A Guide to Basalt: A Common, Fine-grained, Dark-Colored Volcanic Rock

Basalt is a hard, fine-grained, dark-colored volcanic or extrusive rock. It has primarily calcium-rich plagioclase and augite. Also, it may have minor amounts of other minerals like quartz, olivine, foids, alkali feldspar, biotite, and amphibole.

This rock is undoubtedly popular, only rivaled by granite. Some reasons for its popularity are its relative abundance and many uses. Did you know that Basalt is the most common igneous rock on the Earth’s surface?

It dominates the ocean’s crust, which covers about 70% of the Earth’s surface. Also, basalt dominates oceanic islands and is widespread on continental surfaces, including large igneous provinces (LIPs), with over half of the world’s volcanoes being entirely or majorly basaltic.

Besides Earth, basalt also occurs on the moon, Mars, Venus, and asteroids. Perhaps it occurs on other planets, too. For now, we don’t know.

In this discussion, we will start by looking at basalt’s appearance (texture, and color) which is essential in its identification. Then we will give you its chemical and mineral composition.

Afterward, we will look at its varieties for those who want to go beyond the basics. Also, there is a part on how it forms, erupts, where it occurs, and some of its uses.

Quick facts and properties

• Name: Basalt
• Rock type: Igneous
• Origin: Extrusive
• Texture: Fine-grained or aphanitic with grain sizes less than 1/16 mm but may be porphyritic, vesicular, subophitic, ophitic, or amygdaloidal.
• Colors: Dark gray,  dark green-gray, greenish black to black.
• Cooling rate or history: Fast on the Earth’s surface
• Chemical composition: Mafic or basic
• Silica content: 45-52 wt.% SiO₂
• Density: 2.9 g/cm³
• Mohs hardness scale: 6-7
• Porosity: 0.10 – 1.0 % (source)
• Compressive strength: 266±98 MPa (source)
• Thermal conductivity: Poor, 1.672 Wm^-1K-¹ at 20°C to 200°C  
• pH:  7 to 9.5, varies with variety and composition (source)
• Melting point: 984° to 1260° (source)  
• Basaltic lava viscosity: Low but depends on temperature and amount of phenocrysts present. Typically, molten and flowing magma or lava will have a viscosity < 10⁴ Pa·s.
• Eruption temperature:  1100° and 1250°C
• Coarse-grained or intrusive equivalent: Gabbro, However, gabbro has a highly variable mineral content. 
• Metamorphic form: Greenschist, amphibolite, or eclogite, depending on temperature and pressure.
• Tectonic setting. All. Divergent (mid-ocean ridges and continental rifts), convergent ( island and continental margin subduction zones), and hotspots (oceanic and continental

What is basalt, and what does it look like?

Basalt rock is fine-grained, dark-colored volcanic rock with essentially Mg-rich plagioclase and augite. It is basic or low in silica (42-52 wt. %) mafic (Mg- and Fe-rich) rock, with relatively low amounts of total alkalis (Na₂O and K₂O).

To identify this rock in the field, consider its colors and texture. Basalts are usually dark colored, i.e., dark gray, brown-black, greenish-black, or black fine-grained rocks. However, they can have other textures and colors.  

Also, weathered hand specimens may have a tan, yellow, brown, or reddish-brown surface. Thus, try to get one with a freshly cut surface.

1. Colors

Since it is relatively high in ferromagnesium minerals, basalt is dark gray, brown-black, greenish-black, or black with a typical color index M > 35. However, some, like the leucobasalts, may be lighter colored. Such will have M < 35.

Also, others like the basalts or olivine basalts traded as bluestones in Victoria, Australia are gray to dark bluish gray to blue-black.

Lastly, a given specimen’s color depends on the various minerals present. Also, some textures like porphyritic or amygdaloidal may impart color variations. For instance, olivine phenocrysts will appear greenish.

2. Textures

Basalt rocks often have a fine-grained (holocrystalline aphanitic with minimal glass) with some porphyritic texture. Those fine-grained without phenocrysts are known as aphyric basalts.

Besides these textures, basaltic rocks may be vesicular, amygdaloidal, subophitic to ophitic, and rarely hypocrystalline (a crystal-glass mixture with more glass than crystals).

Here is more about the various textures.

i. Vesicular basalt

Vesicular basalt is characterized by voids, holes, pits, or cavities known as vesicles, making it very porous. This texture forms when exsolved expanding and escaping gas bubbles are trapped or frozen.

This vesiculation happens during the eruption. As magma rises, pressure drops, resulting in the exsolution and expansion of volatiles or gas bubbles.

Some explosive eruptions involving volatile-rich lava may form highly vesiculated rock known as scoria (scoriaceous).

 ii. Porphyritic basalt

Porphyritic basalt has large crystals or phenocrysts embedded in a fine-grained matrix or groundmass. Usually, it has plagioclase, augite, olivine, and less often hornblende phenocrysts or Ilmenite micro-phenocrysts. These phenocrysts are set in fine-grained plagioclase, augite, magnetite, pigeonite, and enstatite matrix that may have some glass.

This texture indicates a two-stage cooling history. Slow cooling step deep in the crust with modest supercooling that favors larger crystal growth over nucleation and faster near or on the surface that forms groundmass.

Sometimes, plagioclase or pyroxene phenocrysts may occur in clusters in groundmass to form glomeroporphyritic basalt. Such crystals enjoy low crystal-melt interfacial energy compared to dispersed ones.

iii. Amygdaloidal basalt

Amygdaloidal basalt texture forms when secondary minerals fill present vesicles and fissure after the rock has cooled. It is associated with low-grade metamorphism (low-temperature alteration) or hydrothermal mineral phases.

Such rocks will have patches and speckles of secondary minerals (white, creamy, or other colors) in dark gray to black rock. Common minerals in amygdule basalts include zeolites, amethyst, calcite, datolite, pumpellyite, pectolite, axinite, aragonite, apophyllite, pumpellyite, chlorite, glauberite, glauconite, and prehnite.

Some of the zeolites in basalts are analcime, mesolite chabazite, natrolite, thomsonite, stilbite, mordenite, phillipsite, laumontite, and heulandite.

Sometimes, these amygdules may host valuable minerals and gemstones. For instance, Keweenaw amygdaloidal basalts from Portage Lake Volcanic Series, Michigan, USA, have economically viable copper. The copper occurs as vesicle and fissure-filling minerals due to hydrothermal activity. However, those with gold are rare.

Basaltic geodes are hollow, nearly spherical rock structures whose inner surfaces have mineral masses or crystals. Their outer durable wall makes them remain intact as the surrounding rock weathers.

These geodes may host various minerals, including calcite, pyrite, hematite, amethysts, barite, etc.  

v. Subophitic and ophitic basalt

Ophitic basalt has pyroxene and sometimes olivine enclosing plagioclase laths (labradorite or bytownite).

on the other hand, subophitic occurs when olivine or pyroxenes partially surround plagioclase laths.

These two are types of poikilitic texture and involve a larger (oikocrysts) mineral enclosing smaller different minerals (chadacrysts). They form in eutectic intergrowth where oikocrysts have fast crystal growth, and the enclosed different minerals have higher nucleation rates.

Chemical composition

Chemically, basalt is a silica-poor or basic rock with 45-52% SiO₂. It is relatively high in CaO, MgO, and iron oxides, poor in total alkali (Na₂O and K₂O), and has at least 14% Al₂O₃.

A typical percentage weight composition of basalt from Le Maitre (1976) data is 50.84% SiO_2, 2.33% TiO_2, 14.98% Al₂O_3, 4.08% Fe₂O_3, 8.32% FeO, 0.18% MnO, 7.53% MgO, 9.59% CaO, 1.75% Na₂O, 0.22% K₂O, and 0.16% P₂O_5.

Data from various analyses puts an average weight composition of basalt rock as SiO₂: 45–52%, TiO₂: > 2%, Al₂O₃: > 14%,  iron oxides: 5-15%, MgO: 5-12%; MgO: ~10% and Na₂O + K₂O: < 5%.

Basalt rock mineralogy

Basalt is a mafic volcanic rock with primarily calcic plagioclase feldspar and augite. It may also contain smaller amounts of olivine, enstatite, quartz, feldspathoids, alkali feldspar and less often hornblende (amphibole) and biotite. However, this depends on the variety or type.

Accessory minerals and opaque in basalt include chromite, ilmenite, magnetite (titanomagnetite), apatite, and spinel (ulvöspinel).

Calcic plagioclase (usually labradorite with anorthite > 50 mol%) is the main felsic mineral but this rock may have a smaller amount of alkali feldspar. Also, this rock may have minor amounts of quartz, glass, and feldspathoids like nepheline, sodalite, leucite, or analcime which are often interstitial.

On the other hand, augite (clinopyroxene) is the dominant pyroxene mafic mineral, subordinated by calcium-poor enstatite (orthopyroxene) and pigeonite (clinopyroxene).

Also, this rock may have a small amount of olivine. However, olivine may be essential in some varieties, like olivine basalts. Also, some olivine nodules have diopside.

Lastly, micas (biotite) and amphiboles (hornblende) are rare since the higher crystallization temperatures allow the formation of only anhydrous minerals.  

1. Secondary minerals

Basalt may also have secondary minerals, i.e., not crystallized from magma like celadonites, actinolite, prehnite (well-developed in Deccan Traps), calcite, pumpellyite, apophyllite, zeolites, etc. Such results from low-grade metamorphism or hydrothermal fills.

Also, peridot nodules from the upper mantle may occur in basaltic rocks with pyrite (fool’s gold) rare. However, it may form via the substitution of pyrrhotite during contact metamorphism.

Lastly, typical alteration and low-grade metamorphism of basalt include:

• Olivine → iddingsite or serpentine
• Plagioclase → sericite,
• Pyroxene → chlorite or uralite
• Various glass or minerals → smectite, a clay group mineral including nontronite.

2. QAPF and TAS classification

On the QAPF diagram of volcanic rock (based on relative volumes of quartz, alkali feldspar, plagioclase, and feldspathoids), we define basalt as a volcanic rock in which plagioclase is more than 65% of total feldspars and has less than 10% feldspathoids and less than 20% quartz of total QAPF content by volume. It plots in the orange region.

Based on the QAPF diagram, basalt plot on the same area as the andesite. However, it has less than 52 wt. % silica vs 52%-63 wt.% of andesite. Also, it is darker (has more mafic minerals) than andesite. The typical color index for basalt is > 35 and < 35 for andesite

However, leucobasalt has < 35 vol. % mafic, just like mela-andesite; thus, silica content is one way to differentiate these two.

Since it is fine-grained volcanic rock, it is impossible to determine basalt mineralogy in the field. Thus, geologists use a TAS classification that considers the content of Total alkali and silica. In this classification, basalt has 45-52wt.% and less than 5% total alkali oxides (Na₂O + K₂O).

Basalt classification types or varieties

Like all rocks derived from magma, basaltic rocks form a continuous compositional spectrum. Thus, classification is only an artificial attempt to subdivide the otherwise continuum composition.

The common types or varieties of this rock are:

1. Tholeiitic basalt

Tholeiitic basalts are extremely fine-grained quartz-saturated or oversaturated rocks relative to alkalis (Na₂O and K₂O).

These rocks are poor in alkali metals and less enriched with aluminum. Also, they have fewer incompatible minerals, relatively higher silica 48–52%, and iron than alkali basalts. Oceanic varieties have > 2.5% TiO₂, while continental have about 1%.

Like others, these tholeiitic rocks are porphyritic. Their groundmass has calcium-poor pyroxenes like enstatite (orthopyroxene), pigeonites, and normative quartz. Also, interstitial glass is common, but they have no alkali feldspar or olivine in groundmass. 

On the other hand, phenocrysts include clinopyroxene, plagioclase, and olivine with orthopyroxene, which are uncommon. Clinopyroxene is colorless to pale brown since they are poor in iron, and plagioclase forms early.

Olivine tholeiitic basalts are abundant in olivine, which may show resorption or reaction to pigeonite or enstatite.

Going to formation, tholeiitic basalts form from the tholeiitic magma series and are associated with silica-rich trachytes and andesites.

Where do they occur? These rocks comprise most of the oceanic crust and some large igneous provinces (LPIs – oceanic and continental). Also, they occur in the mid-ocean ridge, some large, layered intrusions, and on the moon.

Lastly, we can divide these rocks into high and low titanium as distinguished by Paraná-Etendeka traps and Emeishan Traps.

2. Alkali basalts

Alkali basalts are richer in alkali oxides (Na₂O and K₂O) and poorer in CaO and iron than tholeiitic or other basalts.

Also, they are lower in silica (46-48%) since they crystallize alkali feldspars like albite, which takes more silica than calcic feldspars. Thus, they effectively deplete silica and will have interstitial normative nepheline or alkali feldspar.

Alkali basalts are characterized by olivine and titanium-rich augite phenocrysts in the groundmass. Also, they may have phlogopite and kaersutite. However, they may lack orthopyroxene phenocrysts, and plagioclase is less common and usually appears at a later crystallization stage.

Also, augite phenocrysts are pleochroic due to small amounts of ferric iron and titanium, while olivine phenocrysts are zoned.

Their groundmass often has olivine and interstitial feldspathoids or alkali feldspar. However, they have no quartz, and orthopyroxenes or interstitial glass are rare.

Lastly, alkali basalts occur in oceanic islands associated with volcanism, like Hawaii, on continental rifts and other volcanic fields. Their magma evolved towards nepheline-bearing rocks like phonolites and nepheline syenites.

3. Picrobasalt  or picrite

Picrite basalt is an Mg-rich olivine basalt high in the yellow-green olivine phenocrysts (20-50% by volume). It has less silica (< 45%, i.e., ultramafic) and may have opaques and interstitial glass.

Picrobasalt usually occurs in tholeiitic magma series, especially in Deccan traps in India and Kīlauea volcano in Hawaii, USA

4. Trachybasalt

Trachybasalt is a fine-grained mafic rock whose mineralogy resembles basalt but is higher in alkalis (Na₂O and K₂O). This volcanic rock has calcic plagioclase, augite, alkali feldspar, and olivine with minor amounts of feldspathoids, i.e., analcime or leucite.

5. High alumina basalt

This rock has an intermediate composition between alkali and tholeiitic basalts and > 17% aluminum oxide based on rocks without plagioclase phenocrysts.

It may have phlogopite, plagioclase, olivine (rimmed or not rimmed by pyroxene) phenocrysts, and about 1% TiO₂. Usually, pyroxene in this rock is augite, rarely pigeonite or enstatite.

Lastly, it forms from silica-poor calc-alkaline magma series and occurs in mostly volcanic arcs.

6. Silica-saturated or undersaturated

An alternative scheme is classifying basalts into 1) silica oversaturated with quartz, 2) silica saturated without quartz or feldspathoids but with olivine and enstatite, and 3) silica undersaturated with feldspathoids.

In silica undersaturated, silica isn’t enough to convert all Na₂O and K₂O to orthoclase or albite. Thus, feldspathoids replace some or all feldspar.

7. More types

• Olivine basalt: It contains a substantial amount of olivine, i.e., as an essential element.
• Transitional basalts: Their composition lies between tholeiitic and alkali basalts. These rocks have ca-rich augite, plagioclase, titanomagnetite, olivine, and a varying amount of alkali feldspar. Also, such rocks don’t have calcium-poor pyroxenes and are associated with peralkaline trachyte and rhyolite.
• Subalkali basalt: Represents many varieties without normative nepheline, including high alumina, MORB, tholeiitic, and transitional.
• Primitive basalt: It forms from melts that haven’t undergone evolution/differentiation.

How is basalt formed?

Basalt rocks form when basaltic lavas cool quickly close to or on the Earth’s surface (ocean or land). The fast cooling favors more nucleation over crystal growth, creating a fine-grained textured rock.

Usually, once magma arrives on Earth’s surface, it will take several days to a few weeks to form these rocks, depending on thickness. Should this lava cool superfast it will form basaltic glasses like tachylyte or sideromelane.

Lastly, basaltic magma originates from mantle rock partial melting, i.e., ultramafic or peridotite rocks. The partial melting occurs via 1) upwelling and decompression of the asthenosphere, 2) mantle plumes, and 3) less often from flux melting at subduction zones. Heat conduction partial melting will produce andesite or rhyolite.

Basaltic eruption

Basalt eruptions occur at a relatively higher temperature of 1100°C to 1250°C. Most are effusive, a few explosives, and others start explosively and become effusive.

Furthermore, most of these eruptions happen underwater (mid-ocean ridges and seamounts) and a considerable amount of subaerial on island volcanoes, intracontinental rifts, and other hotspots

Let us look at these two eruptions.

1. Effusive eruptions

The low viscosity of most basaltic magma allows gas escape and inhibits lava fragmentation, favoring effusive eruptions. These eruptions may be submarine or subaerial.

Submarine basaltic effusive eruptions will produce mostly pillow lava and, less often, sheet flow and lobate lava. These eruptions are often accompanied by pillow breccia and hyaloclastites.

On the other hand, those on land (subaerial) will result in pahoehoe and aa lava flow. Some enormous effusive eruptions from a series of fissures may form thick and extensive lava deposits known as flood basalts or plateaus.

These thick deposits may exhibit nearly hexagonal cracks known as columnar joints or basalt columns. Examples are Devil’s Postpile in California, USA, and Giant’s Causeway in Ireland.

2. Explosive eruptions

Explosive basaltic eruptions are less common because high temperatures and low viscosity inhibit pressure buildup. However, they can happen, especially Plinian, or low strombolian. Also, a Surtseyan eruption type may occur when molten lava meets underground or surface water.

Examples of explosive basaltic eruptions are Sunset Crater (Arizona, USA 1085), Eyjafjallajökull (Iceland, 2010), Mt. Etna (Italy, 122 BCE and 1669), Mauna Loa (Hawaii, USA 1843), and Mount Tarawera (New Zealand,1886).

Usually, explosive eruptions will happen when ascending volatile-rich (CO₂ and H₂O) produce a pyroclastic plume of ash, lapilli, bombs, and blocks (including from country rock).

If the pyroclasts cool in flight during these eruptions, they will form cinder cones with scoriaceous rocks like scoria, ash, and other clinkers. The burial and cementation of these pyroclasts will form basalt breccia.

However, you will have a basalt tuff if the pyroclasts fall and consolidate before fully solidifying during explosive eruptions.

Also, you can have maars and a tuff ring with low pyroclastic cones if the lava interacts with surface/underground water.

Lastly, explosive basaltic eruptions may form Pele’s tears and Pele’s hair. These two are common in Hawaii, USA.

Where is basalt found?

Basalt is the most abundant rock on the oceanic crust (0.5–2 km thick). It is also found in oceanic islands associated with volcanism, large igneous provinces (flood basalt), and intraplate continental rifts.

Other places with this rock include volcanic arcs (island and continental arcs but it is less abundant here), greenstone belts, and ophiolites.

Usually, this rock occurs as lava flows, dikes, sills, and in some large, layered intrusions or on some cinder cones. Also, it may occur in shield volcanoes or be a part of stratovolcanoes and central volcanoes together with silica-rich rocks like andesite, dacite, and rhyolite.  

In the USA, basalt is widespread in Hawaii, Snake Plain River, and Columbia Basalt. Also, notable deposits occur in California, Minnesota, Texas, New England, and many other states.

Did you know Lake Superior has basalt and other volcanic rocks 55 kilometers deep into the Earth’s crust? Get more and the exact location where this rock occurs in the USA at Mindat.org.

Let us look at the various basalt tectonic environments giving you examples.

1. Mid-ocean ridge basalts (MORBs)

Mid-ocean ridges create most basaltic rocks and new oceanic crust and are dominated by pillow lava, with some having sheet flow and lobate lava. Examples are the East Pacific Rise, Mid-Atlantic Ridge, and Gakkel Ridge.

MORBs are mainly sub-alkali and tholeiitic basaltic rocks, especially olivine tholeiitic with olivine phenocrysts with or without augite, chromite, or plagioclase. Also, they have smaller amounts of alkali types.

However, some MORBs may undergo hydrothermal metamorphism, with some minerals replaced by chlorite, carbonate, albite, or epidote.

Lastly, MORBs are low in incompatible elements. These incompatible elements vary from depleted to normal to enriched. Also, they may be Low-K, Low-Ti with Mg-olivine, titanomagnetite, and some may have pale brown glass, pigeonite, or ilmenite.

2. Island Ocean basalts (IOBs)

Island oceans (intraplate volcanic islands and seamounts) have overwhelming basaltic rocks, mainly subalkaline or tholeiitic. Also, they have fewer alkali varieties, including olivine-bearing ones, never calc-alkaline. Some may have basanite and nephelinite.

Examples are Hawaii, Easter, Reunion, Canary, and the Azores Islands. However, unlike MORBs, they are incompatible element enriched, pointing to a different magma source, i.e., a non-depleted kind.

3. Large igneous provinces (LIPs)

Large igneous provinces represent the intraplate enormous outpouring of lava, usually basaltic, from mantle plumes. They occur both on the continental and oceanic plates.

a). Continental flood basalt provinces (CFBs)

Continental flood deposits are thick (10s to some over 100 meters) and extensive. Examples are the Columbia River (Idaho, USA), Volyn (Europe), Deccan traps (India), Paraná-Etendeka traps (south America, Namibia, and Angola), and Siberian Traps (Russia).

Others are the North Atlantic Igneous Province, Kalkarindji (Australia), Ethiopia-Afar-Yemen, Rajmahal, and Madagascar.

These CFBs have mainly plagioclase and olivine porphyritic sub-alkali basalts with varying amounts of incompatible elements. However, some, like in Yemen and Etendeka, are more evolved forms.

Lastly, the effusive eruption forming the CFBs presented a major crisis and mass extinctions.

b). Oceanic plateaus

Oceanic plateaus have mainly porphyritic olivine-plagioclase tholeiitic basalts low in potassium. Examples include Ontong-Java, Kerguelen Plateau, Caribbean-Colombian, and Wrangellia.

4. Intracontinental rifts

Intracontinental rifts have mostly alkali and transitional basalts and occur in well-developed rifts like the Kenya-Ethiopia rift system, the Rhine rift valley in Germany, the Baikal rift in Russia, the Rio Grande Valley (New Mexico, USA).

They occur with undersaturated and alkali mafic rocks like basanites, nephelinite, melilite, and numerous phonolite and trachyte.

5. Volcanic arcs basalts

This rock occurs both in the island and continental arc subduction zones. Here magma forms from flux melting.

At volcanic arcs, basalts usually occur with larger volumes of evolved magma, especially andesitic in island arcs, dacites, and rhyolites in continental margins.

However, island arcs will erupt mainly tholeiitic type with variable amounts of potassium and aluminum, i.e., high and low.

Here is more:

5. Lunar and other extraterrestrial basalt

Besides Earth, basaltic lava flows occur on the moon, some with coarse-grained texture. The visible dark surface, i.e., lunar maria (mare in singular), has basaltic floods higher in iron than Earth’s i.e., 17 to 22 wt.%.

Lunar maria rocks have highly variable titanium content in ilmenite (1-13 wt.%), lower alkalis 0.5 and 0.1, and have variable pyroxenes.

Also, these rocks are low in silica (38%) outside IUGS classification, have no detectable water, and are highly reduced in iron minerals. Some samples have trace metallic iron.

Lastly, basaltic lava flow constitutes 80% of Venus and occurs on the surface of the Vesta and Jupiter I (Io) moon. Evidence includes Martian basaltic achondrites (shergottites) and 4 Vesta. These stony basalt meteorites have rock-forming minerals that indicate the possibility of volcanic activities.

What is basalt used for?

Basalt is a hard, dense, safe (non-carcinogenic), durable rock with excellent engineering properties. Also, it is abundant and has poor thermal conductivity, high tensile and compressive strength, and resists chemicals (alkalis or acids).

Uses of basalts include industrial (fibers, casts, and protective coatings), construction, dimension stone industry, and soil amendment. Also, they form unique tourist attractions, may help in carbon dioxide sequestration, etc.

Let us see more of the modern and ancient uses:

Modern uses

• Construction: Basalt is preferred for making aggregate or gravel for concrete and asphalt pavement. Also, it is ideal for road bases, railroad ballast, etc. Similarly, basalt sand and fines have various uses.
• Dimensional stone industry: It is cut and polished to make building blocks or bricks, wall/floor tiles, kitchen countertops, slabs, siding/façade cladding panels/veneers, and pavers for patios, driveways, pathways, etc.
• Basalt fibers: Melting and extrusion of this rock make basalt fibers used as a thermal insulator (fiber tape, textile, ropes exhaust wraps), making mesh and concrete reinforcement. Also, these fibers make boats, car bodies, composites, pressure vessels, speaker cones, windmill blades, etc.
• Cast basalt: This rock makes pipes, pipe liners, wear-resistant valves, cement and coal hoppers, chain conveyors, etc.  
• Basalt CO₂ sequestration: It can permanently trap or capture carbon dioxide from the atmosphere by reacting with it to form stable carbonates like magnesite, dolomite, or calcite.
• Tourism and recreational: Some stunning basaltic landforms and outcrop formations like Devils Postpile (Nevada, USA), Giant’s Causeway (Finland), Fingal’s Cave and Staffa (Scotland), Organ Pipes (Namibia), Svartifoss Waterfall and Stuðlagil Canyon (Iceland), Prisms of Santa María Regla (Hidalgo, Mexico), etc. are recreational and tourist attraction formations.
• Basalt dust fertilizer: It is rich in magnesium, calcium, iron, and other essential nutrients. Also, it helps in soil amendment, increases fertility, and weathers to only soil (clay or silt).
• More uses: Other basalt uses include landscaping, ripraps, gurgler or fountains pillars, metate (grinding stone), cooking/grilling stones, hot stones, fire pit/fireplace veneer, and mortar & pestles. Also, these stones can make retaining walls, benches, statues, monuments, headstones, stepping stones, terminate barriers, and are safe for aquariums.

2. Prehistoric to Middle Ages uses

Basalt made ancient ground stone tools like knives, arrowheads, axes, and adzes. Also, they made statues like basalt stele, Cleopatra, or sarcophagi, and the 17 large Olmec basalt heads.

More uses were making vessels, temple-pyramids (especially flooring), street pavers (setts and cobblestones), and stadia seating.

Frequently Asked Questions (FAQs)

References

• Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
• Frost, B. R. (2014). Essentials of igneous and metamorphic petrology. Cambridge University Press.
• Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
• Winter, J. D. (2014). Principles of igneous and Metamorphic Petrology. Pearson Education.
• Blatt, H., Tracy, R. J., & Owens, B. E. (2006). Petrology: Igneous, sedimentary, and metamorphic (3rd ed.). W.H. Freeman and Company
• Jelinek, E., Holub, F. V., Klapova, H, & Soucek, J. (1989). Basalt. In Bowes, D. R. (ed.). The encyclopedia of igneous and metamorphic petrology (pp. 51-57). New York: Van Nostrand Reinhold.  
• Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals (3rd ed.). The Mineralogical Society.
• Le Maitre, R. W. (Ed.) (2002). Igneous rocks: A classification and glossary of terms (2nd ed.). Cambridge University Press.