Water and conglomerate rocks go hand in hand.
Wherever pebbles and water are found, conglomerate rocks are rarely far behind. Though they form a relatively small portion of the total sedimentary rock record, they do provide a great deal of information about the composition of this world -- the past of Earth as well as its present.
Conglomerate Rocks
A conglomerate rock is a type of sedimentary rock. It is formed of rounded fragments so large -- approximately 0.08 inches or 2 millimeters in diameter -- that they can be seen by the naked eye. Pressure alone cannot hold them together. These fragments, called clasts, are usually cemented together by dissolved minerals. Clasts are categorized by size into pebbles, cobbles and boulders. Conglomerates are classified in numerous other ways as well: by their lithology (rock structure), by their sorting method and by their matrix composition.
Basic Types
The variety of divisions and subdivisions in rock classification is truly remarkable. However, conglomerate rocks can be broken down into two fundamental types: well-sorted, single-lithology, matrix-poor conglomerates; and poorly-sorted, heterogeneous lithology (mixed rock structure), abundant matrix conglomerates. Well-sorted conglomerates were formed by eroded sediment rolled along into rivers, oceans, lakes and even glaciers and deposited slowly, layer by layer, over time. As a result, the unstable minerals contained within were eventually washed out. On the other hand, poorly-sorted conglomerates were distributed much more swiftly by earth movements, floods or slides and, therefore, contain large amounts of unstable minerals.
Constituent Minerals
Curiously enough, the definition of "mineral" is wider than might be expected. According to the Merriam-Webster Online Dictionary, a mineral is any inanimate, homogenous, naturally-occurring, solid compound that possesses a definite chemical composition and a crystal structure. Consequently, the number of potential minerals in a rock formed of anything pulled along by the movement of water, mud or glacial ice and lithified is extensive. However, conglomerates themselves are most often held together by quartz, but also can be cemented by feldspar, mica, silica, iron-oxide, calcium-carbonate, calcite, gypsum or clay.
Potential Uses
Because conglomerate rock clasts are so porous and loosely held together, the number of commercial uses for them isn't high. However, they are sometimes found in building materials and are utilized frequently for ornamental purposes in tiles, grave markers, monuments and as loose, decorative stones. Conglomerate rocks also are used in petroleum and natural gas reservoirs, as aquifers and as educational glimpses into the Earth's environmental past.
Curious Fact
Clusters of conglomerate rock can be excellent indicators of valuable placer ore deposits because they frequently contain large concentrations of heavy minerals such as gold.
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