Sandstone is one of the most common types of sedimentary rock, found all across the world. It is composed mainly of sand-sized silicate grains that have been cemented together over time. The most abundant minerals in sandstone are quartz and feldspar, which resist weathering and thus make up the sand grains that eventually form sandstone. These quartz and feldspar grains are worn away from other rocks, broken down into small pebbles by weathering processes, and then further ground down into sand-sized particles.
Sandstone varies widely in appearance, depending on the minerals it contains. Its color can range from white to yellow, orange, red, brown, or even green. The color depends on the sand grains as well as any impurities present. For example, iron oxide can give sandstone a reddish hue. The texture also varies from smooth to coarse. Sand with more uniform grain sizes produces a finer grained sandstone. The composition and texture both help geologists classify different types of sandstone.
Due to its abundance and durability, sandstone has been used by humankind since ancient times. Prehistoric people used it for basic tools and construction. Throughout history, sandstone has been employed in decorative works like monuments and sculptures. It remains an important building material today. The long history and utility of sandstone demonstrates that it is an enduring, useful rock.
How Sandstone Forms
Sandstone belongs to the clastic sedimentary rock family, meaning it is composed of pre-existing rock fragments and minerals. Specifically, sandstone consists mainly of sand-sized grains, typically of quartz and feldspar. These grains originate through the weathering and breakdown of other rocks.
The process starts when rocks are broken down into small grains through mechanical and chemical weathering processes. These sand-sized particles are then transported by wind, water, gravity, or ice to environments where they accumulate and are deposited as sediment. Typical depositional environments include beaches, river beds, sand dunes, and even glacial outwash plains.
Over long periods of time, usually thousands to millions of years, the quartz and feldspar sand grains are compacted and cemented together to form sandstone. The weight of overlying sediments squeezes the grains together, expelling air and water from pore spaces. Mineral cements like calcite or silica fill the pores and bind the grains, turning the sediment into solid sandstone rock.
Types of Sandstone
There are numerous varieties of sandstone based on differences in composition, texture, color, and other properties. Some major types include:
Based on Mineral Composition
- Quartz arenite: This type contains over 90% quartz grains and very little feldspar or rock fragments. It’s typically light-colored.
- Arkose: This is a feldspar-rich sandstone with a high feldspar content, more than 25%. The composition is similar to granite.
- Lithic/litharenite: This sandstone contains an abundance of rock fragments made up of pre-existing sedimentary, igneous or metamorphic rocks.
Based on Texture
- Well-sorted: Grains are very uniform in size and rounded. This indicates they’ve traveled far before deposition. Well-sorted sandstone is more porous.
- Poorly-sorted: There is a wide range of grain sizes and shapes. The grains did not travel very far before deposition.
Based on Color
- Red sandstone: Gets its color from the presence of iron oxide. Other colors like orange and brown also come from iron compounds.
- White/gray sandstone: Tends to be composed mostly of quartz grains.
- Black sandstone: Contains carbonaceous material or iron sulfides that give it a dark color.
- Green sandstone: Gets its color from grains containing glauconite, a greenish mineral.
- Crystalline sandstone: Cemented with crystalline minerals like calcite and gypsum.
- Conglomeratic sandstone: Contains abundent gravel-sized particles mixed with sand.
- Carbonate-cemented sandstone: Cemented by carbonate minerals like calcite.
Properties and Characteristics
In addition to mineral composition and texture, sandstone exhibits other defining physical and chemical properties:
Texture and Grain Size
- Smooth, coarse, gritty
- Grain size ranges from fine to very coarse
- Well-sorted vs. poorly sorted
- Controlled by composition of grains and matrix
- Common colors: tan, brown, yellow, red, grey
- Calcite, silica, iron oxides act as natural cements
- Cement type influences hardness and durability
Porosity and Permeability
- Varies greatly depending on cement and sorting
- Well-sorted and weakly cemented is more porous/permeable
Weight and Density
- Typically weighs 115-175 lbs per cubic foot
- Density ranges from 1.61–2.76 g/cm3
- Hardness varies significantly but commonly 6 or lower on Mohs scale
Uses and Applications
Thanks to its abundance and range of properties, sandstone has served humans since prehistoric times and remains an important rock today. Some of its core uses include:
- Building/construction: One of the main uses of sandstone. Used both for foundations and facades. Properties like strength and durability make it suitable.
- Decorative: Used for monuments, sculptures, fountains and other decorative constructions in parks, buildings, temples, etc. Easy to carve.
- Paving/flooring: Sandstone tiles and slabs used for patios, porches, sidewalks. Also used for interior flooring.
- Architecture: Used both historically and today in buildings. Some notable examples include churches and government buildings.
- Sharpening/abrading: The coarseness of some sandstones makes them ideal for sharpening tools and blades. Also used for grinding/abrading.
- Aquifers: Thanks to porosity, sandstone forms important aquifers providing water. Wells extract groundwater from sandstone aquifers.
- Natural gas/petroleum reservoirs: The porous nature also makes sandstone a crucial source rock for oil and natural gas resources.
- Gardening/landscaping: Used for products like stepping stones, fountains, gazing balls, and other decorative garden features.
How Sandstone Forms
As a sedimentary rock, sandstone requires a combination of weathering, erosion, transportation, deposition, and cementation to form. This multi-step process can be summarized as follows:
- Weathering: Rocks like granite break down into sand-sized particles due to mechanical weathering and chemical decomposition. Feldspar and quartz resist weathering.
- Transportation: Wind, water, gravity and ice carry the sand particles to new environments like beaches, rivers, or sand dunes.
- Deposition: The sand sediments accumulate in these environments in deposits that can be hundreds of feet thick.
- Compaction: New deposits compress older layers, squeezing out air and water. Compaction is driven by overlying weight.
- Cementation: Dissolved minerals precipitate out and fill the pore spaces, cementing the grains together into rock. Quartz and calcite are common cements.
- Lithification: The sediment is transformed into solid rock through compaction and cementation. Lithification occurs over thousands to millions of years.
- Uplift: Continued geologic forces can raise the sandstone, bringing it above sea level and re-exposing it at the surface.
Different source rocks, transportation methods, depositional environments, and cementing minerals all influence the resulting sandstone’s properties. These diagenetic processes create the range of sandstones we see today.
Interesting Sandstone Facts
Beyond its basic characteristics, formations, and uses, sandstone exhibits many other fascinating qualities:
- Sandstone sometimes preserves exciting fossils of prehistoric plants, animals, footprints, and environments. Rapid burial is key to fossilization.
- It has played a crucial role in human civilization since prehistoric times for construction and tool making.
- Landmarks like the White House, historic European churches, and Egyptian pyramids showcase sandstone’s enduring architectural beauty.
- Spectacular natural sandstone landscapes occur worldwide, from Utah’s Arches National Park to Australia’s Uluru.
- It is one of the main reservoir rocks for oil and natural gas resources, hosting huge deposits.
- Layered bands and striking colors seen in some sandstones make them favorites for decorative uses.
- Sandstone’s natural porosity makes it a great natural water filter. It cleans and holds water in underground aquifers.
- Special types like quartz arenite and arkose influence sandstone hardness, durability, and mineralogy.
- Its high silica content makes sandstone ideal for glass making and industrial silica uses.
Differences from Other Sedimentary Rocks
Sandstone belongs to the clastic sedimentary rocks along with conglomerate and shale. Here’s how it differs from other major sedimentary rock types:
- Shale: Much finer grained than sandstone. Shale is composed of silt and clay-sized particles.
- Conglomerate: Contains larger gravel-sized fragments rather than only sand-sized grains.
- Limestone: Made of calcium carbonate not silica grains. Forms from marine organisms not terrestrial weathering.
- Coal: Formed from plant matter, an organic sedimentary rock. Contains high carbon not silica.
- Chert: Microcrystalline quartz rather than sand-sized quartz grains. Metamorphic not sedimentary.
- Gypsum: A chemical sedimentary rock that forms by precipitation from mineral-rich waters.
Comparison of Sandstone with Other Major Sedimentary Rock Types
|Rock Type||Composition||Grain Size||Texture||Hardness||Color||Uses|
|Sandstone||Quartz, feldspar, lithic fragments||Sand-sized||Clastic||Variable, often 6 or less on Mohs scale||Tan, brown, yellow, red, grey||Construction, building, flooring, sharpening, aquifers|
|Shale||Clay minerals, quartz, feldspar||Clay and silt-sized||Laminated, fissile||1-3 on Mohs scale||Black, brown, grey, green||Structural fill, clay production|
|Conglomerate||Gravel, boulders, sand||Rounded fragments over 2 mm||Clastic||Variable, often hard||Red, grey, green, tan||Decorative stone, concrete production|
|Limestone||Calcite, aragonite||Varies greatly||Crystalline, clastic, organic||3-4 on Mohs scale||White, tan, grey||Construction, cement, sculpture, acid neutralization|
|Chert||Microcrystalline quartz||Cryptocrystalline||Massive, nodular, banded||7 on Mohs scale||White, grey, brown, yellow, red||Flint, silica source|
|Coal||Carbonaceous material, organics||Varies||Banded, organic||1-2 on Mohs scale||Black, brown||Fuel, adsorbent, water filtration|
|Gypsum||Calcium sulfate||Fine crystals||Crystalline||2 on Mohs scale||White, grey, brown||Plaster, fertilizer, drywall|
Frequently Asked Questions about Sandstone
What is sandstone made of?
Sandstone is made of sand-sized grains of minerals and rock fragments, typically quartz and feldspar. These grains are cemented together by minerals like calcite, silica, or iron oxides.
How is sandstone formed?
Sandstone forms through weathering, transportation, deposition, compaction, and cementation of sand grains. Sediments accumulate and are buried, compressed, and cemented into solid sandstone rock over long time periods.
What color is sandstone?
Sandstone can be tan, brown, yellow, red, grey, or even greenish in color. The colors come from the composition of the grains and any impurities present, like iron compounds.
What is sandstone used for?
Sandstone has many uses including building/construction, decorative architecture, monuments, flooring, sharpening tools, and providing natural resources like water aquifers.
How old is sandstone?
Sandstone can range greatly in age from thousands to hundreds of millions of years old, depending on when and how it formed geologically. Much sandstone dates to prehistoric times.
Is sandstone porous?
Yes, sandstone porosity and permeability vary but sandstone is generally porous due to tiny spaces between the sand grains. However, denser cement can make some sandstone less porous.
Where are some famous sandstone places?
Prominent sandstone sites include Arches National Park, Uluru/Ayers Rock, Petra in Jordan, and Machu Picchu in Peru. Britain also has many historic sandstone structures.
What plants grow in sandstone?
Acid-loving plants like heather, blueberries, and rhododendrons grow well in sandstone, which weathers into acidic sandy soils. Conifers also thrive in sandstone areas.
Can you find fossils in sandstone?
Yes, sandstone sometimes preserves remarkable fossils of organisms that were buried quickly during its formation from loose sediments.
In summary, sandstone is one of the most abundant sedimentary rocks, found across nearly every continent. Its unique physical and chemical characteristics come from the combination of quartz/feldspar sand grains and mineral cements that bind it together. Diagenetic processes ultimately transform loose sediment into solid sandstone over long time periods.
Thanks to its prominence and utility, sandstone has long been utilized by human civilizations for construction, art, and tools. It remains an important rock today for buildings, flooring, landscaping, and more. Sandstone’s porous nature also makes it a valuable natural resource as aquifers and petroleum reservoirs. There are many different classifications and types of sandstone depending on origins and properties. Its diverse applications demonstrate that sandstone is a versatile, attractive, and useful rock.