Sandstone

Background |  Name |   Sources |  Uses |  Substitutes and Alternative Sources

Background

Sandstone is a sedimentary rock composed of mostly of quartz sand, but it can also contain significant amounts of feldspar, and sometimes silt and clay. Sandstone that contains more than 90% quartz is called quartzose sandstone. When the sandstone contains more than 25% feldspar, it is called arkose or arkosic sandstone. When there is a significant amount of clay or silt, geologists refer to the rock as argillaceous sandstone. Argillaceous sandstones are often gray to blue and consequently are referred to as bluestone. Because it is composed of light colored minerals, sandstone is typically light tan in color. Other elements, however, create colors in sandstone. The most common sandstones have various shades of red, caused by iron oxide (rust). In some instances, there is a purple hue caused by manganese.

Sandstone began as large deposits of beach or river sands that were later compacted and lithified (“turned into stone”). The grains of sand of which sandstone is composed is the mineral quartz (SiO2). The quartz grains came from the weathering and erosion of igneous rocks, particularly granite, that have high amounts of quartz (granite is an intrusive igneous rock composed of feldspar, quartz and biotite mica).

Sandstone is deposited by water or air and therefore can represent a number of different geologic environments. In many cases, the sand was deposited in shallow lakes or oceans, and beach environments. In others, the sands were deposited by large rivers and therefore represent an inland river environment. Many are deposited in deltas where rivers empty into oceans. Some sandstones were deposited in ancient desert environments by blowing winds.
Sandstone is a detrital sedimentary rock. This means that it is composed of the weathered fragments of other pre-existing rock. In most cases it is stratified, that is, deposited in layers. The layers often run parallel to each other. This is typical in lake and ocean deposits of sand. Rivers and deserts, however, represent environments in which the direction of the water or wind can change regularly. As a result, the layers of sand are deposited in different directions, always at an angle. This creates a special sedimentary structure that geologists call cross-bedding. The direction in which the beds of sandstone dip indicates the direction in which the water or wind was moving at the time of deposition.

Sandstone is a very significant aquifer. An “aquifer” is a rock body that has a high degree of porosity (which means it has a large volume of space between the individual grains of which the rock is composed) and a high permeability (which means the spaces are connected so water can move through the rock). The Ogallala Sandstone, for example, is an immense aquifer (it is called both the Ogallala Aquifer and the High Plains Aquifer) that lies beneath the Great Plains of the Midwestern United States. It covers approximately 174,000 mi2 and is found under portions of South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, Texas and New Mexico. Today the Ogallala Aquifer provides 12 billion cubic meters of water each year. Unfortunately, more water is being removed than is going back into this aquifer (a process called replenishing.)

Name

Sandstone has two major applications, as crushed stone and as dimension stone. “Dimension stone” is any rock material that is cut into specific sizes, typically as blocks and slabs. Dimension stone is used in the construction of roadways and road structures such as bridges, and in buildings, both commercial and residential.

Crushed Sandstone - Crushed sandstone represents less than 6% of the total tonnage of rock quarried and crushed for different construction needs in the United States. Approximately 1.7 billion tons of crushed rock is produced in the United States annually. Therefore, less than 102 million tons of crushed sandstone is produced in the U.S. each year. The majority of this sandstone is used in road and highway construction and maintenance.

Resources for crushed stone in general, and crushed sandstone specifically, are enormous. The world supply will likely never be depleted.

Dimension Stone - Of all the dimension stone quarried in the United States (which includes rock such as sandstone, marble, granite, limestone, and slate), sandstone represents 13% of the 1.5 million tons used annually. 195,000 tons of sandstone is quarried for use in construction every year. Even more is imported from other countries. Resources for sandstone as dimension stone, both in the United States and worldwide, are more than adequate to meet projected needs for the near and distant future.

Uses

Substitutes and Alternative Sources

There are a number of alternative materials that could be used in place of sandstone. As crushed rock, any number of alternative rock materials can be used in road construction. For example, limestone, granite, slate and other rock materials are plentiful and easily accessible. Where there is a need or desire to recycle materials, steel slag and glass slag can be crushed and used in road construction, eliminating both the need to quarry fresh material and to dispose of the slag. As dimension rock, bricks, ceramic tiles, concrete, and resin-agglomerated stone can replace sandstone. (“Resin-agglomerated stone” is a material composed of crushed pieces of stone held together by resin, then cut to the dimensions and shapes needed for each application.) Aluminum, steel and some plastics can also be used in place of sandstone, depending on the application and properties required of the material for that application.