Basalt is a common extrusive igneous rock. It is a dark volcanic rock formed when mafic magmas reach the Earth’s surface and flow across the landscape. Mafic means that basalt is magnesium-rich and silica-poor. Sometimes gases in the molten magma have not had time to escape and form bubbles or vesicles in the rock, making a porous or vesicular texture called scoria. It is usually grey to black but rapidly weathers to brown or rust-red due to oxidation of its iron-rich minerals into rust. Basalt is high density and is fairly hard being equal to the mineral, quartz. It is generally used in construction as building blocks. Columnar basalt is used for making cobblestones.
Bluestone. The common commercial name of “bluestone” is applied to a variety of building stones. Basalt in Victoria, Australia and in New Zealand is called bluestone. The dolorite of Stonehenge in Britain is also called bluestone. (Diabase or dolerite is very similar to basalt.)
Black Granite. Some dark colored igneous rocks, which are actually basalt, gabbro, dionite, diabase and anorthosite, are quarried and sold as “black granite.” These stones contain little or no quartz or alkali feldspars, but, for all practical purposes, they are used interchangeably with true granites.
Theoleitic basalt is rich in silica and poor in sodium whereas Alkali basalt is the reverse — poor in silica and rich in sodium. Alumina basalt is high in alumina and Boninite basalt is high in magnesium.
The most famous basalt flow in the world is the Giant’s Causeway on the northern coast of Ireland. Natural geological formations of native Oregon columnar basalt can be seen along the Columbia River. A famous example of buildings constructed with basalt blocks is the ancient city of Ummel-Jimal in Southern Syria. Basalt columns in western Scotland inspired the entrance to the world famous Waterhouse Building, a London landmark.
- Basalt, a mafic extrusive igneous rock. www.flexiblelearning.auckland.ac.nz
- Basalt Building, Loa, UT. www.gly.uga.edu
- Black Basalt Slab: “Antique Flamed” Finish. www.classictileandmosaic.com