The following study guide is provided as an aid to help you identify the major concepts you should have learned concerning topics covered during the first few weeks of class. Key vocabulary with which you should be familiar is highlighted in UPPER CASE GREEN throughout this document and on future documents of this type for the course. You should, at the very least, be able to define the terms given in UPPER CASE GREEN in order to complete the exam. Ideally, however, I hope you will be able to do more than simply respond to definitions.

You will also find words underlined in blue - these can be clicked with the mouse and will transport you to images or various sources of additional material I think you might find interesting and helpful to your studies. I hope you will find the time to explore these links, as they are often quite fascinating and (I hope) will pique your interest in this course and Geology as a discipline. It has been my experience both as a student (remember, I was a student once, too! - and it wasn't that long ago!) and professor that the students who enjoy the course most are those who spend a little extra time getting to know it better. This is especially true of Geology - I have found that most people have a natural curiosity about Geology and geologic processes. My aim is to enhance your curiosity, provide you with information regarding the basic principles of Geology, and gain sufficient knowledge of these principles to be able to apply the definitions and the concepts they represent to a fuller understanding of the Earth as a planet and our role as residents of it!


Mankind has been fascinated by Earth and its surrounding planets in the SOLAR SYSTEM for centuries. However, it is only recently (over the last 40 years) that mankind has been able to venture off of this planet to our nearest neighbors. Amazingly, in the last 40 years, mankind has managed to send space craft to ALL of the PLANETS except Pluto. Even more amazing, we've succeeded in placing an assortment of exploratory space craft on the surfaces of three of our nearest neighbors in space, VENUS, THE MOON, and MARS.

We now have more detailed maps of the surface of Venus than we do of Earth. This is due primarily to the success of the Magellan Mission to that planet.

Of course, mankind's epic journey to the Moon is now the stuff of high school history classes.

Our recent fascination is with Mars because it bears many similarities to Earth. Coupled with the spectacular success of the Mars Pathfinder Mission, excitement within the field of PLANETARY GEOLOGY is greater than it has ever been, and this has caused all Earth scientists to reconsider Earth's place in the SOLAR SYSTEM and the UNIVERSE.


The acquisition of tangible data from other planets has permitted scientists to engage in the study of COMPARATIVE PLANETOLOGY. COMPARATIVE PLANETOLOGY is the study of planets (primarily in our own solar system, though planets outside our system are now confirmed) and analysis of their similarities and differences. By comparing similarities and differences among the planets, Planetary Geologists hope to learn more about important processes in the individual histories of planets and the implications of these processes for planetary evolution.

For starters, we can compare Earth to her nearest neighbors in space, termed the TERRESTRIAL PLANETS. The TERRESTRIAL PLANETS are those which are grossly similar to Earth in that they are composed of ROCKS similar to those which might be found on Earth and they are not too dissimilar in SIZE WHEN COMPARED TO EARTH. Some basic data on the terrestrial planets:

PLANET DIAMETER MASS (relative to Earth) DENSITY (g cm-3)
MOON 3476 km 0.01 3.3
MERCURY 4680 km 0.05 5.4
MARS 6796 km 0.11 3.9
VENUS 12,320 km 0.90 5.0
EARTH 12,756 km 1.00 5.5

A convenient measure of a planet's size is its DIAMETER. Note that EARTH is the largest of the terrestrial planets. However, the diameter of a planet can be misleading. Mars has a diameter about 50% as large as Earth, but is only about 10% (i.e. 1/10th) as MASSIVE.

Point to ponder: What is MASS? What information can be determined from the mass of the planet?

The MASS of a planet refers to the TOTAL QUANTITY OF MATTER making up the planet. The VOLUME of a planet is the TOTAL QUANTITY OF SPACE occupied by the planet and can be estimated mathematically by inserting the value of the planet's diameter into an equation for the volume of a sphere.

MASS is important when comparing planets because if we divide the MASS OF THE PLANET by the VOLUME OF THE PLANET, we are left with a term called the AVERAGE DENSITY. That is:


Thus, the AVERAGE DENSITY is THE MASS PER UNIT OF VOLUME (usually expressed in GRAMS PER CUBIC CENTIMETER or g cm-3) of the materials comprising the planet. DENSITY provides scientists with clues to the nature of material making up a planet because we have knowledge regarding the density of many materials, including most rocks, on Earth.

Note that the Moon has a diameter about 25% of Earth, but is only 1% (i.e. 1/100th) as massive.

Mars is only 10% (i.e. 1/10th) as massive as Earth.

The fact that these planets have such small total masses for their total volumes means that they have relatively LOW DENSITY. Indeed, the Moon and Mars are substantially less dense than Earth and this tells us something about their geologic histories. Venus, on the other hand, is almost as dense as Earth, suggesting that it may be composed a rather similar materials.

In what other ways is Earth the same of different from the other planets? A simple table can help us compare:

LIFE     X

Let's look in detail at some of these features:

As already discussed, Earth is a planet whose DENSITY is greater than some of the other Terrestrial Planets (Moon, Mercury, Mars) but close to that of Venus. By itself, this characteristic does not discriminate Earth from the other planets, so we indicate this as a non-decisive factor.

WATER is abundant on Earth. The OCEAN covers 70% OF EARTH'S SURFACE to an AVERAGE DEPTH OF 3.7 KM. In addition, a substantial quantity of water is stored a ice at both poles of Earth. This appears to be unique among the terrestrial planets - there is no water on Mercury or the Moon. Venus does not have water. Water is present on Mars, but not presently in liquid form and not nearly as much as on Earth. So the prevalence of water on Earth is unusual.

.Point to ponder: The surface of Mars bears evidence of liquid water (river valleys & lake basins) flowing across it at some stage of its history. What happened to it? Why isn't water there today?

The ATMOSPHERES of those planets having an atmosphere are interesting features for comparison. We have actually sample the atmosphere in all of the TERRESTRIAL PLANETS WITH AN ATMOSPHERE. The table below gives the chemical composition of those atmospheres:

CARBON DIOXIDE 97% 0.035% 95%
NITROGEN 3% 78% 2.7%
OXYGEN trace 21% 0.1%
ARGON 0.01% 0.9% 1.6%

Earth's atmosphere is unique among planets. The very low level of CARBON DIOXIDE (only 3/100ths of a percent) and extraordinary levels of NITROGEN and, especially, OXYGEN are unknown in other planetary atmospheres.

The SURFACE TEMPERATURE OF EARTH is somewhat odd, but this is a relative oddity. It could easily be argued that the SURFACE TEMPERATURE OF VENUS (about 750 Kelvin or 477oC, which, by the way is almost hot enough to melt rocks) or SURFACE TEMPERATURE OF MARS (click here for today's' Mars Weather Report) are equally odd. So temperature by itself is not very distinctive. In fact, the broad range of observed planetary temperatures has led scientists to use the term "Goldilocks Syndrome" in reference to Earth's rather mild temperature (Venus is too hot, Mars is too cold, but Earth is just right!).

Point to ponder: Why would anyone care about the weather on Mars?

The issue of LIFE and LIVING SYSTEMS is an important one in comparative planetology. Though there has been much recent discussion (even one very well publicized claim) that there may be Life on Mars, there remains no definitive, conclusive proof of this. Indeed, the present state of knowledge on this issue is that Life is known only from Earth. So far as we know, this is unique in the Solar System and the Universe.

A final piece of the comparative planetology puzzle is to examine the HYPSOMETRY of the terrestrial planets. By definition, HYPSOMETRY is THE DISTRIBUTION OF A PLANET'S SURFACE AT VARIOUS ELEVATIONS (for example, a planet might have 10 % of its surface is between 0 and 2000 meters, 10% between 2000 - 3000 meters, etc.).

Since mankind has surveyed a number of other planets, we now have good information regarding the DISTRIBUTION OF ELEVATIONS (HYPSOMETRY) of Venus, the Moon, Mars and Earth. The figure below compares the hypsometries of the planets.

The hypsometries of several of the terrestrial planets are shown here. Note that the HYPSOMETRIC CURVE of Earth is unique in that it is BIMODAL. The peaks on Earth's curve represent the MEAN ELEVATION OF THE CONTINENTS (about 800 m) and the MEAN ELEVATION OF THE OCEAN BASINS (about 4000 m deep).

The HYPSOMETRIC CURVES of terrestrial planets other than Earth are UNIMODAL, meaning that they have only one peak. Earth's HYPSOMETRIC CURVE is BIMODAL - it has two peaks. The peaks on Earth correspond to the continents (high elevations) and ocean basins (low elevations) and are a clue to earth's earliest geologic history.


The bimodal hypsometry of Earth occurs because the continents and ocean basins are formed of rocks with different chemical compositions, different minerals, and therefore, have different densities. These materials have separated from one another throughout the geologic history of the planet through a process called DIFFERENTIATION.

To a first approximation, DIFFERENTIATION is controlled by the influence of GRAVITY on the planet. As such, materials will DIFFERENTIATE or SEPARATE according to their DENSITY (remember - MASS VOLUME = DENSITY). Materials of HIGH DENSITY will slowly migrate to Earth's center whereas materials of LOW DENSITY will migrate towards Earth's surface.

The result of DIFFERENTIATION has been to produce a planet with a LAYERED INTERIOR STRUCTURE, where materials are arranged from the center outward in layers of DECREASING DENSITY.

The process of DIFFERENTIATION continues to the present day - Earth is still evolving and many of the GLOBAL GEOLOGIC PROCESSES which we observe (such as earthquakes and volcanic eruptions) are, in part, related to DIFFERENTIATION.

The outermost rocky layer of the Earth is very thin (averaging about 35km thick) and is called the CRUST. The CRUST is generally divisible into two primary components:

One component which is enriched in SILICON and ALUMINUM called SIAL. The SIAL is the material from which the CONTINENTS are made and is represented on the HYPSOMETRIC CURVE OF EARTH (below) by the upper peak. The average density of the SIAL is about 2.7 g cm-3.

The other component of the crust is enriched in SILICON and MAGNESIUM and is called SIMA. SIMA is the material from which the floor of the ocean basins is made and is represented on the HYPSOMETRIC CURVE OF EARTH (below) by the lower peak. The average density of the SIMA is about 3.0 g cm-3.

The next layer of the Earth is a liquid layer composed of WATER. This layer is represented by the OCEAN and is sometimes referred to as the HYDROSPHERE. It has a density of 1 g cm-3.

Finally, the outermost layer or our planet is the gaseous envelope which surrounds us, the ATMOSPHERE.


All matter is composed of ATOMS. Atoms are composed of PROTONS (sub-atomic particles with POSITIVE CHARGES), ELECTRONS (subatomic particles with NEGATIVE CHARGES) and NEUTRONS (subatomic particles with NO ELECTRICAL CHARGE).

Protons and neutrons are found in the NUCLEUS (center) of the atom. Electrons are found whirling around this nucleus.

It is the number of PROTONS IN AN ATOM which defines the ELEMENT. (For example, if you have an atom with 14 protons, this atom is the element SILICON).

ELEMENTS are substances which cannot be broken down by ordinary chemical methods. There are 118 ELEMENTS which have been discovered by chemists. However, of those 118 elements, only a few occur in abundance on Earth. All the others are relatively rare.

The table below lists the 8 MOST ABUNDANT ELEMENTS IN THE EARTH'S CRUST. Note that these 8 elements account for almost 98% of all matter in the crust!

IRON 5.0

Within certain limits dictated by laws of nuclear physics, variable numbers of neutrons can be added to an element without altering its chemical behavior. Atoms of an element containing DIFFERENT NUMBER OF NEUTRONS are called ISOTOPES. As an example, the element oxygen always contains only 8 protons in the nucleus, but commonly occurs with either 8, 9 or 10 neutrons.

The result of adding neutrons to an atom is to vary its atomic mass slightly.

Some atoms will BOND (or join) by sharing an electron or two. Such sharing of electrons is termed COVALENT BONDING.

Another type of ATOMIC BONDING is called IONIC BONDING. In nature, it is not uncommon for atoms to either GAIN or LOSE ELECTRONS. Of course, GAINING or LOSING ELECTRONS alters the ELECTRICAL BALANCE of the atom. Atoms with RESIDUAL ELECTRICAL CHARGE are called IONS.

Atoms which GAIN ELECTRONS (i.e. electrons have been added) will be observed to have a NET NEGATIVE CHARGE because electrons have negative charges associated with them. Adding an electron is adding a negative charge to the atom. IONS with NEGATIVE CHARGE are referred to as ANIONS.

Atoms which are DEFICIENT IN ELECTRONS (i.e. electrons have been removed from the atom) have a NET POSITIVE CHARGE. This is because particles carrying negative charge have been removed, leaving an excess of positively charged particles behind. IONS with POSITIVE CHARGE are referred to as CATIONS.

Generally, cations and anions will join in such a way as to cancel their net electrical charges, forming an electrically neutral COMPOUND (i.e. a substance with no net electrical charge). The resulting chemical bond is said to be IONIC (see Box 2.2, p.32 of your text).


Of all the compounds that exist in nature, some of these compounds are classified as MINERALS. What is a MINERAL? Why is it different from any other compound? Geologists have a very strict definition of a MINERAL.

In order for a material to be considered a mineral, it must meet five criteria:

1. The material must be a COMPOUND (i.e. formed from two or more elements).

2. The compound must be NATURALLY OCCURRING. Any synthetic or man-made compounds are excluded.

3. The compound must be SOLID. Liquids and gases, while natural substances are not solid and thus excluded.

4. The compound must have a DEFINITE CHEMICAL COMPOSITION.

5. The compound must have a DEFINITE CRYSTALLINE STRUCTURE.

When atoms form either covalent or ionic bonds and join in an ORDERLY THREE-DIMENSIONAL ARRANGEMENT, the substance formed is said to be CRYSTALLINE.

Point to ponder: Is water a crystalline substance? Is ice a crystalline substance?

NOTE: Your textbook declares one additional criterion, minerals are INORGANICALLY FORMED. However, this last specification is not strictly true, since SEA SHELLS are MINERALS but are not formed through inorganic means. There are also other minerals formed by biological organisms (e.g. PYRITE, MAGNETITE, APATITE), and these minerals are indistinguishable (at the atomic scale) from inorganic forms of the same mineral.

Points to ponder: Is water a mineral? Is ice a mineral? Is coal a mineral? Is petroleum a mineral? Are cubic zirconia minerals?

There are over 2500 known minerals on Earth.

The most important minerals are the SILICATES, formed primarily of the elements SILICON, OXYGEN and lesser amounts of ALUMINUM, IRON, CALCIUM, SODIUM, POTASSIUM, MAGNESIUM.

Coincidentally (or, perhaps not so coincidentally), the elements listed above are also the 8 MOST ABUNDANT ELEMENTS on Earth.


In hand specimens, minerals can be identified by their PHYSICAL PROPERTIES. There are a variety of physical properties of minerals which are useful in identifying an unknown mineral.

FRACTURE is a physical property displayed by all minerals and is used to describe the manner in which a mineral breaks. Types of fracture include IRREGULAR or HACKLY, CONCHOIDAL, and FIBROUS.

CLEAVAGE is a special type of fracture exhibited by many (but not all) minerals. It is a fracture which will occur along a distinctive plane or planes through a mineral and is related to the crystalline arrangement of atoms within the mineral. Because minerals have a distinctive crystalline structure, cleavage is a diagnostic aspect of a particular mineral or mineral group.

Also, because minerals have a distinct arrangement of atoms within them, the EXTERNAL CRYSTAL FORM of a given mineral will be unique and is a useful physical property. However, it is not that common to find well formed crystals of minerals and this is why items such as the quartz crystals mined in Arkansas are prized by collectors.

The HARDNESS of a mineral is a relatively reliable physical property and is measured on a scale from 1 to 10 with 1 (TALC) being the softest and 10 (DIAMOND) being the hardest.

Other minerals on the MOHS HARDNESS SCALE are GYPSUM (2), CALCITE (3), FLUORITE (4), APATITE (5), FELDSPAR (6), QUARTZ (7), TOPAZ (8), and CORUNDUM (9).

LUSTER is a physical property of minerals describing their relative "shininess". Typical terms used to describe mineral luster are METALLIC, NON-METALLIC, VITREOUS (glassy), RESINOUS, SILKY, PEARLY, WAXY, DULL.

Mineral COLOR can sometimes be used to identify a specimen. However, color often varies in extreme ways due to small amounts of impurities in minerals and geologists learn early not to trust color as an identifying feature.

Mineral STREAK on the other hand, is a very useful physical property and does not vary nearly as much as color. Streak is the color of a mineral in its powdered form. To obtain a streak, geologists scratch the mineral over a small piece of unglazed porcelain called a STREAK PLATE.

Finally, SPECIFIC GRAVITY is a very consistent physical property used to identify minerals. In words, the specific gravity of a mineral is the RATIO OF A MINERAL'S WEIGHT IN AIR TO THE DIFFERENCE OF ITS WEIGHT IN AIR AND ITS WEIGHT IN WATER.

Mathematically, specific gravity is calculated by weighing the mineral on a balance in air, then weighing the same mineral on a balance in beaker of water. Then, these two weights are compared in the following way:


The number represented by this quantity is the specific gravity of the mineral.


The basic building block of the silicate minerals is the SILICA TETRAHEDRON.

This is a pyramid-shaped molecule composed of 1 silicon atom (cation) surrounded by 4 oxygen atoms (anions). In the linked image above, the green spheres represent oxygen anions. The single silicon cation is not visible, but beneath the central oxygen anion (see Fig. 2.7, p.34 of your text).

The silica tetrahedron can be assembled in a variety of ways to produce a great number of SILICATE MINERALS which have been categorized into 5 CLASSES OF SILICATE MINERALS:


Some silicate minerals are formed of ISOLATED SILICA TETRAHEDRA loosely bound by cations (such as magnesium and iron) to form a mineral. The most common example of the isolated tetrahedral silicates is OLIVINE.


By recombining silica tetrahedra slightly, a new silicate structure can be formed. By allowing one corner of each tetrahedron to be joined to another tetrahedron, a chain of tetrahedra is formed (see Fig. 2.11, p.35 of you text). The family of silicates with this particular structure are known as the SINGLE-CHAIN SILICATES. The most common group of minerals with this structure are called PYROXENES and one of these which you will see in the lab is AUGITE.


Joining 2 rows of tetrahedral chains will produce the DOUBLE CHAIN SILICATE structure (see Fig. 2.9, p.34 of your text). Notice in this structure that there are large hexagonal spaces between the chains into which many larger cations (such as potassium or sodium) will fit. The most common group of minerals with this structure are called AMPHIBOLES and one of these which you will see in the lab is HORNBLENDE.


Increasing the complexity of the silicate structure by joining the bases of tetrahedra will produce the SHEET SILICATE structure (Fig. 2.9, p.34). This structure is characteristic of CLAYS and a group of silicates known as MICA. Common forms of mica include BIOTITE (shown here) and MUSCOVITE


Finally, the most complex silicate structure is that of the FRAMEWORK where each individual tetrahedron is joined at every corner to another tetrahedron (Fig. 2.9, p.32). Good examples of FRAMEWORK SILICATES are the minerals PLAGIOCLASE, POTASSIUM FELDSPAR, and QUARTZ.

Click on the button to view images of common rock-forming minerals

Click on the button to view truly amazing Virtual Reality images of different mineral structures! (Note: to view this site, you will need a Virtual Reality plug-in for your Web Browsing software - you can download a free one here.

Many people become fascinated with the intricate world of minerals and crystals every year. The links below provide you with just a few sites so that you might discover the hobby of "rock hounding" for yourself. Enjoy! :-)






Copyright 2002 Dr. Stephen K. Boss All Rights Reserved