What Colour Is Basalt Basalt stands as a robust, dark-hued, fine-grained volcanic rock, extruded from calcium-rich plagioclase and augite. This mafic rock, high in magnesium and iron, emerges from the rapid cooling of low-viscosity lava near or on the Earth’s surface.
Ubiquitous in occurrence, basalt dominates the Earth’s crust
spanning oceanic domains, commanding oceanic islands, and scattering across continental landscapes, even forming extensive igneous provinces. It extends its reach to the moon, Mars, Venus, and asteroids.
A majority of the world’s volcanoes
more than half in fact, prominently feature basaltic composition. Its manifestations encompass lava flows, dikes, sills, layered intrusions, cinder cones, and shield volcanoes. It can even be a component of stratovolcanoes and central volcanoes when paired with silicic magmas like andesitic, dacitic, and rhyolitic compositions.
For an in-depth understanding of basalt, encompassing its textures, chemical composition, mineral structure, and formation mechanisms, continue reading. We’ll also delve into the different variations, global occurrences, and its manifold applications. Given the extensive nature of this information, your patience is greatly appreciated.
Key Points and Properties of Basalt
- Rock Type: Igneous
- Origin: Extrusive
- Texture: Fine-grained or aphanitic, with potential variations such as porphyritic, vesicular, subophitic, ophitic, or amygdaloidal.
- Colors: Ranging from dark gray to dark green-gray, and greenish black to black.
- Cooling Rate/History: Rapid cooling 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: Low, 1.672 Wm-1K-1 at 20°C to 200°C
- pH: Ranges from 7 to 9.5, varying with variety and composition (source)
- Melting Point: 984° to 1260°C (source)
- Basaltic Lava Viscosity: Low, dependent on temperature and phenocryst content.
- Eruption Temperature: 1100° and 1250°C
- Coarse-grained Equivalent: Gabbro, although gabbro’s mineral content varies considerably.
- Metamorphic Forms: Greenschist, amphibolite, or eclogite, contingent on temperature and pressure.
- Tectonic Settings: All settings, including divergent (mid-ocean ridges and continental rifts), convergent (island and continental subduction zones), and hotspots (oceanic and continental).
Understanding Basalt and its Characteristics Basalt rocks manifest as fine-grained, dark-colored, or mafic volcanic or extrusive igneous formations. They showcase richness in magnesium and iron content, with silica levels ranging from 42-52 wt.%, and relatively lower alkali concentrations (Na2O and K2O).
The distinctive dark coloration of basalts, such as dark gray, brown-black, greenish-black, or black, emanates from their elevated ferromagnesium mineral content. However, exceptions like leucobasalts display lighter hues. Weathering of the surface can lead to tan, yellow, brown, or reddish-brown tones.
Various textural variations of basalt abound, from fine-grained aphanitic types to those exhibiting porphyritic features. Aphyric basalts are devoid of visible phenocrysts. Certain variations may present vesicular, amygdaloidal, subophitic, ophitic, or even hypocrystalline textures, with varying mixtures of glass and crystals.
Notable Basalt Textures
- Vesicular Basalt: Notable for its voids or vesicles formed by trapped gas bubbles during magma ascent.
- Porphyritic Basalt: Contains larger phenocrysts embedded in a fine-grained matrix due to two-stage cooling.
- Amygdaloidal Basalt: Forms when secondary minerals fill vesicles and fissures in cooled rock.
- Basalt Geode: Hollow spherical structures with mineral masses lining their inner surfaces.
- Subophitic and Ophitic Basalt: Involves encasing of plagioclase by pyroxene, with subophitic featuring partial encasement.
Basalt’s Chemical Composition and Mineralogy With a silica content of 45-52%, basalt emerges as a silica-poor rock. It is rich in calcium, magnesium, and iron oxides, with a notable scarcity of total alkali (Na2O and K2O). Augite and calcic plagioclase feldspar dominate its mineralogy, while olivine, enstatite, quartz