HR: This question is possibly the most difficult of those I have assigned as it asks you to describe a subject you have yet to study. {Please note: the following dialogue did not take place.}

Scott: No problem.

The difference between physical and historical geology is that physical geology examines earth's rocks and minerals and seeks to understand the hundreds of processes that operate beneath or upon its surface.

HR: Amazing. Although I have a lot to criticize about that sentence, these are your thoughts?

Scott: No, I thought you would guess. The sentence is lifted straight out the textbook you assigned for this course.

HR: To avoid any hint of plagiarism, you should use quotes and credit the source or at least declare your references.

Scott: Right. I resubmit the line as:

To quote from Essentials,

"The difference between physical and historical geology is that physical geology examines earth's rocks and minerals and seeks to understand the hundreds of processes that operate beneath or upon its surface."

I have now made these my thoughts.

HR: The problem with any quote from a textbook, at the introductory level, is that the authors of these text themselves do not credit their sources. To avoid plagiarism, originality for these authors requires that what should be boilerplate (to use a legal term for text that has been well tested in court to be unambiguous - so that no further change to it should be made - and its repeated use has placed the phrase in the public domain to be used without citation) is mangled to the point of being plain misleading. For a start "rocks and minerals" in the sentence you quoted. Rocks are not all made of minerals, but all minerals are rocks. Does the author mean to imply that rocks and minerals are completely different things? So best use an inclusive term such as "earth materials."

Scott: (After discussion with HR I have decided that)

The difference between physical and historical geology is that physical geology examines earth materials and seeks to understand the hundreds of processes that operate beneath or upon its surface, and ....

HR: Woah! Geology, literally "earth science" is the study of earth materials and so both physical and historical geology study earth materials.

Scott: OK.

Geology is the study of earth materials (geo, or ge means earth, ology means study). Geology can be subdivided into physical and historical geology.

The difference between physical and historical geology is that physical geology seeks to understand the hundreds of processes that operate upon and beneath its surface.

HR: Good, I see that you have switched the order of "upon and beneath." Historically, it was the study of the landscape and what processes seen operating changes it, that led Hutton (the founder of geology) to an understanding that the scenery and, by inference, rocks as well, had a natural (not supernatural) explanation.

Scott: To continue (about physical geology),

It deals with the nature of the earth, the development of its surficial features and the results of the interaction of chemical, physical, and biological factors on the planet.

HR: The problems I have with that are in {}

It deals with the nature {What do you mean by this word? I think, a given is that science does not deal with the unnatural and so, if that is what you mean, then the phrase is at best redundant}of the earth, the development {What do you mean by that word? If you use it, for it to be meaningful to your readers, you should state from what to what.) of its surficial features and the results of the interaction of chemical, physical, and biological factors on the planet. {A high sounding essentially meaningless phrase. None of the information can be remembered as it is attached to nothing. The mind is left spinning.}

Scott: OK. I will drop all those statements. To quote from Essentials,

“Historical geology on the other hand seeks to understand earth's origin and how it is changed over time and strives to establish the chronology of physical and biological changes of the past 4.5 billion years.”

HR:

Historical geology on the other hand seeks to understand earth's origin {It does not, only astronomy can have anything to say about what that might have been.} and how it is changed over time {It does not, only physical geology can have anything to say about "how."} and strives {Please!!} to establish the chronology of physical and biological changes of the past 4.5 billion years {since when or what, you must say}[since the earth became a stony body].

Scott: In conclusion, to quote from Essentials:

They [physical and historical geology] are related in that: in order to study earth's history we must first understand how earth works before attempting to unravel its past. In other words, in order to understand historical geology we must have a strong knowledge of physical geology.

HR: {Those last two sentences have some truth: the same in each and in as many words in each. So, why, unless this is a mantra, say it twice?}
     

If you are "modern" you are so because you understand that the natural world is not just old but that it is incredibly so. For people of Western traditions, this profound truth was first published in 1788 by James Hutton. He had arrived at his insight by inductive reasoning from what can be seen of the world as it exists.

Given an old earth, the science of geology becomes possible. In earlier times, the distribution of the earth materials and their discovery had been a matter of serendipitous prosecuting. Nor had there been any reason to suppose, before Hutton, that we live in a world in which the landscape is not original.

Geology developed slowly for the world is a large, complex, place and field work is arduous. Charles Lyell’s Principles of Geology (revised 11 times between 1830 and 1872) advertized the truth that ongoing geological process, by the effects they have had on the landscape, can be known.

Geology developed as a mature science by the end of the last century. For us, most significant is that where we live, our farms, mines, cities and their infrastructures, date to the before. As a result, we find ourselves in a world that is woefully planned for the now known geological realities. In almost its every aspect, our built world needs to be drastically retrofitted, or be differently planned. This is why all who wish to live in a successful democracy (in which the public makes informed choices about what serves the public best) should be educated in geology.
     

Raymond offers the following in the style of Jacques-Yves Cousteau

Geology affects our everyday lives in the following way: the air we breathe, rocks and mountains, and oceans. Even though we don't hear much about volcanoes, they too have an effect on the people who live near them. Rocks are all over the place; they are underneath the ground, in the parks, the zoo's, forests, and on the highways. Mountains are also a beautiful sight, and it makes you wonder where did they actually come from? The way it affects us is if you drive upstate or anywhere else that has a mountain view, it will be right in front of you. The air we breathe is called oxygen, which is very important to all of us. Without oxygen we wouldn't be alive at all. Oceans are everywhere really, I think it is the second most important element of geology other than oxygen. We see oceans everyday either through the media (tv news, newspapers) or visualize them ourselves. Maybe I misunderstood the question, but to me that is how geology affects everyday lives.

Geology benefits modern society though discovery of no longer easy to find minerals and fuels and contributes to the responsible mining, extraction and production of these; has application to engineering site choice and development projects; location, production and conservation of ground water; warnings of natural hazards; reduction of risks from natural hazards; safe waste disposal; and environmental monitoring and remediation.
   

Two industries that employ geologists are:

1) land development and construction

The land is surveyed and, to know how safe it is to build on, the geologist determines what processes have shaped the land and evaluates in terms of ground stability, site modification, and setting which in the short and long term will be ongoing and advises on how they should be addressed. The land development and construction geologist will be directly involved in the assessment of the saftety, the insurability, and the environmental impact of the project.

2) exploration and mining

To find more of what prospectors have found, the exploration geologist studies the circumstance of the known to formulate strategies to find that which is hidden in the vicinity or at remote locations. Mine geologists keep current geological maps of the workings which they sample and assay to project ore reserves and determine directions for further mine excavationing. Exploration and mining geologists are not directly involved in environmental impact assessments of projects. The exploration geologist will plan, with permission, where to trench and drill to verify the potential mining value of places. The mining geologist will indicate where ore is located to the mining engineer who takes on the responsibility for its safe and environmentally sound extraction.
           

The earth is layered. This is known mostly from the way that layers effect the transmission of earthquake disturbances. The layers have different chemical compositions and mechanical properties.

The different compositional layers are the crust, the mantle, and the core. The crust and mantle are rocky. A common mineral of the crust is feldspar. The composition of the mantle is that of the rock peridotite. The core is almost pure metallic iron.

Early in the history of the earth, the accumulating primordial whole separated into mantle and core. The heat required to partly melt the whole, and allow this to happen, would early have been produced by gravitational compression. Likely, the downward separation of the material of the core released enough energy to completely melt the earth. Since that early time, the earth has been slowly cooling. Since the mantle became a solid, the crust has been accumulating. The process is: Local partial melting of the mantle occurs wherever, in addition to the heat left over from the formation of the planet, radioactive isotope produced heat raises the temperature sufficiently. The magma so produced moves upward (is buoyant) and the adds to the crust.

The crust is the very thin, outermost rocky layer of the earth. It's thickness is from 5 to 10 kilometers thick in oceanic areas. In continental area it's thickness is about 35 kilometers beneath plains to over 70 kilometers in some mountain belts. Below the crust, the mantle is a 2885 kilometer thick rocky layer. Below the mantle is the core. The core is divided into an outer- and inner-core. The outer-core is a layer of still molten iron. It is about 2270 kilometers thick. As it cools iron crystal that form sink to add to the inner core. The inner core is a solid sphere of accumulated iron crystals. It has a radius of 1216 kilometers.

The different mechanical layers, of greatest importance to geology, are the lithosphere and the asthenosphere. The lithosphere is outermost rigid shell of rock comprised of crust and upper mantle to a depth of about 100 kilometers. The lithosphere (Gr. Lithos means rock) is underlain by the asthenosphere. As its name implies (Gr. asthenos means weak), the asthenosphere a is relatively soft layer in the upper mantle. It is about 100 kilometers thick. However, beneath oceanic ridges, where the lithosphere is only a few kilometers thick, the upper surface of the asthenosphere (occupying what would otherwise be a gap) is at high elevation.
          

Charles Lyell’s Principles of Geology, 1830 gives example after example of evidence of changing rock formations and life in existence for eons of time before (historical) time. On the progress of geology he could write: "Many appearances, which had for a long time been regarded as indicating mysterious and extraordinary agency, were finally recognised as the necessary result of the laws now governing the material world; and the discovery of this unlooked-for conformity has at length induced some philosophers to infer, that, during the ages contemplated in geology, there has never been any interruption to the agency of the same uniform laws of change."

In short, geology demystifies the earth
              

Up to the end of 18th Century, science was inductive, and technology was by trial and error. The one was theortical, the other was practical.

The industrial revolution came about when scientific laws of thermodynamics were formulated and the technologies of converting energy from one form to another became a reality. What had begun was  technology based on science, and science being advanced by these technologies (Watt's steam engine and later Tesla's electrical generator and transformer). Also arose was the hypothetico-deductive scientific method whereby induction led to an hypothesis from which deduction provides for observations to be made that if found establish the hypothesis as theory or if not found falsify the hypothesis.
              

Geology, the study and understanding of the geological working of the world,  has become a reliable applied science, in the last two hundred years, and dramatically so, since the 1960's.  Significantly where we live, our farms, mines, cities, and their infrastructures, date to before the now known geological realities and so need to be drastically retrofitted, or be differently planned. This is why all who wish to live in a successful democracy (in which the public makes informed choices about what serves the public best) should be educated in geology.
               

Gaia is also called geophysiology. Unlike ecology, in which the living system studied plays in an environment, in geophysiology the two, the organic life and the inorganic environment, are studied as a coevolving whole.
          

There are at present seven major plates along with other intermediate and smaller plates.

The plates move with respect to each other in different directions at slow rates amounting to a few centimeters a year. On a human time scale, the effects of the plate movements are earthquakes and volcanoes. On a geological time scale, the effects of the plate movements are the coming to being of continents, continental mountains, continental drift, and the opening, transcurrent faulting and closing of oceans.
       

Plates move as rigid units relative to all other plates. If so, to either side of a line of contact, two plates must either be moving away from each other, or toward each other, or are sliding past each other. Correspondingly, each plate itself must have three types of boundaries 

1. Divergent boundaries: where plates, to either side, are moving apart. The result would be a gap but for the upwelling of material from the mantle which cools laterally to become the trailing edge of each plate. The new seafloor produced spreads away from the line of separation of the plates

2. Convergent boundaries: where plates, to either side, are moving together. The result is that one plate passes beneath the other The plate that moves over is said to obduct, The plate that moves under is said to subduct. The subducting plate also sinks along its leading edge into the mantle. As it does do it heats and becomes indistinguishable from the surrounding mantle below a depth of about 600 kilometers

3. Transform fault boundaries: where plates, to either side, are moving horizontally past each other. 

The three types of boundaries are different in that at:  

1. Divergent boundaries: lithosphere is produced

2. Convergent boundaries: lithosphere is consumed 

3. Transform fault boundaries: lithosphere is neither produced nor consumed. 

Physiographic features, long known, and now understood to mark the three different types of plate boundaries, are:

1. Mid ocean Ridges (divergent plate boundaries). 

2. Oceanic Trenches (convergent plate boundaries). 

3. Strike-slip Faults that link between Ridge and Trench ends (transform fault boundaries).

Metamorphic rocks are formed by the recrystallization of a previously existing rock be it sedimentary, igneous, or metamorphic. Recrystallization usually results in metamophic rock with a different texture, and often different mineral composition, than the original rock. In what is called a metavolcanic, or a metasediment, some of the textural features of the original rock survive. If the chemical composition of the metamorphic rock indicates the original rock, the metamorphic rock name is prefixed with ortho- (for igneous) or para- (for sedimentary).
          The age of a metamorphic rock is when its recrystallization ended.
          

Three types of rock are igneous, sedimentary, and metamorphic. Each has a distinctly different origin (as wll be discussed below). Hybrids occur. Their provenance is described in terms of the three rock types. Other earth materials (not rocks) are, for example: soil, water, salts in solution, air, colluvium (unconsolidated particulate material on a slope), sediment (solids in suspension or in transport as in dunes or stream bed gravels), and sediments (accumulations of transported materials). The rock cycle does not help us understand the origin of these rocks. The origin of rocks was inferred from their texture, composition, and field associations.

The rock cycle is a concept that does help us organize our understanding of rocks in ways that suggests how the materials of the earth can cycle though such states if the earth is indeed very old. That latter hypothesis (that the earth is exceedingly older than historical or human time) is tested by deducing how the variety of rocks in each type can be explained in terms of evident processes. This enquiry ongoing in the two hundred years since James Hutton conceived of the rock cycle, has established that its every part requires, for the circulations of earth materials though these, the passage of geological lengths of time.

The explanatory power of the rock cycle early proved to most peoples’ satisfaction that the earth is millions of years old. Corroboration, for the earth’s great age, was the discovery, near the beginning of the 20^th century, of radioactive elements that act as clocks and give absolute geological ages. The age of the earth was so calculated to be 4.6 billion years old. This is certainly plausible as astronomers have since found that the earth is in a universe that is at least three times older.

A cycle has neither beginning nor ending but a description of the changes that recur can begin with any of them and we can choose to begin a description of the rock cycle with rock exposed at the earth’s surface.

Rock weathers to soil [rock does not weather to sediments] when exposed to the gases of the atmosphere and surface waters. It weathers mechanically by physical breakup and chemically by chemical reactions, such as hydrolysis, to soil (the physical components of which are fragments of the original rock and minerals, and new products, which are clay and salts).

The removal of soil, or the direct wearing away of bed rock by such agents as mass wasting, running water, wind, glacial ice, is called erosion. Agents of erosion transport materials as sediment, and deposit them as sediments.

An accumulation off sediments can, in time, lithify (means "become stone") to sedimentary rock (The word harden is not used as it can apply to igneous rocks),

Lithification processes, collectively referred to as "diagenesis," include: compaction (example: clay to shale), cementation (example: gravel to conglomerate), and partial recrystallization (example: lime mud to limestone).

[Note: Pressure by itself does not lithify sand to sandstone: its diagenesis is mostly by cementation and also, sometimes, by partial recrystallization and crystal overgrowths.]

Burial of sedimentary rock subjects it to an increase of pressure of the overburden and to increase of temperature by the blanketing effect of the overburden that allows the sediment to heat by conduction towards equilibrium, at its depth, with the temperature gradient of the whole earth.

The minerals of a rock are stable only within certain ranges of temperature and (depth related hydrostatic and, imposed from without, directed) pressure, and chemical environment (their contact with other minerals and mineralizing fluids in the rock). Change of any of these beyond the limit of stability for a rock's minerals can cause them to recrystallize. Recrystallization, without melting, of a preexisting rock as a result of such environmental change(s) is metamorphism: called specifically contact metamorphism when due mainly to a change of temperature (example: hornfels), dynamic metamorphism when due mainly to shear (deforming) pressure (example: mylonite), and regional metamorphism when due to both a change of temperature and directed pressure (examples: slate, schist, gneiss).

A rock can heat to the point of melting. The heat can be from radioactive sources in the rock, by advection into the rock from igneous sources, or by conduction into the rock when it has been carried deeper into the earth.

[Note: Increase of pressure cannot cause a rock to melt. Quite the reverse! Increase of pressure can cause a melt to solidify (An example of a solid due to pressure is methane hydrate at water depths greater than 300 meters in continental rise sediments). Decrease of pressure can reduce the melting temperature of a rock so that at a given temperature within the earth a decrease of pressure can cause melting. Decompression melting explains the origin of basaltic magmas that result from partial melting of asthenosphere peridotite in its rise beneath oceanic ridges.]

Recrystallization of crustal rock requires the presence of a fluid (such as H₂O, or CO₂) in the rock to allow for the reorganization of its crystalline materials. When components of this mediating fluid, and/or salts brought in by it, become part of the crystals of the recrystallizing rock, the process is called metasomatism. To indicate that a bulk composition is different from that of the original, the resulting metamorphic rock can be referred to as a metasomatic rock.

Partial melting of a foliated metamorphic rock produces a magma that, if still this mixture, solidifies as an igneous rock called a migmatite.

Magma where it forms is usually less dense that the country rock and, when it more than merely wets the material it is between, gravity causes it to rise buoyantly. Rising magma intrudes and makes room for itself at a level where it achieves neutral buoyancy. At this higher level in the earth, it will cool and begin to crystallize. When the magma between crystals grown from it can no longer keep volatiles in solution. so called "second boiling" begins. The separated volatiles exert gas pressure that can result in volcanic activity.

Extrusions of lava cool and degas to solidify as volcanic rock. Volcanic gas explosions produce pyroclastics that accumulate and compact as tuff deposits.

[Note: Intrusions of magma and extrusions of lava solidifies (hardens) to igneous rock (they do not only cool to as they may degas to, nor do they necessarily crystallize to) The wording, "crystallizes to igneous rock" is wrong as this is not so for vast volumes of natural glasses - such as obsidian, pumice, scoria, and tuff (these amorphous "solids" are not crystalline)].

The energy that drives rock cycle at the earth’s surface is from the sun and within the earth is from primordial and radioactive decay produced heat.
           

Uniformitarianism is the philosophy that the known world can be accounted for by processes still in operation. This principle, formulated by James Hutton allowed him to offer explanations for the features of rock strata that required only that the earth is extremely old. That possibility, at the time, was entirely against Church canon and was unanticipated by other naturalists who, to explain similar features of the earth, had imagined catastrophic causes. This alternative, the then paradigm of natural sciences, which allows for a young earth and requires miraculous explanations, is known as Catastrophism.

The philosophy of uniformitarianism includes a narrower principle, called Actualism, which is that the laws and constants of physics, chemistry and biology are unchangeable, and were so, also, in the geological past.

Uniformatarianism made modern geology possible in that, given a great length of time, natural explanations for the origin of the landscape, rock formations, and organic fossils can be formulated. The success of modern geology justifies the philosophy of uniformatarianism which has since become the paradigm of all the natural sciences.

Uniformitarianism thinking is to assume the whatever forces and processes working now in or on the earth, have been at work for a very long time and have remained unchanged. To understand the landscape and ancient rocks, we must first understand present day processes and their results.

However, we do need to consider how, and to what extent, environmental constraints in the past could have been different, and so processes and rates, without violating actualism that physical, chemical and biological laws stay constant. Also within these constraints, natural catastrophes, however remarkable in human terms, are uniformitarian in that they can be assumed to have occurred frequently in the course of geological time.
      

Science gives us a logical understanding of how and why things happen or don't happen. For example, it lets meteorologists predict the weather, doctors treat people that are sick, and lets geologists move forward with their study of the earth, and assure, say, that, although LA should be prepared for a great earthquake, California will not then vanish as did mythical Atlanitis.

Scientifically literate people contribute to all by coming up with new ideas and thoughts of how to do things and how to make things. A liberal scientific education helps us identify follies and triumphs, past and present, of our and other societies. Having a scientifically literate populace is the best guarantee that laws governing implementation and funding of scientific research and technologies will be democratic. Such lets us improve physical comforts and find leisure to enjoy life in this world.
       

Elements are those substances that alchemists, try as they might, could not divide into other substances. The different elements have different weights. Listed in order of increasing weight, the elements occur as groups of metals and nonmetals. In 1869, Dimitry Ivanovich Mendeleyev, famously dreamed that (and on awaking saw that) the line up of 63 then known elements could be broken so as to form a table with a column of metals and one of nonmetals. Gaps in the pattern could then indicate, he suggested, what was still to be found. He was right.

Today, we can talk of a list of naturally occurring elements in the Periodic Table of Elements that runs from 1 (hydrogen, the lightest) to 92 (uranium, the heaviest). Of this list, missing in nature on earth is 42 (technetium, identified in the spectra "late-type" stars) and 61 (promethium, made only by physicists who, with atom smashing devices, can do what alchemists never could).

Atomic theory gives a simple explanation for the different elements. Elements are made of atoms. The atoms of different elements are different by the count of protons in their nuclei. An element’s place in the list of elements is the same as number protons in the nuclei of each of its atoms. Thus, the number of protons in each atom of the element H is one, ... in Tc forty two , ... in Pm sixty one , ... and in U ninety two.

Elements, except for the inert gasses, are chemically reactive and tend to form compounds. Compounds, as their name implies, are composed of two or more elements. Compounds are not mixtures as the different elements in them are chemically bonded to each other. Thus compounds can be separated into their component elements only by processes that break chemical bonds.

On earth, special circumstances need to be for elements, other than the inert gases, to be found as pure substances. A native element is a naturally occurring mass of a pure element. Examples are workable masses of pure sulfur, graphite, carbon, copper, zinc, lead, tin, mercury, silver, platinum, antimony, arsenic, gold. To find such a deposit is double surprising for these elements are chemically reactive and are among the rarest elements in the earth's crust. Examples of gaseous native elements are oxygen, argon, helium.
          

A model of an atom due to Bohr is of a planetary system with electrons, each with a single negative charge, in orbit about a positively charged nucleus. (Note: this vision of an atom is very flawed but it is the standard one used for simple discussions of the ways that atoms chemically bond to each other.)

Chemical bonding of atoms involves only their electrons.

In the modern model of an atom (more advanced than Bohr's), electrons occupy shells of successively greater radii (distance measures are statistical means) centered on the nucleus. Protons in the nucleus can attract matching numbers of electrons. These electrons are now known to go into quantum states called electron shells. The Pauli Exclusion Principle states that each electron must have a unique set of quantum numbers. A shell can contain one up to the limit of electrons.that can fit into its orbitals. Innermost is the s-shell (2 electrons maximum), and, sequentially outward from this are p-shells (6 electrons maximum), d-shells (10 electrons maximum), and f-shells (14 electrons maximum). The arrangement of the periodic table exhibits the ocurrence of these different types of shells.

The number of electrons that an isolated atom has is equal to its atomic number (which is the number of positive charges in its nucleus). Not all the electrons are involved in chemical bondings. Only electrons in the outermost shells are, and they are called valence electrons.

Atoms are electrically neutral when the number of positive charges in the nucleus is screened off by an equal number of orbiting electrons. In ionic bonding atoms give up or receive electrons. An atom that loses one or more valence electrons to another atom will itself have a nucleus no longer fully screened and will have an overall positive charge equal to the number of electrons it lost. The reverse is true for the atom that gains electrons.

An atom with an electrical positive or negative charge is called an ion.

Opposite electrical charges attract. Positive ions attract negative ions. Two or more ions held together by electrical attraction are said to have formed an ionic bond. The amounts of ions in an ionic compound are in the proportions that their electrical charges balance (equal). Example: Sodium ions (each with a single positive charge) and chlorine ions (each with a single negative charge) in equal amounts form the ionic compound, and electrically neural, table salt (halite): Na⁺ + Cl^- = NaCl.

The inert gases (in the Bohr model) are elements with filled electron shells. The atoms of all the other elements have electron shells that are not filled. 

In covalent bonding, atoms in close proximity fill their valence shells by sharing pairs of valence electrons (each atom contributes one electron to each pair so formed). The sharing can be between atoms of the same element (example: O₂) or between atoms of the different elements (example: H₂O).
        

A crystalline substance is comprised of one crystal or a mosaic of crystal grains of the substance. A crystal is a solid in which chemically bonded atoms or molecules are in a definite crystal structure with a pattern that repeats. The smallest subdivison of a crystal with this pattern is called a unit cell. Atoms within a unit cell can be chemically balanced by bonding to the atoms that surround them, but those that are at the surface of a unit cell can only be chemically balance if they bond to atoms without. The most complete way for a unit cell to have its surface atoms chemically balanced is for it to bond with atoms that continue its structural pattern in all directions. So although a unit cell is are far too minute to be seen, any can be the template for the ordering of the incredibly large numbers of atoms needed to form a crystal large enough to be visible.

During growth, atoms are added to a crystal's surface in a systematic way evidenced by the growing crystal having a geometric shape. The bounding surfaces of a crystal that is growing (or could continue to grow) are called faces. Crystals grow by the orderly adding of atoms to their crystal faces. The rate at which atoms are added to crystal faces is determined by the reactivity of the crystals atoms exposed to chemical reactions in that surface. This means that a crystal with a cubic arrangement of atoms in its unit cells will not necessarily grow into a crystal with the form of a cube but its chemical composition could cause it to grow with, say, octahedral or tetrahedral form.

If there is not enough space for the crystal to grow unhindered it will increase only until it meets something which gets in its way and then stop. Often many small crystals begin forming at the same time, and they grow until their edges meet at varying angles. They do not join to form a single large crystal but rather an interlocking mosaic of small crystals forming a polycrystalline (poly means many) mass. In such crystalline substances, the adjoining faces of the crystals are called the grain boundaries. Unlike crystal faces, which are ideally flat, grain boundaries are usually irregular and curved in shape.
      

Components of crystals can be atoms (example: Na and Cl in halite) or molecules (example: H₂O in ice) or both (example: Fe and SiO₄ in olivine). In a crystal these components are held in a fixed geometric arrangement by chemical bonding between them. This structure is not rigid. All crystalline substances are elastic, the mean position of the components can be displaced by stress, and heat energy is stored in vibrations of the components about their mean positions in the crystal structure.

The temperature of a crystal can be raised when energy that enters (by conduction or radiation) is stored by more vigorous vibrations of its components. At some point, as heat is applied, the vibrations of the components carries them so far from their mean positions that chemical bonds, which hold the structure elastically together, become broken and translational motions of the component are not all elastically reversed. Supplying more energy to the crystal has the effect of increasing the number of components that store energy in free translational motions. The temperature of the whole remains at this "melting temperature" until enough heat energy has entered to effectively break all the elastic bonds. The amount of heat energy required to change a crystal (a solid) to a liquid at the melting temperature is called the latent heat of melting (to change 1 gram of ice to 1 gram of water, requires the addition of 80 calories of heat. The reverse also.).

In a liquid, chemical bonding forces that would hold together two or more components are exceeded by components' motions when these are in away directions. The temperature of a liquid is a parametric measure of the average motions of the components.

Liquids (or solids) exert a vapor pressure. What this means is that the fastest of the some of the speeding (or vibrating) components escape their surfaces. The phenomena is called evaporation. On average more fast moving components exit than those with slow speeds (or vibrations). Evaporation decreases the temperature of the liquid (or solid) as the average speed (or vibrations) of components not exited will be less..

Wide separations between the escaped components has the effect that they are not chemically attracted to each other. The components in this state are a gas.
       

Crystalline materials are macromolecules and, like a molecule, the composition of any can be expressed as a chemical formula. However,  crystalline materials do not all have a definite composition. Some crystalline materials have sites that can be occupied by different elements. For example, plagioclase has sites in its structure that can be occupied by either Ca or Na. This is indicated by writing (Ca, Na) in the formula for plagioclase. The proportion of Ca to Na that occupy these sites in a plagioclase is determined by temperature at the time of crystallization. Deceasing temperature, favors Na over Ca. Plagioclase can be described as a solid solution with respect to Ca and Na in its structure.

Liquids and gasses can be mixtures and, when so, their composition cannot be expressed by a chemical formula.
      

The crystal habit of a mineral refers to the way in which the mineral commonly occur. Crystals habits include: equi-dimensional crystals, elongate crystals(blades or nedles), flattened crystals (plates).massive formless lumps(massive), rounded lumps (botryoidal), spherical (spherulitic or "oolitic"), fibrous (asbestiform), branching crystals (dendritic), pairs of crystals with fixed angles between them (twins); multiple sets of crystals with fixed angles between them (multiple twins or "polysynthetic" twins). 
     

The earth, outside of the core, is mostly made of oxygen, silcon, and aluminum.  For example, the atoms of the continental crust are 62 % O, 22% Si, 7% Al, and atoms other elements.

Ions of silicon Si⁺⁴ strongly attract ions of oxygen O^-2. Each ion of silicon packs about itself as many (of the abundantly availble) oxygen ions as will fit. The closeness of packing is determined by the size of the oxygen and silicon ions  Oxygen atoms are comparative large compared to the size of silicon atoms and are a little more so when they are ions. The packing of the oxygen about a silicon ion at the center, with no room left for additional oxygens to fit, is a tetrahedral arrangement of four oxygen atoms.

The attracting silicon ion forms ionic bonding with the four oxygens and the oxygens on being bought close together bond covalently with each other. The resultant ionic and covalently bonded silica tetrahedron [SiO₄]^--4 is an essentially indestructible but highly reactive compound ion. By their abundance they dominate the chemistry of the earth's crust and upper mantle (below a depth of 600 km, pressure forces a different arrangement).
            

A single silica tetrahedron [SiO₄]^-4 is called a monomer in that it can polymerize. That is, one monomer can chemically link to another. In the case of silica tetrahedron polymers, covalent bonds are form between an oxygen of one silica tetrahedron with an oxygen of another. Examples of silica tetrahedron polymers are: single chain  [SiO₃]^-2, double chain  [Si₄O₁₁]^-8, sheet silica  [Si₂O₅]^-2,  and  framework [SiO₂].

------------> to be continued      
          

Silicate minerals are those that contain silica tetrahedrons. Of the eight common rock forming silicates minerals, olivine has only monomers ( [SiO₄]^-4 ). In the others are polymers: single chains in pyroxene, double chains in hornblende, sheets in the micas, and three dimensional open structures in the feldspars and a three dimensional closed structure in quartz.

The common rock forming silicates can be divided into two groups: those with iron and magnesium, the ferromagnesians: olivine, pyroxene, amphibole, and biotite mica and those without iron and magnesium, the nonferromagnesians: muscovite mica, feldspars, and quartz.

When viewed in thin section under a microscope in unpolerized transmitted light, the ferromagnesians are melanocratic (have color) and the nonferromagnesians are leucocratic (are colorless).
       

Carbonate minerals of sediments, sedimentary rocks, or marble are typically: transparent, white, lightly colored, with a white streak, (less common, hydrothermally produced carbonate ore minerals have intense colors with a pale streak of that color), average or slightly above average heft, soft (the mineral Calcite is the reference mineral for Hardness 3 on Mohs Scale), with good to perfect cleavage, and soluble to some degree in acidic solutions.

The ion [CO₃]^-2 is a triangle of three oxygens surrounding a carbon. This threefold symmetry explains the trigonal symmetry of carbonate minerals.
      

Crystals are made of chemically bonded atoms. The strength of bonding between components of a crystal can favor the propagation of cracks in some planar directions. Such minerals have the property of cleavage. Cleavage shows as planar partings or flat shiny surfaces where a mineral has been stressed or has been broken.

Our knowledge, or shall I say our society's knowledge of the formation of the earth and its properties, specifically the mineral composition of rocks, would not be there were it not for the study of geology

The solid part of earth is made up of rocks. Rocks in turn, are composed of minerals, and all minerals, except for imaginary Kryptonite, are made of chemical elements.

The earth is comprised of surprisingly few minerals (see below). The properties of minerals are independent of the history of how they were formed. Mineral properties are explainable in terms of the crystal arrangement and chemical nature of the atoms in them. Minerals are compounds and a few are elements. Minerals that are elements are called native minerals. Examples of native minerals are sulfur, diamond, graphite, gold, and copper.

Both elements and compounds have definite chemical and physical properties that are unique enough to enable us to distinguish one from another. All consist of particles called atoms. The elements in the earth's crust rarely exist by themselves as (except for inert gases) being chemically active, they occur combined with other elements as compounds. Compounds consist of molecules, which are groups of atoms joined together in definite proportions.

There are more than 4000 known minerals (added to by one or two discoveries a year) but common minerals number about twenty (20) and of these, only nine (9) constitute 95% of the earth's crust. These latter nine (9) minerals (see below), referred to as the "rock-forming minerals," are all silicates (that is, they contain silica-oxygen, SiO₄ ^-4, tetrahedrons).

The silicates that are called the rock-forming minerals can be subdivided into two groups: mafic and felsic

Mafic rock-forming minerals are : Biotite (dark mica), Hornblende, Augite, Olivine, Ca-Plagiogclase (feldspar). The first three are dark (almost black to greenish black in color. Mafic rocks that have an abundance of one or more of these dark minerals are consequently dark in color (as is so for dunite, an igneous rock). Ca-Plagiogclase (feldspar) is gray light, white, or transparent. As a result, mafic rocks that have an abundance of Ca-Plagiogclase can be light in color (as is so for Anorthosite, an igneous rock)

Felsic rock-forming minerals are: Quartz, Muscovite (white mica), Feldspar, Orthoclase (K-feldspar), Plagiocase (Na, CA-feldspar)/Albite (Na-feldspar). The felsic minerals are light in color and felsic rocks are therefore typically a light color (as is so of granite, a felsic, phaneritic, igneous rock in which the quartz is visible. However, the feldspars in granite can occur as dark red or black varieties. When so, the rock is dark in color.

A crystalline substance is comprised of one crystal or a mosaic of crystal grains of the same substance. A crystal is a solid in which chemically bonded atoms or molecules are in a crystal structure (with a geometric pattern that repeats). The smallest subdivison of a crystal with this pattern is called a unit cell. Atoms within a unit cell can be chemically balanced by bonding to the atoms that surround them, but those that are at the surface of a unit cell can only be chemically balance if they bond to atoms without. The most complete way for a unit cell to have its surface atoms chemically balanced is for it to bond with atoms that continue its structural pattern in all directions. So although a unit cell is far too minute to be seen, any can be the template for the ordering of the incredibly large numbers of atoms needed to form a crystal large enough to be visible. During growth, atoms are add to a crystal in a systematic way evidenced by the growing crystal having a geometric shape. The bounding surfaces of a crystal that is growing (or could continue its growth) are called faces. Crystals grow by the orderly adding of atoms to their crystal faces. The rate at which atoms are added to crystal faces is determined by the reactivity of the crystals atoms exposed to chemical reactions in that surface. The means that a crystal with a cubic arrangement of atoms in its unit cells will not necessarily grow into a crystal with the form of a cube but its chemical composition could cause it to grow with, say, octahedral or tetrahedral form.

If there is not enough space for the crystal to grow unhindered, it will increase in size in a direction only until it meets something which gets in its way and then stop. Often many small crystals begin forming at the same time, and they grow until their edges meet at varying angles. They do not join to form a single large crystal but rather an interlocking mosaic of small crystals forming a polycrystalline (poly means many) mass. In such crystalline substances, the adjoining faces of the crystals are called the grain boundaries. Unlike crystal faces, which are ideally flat, grain boundaries are usually irregular and curved in shape. Such boundaries are particularly evident in metals which have formed by fairly rapid cooling of the molten form. During the cooling process innumerable small crystals form and grow until they bump into a neighboring crystal. Crystals can form from the cooling or evaporation of solutions, or from the cooling of molten solid, or the cooling of vaporized substances.
     

Lavas can loose heat by both radiation from their upper surface and by conduction with cooler rock over which they flow or in chance contact with rain, lake or ocean water. Aiding their ability to loose heat, lavas can flow to spread out into thin sheets. The rate of cooling of lavas is likely to be relative rapid compared to the circumstances of intrusive magmas. Unsurprisingly, the crystal size of volcanic rocks is likely to be finer grained than that found in plutonic rocks of the same composition and volume. What is found is that component crystals of crystalline volcanic rocks are usually too small to be seen by the naked eye. As a result, the volcanic rock looks dull. That igneous rock texture is referred to as aphanitic (Gr. phaneros means apparent, aphanitic means not apparent).
           

When unordered ions of a melt freeze in position, before they can move and come together in an orderly crystalline structure, the cooling product is a solid with a glassy texture. Natural glass is a naturally occurring inorganic substance that solidifies from magma (molten material wihich forms inside the earth) when this has cooled too quickly to crystallize.

Volcanic eruptions extrude lava. Volcanic glass is formed by rapid cooling of lava. The surface of a lava flow, which can loose heat easily (by radiation or by conduction in contact with water), and so chill rapidly, will invariably be glassy. However, for thick thick flows to solidify as glassy masses throughout, the  lava must, as a cofactor, have high viscosity. High viscosity slows the rate of crystallization and allows for slower cooling to still result in a glass, as the final product, rather than a crystalline mass. However, in addition to some trapped gas bubbles, volcanic glass often contains crystallites (tiny crystals) that show skeletal-crystal form. 

Types of natural glass:

Obsidian is a black variety with a brilliant luster. 

Pitchstone is dull and is frequently brown, green or gray. 

Pumice is a light rock and consists of a frothy glass with tiny gas bubbles about 1mm in diameter.

Perlite is a gray to green glass with spherical cracks which cause it to crumble into small pelletlike masses.

The water content of natural glass varies. It is usually 1% in obsidian, 3-4% in perlite and 4-10% in pitchstone. Much of the water is lost as vapor when the lava erupts. Some water in perlite and pitchstone is believed to come from the absorption from the sea. The formation of glass from magma comes from rapid cooling and (if its volume is large) high viscosity. The composition of most natural glass is granitic.

An interesting piece of information I found is that because of its conchoidal fracture volcanic glass was highly prized among cave men as material for their tools and weapons.

True oceans have abyssal depths (seas are comparatively shallow). The Mohorovicic discontinuity [see Chapter xx.x], which demarks lower boundary of the crust, shows that, wherever the ocean is a true ocean, the oceanic crust is a rocky layer 5 to 10 km thick.

Deep sea drilling has confirmed that the oceanic crust beneath relatively thin layer of covering sediments is igneous and is basaltic in composition. Seismic studies have shown oceanic igneous crust is two layered. Remarkably in some places the entire ocean crust and mantle on which it rests has been raised above sealevel (by obduction) and the up-ended sequence of rocks (the whole called an ophiolite) is exposed in horizontal section (by erosion). These exposures show that beneath oceanic sediments, the upperpart of the oceanic igneous crust is made of basalt flows and feeder dikes and the underpart is a thicket of gabbro feeder dikes and interspersed sills of gabbro.

The details of the oceanic crust are explainable in terms of plate tectonic theory. In the model, the oceanic crust is added to the top of lithospheric plates where these diverge (the line of divergence is marked by an oceanic ridge). Below there, hot asthenosphere rises buoyantly to occupy the gap that would otherwise be left by the movement away of the lithospheric pates to either side. The rising lithosphere remains hot as it rises but the pressure on that which rises in the earth must decrease (as the overburden thickness lessens). Less confining pressure means lower melting temperature. Beneath ridges, rising asthenosphere undergoes decompression melting. The magma produced by the partial melting of the its ultrabasic mantle rock (peridotite) is what rises to accumulate as the igneous basaltic oceanic crust.

The lithosphere is created by the cooling of asthenosphere to either side of at the line of divergence. The oceanic basaltic crust that accumulates as its upper part does not reach a thickness of more than about 5 km before it is moved away from where basaltic magmas are rising. In some places divergence is so rapid that the upper part of the diverging pates is basalt free and the seafloor crust there is hydrated peridotite. Where the ocean crust is more that 5 kilometers thick (which is so for much of the northwest Pacific) the additional thickness of basaltic igneous rock there is due to later additions of material from hot spot sources (see Chapter xx.x)
       

In sedimentary strata, beds of volcanic ash are called tuffs. The degree of induration need not be great. The tuffs are recognized by their containing glass or (because devitrification will have removed this attribute in ancient tuffs) by their chemical composition. Tuff layers, which geologically accumulate instantaneously across all environments, have great value in establishing time horizons in sediments.
          

Central-vent volcanoes that are active, or dormant, have a conical form that makes them easy to recognize to be volcanoes. All volcanoes eventually go extinct. But how can we know that a volcano is extinct and not just dormant? The answer is that an extinct volcanoes is one that has been inactive for so long that prolonged erosion has altered its appearance to the extent that only a trained observer sees any evidence of a volcano.

The flanks of conical volcanoes are made of flows of lava rock and layers of ash. When the volcano is in the process of being built, magma rises up a tube-like pipe from a magma chamber deep beneath the volcano to vent as lava or ash. This pipe is called the neck of the volcano. In between times of activity, magma not poured out, which fills the neck, begins to solidify. But, while the volcano is dormant, there is not usually not enough time for the neck filling magma to become completely solid. The next eruption pushes this previous magma out of the neck of the volcano. When the volcano goes extinct, magma in the volcanic neck solidifies completely.

Erosion usually has an easier time in removing the mechanically weaker flank lava and ash layers than the rock that occupies the volcanic neck. This is evident as the rock that crystallized from the magma that filled the neck often stands spire-like above the surrounding terrain from which any once cone has long vanished. Examples are: Devils Tower National Monument, northeastern Wyoming [1] [2] [3], Shiprock, New Mexico [4] [5].
    

A batholith is a vast granitic pluton with an outcrop area of more than 100 square km (40 square miles). The name batholith (Gr. bathos means deep) is because the bottom of these are nowhere exposed, nor has mining or drilling reached their base. However, imaging techniques revealed that the batholith's rock bottoms at depths of less than 15 kilometers. The igneous rock of the batholiths is commonly granite, granodiorite, quartz diorite (tonalite), or quartz syenite (monzonite) and is never as mafic, as say, diorite or gabbro (although these can be present as inclusions). Batholiths are not monolihic but are comprised of many large bodies of magma that rose as buoyant plebs which then spread out at levels of neutral buoyancy. Those that rose to shallow depths stoped their way into the brittle roof rock (as roof pendants, where they exist, show, as do the common inclusions of country rock fragments—zenoliths). In mobile belts, the emplacement of batholiths is post-orogenic, as their discordant contacts evidence (example: Sierra Nevada Batholith, California), and in shield areas, is anorogenic (example: Wolf River Batholith, Central Wisconsin). The granitic rock of a batholith is usually foliated near its boundary. The last magma to cool in each can be dry, as evidenced by dikes of this material, which cut the outer part of the pluton and country rock, that are fine- to medium grained suggary apearing igneous rock of quartz, potassium feldspar and sodic plagioclase, called aplite.

Batholith magmas, when emplaced into the ductile lower part of the upper crust, spread and can drag aside the roof rocks to fold these into marginal synclinoria (examples: Archean gneiss mantled domes, Pilbara, Australia.)
       

During the crystallization of a magma, the common rock forming minerals that crystallize have sites in their structures for O Si AL K Na Ca Fe Mg. Left out of the growing crystals are all the other elements present in the magma. These, therefore, become more and more concentrated in the remaining magma.

In the later stages of crystallization, volatiles like H₂O and CO₂ become so concentrated in the lessening and cooling melt that they cannot be held in solution in the magma and separate out. These fluids are very good solvents. From the outset they are saturated by all the components of the magma. The amount of each material that the separating fluids can have in solution decreases as the temperature falls. This happens as the fluids, driven off by the heat of the magma, make their way along fissures that extend out through already crystalline igneous rock and into the country rock. Not surprisingly these hydraulically opened fissures start to become filled with materials that crystallize out of the fluid flowing though them. As the fluid has little viscosity to impede crystal growth and the growing crystals are bathed in the fluid that is passing over them (and which has the materials for their continued their growth) the crystals in the fissures usually attain a large size. The  igneous rock so produced has course texture, called pegmatitic, and the igneous rock itself is called a pegmatite.

It would be nice (but nature has no heart) if only the material that cannot fit into the common rock forming minerals was in solution in the driven off fluids, but this is not the case. The fluids move off essentially saturated with the common rock forming elements along with what cannot fit into the common rock forming minerals. Pegmatites are predominantly large crystal of common rock forming minerals but, far along a fissure, that material having crystallized out no longer arrives. The last of the common rock forming material still in solution at low temperature is usually just quartz. This crystallizes out as vein quartz (an igneous rock). This is where, if present in solution, gold can become concentrated enough to also crystallize out of solution. The product is vein quartz with a disseminated crystals of gold. Example: Homestake Mine (gold), South Dakota.

The fluid moves on now depleted in what it started out with but quite capable of dissolving material from the country rock it passes though and carrying these forward to deposit them elsewhere. Example: Comstack Lode (silver), Nevada. (see also Epithermal Ore Deposits, Chapter xx.x)
    

Lavas extruded from volcanoes have been witnessed to congeal as rock. The origin of such "extrusive rocks" is not in doubt. In such a location, rocks not witnessed to have formed, but which have the same composition, texture, and field associations as the such known volcanics, were inferred to be of the same origin by Abbé Anton Moro (1687-1750).

In areas far from where there is active volcanism, rock appearances was not sufficient to convince. In Saxony, Werner in his Short Classification and Description of Rocks (1787) formalized his theory that out of a once universal ocean separated crystalline chemical precipitates and that, on these oldest rocks, younger layers are alternations of chemical precipitate and detrital sediments. In his scheme, layers of coal are only a little older than the alluvial deposits which rivers and floods are presently witnessed to deposit. He incorrectly surmised that volcanism is caused by the subterranean burning of coal. Volcanic rocks, as a result, are all young.

In Britain, Hutton in Theory of the Earth, 1795, used field observations to arrive at the conclusion that masses of finely crystalline basalt, of what ever age, are igneous.

Fined grained rocks are hard to study for their mineral differences. Some appearing-to-be- basalts (in fact indurated shales) contain fossil shells and this bolstered what came to be called the Neptunists’s case.

Hutton’s friend James Hall, was able to show by experiments that he began in 1805, that basalts can be melted and on cooling the basalt recovered and this bolstered the what came to be called Vulcanists’s case.

The Neptunist vs.Vulcanist (or Plutonist) debate had no resolution other that it died away as basalt is patently insoluble in water whereas igneous theory provides for tests and observations that support a volcanic origin for basalts and a plutonic origin for rocks of similar mineralogy, called gabbros, that have solidified (out of site) as intrusive bodies.