December 8, 2010

Geology Word of the Week: F is for Fabric

Posted by Evelyn Mervine

Poikilitic texture. Image from (Geology).
def. Fabric:
The arrangement of the elements (minerals, textures, fossils, layers) that make up that rock. The fabric of a rock is basically the pattern of the rock.

In geology, like in many scientific and non-scientific specialties, everyday words often take on new meanings. Everyday words such as fault, joint, layer, reef, slip, mantle, blast, cauldron, dip, strike, exposure, and many others have specific meanings in geology. These meanings are often vaguely related to their more conventional meanings, but not always.

For example, take the word “reef”. This word has two definitions, both of which are geologic in nature. The more conventional definition of reef is a strip of rocks, sand, and/or coral that rises near the surface of water. This is a place you go snorkeling or scuba diving to look at pretty fishes and other biological-thingies (hey, I know rocks, not animals). However, another definition of the word reef in geology is a vein of rock (or ore) that is rich in a particular element or mineral. One of the most common ways to use reef in this context is to refer to the reefs of gold-rich ore located in South Africa.

My geologist fiance (who now works for a South African-based gold company) taught me this second definition of reef several years ago when we first started dating. He thought I was an idiot for not knowing the definition already, but he was too polite to say so at the time. Before I figured out the second definition, we had a very confusing conversation that went something like this:

Future fiance: “So, I think we’re going to visit some reefs for our honours geology field trip.” Note that he’s South African so he always speaks in “ou”s.

Me: “Oh, that’s great- so will you be staying along the coast then?”

Future fiance, sounding slightly befuddled: “What? No- we won’t be along the coast. We’ll be inland, near Joburg.”

Me: “Oh. I see- they’re fossil reefs. I didn’t realize there were fossil reefs inland. How old are they?”

Future fiance, talking slowly as if to a child: “Why, Precambrian of course. Most of the reefs in South Africa are Precambrian.” For the non-geologists, Precambrian is very old- older than 542 million years.

Me: “Wait… Precambrian? How can reefs be preserved that long?”

Future fiance, now clearly thinking me an idiot: “Well, most of the reefs are that old.”

Me: “So, wait, do these reefs record paleo-sealevel? They can’t be Precambrian.”

Future fiance: “Sea level? What? These are gold reefs.”

Me, utterly confused: “Gold reefs?”

You get the idea… shortly after this, I figured out that there is a second definition of the word reef. I should have known this, but then again it is a much more common word to teach in a South African geology class than in an American geology class.

Anyway, this week’s Geology Word of the Week is actually not “reef” (primarily because I’m at the letter F this week) but rather “fabric”. This is a word that has a conventional meaning and a geologic meaning. In geology, fabric refers to the arrangement of the elements (minerals, textures, fossils, layers) that make up that rock. The fabric of a rock is basically the pattern of the rock.

There are hundreds of words describing specific rock fabrics. This is not so different, I suppose, from the dozens (maybe hundreds?) of words used to describe fabrics in a conventional sense. We have words such as plaid, striped, paisley, gingham, and checked to describe conventional fabrics. Geologists have words such as euhedral, perthite, ophitic, holocrystalline, poikilitic, glomeroporphyritic, cross-bedding, lenticular beds, flame structure, foliation, and- a favorite of mine- schistosity to describe rock fabrics. These words are more complex-sounding and esoteric than words such as gingham, but the idea is the same. These geologic words describe rock patterns just as the conventional words describe cloth (or fabric) patterns.

Below are a few more pictures of rock fabric. I won’t explain all of these fabric names in this post, but perhaps some of these fabric words will become future Geology Words of the Week!

Braided stream cross-bedding, South Africa, September 2010.
Cross-bedding in sand dunes, South Africa, December 2009.
Schistosity and porphyritic texture in thin section. Image from wikipedia commons.
Perthite fabric in feldspar. Image from wikipedia commons.
Brightly-colored perthite in feldspar (in thin section, microscope image). Image from wikipedia commons.