Basalt

Basalt is the most common volcanic (extrusive) rock on Earth, a dark, fine-grained mafic rock made mostly of plagioclase feldspar and pyroxene, often with olivine and magnetite. Its iron and magnesium content gives it its characteristic black to dark grey color.

Because it erupts at the surface and cools quickly, its crystals are usually too small to see without magnification. Basalt makes up the bulk of the oceanic crust and forms vast continental flood-basalt provinces.

It frequently displays gas bubbles (vesicles) and, where thick flows cool evenly, can fracture into the famous hexagonal columns seen at sites like the Giant's Causeway.

Basalt forms from the rapid cooling of low-silica, iron- and magnesium-rich (mafic) lava at or near Earth's surface. This fluid lava erupts from mid-ocean ridges, hotspots, and fissure eruptions.

At mid-ocean ridges, basalt is continuously created as seafloor spreads, making it the dominant rock of the oceanic crust. On land, enormous outpourings produce flood basalts and lava plateaus.

Famous occurrences include the Columbia River Basalts and Hawaiian volcanoes (USA), the Deccan Traps (India), the Giant's Causeway (Northern Ireland), Iceland, and the Siberian Traps. Basalt also covers much of the Moon's maria and the surface of Mars.

Look for a dense, hard, fine-grained rock that is dark grey to black and feels heavy for its size. Individual crystals are usually invisible to the eye; sometimes scattered larger crystals (phenocrysts) of olivine or pyroxene are present.

Many basalts are vesicular, peppered with small round gas holes, or show columnar jointing when seen in outcrop. The streak is grey to white and there is no layering or banding.

Look-alikes include andesite (lighter grey, more intermediate), gabbro (same composition but coarse-grained and intrusive), and obsidian (glassy, conchoidal, not crystalline). Basalt's combination of dark color, fine grain, weight, and often vesicles or columns is distinctive.

Basalt is a major construction material: crushed basalt ('trap rock') is widely used as concrete aggregate, road base, and railroad ballast because it is hard, tough, and abundant. Dimension basalt is used for paving, cladding, and cobblestones.

Melted and spun, basalt becomes rock wool insulation and basalt fiber, an emerging reinforcement for composites. Basalt is also studied for carbon storage, since it reacts with CO2 to form stable carbonate minerals.

Scientifically it is invaluable for understanding plate tectonics, mantle composition, and planetary geology, since it is the principal rock of ocean floors and other rocky worlds.