Basalt. It’s the fine grained igneous rock that makes up the sea floor and sometimes spews forth from volcanoes on the continents. End of story right? Wrong! That black rock sample I remember sitting in my classroom draw was interesting but it was about to become a whole lot more interesting when I first visited Hawaii back in the 1980’s.
Basalt is quite a common rock on Earth. It makes up all the ocean floors as well as some huge areas of some of the continents. It is a dark fine grained rock made up of the minerals olivine, pyroxene and plagioclase feldspars (notice…no quartz). Almost always these minerals are too fine grained to be seen with the naked eye. However sometimes Olivine can be seen as small sand-sized grains of olive-green glass in the specimen.
At other times larger crystals of plagioclase feldspar can be seen.
But on an active basaltic volcano, basalt can look very different from what most of us have seen in our classroom samples. Basalt can be glassy black with amazing irredescence. It can be bright red. It can also be rough chocolate brown. Basalt can be solid or full of gas bubbles. It can be little tear shaped pieces (called Pele’s Tears) or thin drawn out glass fibers (called Pele’s Hair). It can look like rope or even intestines!
Ropey pahoehoe basalt flow
Dark grey pahoehoe flow over chocolate brown aa flow – all basalt of approximately the same age.
Basalt cinder from a fire fountain
Red colors of basalt from close to vent
A delicate form of basalt froth – Reticulite
All this variation is due to the way the basalt has been erupted onto the Earth’s surface. So while it is all the same rock type chemically, it’s appearance can be so varied.
Want to discover all this and more about basalt? Why not join us on our Discover Hawaii Adventure.