Minerals and Rocks: Rocks and their weathering products are main components of the inorganic part of the earth . A rock, by definition, contains one or more minerals, and a mineral is defined by Hurlbut (1970) as follows:
“In addition to being natural and inorganic, a mineral must meet another requirement: it must be a chemical element or compound. It cannot be a random mixture of elements; the atoms that make it up must have definite ratios to each other, so that its composition can be expressed by a chemical formula. Not only are the proportions of the various atoms of a given mineral fixed, but so are their relative positions. These attributes give to each mineral a set of properties that characterize it so uniquely that one can distinguish it from all other minerals”.
A rock can be composed of one mineral or a mixture of several. For example, sandstone may contain grains of the mineral quartz (silica or SiO2), and a cement between the quartz grains composed of the mineral calcite (CaCO3). Granite is normally composed of crystals of the minerals feldspar, quartz, mica, and others. By definition then, frozen water (ice) is both a mineral and a mono-mineralic rock. The most fundamental of the three classes of rocks are igneous rocks, which are formed as cooling products from the molten state. Igneous rocks are primordial in many parts of the world and are some of the world’s oldest, exceeding three billion years in age. One way they can be formed is when molten rock is intruded into other rock formations and then cooled to the solid state. If the cooling is slow enough, various minerals will crystallize into an interlocking solid mass that is characteristic of particular rocks such as granite.
Extrusive igneous rocks like dense basalt are forced from fissures in the earth’s crust and harden into vast sheets of solid material, usually containing very small crystals (due to the rapid cooling), or perhaps no crystals at all (obsidian glass). Other extrusives like lighter lava or pumice are ejected during volcanic eruptions and are highly charged with gases to form very porous and even frothy glasses resembling a sponge. Some of the ejecta may fall from the air to settle as a sediment. This particular kind of deposit is known as a pyroclastic rock, i.e., both igneous and sedimentary.
A sedimentary rock, the second class, is deposited from either air or water as grains of rocks and minerals. These sediments in turn may have been derived from the weathering of igneous, metamorphic, or other sedimentary rocks.
If any kind of rock, igneous, metamorphic, or sedimentary, is subjected to intense heat and pressure, such as exists at great depths in the earth’s crust, at the edge of tectonic plates, or in rising mountain ranges, the parent rock will be transformed into the third class of rock — a metamorphic rock. A metamorphic rock may contain the same chemical composition as the parent rock, but the mineral composition and structure may be changed drastically from the parent. For example, limestone containing amorphous or cryptocrystalline calcite (CaCO3) is often metamorphosed into marble that has a definite crystal structure and is much harder than the original limestone. Granite may be metamorphosed into gneiss with the same overall chemical composition as the original granite, but with new minerals and mineral structures.
Groundwater can be found in all three classes of rocks, but in general, the sedimentary rocks contain by far the greatest amounts of water due to their greater porosity.
Unconsolidated materials are those earth materials which have not been indurated. That is, the grains have not been fused together by heat and pressure, as in the cases of igneous granite or metamorphic gneiss; or cemented together, as in the case of sedimentary rocks. Unconsolidated materials can be the non-indurated products of weathering of all three classes of rocks, or sediments laid down by running water, ponded water, the sea, or ejecta from volcanoes.
Most unconsolidated materials are young, geologically speaking, and are at or near the earth’s surface. Thus, they have not been exposed to pressure, heat, and migrating cementing fluids long enough to become consolidated or hard. Hence, they generally have high porosity and are the sources for much groundwater.
A common unconsolidated deposit in glaciated areas of the world is glacial drift. It is any kind of earth material that was deposited directly by glaciers or by meltwater from glaciers. As such, it can range in size from the finest silt to the largest boulders, and can be mixtures of all sizes. The name “drift” was given to this material when it was believed that it was depositional material, or “drift,” from the great Noachian Flood, described in the Book of Genesis. Drift can generally be subdivided into the more specific lithologies such as till and outwash.
Glacial till is a generally heterogeneous mixture of many different lithologies and particle sizes. Typically in the midwestern U.S., till contains a preponderence of clay and silt with additional amounts of groundup rocks and boulders that may vary in size from small pebbles to erratics the size of a house or larger. On rare occasions, geologists find tills composed of one lithology, indicating local sources of material. Glacial till is not generally utilized as a source of groundwater because of its low permeability.
Glacial outwash is material deposited from high-energy streams of water that originated from melting glaciers. This process can be seen today at the toe of any mountain glacier on different continents.
Glacial ice, more often than not, contains entrained rock and soil material which it has eroded from the surrounding valley sides or the ground beneath it. This material varies widely in size from clay to large boulders. As the ice melts and leaves the toe of the glacier, normally in great flow rates and very turbulent, this material is moved with the flowing water. As the stream loses energy, materials settle out, with larger material coming out first, followed by gradually smaller material. Therefore, along a stretch of an outwash stream one may find coarse gravel and boulders settling out first, followed by finer gravel, then sand and gravel mixtures, then sand, silt, and finally clay (in still water).
Many present-day stream courses were glacial spillways for outwash water and sediment during the Pleistocene Epoch (Ice Age). For example, the Wabash River Valley in Indiana contains outwash sand and gravel deposits in excess of 300 feet in thickness which were deposited by the melting of two and possibly three different ice sheets. The Big Sioux River valley in South Dakota and Iowa is another example. A very extensive deposit of outwash sand is the Cape Cod peninsula which was deposited in an interlobate outwash between two lobes of ice — one to the east and one to the west of the site. Outwash sand and gravel deposits are frequently exploited for groundwater because of their high porosity and permeability. Well yields in many of these deposits often exceed 5500 m3/d.
Other water-lain deposits not directly deposited from glacial meltwater may be comprised of reworked glacial detritus (glacially transported material), or they may be found in areas where glaciers never occurred. The most common example of such deposits is alluvium. Flood plains along large streams are created of this material as the streams flood over their banks and deposit the material. Stream beds also contain alluvium.
In fast-flowing streams with high gradients, as in mountainous areas, alluvium may be absent because the stream is eroding rather than depositing material. If alluvium is found in and along such streams, it is generally very coarse-grained gravel with large boulders. On the other hand, mature streams such as the Ohio, Missouri, and Mississippi Rivers, deposit their loads of fine silt and clay over broad flood plains.
Alluvium deposits may serve as important groundwater sources, but in large river valleys, the yield of such deposits may be low to moderate, depending on the grain size and the resulting permeability and
Lacustrine materials are silts and clays that are deposited from relatively still bodies of water such as lakes and lagoons. This material, being so fine-grained, is not utilized for groundwater supplies because of its low permeability.
Peat is the remains mostly of water plants that die and accumulate in ponded water and marshes over long periods of time. The top part of a peat deposit is very porous and permeable, but it becomes more compact with depth. The lower layers of peat are often sticky masses of black organic material with little resemblance to the original plant material.
Peat is not generally utilized as a groundwater source, but it can serve, under the right conditions, as a natural cleansing agent to remove organics and heavy metals from water that passes through it — the large and complex organic molecules in the peat attract such contaminants.
Chemical precipitates of most importance include limestone and marl which precipitate directly from sea water or even from fresh water bodies. Major deposits of limestone were deposited in many parts of the world during the Cretaceous Period (the “Chalk Period”) of the Mesozoic Era. Examples include the Chalk Beds of Dover, England; the limestones of the Balkan Peninsula; and limestones of the High Plains in the United States. Other vast limestone deposits were formed in earlier times and are found across most of the midwestern U.S. and the Appalachians.
Limestone is composed primarily of calcite. Entrained silt and clay and other materials may also be present. After deposition, a process known as diagenesis often takes place in which the rock incorporates magnesium to become dolostone, or CaMg(CO3)2; this is also the formula for the mineral, dolomite. Pure MgCO3 is the mineral, magnesite.
Collectively, limestone and dolostone are called “carbonate rocks,” or “carbonates.” Carbonates, especially limestone, often undergo solution along bedding planes and fractures to form caves and sinkholes, which in an interconnected system, is known as karst terrane, after the Karst region in Yugoslavia. Networks of such caves and tunnels may exceed hundreds of miles in length and may contain large streams which emanate from springs in the rock.
Examples of such systems are found in the Balkan Peninsula, the Mediterranean area, France, Kentucky (Mammoth Caves), and New Mexico (Carlsbad Caverns), to name just a few. Karst systems are often sources of very large quantities of groundwater, and due to the very high permeability, can be productive aquifers. A drawback, though, is the ease with which water in karst systems can be contaminated by surface sources. Thus, care must be taken to protect such sources.
Aeolian deposits are fine-grained materials, such as silt and sand, which may have been deposited originally from water, but which have been reworked and redeposited by wind. Examples of such active deposits today can be found in sand-dune areas of the Sahara, the Middle East, New Mexico, Nevada, and many other places.
Ancient dunes from the geological past are often found as sandstone bodies and may have some potential for groundwater extraction if coarse enough to allow sufficient porosity and permeability. Generally, wind-blown deposits are fine-grained, and when cemented with precipitates from circulating groundwater, may possess low porosity and permeability. The finest-grained aeolian deposits are composed of silt or “rock flour” known as loess. The silt source is usually a wide river bed with braided channels where large dry areas of fine-grained materials are exposed to the wind. In most cases, the silt was deposited in such rivers as the end product in the long chain of deposition of glacial outwash. Prevailing winds then pick up the silt and transport it downwind where it is deposited on the lee sides of river valleys.
Significant loess deposits are found on the east side of the Missouri River, which acted as a glacial spillway during the Pleistocene. North of Sioux City, Iowa, this material forms bluffs which are tens of meters high. Other noteworthy deposits are found along the Mississippi River (another spillway) and in the Gobi Desert of China.
Loess, after deposition, will often be reworked by frost action to form columnar structures with vertical fractures. The grains of silt are then oriented with their long axes vertically to form such features. With this alteration, it will allow fast vertical movement and drainage of water and is fairly solid material to build upon, but its permeability is too low to utilize it for a groundwater supply.