Geology Cafe

Introduction to Geology

Chapter 5 - Plate Tectonics

This chapter reviews the major concepts of the structure of the Earth and describes 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 mapable 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.
Click on small images for a larger view throughout this website.
Volcanic eruption
Fig. 5-1. Nearly all geologic processes observed on Earth "fit" into "Plate Tectonics Theory."

The Structure of the Earth (revisited)

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 5-2). 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. 5-2. 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.
Seismic waves passing through the earth reveal earth's structure Seismology is the study of earthquake shock waves as they pass through the earth. Seismology is the science that helped resolve many questions about the internal structure of the earth.

Extensive study of shock waves of earthquakes and the global monitoring of underground nuclear bomb testing revealed information about the internal structure of the Earth. Zones of "seismic wave shadows" in the regions between about 105° to 140° on the opposite side of the globe from a seismic shock source revealed that part of the Earth's core is liquid material (molten material)(Figure 5-3). In contrast, an inner core is believed to consist of solid metal, possibly similar in composition of iron meteorites. Earth's magnetic field is believed to be associated with this inner metallic core and possibly hot molten metallic material of the outer core. Between the core and the solid surface crust is the mantle, a zone of mostly solid rock under high pressure and temperature conditions.
Earth cross section
Fig. 5-4. Internal structure of the Earth show thickness of the core, mantle, and crust.
Fig. 5-3. Seismic shock wave provide information about the structure of the Earth.

Behavior of the Lithosphere:
(Crust and Upper Mantle)

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). It is the solid "brittle" zone of the earth where earthquakes occur.

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 5-5). Gravitational convection works as follows—Adding heat causes materials (solid and molten) to expand, loose density, and rise; whereas cooling material shrinks and increases in density, and sinks. The asthenosphere is a semi fluid layer of the earth, between about 40 to 80 miles (100-200 km) below the outer rigid lithosphere (oceanic and continental crust) forming part of the mantle and thought to be able to slowly flow vertically and horizontally, enabling sections of lithosphere to subside, rise, and undergo lateral movement associated with plate tectonics.

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.

ocean crust—part of Earth's lithosphere that underlies ocean basins. Oceanic crust is primarily composed of mafic rocks (rich in iron and magnesium) and are less dense than rocks that underlie continents (continental crust is enriched in silica and aluminum). Ocean crust around the world is significantly younger (less than 200 million years) relative to continental crust which has typically accumulated through the natural refining processes associated with plate-tectonics over many hundreds of millions to several billion years.

continental crust—the relatively thick part of the earth's crust that forms the large landmasses. It is generally older and more complex than the oceanic crust, and dominantly composed of igneous and metamorphic of granitic or more felsic composition.

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.

Isostacy, Mountain Building, and the "Age of Landscapes"

orogenesis—The process of mountain formation, especially by a folding and faulting of the earth's crust.

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 5-6). 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 5-7).

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, see Figure 5-11 below). 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 5-8).

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.

The "Continental Drift Hypothesis"

The Continental Drift Hypothesis was an prelude to the modern Plate Tectonics Theory (discussed below). Many of the observable facts from the early investigations of continental drift are fundamental to the newer theory.
Modern understanding of the evolution of the Earth through time is the culmination of hundreds of years of world-wide exploration and observation. Maps compiled by early global explorations resulted in the observation of the similarities of coastline patterns on opposite sides of the Atlantic Ocean. These similarities were noted from early maps by by a Flemish cartographer named Abraham Ortelius in 1596 who first suggested that it looked like the continents had drifted apart. Also, early exploration brought awareness of the unusual abundance of volcanoes, rugged mountain ranges, and earthquakes in the region bordering the Pacific Ocean, a region described as the "Ring of Fire" (Figure 5-9). In contrast, the Atlantic Ocean basin is surrounded mostly by gentle coastal planes and old, worn down mountain ranges, and by comparison relatively little volcanic or earthquake activity.

The Continental Drift Hypothesis was 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 also supported by paleontological and geological comparisons on the continents bordering the ocean.
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 (Figure 5-10 and 5-11). Whereas the geologic and paleontological evidence (fossils) on continents on opposite sides of Atlantic Ocean basin and parts of the Indian Ocean basin provided fairly conclusive evidence supporting continental drift (Figure 5-12). In contrast, the Pacific and other ocean regions were much less understood. In most places places around the Pacific Rim ("Ring of Fire") the transition zones 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. In most places 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.

Pangaea—a supercontinent comprising all the continental crust of the earth, theorized to have assembled from other continental land masses in middle to late Paleozoic time. The assembled landmass, Pangaea, existed through late Paleozoic and through early Mesozoic times before the component continents separated and gradually migrated into their current configuration.

Continental Drift intrigued the scientific community but was largely rejected because there was no data to explain all the observable facts about how or why continental plates moved across ocean basin. 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. Many other hypotheses existed in the scientific community well into the late 20th century, but these conflicting ideas have faded in significance with the advances of the newer Plate Tectonics Theory.

Plate Tectonics Theory

Today, Plate Tectonics Theory explains the large-scale motions of Earth's lithosphere. Plate tectonics theory builds on concepts of "continental drift." It was the global efforts of seafloor exploration following World War II resulted in the development of seafloor spreading theories in the late 1950s and early 1960s. This exploration effort involved perhaps many thousands of scientists within the "global geoscience community" (geologists, oceanographers, paleontologists, and geophysicists, assisted by world leaders) who systematically gathered information and mapped the world, both on land and underwater. The mechanics of Plate Tectonics Theory were largely resolved as 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 from around the world. Seafloor mapping, along with the study of volcanoes and earthquakes provided the evidence to support plate tectonics theory.

Today, Plate Tectonics Theory 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 rigid “plates” that move relative to one another. Movements along fault systems that define plate boundaries produce most observed earthquakes.
Convection in the mantle
Fig. 5-5. Gravity-driven heat convection within the Earth is the conclusive power source driving plate tectonic motions.
Isostacy and the density of crustal rocks
Fig. 5-6. 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. 5-7. Crustal thickening in the Himalayan Mountains and Tibetan Plateau is illustrated in this photograph from space. (NASA)
Crustal isostacy over time compared with youthful, mature, and old age landscapes
Fig. 5-8. Isostacy and the hypothesis of landscapes evolving through "youthful," "mature" and "old age."
Rig of Fire
Fig. 5-9. The "Ring of Fire" is a zone of volcanoes, numerous earthquakes, and offshore deep trenches.
Pangaea supercontinent
Fig. 5-10. Supercontinent Pangaea as it existed about 300 million years ago.
Plate Motion
Fig. 5-11. The breakup of Pangaea over 300 million years.
Pangaea fossils
Fig. 5-12. Fossil evidence connecting lands of Pangaea about 260 million years ago)(USGS)
This Dynamic Planet
Fig. 5-13. Plate Tectonic Features Map. Learn more at these online resources by the Smithsonian Institution and U.S. Geological Survey:

This Dynamic Planet (World Map of Volcanoes, Earthquakes, Impact Craters, and Plate Tectonics

This Dynamic Earth (The Story of Plate Tectonics
Supporting Evidence for Plate Tectonics Theory:

Paleomagnetism and the study of the seafloor

Earth's magnetic field has been a curiosity since ancient times. The magnetic compass was first invented as early as the Chinese Han Dynasty (about 206 BC). The compass was used during China’s Song Dynasty for military navigation by 1044 AD, and for maritime navigation by about 1117 AD. Today, the source of the magnetic field is presumed to be from the movement of molten iron and metals in the earth's core. The "spinning" of these liquid metals produces electric currents in the same manner as an electric coil produces a magnetic field. The magnetic field extends into space (Figure 5-14). Over time, these currents fade, change direction, or intensify elsewhere, causing the magnetic poles to migrate or reverse the magnetic polarity of the entire planet (events called magnetic reversals).

Magnetometers (devices used to magnetic fields) were used in World War II to search for submarines. It was noted from these investigations that the seafloor preserved mapable magnetic anomalies that parallel the Mid Atlantic Ridge (Figure 5-15). These investigations showed that that the earth's magnetic field has reversed many times through earth history; magnetic reversals happened over periods ranging from thousands to millions of years.

is the study of the fixed orientation of a rock's magnetic minerals as originally aligned at the time of the rock's formation. Paleomagnetism is usually the result of thermoremanent magnetization (magnetization that occurs in igneous rocks as they cool below the Curie Point). Igneous rocks may keep their magnetic orientation they obtain at the time they form (if they are not altered). This magnetic signature is preserved, even if the landmass the magnetic rocks are on is moving. Mapping of the seafloor with magnetometers revealed lines of rock preserving history of "magnetic reversals" running parallel to the mid-ocean ridges (Figure 5-15 and 5-16).

Seafloor spreading is the processes associated with the formation of new areas of oceanic crust, which occurs through the upwelling of magma at mid-ocean ridges and its subsequent outward movement on either side. As new rock forms along spreading centers it becomes attached to the lithospheric plates on either side of the spreading centers.

Paleomagnetic studies of the world ocean basin resulted in the discovery of mid-ocean ridges and spreading centers. These undersea mountain ridges extend for 10s of thousands of miles beneath portions of the global ocean basins (Figure 5-17). Seafloor spreading became a mechanism to explain "continental drift". However, seafloor spreading alone does not explain the formation of continental landmasses through geologic time.

Earth's magnetic field Fig. 5-14. Earth's magnetic field extends into space.
Map of the Atlantic Basin
Magnetic reversals
Fig. 5-16. Mapping of the seafloor with magnetometers revealed lines of "magnetic reversals" on opposite sides of mid-ocean ridges. (USGS)
Magnetic ridge, Juan de Fuca
Fig. 5-16. West Coast magnetic reversals (USGS) reveal the location of spreading centers and fault boundaries in the ocean basin offshore of California, Oregon, Washington, and British Columbia
Fig. 5-15. Seafloor bathymetry of the Atlantic Basin showing the Mid-Atlantic Ridge.(USGS)
Mid Ocean Ridges
Fig. 5-17. Map showing the location of mid-ocean ridges. These undersea mountain ranges are the longest on earth. Mid-ocean ridges (where new ocean crust is forming) is found beneath portions of all the world's ocean basins.
Seismology and Plate Tectonics Theory

Seismology has revealed important aspects of how lithospheric plates interact with each other, how plates form and are destroyed. Over time, as earthquake detection equipment (seismographs) 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 5-18).

Figure 5-19 illustrates how earthquake data reveals the geometry of a subduction zone. This diagram show the location and intensity of earthquakes over a period of time in the vicinity of the Tonga Islands in the South Pacific Ocean. A deep ocean trench runs along the southeast side of the island chain. Earthquake data shows that a major fault system descends at an angle, extending eastward beneath the Tonga Island and extends of hundreds of kilometers at a steep angle deep into the upper mantle (asthenosphere) where it is presumed that earthquakes cease because rocks are too hot and under intense pressure that it easier for them to fold and flow plastically than to fracture as brittle rock. The earthquake data suggests that the eastern edge of the Australian Plate is being over run by the western edge of the Pacific Plate, and that rocks of the Australian Plate are descending into the upper mantle.

A global geologic paradox: the differing ages of rocks within continents and beneath ocean basins
  • The oldest ocean crust is about 200 million years.
  • Continental crust is made up of rocks measured into the billions of years, especially in the stable "craton" cores of continental shields (centers).
As shown in Figure 5-20, 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. 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 in the decades following 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 of about 4.4 billion years (from Australia). 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 5-21 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 rocks beneath ocean basins! 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 (Figure 5-23).

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 ( Figure 5-19 and Figure 5-24). 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.

Figures 5-22 and 5-25 illustrate the basic components of the plate tectonics model. These diagrams illustrate where new crust is forming along spreading centers along mid-ocean ridges, and where old ocean crust is being destroyed or recycled into new continental crust along subduction zones. Spreading centers and subduction zones are mapped as plate boundaries, but there are features that are also considered plate boundaries where crust is neither forming or being destroyed but are rather moving past each other or crushing into each other. These regions have earthquakes but little or no volcanic activity. Figure 5-26 is a map showing inferred plate boundaries around the world.
Earthquakes of the world (epicenters)
Fig.5-18. Earthquakes of the world (USGS record for 1978 to 1987).
Earthquakes reveal geometry of the Tonga subduction zone
Fig. 5-19. Earthquake data reveals the geometry of a subduction zone in the region of Tonga.
Mountain belts and stable cratons
Fig. 5-20. 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. 5-21. 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.
Plate Tectonics
Fig. 5-22. Plate tectonic model
Spreading center along a mid-ocean ridge subduction zone Plate Tectonics Map of lithospheric plates and plate boundaries around the world  
Fig. 5-23. Formation of new oceanic crust along a spreading center associated with a mid-ocean ridge. Some spreading centers appear on land. For example, a portion of the Mid-Atlantic Ridge is exposed as Iceland. Fig. 5-24. 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. 5-25. Plate tectonics model 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. 5-26. 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 5-23).

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 5-23).

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 5-12 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 5-12 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. 5-27. Himalayan Mountains are a convergent plate boundary Fig. 5-28. Mid Ocean Ridge in Iceland is a divergent plate boundary Fig 5-29. 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 movements 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. 5-30. Convergent boundary along the west coast of South America Fig. 5-31. A divergent boundary forming in the Red Sea area Fig. 5-32. Transform fracture zones offshore of California are within the Pacific Plate (USGS)
How does plate plate boundaries correlate to the types of mountain building?
Iceland's Rifts African rifts Tibetan Plateau Pacific northwest
Fig. 5-36. Plate tectonics of the Pacific Northwest (as shown in This Dynamic Planet USGS) is a region where the Juan de Fuca Plate is forming along a spreading center far offshore. The new crust is moving east and then being subducted beneath the North American Plate. The Cascade Range is a volcanic arc forming above the subducted Juan de Fuca Plate.
Fig. 5-33. Iceland's is an exposed portion of the North Atlantic spreading center. Iceland is splitting along a rift zone where new crust is forming. Of 130 volcanoes on the landmass, 30 are currently considered active. Fig. 5-34. Africa rift zones are zones where the African continent are being pulled apart. The African Rift basins are spreading centers that are the location of large inland lakes and much volcanic activity. One day in the distant future become seas like the modern Red Sea.
Fig. 5-35. Migration of "India" away from ancient Pangaea has led to the collision of continental land masses resulting in the rise of the Himalayan Mountains. In this region, the continental crust on both sides of the plate boundary are too light to sink into the mantle.

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 5-3, 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 5-37).

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 is 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 5-38 to 5-40). 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. 5-37. 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.
Plate Tectonics Fig. 5-38. Plate tectonic model: Subduction introduces oceanic crustal rocks (including sediments) back into the Asthenosphere. Water and gas helps low-temperature minerals to melt and rise as, forming new continental crust (less dense than oceanic crust). Floating on the Asthenosphere, the continental crustal materials accumulate, forming continents. Subduction is a refining process Fig. 5-39. The processes associated with subduction lead to the accretion (growth) of continents over time. As ocean crust is recycled back into the upper mantle, the lighter material "accumulates" along continental margins. Pieces of lithosphere are sometimes scraped off one plate and crushed onto and added to another plate.
Fig. 5-40. Terranes in the San Francisco Bay area. This cross section of the Santa Clara Valley (south of San Jose, California) shows several fault-bounded terranes. Each of the large crustal blocks formed in locations far south of the Bay Area, but have gradually moved north along the fault system that bifurcates through the region. Over time, California has formed (assembled) by the accretion of terranes (small crustal landmasses) carried in by plate-tectonic processes slowly over geologic time. Cross section of the San Francisco Bay area

Ancient Parts of Continents: Cratons and Shields

craton—the part of a continent that is stable and forms the central mass of the continent. The craton region of North America includes the region between the Rocky Mountains (to the west) and the Appalachian Mountains (to the east) and include the Canadian Shield.

shield—a large area of exposed Precambrian-age crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas. In all cases, the age of these rocks is greater than 570 million years and sometimes dates back 2 to over 4 billion years. For instance: the Canadian is part of the North American craton region.
World Physiographic Provinces Avalon Formation and Breakup of Pangaea

Through geologic time new continental crust forms and accumulates along the margins of continents. The "floating" continental crust eventually crashes into other land masses, and that may assemble into larger continental crustal plates. For instance, the formation of the ancient supercontinent Pangaea assembled through continental accretion. Pangaea later gradually split apart by continental rifting forming the world's continental landmasses that exist today. The geologic story of the formation and breakup of Pangaea are preserved in the rock record all along the Atlantic Margin of North America.
Breakup of Pangaea in the New Jersey region along the Atlantic Coastal Margin
Fig. 5-41. Continental shields and cratons
Continental shields contain the oldest rocks preserved in the cores of continental landmasses. These regions formed by processes associated with continental accretion billions of years ago, long before the continents of the modern world existed. These ancient shield are part of the stable parts of continents (cratons) and in many places are partially covered by younger sedimentary rocks (such as in the Great Plains and Midwestern Low Plateau regions of North America.
Fig. 5-42. Formation of Pangaea Fig. 5-43. Breakup of Pangaea

"Active" Versus "Passive" Continental Margins

Not all continental margins are plate boundaries. Over time, changes crust is created (along spreading centers) and is destroyed (along subduction zones where crustal material sinks into the mantle). As a result, plates move and continents move along with them. Continents can be split apart, moved, and crushed against other landmasses. Typical most continental landmasses will have a active plate margin (such as around the ring of fire) and have a trailing passive margin (such as the continental boundaries along the Atlantic Ocean basin.

active continental margin—a continental margin that is characterized by mountain-building activity including earthquakes, volcanic activity, and tectonic motion resulting from movement of tectonic plates.

passive continental margin—a passive margin is the transition between oceanic and continental crust which is not an active plate margin. Examples of passive margins are the Atlantic and Gulf coastal regions which represent setting where thick accumulations of sedimentary materials have buried ancient rifted continental boundaries formed by the opening of the Atlantic Ocean basin.

North America motion Plate Tectonics Baltimore Canyon Trough Passive margin
Fig. 5-44. Passive & active margins of the North American Plate are related to the dynamics of plate tectonic motion over time. New crust forms along the Mid Atlantic Ridge, and the North American Plate is moving southwestward, crashing against plates in the Pacific. Fig. 5-45. Generalized diagram showing Western North America's active margin located on boundary between the North American and Pacific Plates. Note how subduction results in formation of the accreted terranes and formation of the Cascades volcanic arc. Fig. 5-46. The East Coast is a passive margin in the region of the Baltimore Canyon Trough along the boundary between ocean crust and continental crust that are now attached in stable configuration. This transition is now in the middle of the North America Plate. Fig. 5-47. Before the opening of the Atlantic Ocean that began about 300 million years ago, the western margin of North America was a passive margin (in Mid-to Late Paleozoic time). It is now an active margin because of changes in relative plate motion.

California Geology and Plate Tectonics History
California has been one of most studied geologic region of the world, and for good reasons: earthquakes!

Through its history, California has transitioned from a passive margin (before the breakup of Pangaea) to an active margin with the transition to subduction zone activity and the formation of the Cordilleran volcanic chain (during the Mesozoic Era, the name for the volcanic arc that formed the Sierra Nevada Range and the Peninsula range extending south in to Baja California), and then to the modern transform plate boundary associated with the San Andreas Fault and greater California fault systems and the opening of the Gulf of California.

California Faults California earthquakes Western North America Plate boundary
Fig. 5-48. Geologic map of California shows the complexity of the different regions within the state. Fig. 5-49. California earthquakes demonstrate that the region is an active margin Fig. 5-50. The San Andreas Fault system is part of a complex transform plate boundary along the West Coast

Farollan Plate Assembling California:
California formed gradually over a billion years though processes involving subduction (forming island arcs) and by accretion (attachment of small land masses carried in for other parts of the Pacific Ocean basin). Before the opening of the Atlantic Ocean Basin, California was sometimes a passive margin.
Information about the geologic evolution of California:

Geologic History of Central California

A technical report about the San Andreas Fault System:

Wallace, Robert E., 1990, The San Andreas Fault System, California: U.S. Geological Survey, 283 p.

This historically significant report provides an overview of the history, geology, geomorphology, geophysics, and seismology of the most well known plate-tectonic boundary in the world.

General Summary of California Plate Tectonic History

No rocks older than ~1 billion years exist in CA - all materials in the CA region were subducted or moved elsewhere...

~1 billion to ~250 million: CA was a mostly a passive margin

~250 to ~30 million: subduction dominated the CA coast,
a great volcanic arc formed the core of the Sierra Nevada

~30 years ago to present: the San Andreas Fault System began to modify the coastline - transform faulting replaced subduction

Fig. 5-51. Formation of the San Andreas Fault

Supporting evidence of long-distance movement along the San Andreas Fault System.
Pinnacles volcano California Rocks California conglomerate
Fig. 5-52. The Pinnacles Volcano originally formed near Los Angeles nearly 23 million years ago. The western half (Pinnacles Formation) is now about 215 miles north of the eastern half (Neenach Formation). Fig. 5-53. Granitic basement rocks in the Coast Ranges originally formed as part of an volcanic arc complex in the Mesozoic Era. They were ripped off of SoCal and carried northward by plate tectonics motion. Fig. 5-54. Cretaceous-age gravels deposited by an ancient river system in southern California were carried northward from their source area and are now scattered throughout the Coast Ranges.

Many Types of Mountains

In summary, it is important that there are many factors affecting the character of landscapes. For land to be exposed above sea level requires some uplifting forces, whether it be tectonic or volcanic in origin, or a combination of both. Land that has been uplifted is also exposed to erosion that wears down the landscape. Running water and glaciers carve canyons, and wind can pile up mountains of dust and sand. It takes careful observations to determine the origin of landscapes. Often the causes are obvious, but in most places it tends to be complex, involving aspects of tectonism and climate influences over time. Figure 5-55 illustrates a few of the kinds of mountains found in the western United States.
Types of Mountains in Wyoming
Fig. 5-55. Types Mountains in Wyoming (and throughout the western United States)

Quiz Questions