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Chapter 4 - Structure of the Earth

This chapter follows on the "basic principals of geology" presented in the previous chapters. This chapter introduces the major concepts of the structure of the Earth and the dynamic processes associated with Plate Tectonics, a fundamental theory that captures the science of how the earth works: why "things" are where they are, how they formed, and how they evolved, over time, to become features within the world that we see today.

The appearance of the world as we see it today is a result of the accumulative effects of all geologic processes that have happened in the past. Although some of these processes occur rapidly (such as volcanic eruptions, earthquakes, great storms and flood, and occasional asteroid impacts). However, most features we see on the landscape or in a region (or larger features like continents mappable on a global scale) involve processes that are far grander, operating both near and deep below the surface, and taking place gradually over long periods of time (in periods measured in millions to hundreds-of-millions of years). For instance, the coast lines of northwest Africa and the eastern United States are currently moving apart at a rate of about 2-4 inches a year. However, about 200 million years ago the two continents were joined together before the opening and formation of the Atlantic Ocean basin! Plate tectonics theory helps explain most of the processes and grand landscape features we observe around the world today, both on land and beneath the oceans.

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Precious gemstones
Fig. 4-1. Gemstone form in many different geologic environments.

The Structure of the Earth

While much has been discovered about the character and natural resources of our planet since the time of Christopher Columbus's first voyage, little was know about the internal character of the Earth until the Cold War era following World War II. Although studies of the internal structure of the earth were first reported in the late 19th century using seismic wave data from great earthquakes, it was the data from testing, spying, and verification of underground nuclear explosions that provided a clearer, more detailed picture of the internal structure of our planet. The earth is composed of several zones, including a central core, a mantle, and a crust (Figure 4-2). Oceans (hydrosphere) and atmosphere rest on the surface of the crust. All parts are held together and have their character based on the force of gravity, their chemical composition, and largely how they formed and changed through geologic time. These same factors apply to other planets and moons as well.
Structure of the Earth
Fig. 4-2. The structure of the Earth.

Subdivisions of the Structure of the Earth

The Earth consist of several parts. Other planets and moons in our solar system share some of these characteristics:
  • core—based on geophysical studies, the innermost part of the Earth is believed to consist of a 758 mile thick magnetic metallic inner core overlain by a 1400 mile thick zone of molten material of the outer core. This is overlain by the Earth's 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.

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

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

  • 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.
Other subdivisions used in geologic discussions relating to "Plate Tectonics Theory" (discussed below) include:

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).

asthenosphere—the upper portion of the mantle underlying the lithosphere where heat and pressure is great enough for materials to flow "like plastic." This movement is driven by the heat derived from within the deeper parts of the mantle and core that cause materials to flow by gravitational convection (see Figure 4-3).

Another important distinction within the lithosphere are the differences between what is known as "oceanic crust" and "continental crust." The rocks exposed on continental land masses are different than those found beneath the ocean basins. In general, rocks found within continental landmasses "collectively" are less dense than rocks recovered from beneath the ocean basins. This difference helps explain the geography of the planet as well as explaining many aspects of the tectonic forces changing the landscapes of our planet over time.
Convection in the mantle
Fig. 4-3. Gravity-driven heat convection within the Earth is the conclusive power source driving plate tectonic motions.

Isostacy and the "Age of Landscapes"

In the early days of "modern geology" the variations in elevations on land (topography) and the depth of the oceans (bathymetry) were mapped around the globe. Investigations lead to the hypothesis of isostacy, that continents were floating on a more fluid mantle, much the way that wood blocks or icebergs float on water. With wood or ice blocks, the thicker they were, the higher they rose above the water. This lead to the belief that the crust beneath the continents—especially beneath mountain ranges—is "thicker" and "less dense" than the crust beneath the ocean basins. To maintain an isostatic equilibrium there had to be an equivalent amount of "lighter" crustal material beneath a mountain range in order for it to rise to higher elevations (see Figure 4-4). For example, the crust beneath the Himalayan Mountains must be much thicker that the crust beneath the Indian mainland, and much thicker than the crust beneath the Indian Ocean (see Figure 4-5).

Isostacy is the state of balance, or equilibrium, which sections of the earth's lithosphere (whether continental or oceanic crust) are thought ultimately to achieve when the vertical forces upon them remain unchanged. The lithosphere floats upon the semi-fluid asthenosphere below. If a section of lithosphere is loaded, as by ice, it will slowly subside to a new equilibrium position. Also, if a section of lithosphere is reduced in mass, as by erosion, it will slowly rise to a new equilibrium position.

Many hypotheses were put forward to try to explain the evolution of landscapes—isostacy was one of them. Early hypotheses focused on what was easily observable. Continents around the world shared a variety of large physiographic features: mountain ranges, coastal plains, plateau regions, and inland lowlands. Some of these lowland regions are underlain by what appeared to be ancient rocks that were once to core of mountain ranges in the distant past. These regions were located near the center of most of the continents and have became known as "shields" (such as the Canadian Shield of North America). In most cases, these "shields" are surrounded by belts of mountain ranges that were composed of rocks that appeared younger than the shield regions. Also, some of these mountain ranges appeared much "younger" than other mountain ranges. This lead to conclusions that landscapes could be classified as "youthful," "mature," or "old age" - assuming that all mountain ranges form about the same way, and that "youthful" mountain ranges, like the Himalayan or Rocky Mountains eventual erode way (becoming more "mature" with age, like the Appalachian Mountains). Eventually almost all elevated features (mountains, hills) completely erode away, producing "old age" landscapes, similar to what is seen in shield regions (see Figure 4-6).
Isostacy and the density of crustal rocks
Fig. 4-4. Isostacy of crustal rocks compared with wood blocks floating on water. There would have to be more "light" crust under mountain ranges than under oceans.
Himalayas and Tibet
Fig. 4-5. Crustal thickening in the Himalayan Mountains and Tibetan Plateau is illustrated in this photograph from space. (NASA)
The assumption is that as materials erode away, the crust readjusts itself to maintain an isostatic equilibrium. As material is removed the crust rises. Over time, material that were once deep within mountain ranges eventually becomes exposed at the surface by erosion. Over time, the assumption was that isostatic adjustments eventually cease, and the mountains would completely erode away to a flat plain and eventually sink below the waves. Unfortunately, there were too many cases where the isostatic adjustment hypotheses didn't match all the observable facts. Not all old shield regions were low plains (as illustrated with the Scandinavian region of Europe and much of Africa). In addition, some regions, such as the Colorado Plateau, had characteristics that fit into all three categories, youthful, mature, and old age, all at the same time. In addition, there was very little to explain how mountain ranges and continents formed in the first place! Why do some mountain ranges have volcanoes and other don't? What would explain the composition and distribution of volcanic mountain ranges around the world, and what in the world could explain what chains of volcanoes like the Hawaiian archipelago were doing in the middle of the Pacific Ocean? These questions (and more) were finally resolved with the development of Plate Tectonic Theory. Crustal isostacy over time compared with youthful, mature, and old age landscapes
Fig. 4-6. Isostacy and the hypothesis of landscapes evolving through "youthful," "mature" and "old age."

Plate Tectonic Theory

Plate tectonics is the modern unifying theory that has been resolved to explain to some degree almost "all things geological" in the observable world, past and present. Plate tectonics expounds that Earth’s outer shell (lithosphere) is composed of several large, thin, relatively strong “plates” that move relative to one another. Movements along fault systems that define plate boundaries produce most observed earthquakes. The mechanics of Plate Tectonics Theory were largely resolved starting in the 1960s when large quantities of data about the age and distribution of rocks beneath the ocean basins were compiled from ocean drilling programs and geophysical studies of the ocean crust around the world. This data added essential components to an earlier "Continental Drift Hypothesis"—proposed by a German meteorologist named Alfred Wegener (1880-1930), but based on research by other earlier observers. The Continental Drift Hypothesis was based on observations that the continental coastlines on either side of the Atlantic Ocean seemed to match up and was supported by paleontological and geological comparisons on the continents bordering the ocean. Continental Drift Theory intrigued the scientific community but was largely rejected because there was no data to explain all the observable facts. This was largely because in the early 20th century very little was known about the nature of the world's ocean basins nor the physical characteristics of the structure of the Earth's asthenosphere and lithosphere.

From Alfred Wegener's Continental Drift Hypothesis came the "slow to be accepted" theory that all the observable continents had once assembled into a single supercontinent called "Pangaea," and that this great landmass began to break apart about 300 million years ago (see Figure 4-7). Whereas the geologic and paleontological evidence on continents on opposite sides of Atlantic Ocean basin and parts of the Indian Ocean basin provided fairly conclusive evidence supporting continental drift, the Pacific and other ocean regions were much less understood. The margins of the Pacific Ocean basin became known as the "Ring of Fire" (see Figure 4-8). In most places places around the Pacific Rim the the transition of the continents to the deep ocean has large numbers of active or "recently" active volcanoes. This region also experiences large numbers of sometimes tremendous earthquakes. Where volcanic arcs (island belts and mountain ranges composed of volcanoes) appear on land, there are also very deep-water trenches located not too far offshore of the coastline.
Fig. 4-7. Supercontinent Pangaea as it existed about 300 million years ago.
Rig of Fire
Fig. 4-7. The "Ring of Fire" is a zone of volcanoes, numerous earthquakes, and offshore deep trenches.
Over time, as earthquake detection equipment (seismographs) stations were set up around the world and data collections were compiled, it became apparent that there were patterns that showed that nearly all earthquakes occurred in zones where chains of volcanoes and mountain ranges were most actively forming around the Ring of Fire, across southern Europe into east Asia, and along narrow belts beneath the oceans associated with mid-ocean ridges (Figure 4-8).

Another paradox is the difference between rocks that are exposed on continents and rocks that make up the crust under the oceans. As shown in Figure 4-9, the map shows the bathymetry of the ocean basins, highlighting long undersea mountain ranges (mid-ocean ridges) that extend thousands of miles near the middle of the Atlantic and Indian Oceans, and part of the eastern Pacific Ocean basin. Although early oceanographic studies revealed mountains hidden beneath the oceans, a complete map of the ocean floor wasn't compiled in detail until starting in World War II as part of naval research for submarine warfare. In addition, although some data regarding the age of continental rocks was partly known before the war, much detail of the geology of continental regions wasn't available until global energy and mineral resource mapping was conducted, mostly after the war. What was discovered was that most of the oldest rocks found in the Earth's crust occur in the center of continental landmasses, such as in the Canadian Shield region of North America, Greenland, the central parts of Africa, South America, Australia, and Siberia, and the peninsula of India.Thee regions have rocks that range in age to typical over a billion years to the oldest know rocks about 3.5 billion year. These regions are called continental shield. Note that It is within these regions that most of the world's most economically significant gem and precious metal deposits are found!

Surrounding the continental shields on most of the continents are belts of mountain ranges and coastal plains that contain rocks younger that a billion years in age. The higher mountain ranges, including the Himalayan, Andes, Alps, and Rocky Mountains are considered to be actively forming and are dominated by rocks that have formed after the breakup of the supercontinent Pangaea (mostly after about 300 million years ago). There are some older mountain ranges, like the Appalachian Mountains in eastern North America, that appear more worn down, and the areas are relatively inactive geologically (having fewer earthquakes and little recent volcanic activity). By comparison, the landscapes within the shield regions are nearly completely worn down and are no longer "geologically active." However, these shield regions display characteristics of having once been parts of mountain ranges that existed a billion or more years ago. In many areas parts of the shield regions, ancient mountain ranges have formed, eroded away, and reformed again and again, but today, in contrast, there is very little geologic activity (volcanoes or earthquakes).

Figure 4-10 is a map showing the age of rocks found in the crust beneath the ocean basins of the world. Again, beginning in ernest during World War II and culminating in the Cold War, geophysical mapping and sampling of materials from the sea floors around the globe showed that rocks on the ocean basins were very significantly younger that rocks found on the continents, with ages ranging in only about 200 million for the oldest. In all cases, the age of seafloor grows progressively younger approaching the mid-ocean ridges. Using seismic data and deep-sea submersible exploration craft, the mid-ocean ridges were discovered to be belts of undersea volcanic areas. New ocean crust was (and is) forming along the mid-ocean ridges. Over time, the newly formed ocean crust cooled and moved slowly away from the mid-ocean ridges. These areas where new crust is forming and moving apart are called spreading centers. Since new crust was forming, old crust had to be disappearing somewhere, and it turned out that the old crust was sinking back into the mantle along extensive fault zones associated with the deep ocean trenches. These great fault systems are called subduction zones (see Figure 4-12). Subduction zones are locations where cool ocean crust sinks back into the mantle, as it sinks it heats up. Water and gases trapped in the sinking crust cause partial melting (forming magma) which rises through zones of weakness in the lithosphere where it accumulates in magma chambers or may actually rise all the way to the surface to form volcanoes. Earthquakes caused by friction along the subduction zone reveal that crust is slowly sinking back into the mantle.
Earthquakes of the world (epicenters)
Fig.4-8. Earthquakes of the world (USGS record for 1978 to 1987).
Mountain belts and stable cratons
Fig. 4-9. Map of the world showing continental mountain belts (brown) and stable ancient cratons or "shield" regions (orange and red, the oldest rocks being red). Ocean bathymetry (in shades of blue) show mountain ranges (mid-ocean ridges) beneath the oceans.
Age of the seafloor
Fig. 4-10. Geologic and geophysical mapping show that the crustal rocks beneath the modern oceans are less that 200 million years, with the youngest rocks (and some actively forming) occur along mid-ocean ridges.
Spreading center along a mid-ocean ridge subduction zone Plate Tectonics Map of lithospheric plates and plate boundaries around the world  
Fig. 4-11. Formation of new oceanic crust along a spreading center associated with a mid-ocean ridge. Some spreading centers appear on land. Fore example, a portion of the Mid-Atlantic Ridge is exposed as Iceland. Fig. 4-12. Subduction zone geometry is revealed by the location of earthquakes and volcanic activity. Subduction zones are where oceanic crust is destroyed and new continental crust forms. Subduction zones associated with ocean trenches surround much of the Pacific Ocean basin. Fig. 4-13. Plate tectonics explains many aspects of the geometry of continents and ocean basins and the processes creating new oceanic and continental crust. Material that does not become incorporated into the lithosphere sinks and becomes incorporated back into the mantle. Fig. 4-14. Map showing the location of tectonic plates and plate boundaries of the world. Boundaries shown in yellow are divergent boundaries, Those in orange are convergent boundaries. Note that some plates include both continental and oceanic crust.  
Summary of Plate Boundary Features

divergent boundary—When plates diverge, spreading centers form creating new oceanic crust. Examples include mid-ocean ridges in world's ocean basins. Spreading centers occur where continents are pulling apart. Examples include the Africa rift zones, Red Sea basin, Iceland, and North America's Great Basin region including the Gulf of California (see Figure 4-11).

spreading center— A linear area where new crust forms where two crustal plates are moving apart, such as along a mid-oceanic ridge. Spreading centers are typically seismically active regions in ocean basins and may be regions of active or frequent volcanism (see Figure 4-11).

convergent boundary—When continents collide... mountains belts form - examples include the Himalayas, Alps, and ancient Appalachian Mountains when the ancient continent of Pangaea formed. When continents collide with ocean crust... subduction zones with deep ocean trenches and volcanic arcs form - examples include the Andes Mountains, Aleutian Islands, Japan, Philippines, Indonesia, the ancient Sierra Nevada and modern Cascades Range (see Figure 4-12 and 4-13).

subduction zone
—A plate boundary along which one plate of the Earth’s outer shell descends (subducts) at an angle beneath another. A subduction zone is usually marked by a deep trench on the sea floor. An example is the Cascadia Subduction Zone offshore of Washington, Oregon, and northern California. Most tsunamis are generated by subduction-zone-related earthquakes (see Figure 4-12 and 4-13).

transform boundary
—When plates slide past each other creating fault systems along plate margins. Examples include the San Andreas Fault and major faults in Pakistan, Turkey, and along the Jordan River/Dead Sea.

Examples of Plate Boundaries

Convergent boundaries Divergent boundaries Transform boundaries
When continents collide mountains belts form. Examples:
  • Himalayas
  • Alps
  • ancient Appalachian Mountains
When plates diverge, spreading centers form creating new oceanic crust. Examples:

  • mid ocean ridges in world's ocean basins
When plates slide past each other creating fault systems along plate margins. Examples:
  • San Andreas Fault
  • Pakistan
  • Turkey
  • Jordan River/Dead Sea
Convergent boundary along Hinalayan Mountains Divergent boundary of Mid Ocean  Ridge in Iceland San Andreas Fault in Central California
Fig. 4-15. Himalayan Mountains are a convergent plate boundary Fig. 4-16. Mid Ocean Ridge in Iceland is a divergent plate boundary Fig 4-17. San Andreas Fault system is a transform plate boundary
When continents collide with ocean crust
trenches with subduction zones and volcanic arcs form - examples:
  • Andes Mountains
  • Aleutian Islands
  • Japan, Philippines, Indonesia, etc.
  • Ancient Sierra and modern Cascades
Spreading centers occur where continents are pulling apart. Examples:
  • Africa rift zones
  • Red Sea
  • Iceland
  • North America's Great Basin
Transform faults also occur within plates, but are related to moves that shape the seafloor. Examples:
  • Dead Sea fault zone, Jordan
  • India/Pakistan boundary fault
  • North Anatolian Fault, Turkey
South America plate boundar Divergent boundary forming in the Red Sea fransform faults on the seafloor; North American western plate boundary
Fig. 4-18. Convergent boundary along the west coast of South America Fig. 4-19. A divergent boundary forming in the Red Sea area Fig. 4-20. Transform fracture zones offshore of California are within the Pacific Plate (USGS)

How does Plate Tectonics explain why continental landmasses are so old (compared to ocean crust)?

The interior of the earth is very hot. The source of this heat is thought to be left over from the formation of the planet several billion years ago. Heat is also generated by the radioactive decay of elements, tidal forces between the Earth, Moon, and Sun, and possibly other sources yet to be determined. As shown in Figure 4-3, the combined effect of the internal heat of the earth and the force of gravity drive convection currents within the mantle. Heat things up, they expand, become less dense, and the material rises. Cool things down, they condense, increase in density, and the material sinks. This can be easily demonstrated the way hot air balloon rise and fall, or the way currents move when water is heated, or the way currents within a boiling pot of soup rises and sinks when it cools (Figure 4-21).

When new ocean crust forms in spreading centers, it is still hot for a time, but it eventually cools by having contact with the cold ocean waters. The ocean crust is enriched in dense minerals. As it ages, it absorbs water from the ocean and is becomes blanketed with marine sediments. Where subduction takes place, cold, dense ocean crust sinks back into the mantle. However, as the old crust sinks, it heats up and some of the materials within it melts. The materials that melt rise rise through the overriding continental crust, forming large magma filled chambers that eventually crystallize into rock at depth, some of which erupts at the surface to form volcanoes. The new rocks that form along the continental margins is less dense than the original oceanic crustal rocks, therefore they eventually iso statically float and rise above the ocean surface, becoming land. Over time, more and more of this lighter rock accumulates first forming volcanic island chains. These volcanic arc and the sediments they shed eventually becomes scraped of and crushed onto the margin of continents, often pushed up as mountain ranges. It this manner, continents grow slowly around their margins in a process called accretion. This process explains why the oldest rocks occur in the shield regions of continents.

accretion—A process by which material is added to a tectonic plate or a landmass. This material may be sediment, volcanic arcs, seamounts or other igneous features, or blocks or pieces of continental crust split from other continental plates (Figure 4-22). Over "geologic time" (measured in millions of years), volcanic arcs form and may be crushed onto (or between) colliding continents with plate boundaries. Pieces of continental land masses may be ripped away and carried to other locations. For instance, Baja California and parts of southern California west of the San Andreas Fault are being ripped away from the North American continent and are slowly being carried northward. These rocks may eventually pass what-is-now San Francisco, and perhaps 70 to 100 million years from now will be crushed and accreted into the landmass currently known as Alaska!

Boiling pot of soup
Fig. 4-21. Currents in boiling soup demonstrates convection. Bubbly froth builds up in patches over where cool soup sinks back into the pot. The buildup of froth in patches is similar to the way continents build up over time.
Subduction is a refining process
Fig. 4-22. The processes associated with subduction lead to the accretion (growth) of continents over time.
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