AGGREGATES - Materials for Civil and Construction Engineers - page 160

FIGURE 5.1 Aggregate stockpiling.

5.2 Geological Classification

All natural aggregates result from the breakdown of large rock masses. Ge-
ologists classify rocks into three basic types: igneous, sedimentary, and
metamorphic. Volcanic action produces igneous rocks by hardening or
crystallizing molten material, magma. The magma cools either at the
earth's surface, when it is exposed to air or water, or within the crust of
the earth. Cooling at the surface produces extrusive igneous rocks, while
cooling underground produces intrusive igneous rocks. In general, the ex-
trusive rocks cool much more rapidly than the intrusive rocks. Therefore,
we would expect extrusive igneous rocks to have a fine grain size and po-
tentially to include air voids and other inclusions. Intrusive igneous rocks
have larger grain sizes and fewer flaws. Igneous rocks are classified based
on grain size and composition. Coarse grains are larger than 2 mm and
fine grains are less than 0.2 mm. Classification based on composition is a
function of the silica content, specific gravity, color, and the presence of
free quartz.
Sedimentary rocks coalesce from deposits of disintegrated existing rocks
or inorganic remains of marine animals. Wind, water, glaciers, or direct
chemical precipitation transport and deposit layers of material that become
sedimentary rocks, resulting in a stratified structure. Natural cementing binds
the particles together. Classification is based on the predominant mineral