Aggregate quarries are excellent places for examining the three
main types of rocks; igneous, sedimentary and metamorphic.
Igneous rocks, formed from molten magma derived from deep within
the Earth, have a range of composition and grain sizes, from
the finest grained volcanic lavas to coarse-grained granites
and even coarser-grained pegmatites. The variation in grain
size depends on on the rate of cooling of the magma, with the
largest grains occurring in the rocks which cooled the most
slowly such as large bodies of granite.
Sedimentary rocks also show a great variety of types, from
fine-grained mudstones through to sandstones and coarse-grained
conglomerates and breccias. Other types of sedimentary rock
include limestones and even coal. Quarries are ideal for demonstrating
the layering or stratigraphy of sedimentary rocks.
Metamorphic rocks are of two types; one, contact metamorphosed,
is relatively localised, formed by the affects of heat from
an adjacent igneous intrusion; the other on a larger scale,
regionally metamorphosed rocks formed by pressure and heat
producing slates, schists and gneisses. Quarries provide a
great opportunity to note the similarities and differences
between the two types.
Example of a coal seam.