Geology Cafe

Introduction to Geology

Chapter 1 - Introduction to Geology


This Geology class is an introduction to basic knowledge about the physical processes and materials of planet Earth. This website is a reference source to geology students, and serves as a companion (course notes) to class lectures and presentations. Many links to other web-based resources are provided—most links are to (hopefully) stable government websites intended for general science education.

This chapter disucsses the origins of geology as a science and examines Earth's place in space and time within the Universe. Discussions focus on the processes that, through geologic time, change the landscape around us— examining the Earth from a global perspective as well as features near our homes in our local region of southern California.

Students are encouraged to explore beyond the materials presented herein by searching topics of interest on the Internet or searching through books and other materials found in the QE and QH section of the local library collections."
Click on images throughout this website for a larger view.
Rock Cycle Illustrate
Fig. 1-1. "The Rock Cycle Illustrated" shows many key concepts presented in this course.

1. Define geology, physical geology and historical geology.

2. Define science, observation, hypothesis, fact, theory, scientific law, and scientific methods.
3. Name the people and ideas involved with the early development of geology.
4. Describe the structure and general processes occurring in the Earth.
5. Described the basic ideas of geologic time and the origin of the Earth.
6. Describe and illustrate the "rock cycle"
as it relates to processes and products observed on Earth and other planets.
7. Origin of the Earth within the Universe.

Keywords and Essential Concepts

1. Define geology, physical geology and historical geology.

geology—the study of the Earth.The scientific study of the origin, history, and structure of the earth. The structure of a specific region of the earth's crust. And, the scientific study of the origin, history, and structure of the solid matter of a celestial body.

physical geology—that branch of geology concerned with understanding the composition of the earth and the physical changes occurring in it, based on the study of rocks, minerals, and sediments, their structures and formations, and their processes of origin and alteration.

historical geology
—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.

What do geologists do? * Geologists study earth processes: floods, earthquakes, landslides, volcanic eruptions
* Geologists explore and manage resources: oil & gas, water, metals, rock byproducts
* Geologists work in environmental fields: climate change, waste management, resource protection
* Geologists work in construction and engineering: archeology, foundations, marine
* Geologists serve in national security and are involved in public health and safety
* Geologists study ocean basins, glaciers, and are involved in space exploration programs
* Geologists work in education: schools, parks, and museums.

Many Federal organizations employ geologists (or earth scientists). Can you name these?
Abbreviation website
USGS www.usgs.gov
Many geologists find employment through the Federal Goverment's employment website: www.usajobs.gov

Many states and cities also have agencies that employ geologists.
For instance, in California, many geologists are employed within the branches of the Department of Conservation, and are involved in all aspects of water resource management, earthquakes and other natural hazard investigations, coastal and marine resources, mines and mineral resources, etc.

Many science teachers in public schools have degrees in geology!
NOAA www.noaa.gov
BLM www.blm.gov
NPS www.nps.gov
NASA www.nasa.gov
DOA www.usda.gov
DOD www.dod.gov
DOE www.doe.gov
EPA www.epa.gov
CDC www.cdc.gov

2. Define science, observation, hypothesis, fact, theory, scientific law, and scientific methods.

science
—she systematic knowledge of the physical or material world gained through observation and experimentation. The overall goal of science is to discover underlying patterns in the natural world. The fundamental assumption of science—"the natural world behaves in a consistent and predictable manner."

scientific method—the principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation, generally involving the observation of phenomena, the formulation of a hypothesis concerning the phenomena, experimentation to demonstrate the truth or falseness of the hypothesis, and a conclusion that validates or modifies the hypothesis.

observation—the act of noting and recording something, such as a phenomenon, with instruments, in order to gain information.

hypothesis—
a tentative explanation for an observation, phenomenon, or scientific problem that can be tested by further investigation.

fact—
Knowledge or information based on real occurrences; something demonstrated to exist or known to have existed.

theory—
A set of statements or principles devised to explain a group of facts or phenomena, especially one that has been repeatedly tested or is widely accepted and can be used to make predictions about natural phenomena. (A theory is also defined as an assumption based on limited information or knowledge.)

Data: Grades vs. Attendance
Try out the Scientific Method! (A very valuble start to a college course!)

Example 1:
Use the scientific method to evaluate the data on this table comparing two variable factors: student attendance (number of classes missed in an introductory geology class) compared with final grades of students in three classes. Discuss observations, facts, hypotheses, and theories. How can these hypotheses be tested? What other factors not listed might explain observable facts?

What would it take to make these hypotheses into a proven theory?
Fig. 1-2. Data: Attendance vs. Grade

stream sand beach sand   Example 2: Use the scientific method to make observations using these four microscope images of sand samples (2 from river deposits, 2 from beach deposits). Use observations to make hypotheses about why the sample have identifiable characteristics unique to their origin.

Can you come up with suggestions for experiments to test these hypotheses that would support a theory of the character and origin of sand in different environmental settings?
Fig. 1-3. River sand (CA) Fig. 1-4. Beach sand (TX)
river sand beach sand
Fig. 1-5. River sand (OH) Fig. 1-6. Beach sand (CA)

3. Name the people and ideas involved with the early development of geology.

Early contributions to geology

Records of geologic observations are known from antiquity, including from ancient Egypt, Greece, China, Rome and other cultures that needed resources and basic engineering to mine minerals and build cities. However, without the knowledge available to the world today, interpretations of much that was observed was inferred through the filter of religious ideology. One of the oldest records concerning the origin of the Earth and basic geologic concepts are attributed to Aristotle in the 4th Century BC who made critical observations the slow rate of geologic processes changing the landscape. He noted that changes took place at rates much slower than can be observed in a single life time. Although rather simplistic, Ancient Greek philosophy observed the material world consisting of 5 elements that included earth, air, fire (plasma), water, and “aether” (an unobservable unknown force), and that interactions between these material forces accounted for all that was observable in the world.

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 associated with the science of stratigraphy.

fossil—A remnant or trace of an organism of a past geologic age, such as a shell, skeleton or leaf imprint, embedded and preserved in the earth's crust.

stratigraphy—A branch of geology concerned with the systematic study of bedded rock layers and their relations in time and the study of fossils and their locations in a sequence of bedded rocks.

stratum— 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."

uniformitarianism—the 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. Uniformitarianism is commonly summarized: "The present is key to the past."

Hutton 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.

catastrophism—the theory that major changes in the earth's crust result from catastrophes rather than evolutionary processes. The theory of catastrophism was more in line with religious doctrine common in the 17th and 18th centuries.

It is interesting that today, uniformitarianism still applies to most geologic and landscape features, but discoveries have show that the Earth, or large regions of it, have experience great "catastrophes," such as asteroid impacts, great earthquakes, super storms, great floods, or volcanic events. However, 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."

rock formation—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 is small compared with other locations around the world, particularly South Africa.

Lava flowing into the sea in Hawaii
Fig. 1-7. Lava flowing into the sea illustrates the Aristotelian material elements of "earth, wind, fire, and water."
Grand Canyon
Fig. 1-8. Rocks exposed in the Grand Canyon of Arizona range in age from 2.8 billion years old to some recently formed volcanic rocks.
Geologic time scale
Fig. 1-9. The geologic time scale provides names to a chronology of established periods of time in the history of Earth and the Universe—time periods range from thousands, millions, and billions of years.
William Smith's 1815 map of England, Wales, and parts of Scotland
Fig. 1-10. 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. 1-11. The "Lucas Gusher" in 1901 started the rush to the Spindletop Oil Field in Beaumont, Texas.
(Beaumont Historical Society)
 

4. Describe the structure and general processes occurring in the Earth.

structure of the earth—The earth has a central core (solid & liquid), the mantle (including asthenosphere), and the crust.

Structure of the Earth Structure of the Earth
Fig. 1-12. Structure of the Earth Fig. 1-13. Structure of the Earth

atmosphere—the gaseous mass or envelope surrounding a celestial body (including the one surrounding the Earth), and retained by the celestial body's gravitational field. The Earth's atmosphere is subdivided into levels: the troposphere is the lowest portion (up to about 6-8 miles) where all weather takes place and contains about 80% of the air's mass and 99% of water vapor. The overlying stratosphere contains an abundance of ozone which absorbs ultraviolet radiation, protecting life on land and in the shallow ocean extends up to about 31 miles. The upper atmosphere extends upward to the transition into space above about 60 miles where the charged atomic particles of the solar wind begins to interact with atmospheric gases.

hydrosphere—all the waters on the Earth's surface, such as oceans, lakes, rivers, and streams.

lithosphere—the rocky outer portion of the Earth, consist of the crust and upper mantle (about the upper 60 miles below the Earth's surface).

biosphere—the regions of the surface, subsurface, and atmosphere of the Earth (or possibly other planets) occupied by living organisms.

crust
— the outermost solid shell of a rocky planet or moon, which is chemically distinct from the underlying mantle.

mantle
—an inner layer of a terrestrial planet or other rocky body large enough to have differentiated in composition by density. On Earth, the mantle is a highly viscous layer between the crust and the outer core.

core—based on geophysical studies, the innermost part of the earth is believed to consist of a 758 mile thick magnetic metallic inner core that is overlain by a 1400 mile thick zone of dense molten material in the outer core. This is overlain by the Earth's mantle.

5. Described the basic ideas of geologic time and the origin of the Earth.

Geologic Time Scale

geological time—the time of the physical formation and development of the earth (especially prior to human history). Geologic time also applies to the age and history of the Universe.

geologic time scale—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 mountain range formation), the formation of rock units (such as the age of a lava flow), the age of fossils, organizing geologic map units, and other purposes. Below is a standard geologic time scale listing names of major time periods with time span information.

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. 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 has a history!
Geologic Time Scale
Fig. 1-14. Geologic Time Scale showing major geologic events in Earth history and the evolution of life on earth. New scientific discoveries are refining knowledge about the chronology and impacts or significance of events through deep Earth time.
Time Scale Clock
Fig. 1-15. "If a second were 100,000 years" - this classic diagram show the distribution of a different ages of time as if it were all squeezed into a 24 hour day. All of human history would fit in the last fraction of a second!

6. Describe and illustrate the "rock cycle" as it relates to processes and products observed on Earth and other planets.

rock cycle—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 used to illustrate common rocks and earth materials and the processes that form or change them.

The Rock Cycle

The Rock Cycle

The “rock cycle” is a conceptual model of how earth materials form and change in the earth’s crust over time. Both products (rocks and sediments) and processes (such as melting, cooling, erosion, deposition, metamorphism, remelting) are part of this idealized cycle. The passage of geologic time is the 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 crust. It is important to note that "rock cycle" processes occur on other "rocky planets or moons" but rates may vary due to the presence (or lack of) atmospheric gases or fluids (including water) or availablitity of heat enough to melt rocks.
Fig. 1-16. The Rock Cycle is a conceptual model that portrays "processes and products" changing over time

General Classification of Solid Earth Materials

igneous rocks
—A rock formed from molten materials, 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).

sediments—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.

sedimentary rock
—rock that has formed 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.

metamorphic rocks—Rock 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).

7. Origin of the Earth within the Universe.

Meteor Crater near Flagstaff, Arizona The Astronomy Connection to Geology

Many aspects of astronomy have contributed to the knowledge of the origin and geologic history of our planet. Meteorites have been found, collected, and studied for centuries. Telescopes on the ground and now in space, satellites, robotic and manned missions to space, moon, and other planets and solar system objects have greatly expanded the collective knowledge about the origin of our planet and objects in the solar system—all of which have evolved to their current state over billions of year. So far, life is only known to exist on Earth, but it could possibly exist elsewhere, even within our solar system. We just haven't proven it yet.
Giant redwood in Big Basin State Park, CA
Fig. 1-17. Meteor Crater near Flagstaff, Arizona is a bollide and meteor impact site that happened about 50,000 years ago. Erosion eventually destroy or buries craters, but many have been found. Fig. 1-18. Like this giant redwood in Big Basin State Park, CA...
All materials, including life on Earth, have an origin connection to events that happened in space billions of years ago!

Big Bang Theory—a cosmological theory holding that the observable universe originated approximately 13.8 billion years ago from the violent explosion of a very small agglomeration of material of extremely high density and temperature. See a NASA website about the Big Bang Theory.

galaxy—a system of millions to billions of stars, together with gas and dust, held together by gravitational attraction. Earth is in the Milky Way galaxy. See a NASA website about Galaxies.

star—a self-luminous celestial body consisting of a mass of gas held together by its own gravity in which the energy generated by nuclear reactions in the interior is balanced by the outflow of energy to the surface, and the inward-directed gravitational forces are balanced by the outward-directed gas and radiation pressures. Our sun is our star in the center of our solar system. See a NASA website about Life Cycle of Stars.

nebula—an interstellar cloud within a galaxy consisting of gas and dust, typically glowing from radiant energy from stars nearby within them. Nebulas are the birth place of stars and solar systems, and can form from the explosion of stars at the end of their life cycle.

solar system—the system containing the sun and the bodies held in its gravitational field, including the planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto), planetary moon, the asteroids, and comets, and other interstellar matter. See NASA websites about Planets and Moons within the Solar System and Earth's Moon. (images below from NASA).

Studies of meteorites and samples from the moon suggest that the Sun and our solar system condensed and formed in a nebula about 5 billion years ago. Currently, the oldest samples of "Early Earth" rock samples from the Jack Hills region of Australia that contain crystals of the mineral zircon dated to an age of about 4.4 billion years. A recent Scientific American article places the current assume age of the earth is 4.55 billion years old.
Brief story of the Big Bang Galaxy
Fig. 1-19. A very brief story of the Big Bang and the evolution of the Observable Universe over ~13.8 billion years Fig. 1-20. A spiral galaxy: a typical galaxy may have hundreds of millions of stars
Galaxies Sun
Fig. 1-21. Deep-space observing telescopes show distant field of galaxies: galaxies can be seen in all directions in distant space. The distance to these objects are in the range of thousands to billions of light years away. Fig. 1-22. The Sun (our star), is one of billions of stars in our Milky Way Galaxy. There are many types of stars and other objects ranging from dust and gas clouds to super massive objects called Black Holes.

Subernava Nebula Nebula Carena Our Solar System
Fig. 1-23. Supernovas are great explosions that partial to complete demolish aging stars, releasing new matter and gas to create a new generation of stars Fig. 1-24. Nebula, the birthplace of stars; some are formed from the explosion of other more ancient stars Fig. 1-24. Carina Nebula, a part of our Milky Way Galaxy where new stars are forming and emerging from a gas and dust cloud "stellar nursery." Fig. 1-25. Our Solar System originated from gas, dust, and other matter that gravity pulled together in a stellar nebula about 5 billion years ago.

Planets and Other Objects in our Solar System
Mercury Venus Earth Mars
Fig. 1-26. Mercury - rocky planet, no moons, no atmosphere Fig. 1-27. Venus - rocky planet with hot atmosphere, no moons Fig. 1-28. Earth - rocky planet with oceans, atmosphere, life!, and one moon Fig. 1-29. Mars - rocky planet with ice caps at poles, thin atmosphere, 2 small moons
Jupiter Saturn Uranus Neptune
Fig. 1-30. Jupiter - largest of the gas planets, has 67 moons, of which 4 are large Fig. 1-31. Saturn - a gas planet famous for its visible rings. Currently has 62 moons, including Titan, the largest in the solar system. Fig. 1-32. Uranus - another large gas planet with 5 medium-sized moons (many smaller ones too) Fig. 1-33. Neptune - the largest, outermost gas planet, has 13 known moons
Pluto Moon Asteroids
Fig. 1-36. Asteroids are solid object in space consisting mostly of rock, dust, some metals, and possibly ice. Most asteroids orbit the sun in the Asteroid Belt located between Mars and Jupiter.
Comet Haley
Fig. 1-37. Comets are like asteroids (mostly frozen gases and ice dust, some rocky material) that leave a trail of material as they are heated as they approach the sun. There may be more than 100 million comets in the outter Solar System.
Fig. 1-34. Pluto - "formerly a planet," now it is called a "planetesimal," a "dwarf planet." There are 7 planetary moons larger than Pluto. Fig. 1-35. Earth's Moon - one of the largest of at least 168 moons orbiting planets. Jupiter has the most: 67, only 4 are large.

asteroid—any of the thousands of small irregularly shaped bodies of stone, metal, and ice that revolve about the sun. In our solar system, asteroids typically range in size from about one-mile (1.6 km) to about 480 miles (775 km) in diameter. Most asteroids lie in in orbits between those of Mars and Jupiter, however many large objects have been observed passing through Earth's orbital path. Asteroid collisions with earth were frequent in Earth's early history, but are now extremely rare events. The extinction of the dinosaurs and many other species is mostly blamed on the environmental catastrophe created by an asteroid impact about 65 million years ago, defining the end of the Cretaceous Period (and Mesozoic Era). See a NASA website about asteroids.

comet
—a celestial body, observed only in that part of its orbit that is relatively close to the sun, having a head consisting of a solid nucleus surrounded by a nebulous coma up to 2.4 million kilometers (1.5 million miles) in diameter and an elongated curved vapor tail arising from the coma when sufficiently close to the sun. Comets are thought to consist chiefly of ammonia, methane, carbon dioxide, and water. See a NASA website about comets.

meteor—a bright trail or streak that appears in the sky when a meteoroid is heated to incandescence by friction with the earth's atmosphere.

meteorite—a stony or metallic mass of matter that has fallen to the earth's surface from outer space. See a NASA website about
meteors and meteorites.

bollide—a large meteor (or asteroid or comet) that explodes in the atmosphere. A recent bollide explosion involved the Chelyabinsk meteor that blew up over Russia on February 15, 2013. The explosion occurred high in the atmosphere, but the atmospheric shock wave blew out windows, doors, and injured over a thousand people on the ground (see YouTube video). See a NASA website about bollides.

So... Can you explain why are there so many craters on the surface of the moon but not on surface of the earth?

Plate Tectonics Theory

A unifying collection of concepts that explains most things geological on Earth, and other planets and moons as well.

Plate Tectonicsis a theory that explains the structure of the earth's crust and many associated phenomena as resulting from the interaction of rigid “lithospheric plates” that move slowly over the underlying more fluid mantle. (Plate Tectonics Theory iscussed in greater detail in Chapter 5.)
Plate Tectonics
Fig. 1-38. Model representing essential concepts presented by Plate Tectonics Theory


Quiz Questions
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1/13/15