Basalt is a mafic or dark-colored, volcanic or extrusive igneous rock. Mafic refers to rocks high in dark-colored minerals rich in iron and magnesium. These rocks are relatively low in silica, sodium, and potassium.
On the other hand, volcanic or extrusive rocks form on or near the Earth’s surface. Therefore, they cool fast, forming a fine-grained or aphanitic texture. However, the fine-grained texture can develop alongside other textures, too.
Basalt mainly has calcic plagioclase and augite, which is a pyroxene. However, it can have other minerals. These minerals can include quartz, feldspathoids, olivine, enstatite, etc.
Lastly, basalt is the most common rock on the Earth’s surface. It covers the ocean and occurs in the large igneous provinces. Also, more than half of the volcanoes are predominantly basaltic.
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Basalt description and properties
Basalt is a massive, dense, hard, dark-colored rock with a density of about 2.9g/cm3 and a Mohs hardness scale of 6-7.
It is often dark gray to black on a fresh surface since dark-colored mafic minerals. However, it can have brown-black or greenish-black hues dominate it. Also, rare kinds can have lighter colors and are known as leucobasalts.
However, this rock will be tan, brown, reddish brown, or have a yellowish hue on a weathered surface.
Basalt’s texture is fine-grained or aphanitic. In addition, some basalts can have other textures like porphyritic, vesicular, poikilitic (subophitic or ophitic), or amygdaloidal.
Porphyritic will have large crystals or phenocrysts in a fine-grained matrix, while vesicular will appear pitted or have cavities. If other secondary (not from erupted magma) minerals later fill the vesicles, you will have an amygdaloidal texture.
Lastly, ophitic means pyroxenes or sometimes olivine enclose laths of plagioclase fully, and if they are only partially enclosed, you have a subophitic texture.
Chemical composition
Basalt is a basic rock low in silica, sodium, and potassium oxide and relatively high in calcium, iron, and magnesium oxides.
The typical weight percent chemical composition range of basalt is 45-52 % silica, 5-15% iron oxides, more than 14% aluminum oxide, 5-12% magnesium oxide, about 10% calcium oxide, less than 5% combined sodium and potassium oxides, and 2% titanium oxide.
Basalt mineral composition
Basalt is a mafic rock with calcic plagioclase feldspar, augite, and small amounts of other minerals. These minerals are olivine, quartz or feldspathoids, enstatite, pigeonite, and rarely hornblende and biotite. Also, it can have a small amount of interstitial glass.
Common accessory minerals are ilmenite, magnetite, spinel, apatite, and chromite.
Quartz and feldspathoids don’t coexist and occur most in interstitial spaces together with glass, while hornblende (amphibole) and biotite (mica) are rare since this rock forms at high temperatures.
On the QAPF classification, basalt is a fine-grained igneous rock with less than 10% feldspathoid and less than 20% quartz of QAPF content by volume with plagioclase more than 65% of the total feldspar. It occupies the same spot as the andesite.
On the Total Alkali-Silica (TAS) content diagram, basalt has 45-52% silica and less than 5% combined sodium and potassium oxide.
Considering its chemical and mineral composition, basalt is the extrusive equivalent of gabbro, a coarse-grained intrusive rock or medium-grained microgabbro (diabase or dolerite).
Note: The mineral composition in some basalt varieties like picrobasalt, trachybasalt, high alumina, tholeiitic, alkali, silica saturated, and undersaturated can vary.
However, for rock to be considered basalt, it must be:
- Have primarily calcic plagioclase and augite
- Anorthite is more than 50 mol% in calcic plagioclase, and it is usually labradorite
How is basalt rock formed?
Basalt rock forms when basaltic magma or lava flows quickly cool on or near the Earth’s surface. This can be underwater, i.e., submarine, or subaerial if on the land’s surface.
The quick cooling causes a fine-grained texture. However, if the lava is quenched too quickly, it will instead form sideromelane and tachylite. These are volcanic glasses of basaltic composition.
The magma that forms basalt is mafic. They form when mantle rocks, especially peridotite, partially melt.
These magmas are of relatively low viscosity (flow easily) and are hot, i.e., they erupt at higher temperatures of 1000-1250°C.
They erupt mostly effusively but can also erupt explosively. Their low viscosity allows gases to escape with ease.
The effusive eruption will form lava flows like pahoehoe or aa on land, pillow lava, and sheet flows under the water. Highly voluminous effusive eruptions will form basalt floods or plateaus in large igneous provinces.
On the other hand, explosive eruptions may occur if basaltic magma is high in volatiles, a plug prevents gas escape, or it interacts with water.
Explosive eruptions will form pyroclasts (ejected fragmented rock or magma) like scoria, ash, volcanic bombs, and blocks. These eruptions can form scoria cones, tuff cones, or, if from water interaction, maars and tuff rings.
Where is basalt found?
Basalt is the most abundant rock on Earth’s surface. It covers the ocean and dominates oceanic islands, continental flood basalts, and oceanic plateaus.
Also, it occurs in continental rifts, back-arc basins, and volcanic arcs but in smaller amounts.
Specific examples of each of these places include:
- Mid-ocean ridges (MOR): MORs are oceanic spreading centers where new oceanic crusts form. They include the Mid-Atlantic Ridge, Gakkel Ridge, and the East Pacific Rise
- Oceanic islands: Hawaii, Azores Islands, Canary Islands and Reunion Island
- Continental flood basalts: Columbia River, Deccan traps, Paraná-Etendeka, and North Atlantic Igneous Province
- Oceanic plateaus: Ontong-Java, Wrangellia, Kerguelen Plateau and Caribbean-Colombian
- Continental rifts: Rhine rift, Kenya-Ethiopia, Rio Grande Valley, and Baikal rifts.
- Volcanic arcs: Tonga, Izu Island, South Sandwich Island, North and Southern volcanic zones on the Andes.
The Columbia River flood basalt in Washington, Oregon, Northern Nevada, and western Idaho is one of the largest basalt deposits in the US. Also, states like Hawaii, California, Texas, New England, Minnesota, etc., have this rock.
Besides Earth, it occurs in other celestial bodies like on the moon lunar Maria, Venus, Vesta, and Io moon.
Uses
Basalt is a hard, durable rock with a high compressive strength of 266±98 MPa and resists chemicals and acids. Also, it is safe and not carcinogenic, i.e., it doesn’t cause cancer and is a poor conductor of heat.
These excellent properties make it a number one choice of rock for many constructions, industrial, and other uses.
Crushed basalt makes aggregate for concrete, road, railroad ballast, concrete, sand, and fines in the construction industry. You can also use this gravel on pathways, patios, etc.
As a dimensional stone, cut and polished basalt will make tiles, pavers, building blocks, kitchen countertops, etc.
In the industry, basalt is melted and extruded to make fiber for heat insulation, reinforcing concrete, and use in automobile bodies, speaker cones, boasts, etc. Also, basalt extrusion will make pipes and pipe liners, hoppers for cement or coal, valves that resist wear, etc.
Also, it has the potential to capture carbon dioxide (CO2 sequestration), and its dust can condition and make soils fertile.
That is not all. Some breathtaking basaltic rock outcrops are scenic landforms that will attract tourists. Examples include Giant’s Causeway in Finland, Devil’s Postpile in Nevada, USA, and Svartifoss Waterfall and Stuðlagil Canyon.
Lastly, you can use basalt rocks for landscaping, such as grinding, cooking, hot stones, stepping stones, etc., or decorative items like fountain pillars, statues, monuments, headstones, etc.