What is basalt?

..and for those who were curious of what is basalt fiber, and where it comes from / what is basalt: it is the most common volcanic rock type on Earth. Oceanic tectonic plates are composed predominantly of basalt. It also commonly erupts on Io, the third largest moon of Jupiter, and has also formed on the Moon, Mars, Venus. (In the photo below: Reynisdrangar, Southern Peninsula, Iceland)