The Formation of Basalt
Basalt is an aphanitic (fine-grained) igneous rock with less than 20% quartz and less than 10% feldspathoid by volume, and where at least 65% of the feldspar is in the form of plagioclase. Basalt features a glassy matrix interspersed with minerals. The average density is 3.0 gm/cm3.
Basalt is defined by its mineral content and texture, and physical descriptions without mineralogical context may be unreliable in some circumstances. Basalt is usually grey to black in colour, but rapidly weathers to brown or rust-red due to oxidation of its mafic (iron-rich) minerals into rust. Although usually characterized as “dark”, basaltic rocks exhibit a wide range of shading due to regional geochemical processes. Due to weathering or high concentrations of plagioclase, some basalts are quite light coloured, superficially resembling rhyolite to untrained eyes. Basalt has a fine-grained mineral texture due to the molten rock cooling too quickly for large mineral crystals to grow, although it is often porphyritic, containing the larger crystals formed prior to the extrusion that brought the lava to the surface, embedded in a finer-grained matrix.
Bluestone is a cultural or commercial name for a number of dimension or building stone varieties, including: basalt in Victoria, Australia, and in New Zealand.
There are two distinct building materials called “bluestone” in Australia. In Victoria, what is known as bluestone is a basalt or olivine basalt. It was one of the favoured building materials during the Victorian Gold Rush period of the 1850s. In Melbourne it was extracted from quarries throughout the inner northern suburbs, such as Clifton Hill, Brunswick and Coburg, where the quarry used to source the stone for Pentridge Prison is now Coburg Lake.