WASH Cluster – Groundwater Development and Drilling
GWD1
Handout GWD1 – Occurrence
TABLE OF ROCK TYPES AND POROSITY
Aquifer
category
Crystalline
Basement
Volcanic rocks
Consolidated
sedimentary
rocks
Unconsolidated
sediments
Example Rock
types
Granite,
granodiorite,
gabbro
Gneiss, schist,
hornfels
Basalts
Scoria
Means of Formation
Arrangement of
minerals
Interlocking crystals
Fractures
Metamorphic (heat
and pressure)
Igneous (volcanic)
Interlocking crystals
Fractures
Interlocking crystals
Sometimes granular
Rhyolite,
rhyodacite,
pumice
Igneous (volcanic)
Particles often
welded together,
Sometimes granular
Fractures, layers
between different
lava flows
Fractures, layers
within different lava
flows sometimes
granular
Conglomerate,
sandstone,
siltstone, shale
Limestone
Deposition of particles
by rivers, sea, wind
Particles compacted
and cemented
together
Particles cemented
together
Coal
Deposited mainly in
swamps
Particles bound
together
Gravel, sand, silt,
clay
Deposition of particles
by rivers, sea,wind
Limestone
Deposition in the sea
Particles generally
uncompacted very
little cementation
Particles weakly
cemented together
Igneous (plutonic)
Deposition in the sea
Type of porosity
Fractures, bedding
planes (depositional
layers)
Fractures, bedding
planes.
cavities in caves and
solution cracks
Fractures, bedding
planes
Between the
particles
Between particles,
Cavities in caves and
solution cracks
Aquifers are often referred to by the characteristics of the porosity. Thus:


Fractured rock aquifers comprise: crystalline basement rocks , volcanic rocks and
consolidated sedimentary rocks
Porous media – essentially unconsolidated sedimentary aquifers such as gravel and sand as
well as some non consolidated volcanic rocks such as scoria or pumice
A more complex yet widespread aquifer type associated with limestone deposits and cave systems is
known as “Karst”.