You know how some words can be so confusing? Well, gneiss is one of them.
A Gneiss is a type of rock, but not just any rock, it’s a ZEBRA rock. Trippy I know, an animal rock. By the way, it is pronounced like the English word “nice”, no not “niece” as in the opposite of nephew, but “nice” as in, “Nice car dude.” The word Gneiss derives from a German origin and directly means bright or spark alluding to the sparkly appearance from the crystal grains within the rock called schistocity. Below are a couple examples of gneiss rocks showing the most illustrative characteristics of banding, or stripes.
A nice illustration of a Banded Gneiss — Image from University of Auckland Geology Department (link)
Gneiss, where do you get your stripes?
When certain types of rocks like sedimentary or igneous rocks are exposed to high pressure and high heat within the Earth’s crust, they change into different types of rocks, like a gneiss. A sedimentary rock is typically formed by small grains of mud, sand, or small rocks that become cemented together into a different form such as sandstone and mudstone. Many times, the parent rock, or the kind of rock prior to metamorphism (process of changing under heat and pressure), of a gneiss is a sedimentary rock, the sandstone or mudstone mentioned above. When rocks like sandstone and mudstone, which have various mineral distributions, are subjected to the metamorphic process the mineral grains link together and form bands within the rock that vary in size. Similarly to how people with similar interests become friends, the minerals aggregate together.
A gneiss can easily be identified by these bands that vary in size, a feature that sets it apart from other rocks that tend to have semi-consistent band widths. Another term for the mineral distribution bands in the rocks can also be described as foliation, which is a textbook geologic term to describe sheet-like layers or laminae. Sometimes a gneiss rock can even be spotted with “lenses” of color, like a cheetah. More to come on the formation process but look at this banded Gneiss below.
A Banded Gneiss with “foliation”. Image from trail in Pasadena, CA 2021 — photo credit to M.J.M.
Gneiss, what is in you?
Some of the more common minerals, which make up the individual colors evident in the bands, are feldspar (pink/orange), quartz (white), and darker mafic minerals (biotite, amphibole, or pyroxene). Felsic minerals, such as feldspar and quartz, are lighter in both color and density; mafic minerals are darker and heavier due to the presence of metals like magnesium and iron.
The spotted gneiss specimens often contain lenses/spots of garnet porphyroblasts, which are larger recrystallized grains giving it the appearance the specimen has hardcore chicken pox. In short, because of different densities in mineral types and environmental conditions, you will often get an array of stripes, spots; both light or dark, and full of little crystals. The sparkling crystals are formed from the movement of tiny grains joining to grow, a beautiful orchestral rearrangement within the crust, makes this work of art.
Gneiss, how were you formed?
We talked a bit earlier about the happenings in the crust (metamorphism) and briefly on the formation of a gneiss, but what is really happening when we talk of gneiss formation? When a region goes through shearing, such as convergent plate boundaries where two plates meet and slide, the earthen materials are subjected to high pressures and temperatures. How high? Think 6–12 miles/10–20 km deep and at least 608*F/320*C. For a sedimentary parent rock there are layers upon layers of sediment that build up from years of erosion, and for an igneous parent rock a magma or lava body would have cooled down over time to create crystals. Granite is a common example of an igneous rock.
As the parent rocks are introduced to high enough temperatures and pressures, the grains literally cook out the water molecules and chemically transform into new minerals. This transformation is a more detailed way to describe metamorphism, which specifically means “alteration in composition and/or structure.” Depending on the amount and directions of pressure being exerted on the rock, sometimes the bands sometimes can become wavy from folding. Check out the migmatite, the groovy 70’s wavy, curled version of a gneiss in the sequence of formational stages below.
Gneiss, how do I find you in the wild?
As explorers, geologists, or even generalists who are passionate about Earth Science, wouldn’t it be wonderful to be able to depict and diagnose rocks and their stories in the wild? Though that generally may not work all the time, it generally does for a gneiss rock. The consistency when identifying, but variance in characteristics are some of the main reasons why I like them so much. Well, that and when you get a series of perplexed looks from the plethora of puns to be exchanged after telling someone, “That’s a gneiss rock.”
Simply put, you can find a Gneiss in the wild by finding a striped banded “Zebra-style” rock with a completely different mineral composition between the band (that could be folded) and the base rock material. Remember the sparkly crystals and possible chicken pox and cheetah variants!
Now let’s see if you can find a Gneiss at home.
Gneiss, why did I name my website after you?
Earlier we learned that gneiss means “spark” in German, and who doesn’t like sparkly things?! Well, a lot of people, or so they say… but either way sparkly items catch the eye. The sparkling texture, or schistosity, the bands, and individual medium to coarse-sized crystal grains are the main distinctive characteristics of gneisses.
A pun, a saying, a lifestyle. This sparkly, common, but oddly varying-in-characteristics sort of rock that is easy to catch the eye often becomes the figure head of humor and visual search for things that could be ‘nice.’ A rock that regardless of how much heat and pressure it’s been under, is still nice. I named my website after this artful rock; the exuberant form that brings smiles and thoughts to become more aware and appreciative of rocks, earth science, the world, and others.
Be Gneiss is a calling, a form of saying to genuinely be nice, towards everything, but especially in the exploration of ourselves, others, and ROCKS!