Properties of Basalt Stones

The properties of basalt, which is found under the oceanic crust, include mostly dark material, such as hornblende, as well as dark silicate minerals. Find out how to identify basalt stone with information from a geology professor in this free video on rocks.

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Video transcription

Basalt is an example of an igneous rock, and it's one of the most important and abundant rocks in the earth. If we look at our planet its crust, or outer layer, is divided into two types of material. We have continental crust which is underlaid by granite, it has sedimentary rocks on top. But the vast majority of the Earth's crust is what we call "oceanic crust". And so if you take away the two or three miles of ocean that's present, down at the bottom you're going to find that crust and it's going to be made primarily of the rock called "basalt" which looks like this. Another place that you can see basalt is a place that might be a little surprising to you. If you look at the moon through binoculars and see the dark areas on the moon, the regions that make the face of the so called "Man in the Moon", you're looking at basalt lava floats that occurred on the moon, which brings up the point that other planets have somewhat the same rocks and minerals as the Earth does, although they're not all identical. Now if we look at some rocks, and even some igneous rocks, we can see individual mineral grains in them, and distinguish the components of them. But try as you might with the unaided eye, you can't really do this with basalt, it's pretty much uniform and trying to identify the individual minerals in this would be about as hard as identifying the ingredients in a piece of cake if you were eating the cake because it's all blended together and baked. But anyhow, through a microscope we can begin to see some of the crystals on a very small scale, and we do know, of course, what's in basalt. And one of the things that we find in basalt are dark silicate minerals. Silicate minerals contain silica and oxygen, the most abundant group of minerals in the Earth's crust. This is an example of one of them called " Hornblende" and as you can see here it's a fairly dark material which makes sense given the color of basalt. Basalt also contains feldspar which is light in color but there's not enough of that present, generally, to give you the lightness of the mineral. And, by definition, if we look at a piece of basalt it's typically going to be over three-quarters dark material so it's got a very dark color. Now where do we find basalt? Well, other than in places on the ocean floor, we're going to find it in certain areas where mountain building has occurred, volcanic activity in the earth, some areas you will have this material being mined out of the ground, crushed and used for gravel rather than what we do in Ohio, for example, where we take limestone out of the ground and crush that to make gravel. So basalt is not a real valuable substance, it's not used in building or architecture as much as granite, but still an important and interesting example of an igneous rock.