Identifying Rocks

Video Transcript

"A lot of people find as they start getting interested in geology and related sciences that collecting rocks is a good way to get out in nature and develop a little bit of knowledge about the geological world them. But of course if you're going to collect rocks, one of the things you need to know how to do is identify the one's that you have. And in the next few minutes we're going to talk a little bit about some common rocks, what they look like and how to identify them. We don't have time to talk about every rock that you might come across so my best advice to somebody interested in rock collecting is get a good book on rocks or rocks and minerals. A field guide that's illustrated with photographs and even though you may not find something that looks exactly like what you collect, it might help you to narrow it down what the choices are. Now as far as rocks go, we define all rocks into the igneous group which form from magma and volcanic activity, sedimentary rocks that form either from particles of pre-existing rocks or minerals that were dissolved in water or metamorphic rocks that are rocks that have been changed usually by heat and pressure. Now obviously you're not going to find igneous rocks unless you live in an area where that type of activity has occurred, probably a mountain region and so forth, although glaciers during the ice age have sometimes brought these in from outside. Real common igneous rock is granite. You can distinguish that because of it's overall light color. It has visible crystals that you can identify in various minerals and you see mostly light grains, light minerals but there are also some dark ones present. Another example of an igneous rock, very common one is basalt. Basalt is dark in color but you can't see individual mineral crystals unlike granite which formed by cooling slowly, basalt forms by cooling very quickly. There's not enough time for crystals to form and so it looks pretty much uniformed which it is at least to the level of the eye. Sedimentary rocks, there's only three sedimentary rocks that constitute about 90% of those that you're going to find on the earth. So these are ones that are definitely worth learning to identify and they're not all that hard to distinguish either. Sandstone as you can tell by the name is made of sand sized particles that have been cemented together naturally and if you rub your finger over sandstone it's going to feel sandy and gritty so it's one of the easiest rocks for anybody to identify. Another sedimentary rock made of particles is shale. Now unlike sandstone which is made of bigger sand sized particles, shale is made of tiny little flakes of what we call clean minerals and so if you rub your finger over that it feels very, very smooth. You look at it from the edge you're going to see lineations in it from where all these accumulations took place. Another example is limestone. Now limestone forms when the mineral calcite dissolved in ocean water precipitates and forms a rock at the bottom of the ocean. And the best way to identify limestone is if you put dilute acid on it like vinegar for example, you'll get a little bit of bubbling and fizzing due to the chemical nature of the rock reacting with the acid. And then finally metamorphic rocks, there's many examples of that like there in other groups but here's just one that's a real snap to identify. It's a rock called gneiss, G N E I S S, a German word and gneiss can easily be distinguished because it has bands, parallel bands of light and dark minerals and the reason for that is that the pressure in the earth that caused this rock to form caused the mineral grains that perhaps were randomly arranged at first like this granite, caused them to rearrange themselves into this parallel structure. As I said there's many more rocks but this is just an example of some of the basic rocks that people could find."