that is criss-crossed by fractures (black).  It is important that these fractures intersect as we will see soon.

The third type of porosity only occurs in rocks that can be chemically weathered by solution.  Thus it mostly occurs in limestone, though marble and evaporites can also have pores of this type.  In the diagram below a limestone (brick pattern) has been dissolved in some places to produce CAVERNOUS POROSITY.  The black represents caves that are filled with air.  (Stalactites and stalagmites only grow in empty caves).  The lower cave, shown in blue, is almost completely filled with groundwater.

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