Basalt is a fine grained volcanic igneous rock formed of plagioclase and pyroxene minerals. It is most commonly formed by lava flow and in sills and dykes. Basalt is similar to gabbro in composition but the difference lies in the grain size of the two rocks.
Basalt is the most abundant bedrock forms much of the oceanic crust but in continental crust basalt can be in lava flows and flood basalts.
Composition of basalt
Mineral composition of basalt consists of pyroxene, plafioclase and olivine with minor content of glass in ground mass and in porphyritic the phenocrysts will be any of olivine, pyroxene or plagioclase.
Environments forming basalts
The environments at which basalt is formed are
- oceanic divergent boundaries
- oceanic hotspots
- mantle plumes and hotspots beneath continental crust
Oceanic divergent boundaries
Basalts are formed at oceanic divergent boundaries where plates move apart producing hollow space where convectional current gives away the hot melt from the mantle. These melt erupts at the opening of the divergent boundaries and are deposited as pillow basalt on the ocean floor. Active mid oceanic ridges are the host for the fissure eruption of basalt which spreads through the ocean floor. Basalt are the most abundant rock on the floor of the ocean which makes the large area of oceanic crust at the surface.
Hotspots are underlying fissure eruption at the base of an ocean which results in eruption of basalt at the surface and when its active still, makes the volcanic cone. As volcano erupts at the sea floor it stacks the rock on one another that makes it so large where it comes out of the water and produces islands. Hotspots are currently active where the resulting island is Hawaiian islands where hotspot makes small islands. The hotspot is the great spot of volcanic eruptions that is the reason for forming islands active nowadays.
Mantle plumes and hotspots in continents
Mantle plumes can rise on continents that can be erupted at surface where basaltic lava flows on surface. These can erupt as vents or fissures where lava flow on Earth's surface produces enough rock and if the process is repeatedly occurring it can stack layers and layers of basaltic lava flow and the rock can be of great thickness.
Basaltic rock are used as a crushed material for construction purposes as an aggregate and it can also be used as dimension stone. Other use of basalt is as a thin slabs polished thoroughly for making floor tiles, monuments and other stone objects.