Old Earth Ministries Online Geology Curriculum

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Geology - Chapter 4:  Rocks

    Minerals are the basic building blocks of the earth.  So, what is the difference between a rock and a mineral?  A rock can be a single mineral, or a grouping of several minerals.  Granite, for instance, is a rock which typically consists of three minerals, feldspar, biotite, and quartz.  On the other hand, chalk, which is a rock consisting of calcium carbonate, consists of a single mineral.

Types of Rocks

     There are three basic types of rocks, called igneous, sedimentary, and metamorphic.  A basic overview will be given here, and the next three chapters will each discuss each type individually.

Igneous

     Igneous rock originates when molten material called magma cools and solidifies.  This process may occur deep within the earth, or it may occur on the surface following a volcanic eruption.  Some examples of igneous rocks are  granite, diorite, basalt, and tuff. (Animation)

Sedimentary

     Sedimentary rocks form when the weathered products of previously existing rocks are deposited and solidified.  Geologists use the term lithify to say that a rock becomes hard.  Some of the sedimentary rocks are conglomerates, sandstone, shale, limestone, and gypsum.  (Animation)

Metamorphic

     Metamorphic rocks are formed from igneous and sedimentary rocks by the action of heat, pressure, and chemical change.  Metamorphic rocks typically form during episodes of tectonic movement, such as when mountains are being formed.  They can also be formed when a mass of magma is close to other rocks, with the primary agent of change being heat.  Some examples of metamorphic rocks include slate, schist, gneiss, quartzite, and marble. (Animation)

Rock Forming Minerals

     The vast majority of the earth's crust is made up of a relatively small number of minerals.  One of the most common rocks in earth's crust is granite.  Granite is an igneous rock composed of the following three minerals.

Feldspars.  Feldspar is usually a pink (can be other shades) rectangular mineral.  There is an actual series of minerals in the feldspar group, but the most recognized is orthoclase, which is the pink mineral that is in granite (also known as K-feldspar (k for "potassium")).  Feldspars can be found in igneous, sedimentary, and metamorphic rocks.

Micas.  Mica is black, and shiny, and can be identified easily because it flakes off in sheets.  In other words, it has perfect, one-directional cleavage.  The two common varieties of mica are biotite, which is the common, black variety, and muscovite, which is white or a shade of white.  Micas are common in igneous and metamorphic rocks.

Quartz.  Quartz is a clear mineral that is present in igneous, sedimentary, and metamorphic rocks.  It does come in colors.  As in many minerals, the different colors are caused by impurities in the mineral.  Sand is made of quartz.

     Another important class of minerals are the ferromagnesian minerals.  These minerals contain iron and magnesium.  These minerals range from dark green to black and have a high specific gravity.  We have already discussed one...biotite.  The others are not present in granite, but are common in a rock known as basalt.  The three major classifications of this group are...

1.  Olivines.  Olivenes are a green, glassy-appearing mineral.  It is a high temperature rock.  Usually does not form large crystals.

2.  Pyroxenes.  Pyroxenes are also high temperature minerals found in igneous and metamorphic rocks.  It is usually dark green to black.

3.  Amphiboles.  Amphiboles are a green to black mineral that produces elongate crystals that have two perfect cleavage planes.  Amphiboles are common in a metamorphic rock known as amphibolite.  The most common form of amphibole is called hornblende. 

Clay Minerals

     Clay minerals make up a major portion of the soil.  Clay minerals form at the earth's surface from the erosive actions of air and water upon the silicate mineral group.  They can also form from low temperature hydrothermal processes.  Clay minerals are microscopic, sheet silicates, with more than a dozen varieties identified. 

Calcite

     Calcite is calcium carbonate, and has the formula CaCO₃.  Calcite can be precipitated directly from seawater, or it can be removed from seawater by organisms, or it can be dissolved by water and redeposited, such as you see in caves with stalagmites and stalagtites.  Calcite is the main mineral in limestone.

Dolomite

     Dolomite is also a carbonate, but with some of the calcium replaced by magnesium.

Halite, Gypsum

     These minerals are known as evaporites.  As water evaporates, such as the Great Salt Lake in Utah, it leaves behind the dissolved minerals that were in the water.  Halite is another word for salt, or NaCl (picture at right).  Gypsum is composed of calcium sulfate and water (CaSO₄·2H₂O). 

Collecting Rocks

     Many children become interested in Geology by starting a rock collection.  There are several ways to go about collecting rocks.  Some basic rock collections can be purchased from rock stores, or over the internet, such as this site.  If you don't live near a rock shop, you can mail order from rock shops, or find rocks on Ebay. 

     Of course, the most fun and rewarding way to collect rocks is to go outside and find them yourself.  Many states have guidebooks that tell you about the geology of your state, or even where to find rocks. 

     Once you have a collection of rocks, you will need a method to keep track of them.  When I was young, my father made some wooden boxes, and we glued rocks onto wooden blocks, with the name of the rock on the block.  I don't recommend gluing rocks now, however.  Go to a store such as Wal-Mart and get a plastic container with drawers, usually found in the hardware section (they are used to hold loose screws, nails, etc).  You can also get a plastic box with dividers.  You can find these in the craft section of stores, and they are also sold as fishing tackle box trays.

     One thing I do now is keep track of the rocks with a computer database.  Each of my rocks has a number on it.  The number is applied with some white-out, and then I write the number on the white-out.  If the specimen cannot hold the white-out, or if it is too nice to apply white-out to it, it goes in one of the plastic boxes. 

     I've already mentioned many of the tools needed for collecting and identifying rocks.  Here is a list.

Geologist Pick (Hammer)

Small Chisel

Magnifying eyepiece

Streak Plate

Hydrochloric Acid in Bottle

Hardness kit

Magnet

Black Light

A good rock book (for field use...you can find everything on the internet when you are home)

Maps & Compass (for finding the rocks!)

     Professional geologists and serious hobbyists use the Brunton Pocket Transit compass.  I do not recommend that you get one of these, unless you are going to make Geology your major in college.  You can view them here.  Prices start at $239.  This website features higher-quality geological tools, so have a look around if you are curious.  I have never ordered from them, so I cannot vouch for this store.  Feel free to shop around on the internet.  For most high school learners, the supplies at Home Science Tools will suffice.

End of Chapter

Tuesday - Research

     Research the answers to the following questions about clay minerals.  Your parents may have you simply answer the questions, or they may have you put it in essay form.  Please follow your parents instructions.

What are the groups of clay minerals?

What is kaolinite (kaolin)? 

Why is kaolinite mined?  What is kaolinite used to produce?

Did you put some kaolin in your mouth today?

Is it radioactive?

Wednesday - Quiz

     Today you will complete an 10 question practice quiz.  The link to the quiz will open a new window.  You can come back here and check your answers.  Do not click the Back button on your browser during the quiz.  After the quiz, continue your research project, if necessary.

          Geology Chapter 4 Quiz

Thursday - Review

    Please review the terms in bold in the text, and ensure you have completed your research work from Tuesday.

Friday - Test

     Today you will take the end of chapter test.  Please close all other browser windows, and click on the link below.  During the test, do not click on the Back button on your browser.

          Geology Chapter 4 Test

     After you have completed the test, you may proceed to the next chapter on your next school day.  Please return to the introduction page for the link to the next chapter.

Return to the Old Earth Ministries Online Geology Curriculum homepage.

Helpful Links

Mineralogy Database (webmineral.com)

Amethyst Galleries' Mineral Gallery

Clay Minerals (Wikipedia)

Kaolinite (Wikipedia)