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Chapter 3 - Basic Geologic Principals

This chapter focuses on the basic concepts (background information) fundamental to the modern science of geology. Concepts related to the physical geography of our world are essential to understanding how gem deposits form and where they occur (Figure 3-1). This introductory chapter focuses on basic geologic concepts including the geologic time scale (Earth history as measured in thousands, millions, to billions of years), physical processes and products formed in the natural environment (above and below the observable landscape) as described by the "rock cycle." Perhaps the culmination of geologic research has been the development of Plate Tectonics Theory, a collection of ideas that are useful in explaining nearly all aspects of the geology of our planet, and other planets as well. This chapter introduces fundamentals of geology that will be used over and over in the following chapters.

Why Geology?

Geology defined is "the study of the Earth." It includes the scientific study of the origin, history, and structure of the earth and/or the structure of a specific region of the earth's crust. "Geo" derived from old Greek language is the combining form "of or relating to the Earth" (in such words like geography, geopolitics, geochemistry). However, in modern times, the word "geology" now also applies to the scientific study of the origin, history, and structure of the solid matter of a celestial body (such as "the geology of Mars").

The science of geology is subdivided into many disciplines. Three general subdivisions are presented here, all of which overlap with each other and with other science disciplines including physics, chemistry, oceanography, geography, and others.

Physical geology
is a branch of geology concerned with understanding the composition of the earth and the physical changes occurring within it, and is based on the study of rocks, minerals, and sediments, their structures and formations, and their processes of origin and alteration. Related disciplines include volcanology (the study of volcanoes), seismology (the study of earthquakes), mineralogy and crystallography, sedimentology (the study of sediments), geohydrology (the study of water), geophysics, geochemistry, marine geology, and others.

Historical geology
is the study of the composition, relative positions, etc., of rock strata in order to determine their geological history. Historical geology is dependent on concepts and order of events related to deep time, as defined by a geologic time scale (discussed below). Note that "nothing is written in stone" when it come to understanding earth history. There are great spans of earth history where little information is available. The dynamic processes shaping the earth surface not only produce new rocks but destroy others in the process, erasing whole records of periods of Earth's history. Historical geology has subdivisions including paleontology (the study of fossils), micropaleontology (study of fossil plankton), and petroleum geology (examining the occurrence and origin of energy resources underground). The oil industry employs the greatest number of geoscientists throughout the world.

Geomorphology is the study of the earth's surface including classification, description, nature, origin, and development of landforms and their relationships to underlying structures. Geomorphology is the "geologic companion" to the study of physical geography. Geomorphology now applies to the examination of landscape features on Earth and other planetary objects. It also crosses into the examination of the processes (physical geology) that formed the landscape, but also the changes that have occurred over time to create the feature that are observable today (as with historical geology). Understanding physical geography has always been a primary goal of exploration efforts through history and around the world. Explorers crews typically had a geographer/scientist on board who's job was to map the layout of the land, chart it resources (water, minerals, peoples, etc.), and then decision makers use the data collected to make to interpret what was discovered, and then decide what to do next. This same process continues today, whether it applies to water or mineral exploration, or the exploration of other planets.

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Precious gemstones
Fig. 3-1. Gemstone form in many different geologic environments.
Volcanic eruption of Pu'u'o'o Volcano on Hawaii
Fig. 3-2. Physical geology examines processes that form or alter rocks.
Ammonite fossil
Fig. 3-3. Historical geology examines the evolution of life and changes to the planet over time.
Great Basin National Park
Fig 3-4. Geomorphology is the study of landforms and their origin.

A Very Brief Summary of the History of the Science of Geology

The fundamental concepts of geology have evolved over centuries of research and exploration, involving many tens of thousands of scientists and curious novice observers and mineral prospectors alike. Knowledge of how the Earth formed and how materials are distributed across the landscape come from many sources through history, partly driven by curiosity, but more often by demands created by human need, greed, and desire, sometimes relating to war and conquest. The quest to find riches related to mineral resources (metals, gems, energy, and rocks and minerals for manufacturing) extends back into prehistoric times. The evolving map of the world's continents and oceans through the centuries were an essential component of this effort. The advances of modern ocean-floor mapping was driven by the needs of naval warfare during and following World War II. Exploration of space has lead to expansion of knowledge about the age and origin or the universe and the Earth itself. However, in was perhaps in the 18th Century that many of the basic concepts essential to understanding geology were first worked out. Most of the world's continents have been mapped, and much is known about the submarine geography ocean basins. However, ongoing research (and reinvestigation) is constantly adding new knowledge about our planet Earth.
Map of the world showing continents and ocean basins
Fig. 3-5. The entire world has been mapped—both continents and ocean basins. Space is the final frontier!

Geologic Time Scale

The geologic time scale used today has evolved through the past two centuries as new scientific discoveries have been made and new technologies for dating the age of earth materials have become available. Figure 3-6 shows a simplified version of the Geologic Time Scale for North America. The most recent version of the geologic time scale is released by the Geological Society of America as updated versions become available.

Note that the notions that the Earth being "old" (measured in billions of years) has not been all that popular with some fundamentalist religious groups throughout the ages, but even religions evolve over time. The primary arguments about the age of the Earth and the observable universe have been resolved by the global scientific community, but paradigms have ways of shifting as new discoveries are made and new information becomes available, and those ideas are tested by scientific methods. Vast periods of time in earth history are fundamental parts of understanding biological evolution of life on earth (paleontology), understanding genetics, particularly related to human evolution, and in astronomy explaining the vastness and age of the observable universe. Geologists have subdivided periods in Earth's history is measured periods spanning millions of years (Ma). Segments of time periods have been named to help define the chronology of events (such as the uplift of mountain ranges), the formation of rock units (such as the age of a lava flow), the age of fossils, etc. Names of geologic time periods (like "Late Cretaceous" or "Pleistocene") are used for organizing geologic map units, charting the age or petroleum-bearing rock layers underground, and perhaps hundreds of other purposes.

College courses in historical geology examine what is currently known about the age of the Earth and the events as they are known or inferred to have occurred. For this course, the name of geologic time periods are used to explain the age of when gem deposits formed, and where and how they occur in relation to other rocks and deposits associated with them. For example, some diamond deposits may have formed deep underground in the Precambrian Era (billions of years ago), but were exposed by erosion, transported, and deposited in river gravel beds in the late Tertiary Period (only several million years ago). Every rock (and gem) has a history!
Geologic time scale
Fig. 3-6. Geologic Time Scale (North American) gives names to ages of earth history. The time scale has been refined over time.

Early contributions to geology

Early investigations in geology focused on describing landscape features, classifying rocks, minerals, geologic features, and mapping. As explorers returned with discoveries and maps from missions around the world, libraries and museums began to fill with enough materials for people to begin to recognize patterns in data. Most of the early works in modern geology came out of Europe's scientific community.

Although many thousands of individuals have contributed important ideas, several people stand out for making important early contributions, often at the risk of their own lives and well-being. Italian physicist and astronomer, Galileo Galilei (1564-1642) used an early telescope and discovered four large moons of Jupiter. He promoted the theory the the sun, not the Earth, was to center of our solar system. In 1615 he was subjected to the Roman Inquisition. He was forced to recant his beliefs and subjected to house arrest for the remainder of his life. (Note that the Roman Catholic Church eventually accepted his theory and officially forgave him in 1992!)

Although fossils have been marveled at throughout history, it was heresy to describe them as ancient life forms (Leonardo Da Vinci believe fossils were ancient life forms, older that the stories in the biblical book of Genesis, but he only wrote about it in secrecy). It was was a Danish Catholic bishop, Nicolas Steno (1638-1686), who first promoted science of the origin of fossils and the basic geologic principles. A fossil is a remnant or trace of an organism of a past geologic age, such as a skeleton or leaf imprint, embedded and preserved in the earth's crust. Nicolas Steno's work advanced the science of stratigraphy, a branch of geology concerned with the systematic study of bedded rocks and their relations in time and the study of fossils and their locations in a sequence of bedded rocks. Note that a stratum is a bed or layer of sedimentary rock having approximately the same composition throughout (plural is strata).

A Scottish physician, James Hutton (1726-1797) studied rocks and landscapes throughout the British Isles and promoted "uniformitarianism" a theory that all geologic phenomena may be explained as the result of existing forces having operated uniformly from the origin of the earth to the present time. He fearlessly debated that the Earth was very old, measured in millions of years rather than thousands of years as promoted by the religion organizations of his times. Many scientists of his time promoted a theory of catastrophism, a theory more compatible with established religious doctrine that major changes in the earth's crust result from catastrophes rather than evolutionary processes. It is interesting that today, uniformitarianism still applies, but discoveries have show that the Earth, or large regions of it, have experience great "catastrophes," such as asteroid impacts or great volcanic events, but these events can be scientifically viewed within the greater context of modern geology. Uniformitarianism explains how observable processes taking place over long periods of time can change the landscape. Examples include:
  • earthquakes only happen occasionally, but in an area taking place over millions of years can result in the formation of a mountain range.
  • the deposition of silt from annual floods over millions of years can built a great river delta complex.
  • the slow growth and accumulation of coral and algal material over time can build a great barrier reef.

James Hutton also contributed to a theory of "rock formations." A rock formation is defined as the primary unit of stratigraphy, consisting of a succession of strata useful for mapping or description. A rock formation typical consists of a unique lithology (rock type) that has a relatively defined geologic age and is considered "mapable" (occurs throughout area or region, both on the surface and in the subsurface.

William Smith (1769–1839) used Hutton's theories to create the first geologic map of the British Isles. William Smith's map named Delineation of the Strata of England and Wales with part of Scotland published in 1815 was the first geologic map of an entire country. As nations began to understand the importance of geologic mapping for evaluating their natural resources, the science of began to grow. Once researchers had access to the distribution of materials and landscape features, they began to try to understand how and why landscape features like mountain ranges formed. What could explain the distribution of continents and oceans around the world? Why were some regions rich in certain kinds of mineral resources and others were not?

During the same period, there was an explosion of knowledge was happening in the world of biology. Swedish biologist, Carl Linnaeus (1707-1788) began the biological naming scheme of binomial nomenclature, establishing a logical way to chart and classify life forms. Taxonomy gave later scientist a means to classify both modern and ancient life forms. This helped Charles Darwin (1809-1882) to first propose a theory of evolution, an essential component to explaining the distribution of fossils through the geologic ages.

It can be argued that all the science and discoveries of geology didn't matter much to the average citizen of the world until the discovery of the Spindletop Oil Field in 1901. It was this discovery in Texas that started the modern Petroleum Industry. This happened the year before Henry Ford started his automobile company. This discovery initiated the growth and expansion of the world largest and most profitable industry—Big Oil. The demand for oil and mineral resources over the following century created opportunities for hundreds of thousands of people to be employed as geoscientists.

The explosion of knowledge gain from the Petroleum Industry is only part of the story. The discovery of mineral resources, particular gold and other precious metals, as well as large diamond and other gem deposits have created "rushes" and "Boom to Bust" communities around the world. The advertising of the potential of finding gold in California in 1849 lead to one of the greatest human migrations in modern history, even though the amount of gold found in California pales to other locations around the world.

Grand Canyon
Fig. 3-7. Rocks exposed in the Grand Canyon of Arizona range in age from 2.8 billion years old to some recently formed volcanic rocks.
Highlights of Earth History
Fig. 3-8. A brief summary of selected major geologic events in Earth history.
William Smith's 1815 map of England, Wales, and parts of Scotland
Fig. 3-9. William Smith's geologic map of 1815 was the first attempt to map an entire country.
(British Museum of Natural History)
The Lukas Gusher started the oil rush to the Spindletop Oil Field in Beaumont, Texas
Fig. 3-10. The "Lucas Gusher" in 1901 started the rush to the Spindletop Oil Field in Beaumont, Texas.
(Beaumont Historical Society)

The "Rock Cycle"

Charles Lyell (1797-1875) compiled the first geology textbook entitled "Principles of Geology" in which he promoted concepts of the "rock cycle" (Figure 3-11). The rock cycle illustrates the series of events in which a rock of one type is converted to one or more other types and then back to the original type. The "rock cycle" is a graphic and conceptual model to illustrate common rocks and earth materials and the processes that form or change them. There are 4 classes of rocks and earth materials: igneous rocks, sediments (which are not rocks), sedimentary rocks, and metamorphic rocks.

Igneous rocks are rocks formed from the cooling and crystallization of molten materials. Igneous rocks includes intrusive rocks (rocks cooled below the surface) and rocks formed on the Earth's surface by volcanism (and from melting associated with extraterrestrial impacts). Rocks of igneous origin are discussed in Chapter 5 and 6.

Sediments are solid fragments of inorganic or organic material that come from the weathering of rock and soil erosion, and are carried and deposited by wind, water, or ice. Sediments are discussed in Chapter 7.

Sedimentary rocks
are rocks that have formed over time through the deposition and solidification of sediment, especially sediment transported by water (rivers, lakes, and oceans), ice ( glaciers), and wind. Sedimentary rocks are often deposited in layers, and frequently contain fossils. Sedimentary rocks are discussed in Chapter 8.

Metamorphic rocks are rocks that was once one form of rock but has changed to another under the influence of heat, pressure, or fluids without passing through a liquid phase (melting). Metamorphic rocks are discussed in Chapter 9 and 10.

These rock cycle diagrams illustrate how earth materials form and change over time. Both products (rocks and sediments) and processes (such as melting, cooling, erosion, and deposition) are illustrated. The passage of geologic time is an essential component, although some processes are much faster than others. Note that all these types of processes are taking place simultaneously, but at different locations on and within the planet.
The Rock Cycle
Fig. 3-11. The Rock Cycle: processes are in blue; products are in black.
Rock Cycle Illustrated
Fig. 3-12. Rock Cycle Illustrated. This version of the rock cycle is the same as above, but showing more detail in graphic form. It is good to compare the two diagrams.

Basic Geologic Principles

James Hutton first proposed several basic geologic principles that were later embellished by Charles Lyell. These basic principles are easily observed in geologic outcrops, but have value for any number of scientific and technical applications beyond geology. Figure 3-13 illustrates the three "laws" that are used in resolving the age of rocks and the order in which they formed or geologic events occurred. The three laws are as follows:

Law of Original Horizontality—this law states that most sediments, when originally formed, were laid down horizontally. However, many layered rocks are no longer horizontal.

Law of Superposition—this law states that in any undisturbed sequence of rocks deposited in layers, the youngest layer is on top and the oldest on bottom, each layer being younger than the one beneath it and older than the one above it.

Law of Cross-Cutting Relationships—this law states that a body of igneous rock (an intrusion), a fault, or other geologic feature must be younger than any rock across which it cuts through.
Basic geologic principles
Fig. 3-13. Basic geologic principles illustrated.

Unconformities: Gaps in the "geologic record"

Following on the Law of Original Horizontality and Law of Superposition, both Hutton and Lyell recognized erosional boundaries preserved between rock layers representing "gaps in the geologic record." They named these gaps unconformities. An unconformity is a surface between successive strata representing a missing interval in the geologic record of time, and produced either by an interruption in deposition or by the erosion of depositionally continuous strata followed by renewed deposition. Several types of boundaries are recognized:
  • nonconformity—an unconformity between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock (see Figure 3-15).
  • angular unconformity—an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers (see Figure 3-16).
  • disconformity—an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition (see Figure 3-17).
  • conformable boundary—an arrangement where layers of sedimentary strata are parallel, but there is little apparent erosion and the boundary between to rock layer surfaces resemble a simple bedding plane(see Figure 3-18).
Types of unconformities
Fig. 3-14. Types of unconformities (boundaries between layered rocks)
How do unconformities form?

Unconformities are caused by relative changes in sea level over time. Wave erosion wears away materials exposed along coastlines, scouring surfaces smooth. On scales of thousands to millions of years, shorelines may move across entire regions. Erosion strips away materials exposed to waves and currents. New (younger) material can be deposited on the scoured surface. Shallow seas may flood in and then withdrawal repeatedly. Long-lasting transgressions can erode away entire mountain ranges with enough time.

A transgression occurs when a shoreline migrates landward as sea level (or lake level) rises.

A regression occurs when a shoreline migrates seaward as sea level (or lake level) falls.

Sea level change may be caused by region uplift or global changes in sea level, such at the formation or melting of continental glaciers. Whatever the cause of sea level change, when sea level falls, sediments are eroded from exposed land. When sea level rises, sediments are typically deposited in quiet water settings, such as on shallow continental shelves or in low, swampy areas on coastal plains.
Formation of unconformities
Figure 3-15. Unconformities can form by the rise and fall of sea level. Erosion strips away materials exposed to waves and currents. New (younger) material is deposited on the scoured surface.
Examples of unconformities and conformable boundaries in the Grand Canyon of Arizona
nonconformity in the Grand Canyon disconformities in the Grand Canyon Angular unconformity in the Grand Canyon Conformable contacts in the Grand Canyon
Note: These scenes show named rock units used on the geologic map and legend shown in figures 3-31 and 3-32 below.
Fig. 3-16. Nonconformity in the Grand Canyon (known as the "Great Unconformity") Fig. 3-17. Disconformities between sedimentary formations in the Grand Canyon Fig. 3-18. Angular unconformity between sedimentary rocks of different ages Fig. 3-19. Conformable or gradational contact between sedimentary layers  


The Law of Cross-Cutting Relationships requires the introduction of geologic structures found in rock outcrops. Igneous intrusions are structures that can be found in all kinds of shapes and sizes (discussed in detail in Chapter 4).

Figure 3-14 shows different kinds of folds. Folds are geologic structures formed by the process of folding—the bending or warping of stratified rocks by tectonic forces (movements in the Earth's crust).

anticline—a fold in layers of rock where the concave side faces down, with strata sloping downward on both sides from a common crest.

syncline—a trough or fold of stratified rock in which the strata slope upward from the axis; opposite of an anticline.

plunging folds—folds that are tipped by tectonic forces and have a hinge line not horizontal in the axial plane.

domes—a deformational feature consisting of symmetrically-dipping anticlines; their general outline on a geologic map is circular or oval.

—structural basin is a large-scale structural formation of rock strata formed by tectonic warping of previously flat lying strata. Structural basins are geological depressions, and are the inverse of domes. Some elongated structural basins are also known as synclines.
types of folds
Fig. 3-20. Types of folds: anticlines, synclines, plunging folds, domes, and basins. The patterns created by folds in rock formations are observable on geologic maps.
Examples of kinds of folds
Anticline along NJ Route 23 Plunging syncline in the Fitzgerald Marine Preserve Domes and folds in the Bighorn Basin of Wyoming Upheaval Dome in Canyonlands National Park, Utah  
Fig. 3-21. An anticline and a syncline along Route 23 near Butler, northern New Jersey. Fig 3-22. Example of a plunging syncline,
Fitzgerald Marine Preserve
San Mateo County, CA.
Fig. 3-23. A satellite view of plunging folds in the
Bighorn Basin, Wyoming.
Fig. 3-24. Example of a dome—Upheaval Dome in Canyonlands National Park, Utah.  

Fractures and Faults

Earthquakes are a sudden, often violent shaking of the ground that can cause great destruction. Earthquakes are a result of movements within the earth's crust caused by the release of stress energy when rocks break or may be cause by volcanic activity. Tectonic forces that can fold rocks can also break them, resulting in fractures, or in cases where offset occurs, faults. Rocks deep underground can be shattered by earthquake shock waves. Most rocks exposed at or near the surface display fractures of all sizes. Most of those fractures are do not display significant offset, they are simply cracks. Cracks in rocks that do not show apparent offset are called joints.

A fault is a fracture or crack along which two blocks of rock slide past one another. This movement may occur rapidly, in the form of an earthquake, or slowly, in the form of creep. Types of faults include strike-slip faults, normal faults, reverse faults, thrust faults, and oblique-slip faults. Faults can be small to large complex systems of interlinking faults and may change form one kind of fault in one location to another kind somewhere else.

Faults are classified into several varieties based on their orientation of the fault plane to the surface and the nature of the movement of one side to the other. The two sides of inclined fault planes are called the foot wall and the hanging wall. The foot wall is the underlying block of a fault having an inclined fault plane. The hanging wall is the rocks on the upper side of an inclined fault plane.

Types of Faults

normal fault
—a fault in which the hanging wall appears to have moved downward relative to the foot wall. The dip angle of the slip surface is between 45 and 90 degrees. Many normal faults in mountainous regions form from gravitational pull along mountainsides and may be associated with the headwall escarpment of slumps.

reverse fault—a fault in which the hanging wall has moved up relative to the foot wall.

thrust fault—a fault with a dip angle of 45º or less over its extent on which the hanging wall appears to have moved upward relative to the foot wall. Horizontal compression or rotational shear is responsible for displacement. (See also reverse fault and oblique-slip fault.)

strike-slip fault—a generally vertical fault along which the two sides move horizontally past each other. If the block opposite an observer looking across the fault moves to the right, the slip style is termed “right lateral.” If the block moves to the left, the motion is termed “left lateral.” California’s San Andreas Fault is the most famous example of a right-lateral strike-slip fault.

Not all fractures in rocks are caused by earthquakes. Rocks exposed at or near the surface will split as hillslopes adjust to forces of gravity over time, ice expands in cracks, or even tree roots working their way into cracks.

Over time, earthquake-prone regions experience many earthquakes, perhaps tens of thousands in a span of a million years as the earth continually readjusts to tectonic forces. As a result, the rocks exposed in mountainous areas tend to appear shattered and broken on many scales. Faults can be small, appearing on a single outcrop, or can range to great size, crossing entire regions and splitting into a complex interconnecting system of faults. Fault may be active for a period, then may become inactive, only to become active at a later time in geologic history. For example, the San Andreas Fault extends for nearly 800 miles across across California and has been active for nearly 23 million years, often splitting and changing its path from time to time. Faults can be complex and change from one form to another along their course.

brittle rocks in the crust are prone to fracture, forming faults and joints. Earthquakes occur when rocks fracture from tectonic forces in the earth.
Fig. 3-25. Rocks near the surface of the earth are cool and brittle and are subject to shattering when subjected to tectonic forces associated with earthquakes.
Types of faults
Fig. 3-26. Different kinds of faults display different kinds of displacement and orientation.
Fig. 3-27. Fractures called "joints" exposed on Checkerboard Mesa, Zion National Park, Utah.
Examples of faults
normal fault reverse fault thrust fault strike-slip fault  
Fig 3-28. A normal fault exposed in Anza Borrego State Park, San Diego County, CA. Fig. 3-29. A reverse fault exposed in Arroyo Seco Canyon, Monterey County, CA. Fig. 3-29. A thrust fault exposed in the Andes Mountains, Chili (USGS). Fig. 3-30. A strike-slip fault, a section of the San Andreas Fault exposed in Carrizo Plain National Monument, CA (USGS).  

Continued in Chapter 4.

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