March 3, 2011

Geology Word of the Week: R is for Rock

Posted by Evelyn Mervine

“In spite of the difficulty in defining rocks, most rocks are easily recognized when you see them, and most are made of minerals or mineral-like substances. They are usually solid, hard, and heavy, compared to the other materials you see and use daily.”
-From Rocks and How They Were Formed by, Herbert Zim, Golden Library of Science, 1961.

Carbonate rocks, peridotite rocks, mountains, and field vehicles, Oman, January 2010.

def. Rock:
1. A solid mass of matter– usually composed of one or more minerals or mineraloids– that occurs naturally on the Earth or another planet or extraterrestrial body.
2. What geologists study.

Sometimes, the simplest words are the most difficult to define. This is particularly true when it comes to basic words that have broad definitions. The basic stuff of our universe is innately understood by us but often difficult for us to describe. Words such as water, fire, air, plant, animal, person, rock– and many others– have encompassing, basic meanings that are understood by everyone. These words also have precise– yet still broad– scientific definitions. Yet, if you ask a person to define a simple word such as air or water, this person is likely to struggle somewhat. How do you define a basic, essential word whose meaning you understand instinctively more than intellectually?

When we define complex words, we usually describe them with more simple words.  For example, “petrology” is “the study of all aspects of rocks” according to my trusty Dictionary of Geology (Keary, 1996).  Other complex words can be defined with one-word synonyms. For example, the word “innate” (used earlier in this post) can be defined by synonyms such as instinctive, intrinsic, inherited, native, natural, and intuitive. However, simple words such as water and air and rock cannot be easily defined by one-word synonyms.

Yesterday evening I asked my geologist fiance Jackie for a geology word starting with the letter R. As some of you may have noticed, I am going through the alphabet for my geology words of the week. So far, I have made it from A is for Alluvium to Q is for Quaternary. My conversation with my fiance went something like this:

Me: “I need a geology word that starts with R.”

Jackie: “Huh? Oh- I don’t know. I’m sleepy.”

My fiance and I are temporarily living on different continents, so there’s a six or seven hour time difference. I usually call him late at night and wake him up for work, so he’s often sleepy when I call him.

Me: “I was thinking of regolith, but that’s sort of boring. And I already used the word alluvium, which is similar. I also thought about radioactive decay, but that’s too complicated. I think I want to write about that in some posts about how to date rocks.”

Jackie: “Huh. Well, what about rock?

Me, laughing: “Well, it is an important word for geology! How do you define a rock anyway?”

Jackie: “A silicate network… wait… uh… yeah, a silicate material…”

Me, interrupting: “But what about carbonates?”

Jackie: “Carbonates aren’t rocks.”

Me: “Not even limestone?”

Jackie: “Well, I guess limestone is a rock. Those carbonate alteration products you study aren’t rocks, though. They’re just gardening.”

Earlier in the phone call, Jackie and I had a conversation about how certain high-temperature metamorphic petrologists we’ve met refer to low-temperature surface metamorphism (the stuff I work on) as “gardening.” Not sure why exactly, but it’s become our new term for my thesis research.

Me: “I work on real rocks. And even if my carbonates aren’t real rocks, I also work on peridotites.”

Jackie: “Yeah, okay. Well, I guess a rock is a solid material… that… uh…”

Me: “Do anthropogenic materials count as rocks? Like man-made conglomerate?”

Man-made conglomerate is what we call cement that has little rocks in it.

Jackie: “No, I think rocks have to be naturally-occurring. Like minerals.”

Me: “Okay, so we have to say a naturally-occurring solid material… hmm. I’ll look up rock in my geology dictionary.”

While Jackie muttered some more about how he was sleepy, I consulted my trusty geology dictionary. I was surprised to discover that the word rock is not in my geology dictionary! There are words that contain rock– such as rockburst and rocksalt– but the word rock isn’t in the dictionary.

Me: “The word rock isn’t in my geology dictionary!”

Jackie: “I guess that’s not a very good geology dictionary. Now you definitely have to blog about this word.”

And so this week’s geology word was chosen. I did eventually find various definitions of the word rock in other geology books. From these definitions, I came up with the simple, broad definition above. Geologists, how do you define the word rock?

Below are a few definitions of the word rock from various geology books:

“Look for rocks. They are the materials of which the crust of the earth is made. They form the mountains and underlie the valleys. You see them when they have have been pushed or folded upward or when they jut through soil to form an outcrop. All minerals occur within rocks, and often are components of rocks.”
-From Rocks and Minerals: A Guide to Minerals, Gems, and Rocks (A Golden Nature Guide), 1957.

“Rocks are large masses of material making up the Earth’s crust; many are not solid, like soil and gravel. A rock may consist of just one mineral, like quartz, dolomite, or calcite. Some rocks do not have discreet minerals but are made of glasses. However, most rocks contain several minerals, or were formed from older rocks where these minerals were present.”
-From Collins Wild Guide: Rocks & Minerals, 2000.

“The difference between a rock and a mineral should be clearly understood. Rocks are the essential building materials of which the earth is constructed, whereas minerals are the individual substances that go to make up the rocks. Most rocks, therefore, are aggregates of two or more minerals. Thus, granite (a rock) is composed of at least two minerals (quartz and feldspar), though others are almost certain to be present.

If a single mineral exists on a large scale, it may also be considered a rock, because it may then be regarded as an integral part of the structure of the earth. Thus, a pure sandstone or quartzite rock contains only one mineral, quartz, distributed over a wide area. Other single minerals which are described in this book and are regarded also as rocks by this definition include anhydrite, dolomite, gypsum, magnesite, serpentine, and sulfur– all of which occur in huge beds or masses. Some rocks of this type have a different name from that of the mineral composing them. Thus, the mineral halite makes rock salt; calcite or dolomite can make up the rock called marble. Kaolinite composes many of the rocks we know as clay. Bauxite has been proven to be really a rocky mixture of several minerals, but many geologists still prefer to call it a mineral because of its uniformity.

In addition to these two classifications, rocks include natural glass, though it may be devoid of any actual mineral components. Obsidian, an abundant rock in Mexico and Iceland, is natural volcanic glass. Organic products of the earth, which cannot be called minerals because they are formed from plants and animals, are properly known as rocks. Coal, derived from partly decomposed vegetation, is a rock of this kind.”
-From How to Know the Minerals and Rocks by Richard Pearl, 1955.

“Minerals are the fundamental units of which rocks are composed, homogeneous solids of definite chemical composition, formed by natural inorganic processes. Such a definition includes ice as a mineral but excludes coal, natural gas, and oil… The term ‘mineral’ often has a more extended usage, and may be used to describe anything of economic value which can be extracted from the earth, even clay or coal. A rock, on the other hand, has no fixed chemical composition, is not homogeneous, and has no definite shape of its own. In most cases it will consist of mixtures of several minerals. Granite, for example, is composed of the minerals feldspar, quartz, and mica, but some rocks may be formed mainly from one mineral.”
-From Rocks and Man by Myra Shackley, 1977.

“Rocks make up most of the Earth on which we live. The mountains are built of rocks. The plains and oceans rest on deep layers of rocks. Even in outer space, there are rocks circling the sun like tiny planets.”
-From Rocks and What They Tell Us by, Lester del Ray, 1961.

“What are rocks?

Simply put, rocks are naturally occurring aggregates (collections) of minerals. What makes rocks so different is their diversity– they can range from masses of minerals formed by volcanic action, from eroded sediment,or from great pressures and high temperatures. Rocks are usually composed of more than one mineral, although in rare cases they can be entirely composed of a single mineral, in which case they are called monomineralic rocks.”
-From The Handy Geology Answer Book by, Patricia Barnes-Svarney and Thomas Svarney, 2004.

Finally, below is my favorite definition of rock that I have found in my geology books. Sorry for such a long quotation, but I just love the completeness of this definition of a rock.

“To the geologist, rock is the natural, solid material that makes up the earth. The first word ‘natural’ immediately eliminates man-made materials like cement, glass, brick, and steel, even though these all come from the crust of the earth.

The second word, ‘solid,’ rules out the air and other gases, the oceans, rivers, lakes, and other liquids. However, solids can be changed to liquids and gases by being heated; liquids and gases can be changed into solids by being cooled. The definition of a rock means solids at temperatures which normally occur in the earth’s crust.

Even this does not cover everything, because one of the most common chemical compounds on the surface of the earth may or may not be a rock, depending on its temperature. This chemical compound is water- H2O. Water makes up nearly three fourths of the earth. Most of it is in the form of a liquid, and while liquid water affects the rocks of the earth in many ways, water is not a rock. However, in the arctic and antarctic regions, and in the temperate regions during winter, millions upon millions of tons of water are a hard, frozen solid. In the antarctic, ice occurs in layers nearly two miles thick. Ice is, therefore, a rock, and geologists study the great ice fields just as they study other rock formations.

In speaking of rocks, geologists use the word ‘solid’ in its technical sense. A solid is matter that is not a liquid or gas. What the geologists would call ‘solid rock’ might seem strange to you. The wet sands on the beach and the sifting sands in the desert are a solid– and a rock. This is also true of the layers of mud and muck in the swamps, or the ash and cinders from volcanoes. They are rock also.

The third word, ‘material,’ brings no additional problems to the definition of a rock. But it may be well to point out that materials in the crust of the earth have two distinct origins. Most of the material in the crust of the earth is inorganic. This means that it is no way related to life or living things. Lava pouring from a volcano makes an excellent example of an inorganic material. So do the great masses of granite pushed miles into the air as part of the Rocky Mountains.

While most rocks are made of materials which are not or never have been alive, some rocks are organic– made by living things. Coal and oil deposits, for example, are the remains of ancient plants. Oil, you might say, is a liquid and therefore is not a rock. However, there are no great underground lakes of oil as some people imagine. The oil is usually soaked up in the pores of sand and other rocks. Under special conditions it will drain into wells where it is pumped to the surface. Millions of gallons of oil are locked up in rocks, especially in the oil shales of Alberta, Canada, and other places. Asphalt is another organic rock. Great deposits of it are found on the island of Trinidad.

Less well-known are the rocks which are formed from the remains of sea animals. Shells cemented together form several kinds of limestone. Sometimes these are shells of microscopic animals; sometimes they are larger shells.

Coral is another kind of rock made from living things. Coral animals take lime from sea water and build it into reefs in which millions upon millions of animals live. Islands of coral dot the South Pacific. A few microscopic plants and sponges have silica skeletons. Under certain conditions these, too, form organic rocks.

One final explanation, and the definition of rocks is about as complete as it can be. The definition implies that rocks are large masses of natural, solid material, big enough to form a distinct part of the earth’s crust.

Diamonds are not rocks, even though they are found in the crust of the earth. But if a whole mountain of diamonds was discovered, then it would be correct to call diamonds a rock. There are places where one can see mountains of marble, quartz, granite or limestone. You can find large beds of coal, shale or lava. These are rocks. There are many miles of rich soil, more miles of sand in the deserts and on the shores. They all make up major parts of the earth’s crust, so they are called rocks.

You may have noticed that the definition of a rock does not say anything about minerals. This is odd, for we commonly think of rocks and minerals as going hand in hand. Most often they do. However, all minerals are inorganic. They are all chemical compounds and therefore have definite chemical composition. Mixtures of minerals often do form enough of earth’s crust to be considered rocks. Granite, made mainly of three minerals– mica, feldspar, and quartz– is undoubtedly a rock.

There are also times when a single mineral may form a rock. Quartz is a common mineral. Some forms of sandstone are made up of 99 per cent pure quartz. In this and other cases the rock and mineral are made of the same chemical. This may also happen in the case of the mineral, calcite, which forms a kind of pure marble. Here again the rock and mineral are the same. Gypsum is another rock made of a single mineral. The mineral kaolin makes fine clay and forms still another kind of rock.

However, rocks may be made of materials which are not minerals at all. Volcanic glass or obsidian is not a mineral but frequently forms rocks. Coal, peat, and asphalt are not minerals but they are rocks.

In spite of the difficulty in defining rocks, most rocks are easily recognized when you see them, and most are made of minerals or mineral-like substances. They are usually solid, hard, and heavy, compared to the other materials you see and use daily.”
-From Rocks and How They Were Formed by, Herbert Zim, Golden Library of Science, 1961.