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Chapter 4 - Plate Tectonics

This chapter reviews the major concepts of Plate Tectonics Theory. Concepts of plate tectonics evolved as questions about the "structure of the Earth" and the age of the ocean basins were resolved over time (discussed in Chapter 3). Plate tectonics is 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. 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. 4-1. Nearly all geologic processes observed on Earth "fit" in some way into "Plate Tectonics Theory."
Over time, the newly formed ocean crust cooled and moved slowly away from the mid-ocean ridges (Figure 4-2). These areas where new crust is forming and moving apart are called spreading centers. New ocean crust forms and moves away from spreading centers over time (Figure 4-3). Since new ocean crust is forming, old crust has 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 (illustrated in two plate-tectonics models, Figures 4-4 and 4-5). Subduction zones are locations where cool and dense ocean crust sinks back into the mantle (asthenosphere), as it sinks it heats up. Water and gases trapped in the sinking crust cause partial melting (forming magma) which rises (due to its lower density through zones of weakness in the lithosphere. Some of this rising magma accumulates in magma chambers, whereas some of it 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.

Figure 4-34 is a map of Earth's lithospheric plates (the inferred plate boundaries around the world). Figure 4-35 illustrates 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.
Mid Ocean RidgesFig. 4-2. Map showing the location of the world's 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. Age of the seafloor
Fig. 4-3. 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.
Lithospheric plate boundaries illustrated
Fig. 4-4. Plate tectonic model
Plate tectonics model showing types of plate boundaries
Fig. 4-5. A simplified model of plate tectonics showing types of lithospheric plate boundaries.
In 1962, a classic paper written by Harry Hess (a geologist and Navy submarine commander during World War II) who described that the continents did not plow through the oceanic crust (as proposed by Wegener's [1915] Continental Drift Theory), but instead, proposed that they were riding with the oceanic crust like a conveyor belt. This idea was combined with the works of others including: Vine and Matthews (1963) (see Figure 3-23), and Tuzo Wilson (who first reported his theory about the origin of plate boundaries in the early 1960s). Many any other contributions from scientists around the world to put the Theory of Plate Tectonics together.

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.

Plate Tectonics Theory helps to explains 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 Lithospheric plate boundaries produce most observed earthquakes.

Map of lithospheric plates and plate boundaries around the world
Fig. 4-6. Map showing the location of lithospheric 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.
Watch a video about the Pacific Ring of Fire and Its Earthquakes (and Volcanoes) (YouTube video)

Three Types of Lithospheric Plate Boundaries (see Figures 4-4 to 4-6)

Divergent boundary (where plates are pulled apart by tensional forces)—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 discussions below).

Convergent boundary (where plates are pushed together by compressional forces)—When lithospheric plates 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 in northern California, Oregon, and Washington.

Transform boundary (where plates slide past or are rotational)—When plates slide past each other creating fault systems along plate margins. Examples include the San Andreas Fault in California and major faults in Pakistan, Turkey, and along the Jordan River/Dead Sea.

Divergent Plate Boundaries

Divergent plate boundaries are locations where tensional forces are pulling things apart. In locations where lithospheric plates are diverging, the rates of divergent motion range from 2 to 17 cm/year.

Spreading Centers along Divergent Plate Boundaries

A spreading center is 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 (Figure 4-7). Spreading centers are associated with divergent plate boundaries.
Spreading center along a mid-ocean ridge Iceland's Rifts
Fig. 4-7. 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 on Iceland. Fig. 4-8. 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.

Seafloor spreading and formation of new ocean crust

Seafloor spreading features and processes include:
• Involves oceanic crust (OC) only.
• Tensional forces pull oceanic crust apart (forming faults that split deep into the newly forming ocean crust).
• This forms a mid-oceanic ridge (MOR).
• Volcanic activity along faults and fracture zones allows magma to rise cool on or near the seafloor surface.
• This volcanic activity generates new seafloor/oceanic crust,
• New oceanic crust moves away from spreading center as more is formed.
• Youngest oceanic crust is found near the ridge, oldest (and colder)crust is far away from ridge.
• Uplifts occurs as it is rifted, so MORs are the shallowest part of deep ocean basins.
• The MOR is symmetrical about the ridge.
• Magnetic reversals preserved in volcanic rocks are found along MORs worldwide.
• Sediments thicken away from the ridge (discussed in Marine Sediments chapter).• Hydrothermal vents (ie: vents of hot water

called "black smokers") are found in spreading centers.

Ocean Rises vs. Ridges

Spreading center are active at different rates, resulting in different physical characteristics.
A rise spreads faster and is less steep, a ridge is steeper and spreads slower.
East Pacific Rise (see Figures 4-2, 4-3, 4-6, and 4-9)
Mid-Atlantic Ridge (see Figures 4-2, 4-3, 4-6, and 4-7)

• On Iceland, the MOR rises to the land surface, its rate of spreading is about 15 cm/yr (Figure 4-8).

East Pacific Rise
Fig. 4-9. The East Pacific Rise is a rapidly forming spreading center (a mid-oceanic ridge that separates the Pacific Plate to the west from (north to south) the North American Plate, the Rivera Plate, the Cocos Plate, the Nazca Plate, and the Antarctic Plate. See Figure 4-6.

Continental Rifting

Continental Rifting occurs where divergent boundaries form within continental landmasses:
• Involves rifting (pulling apart) of continental crust (CC) only.
• Forms possibly from convection in the asthenosphere
• Large amounts of sediments are usually produced in continental rifting zones.

• Initially, when a continental rift forms it has continental processes such as the formation of large lakes, rivers, and beaches. Later, as it pulls farther apart it can become an oceanic rift when sea water floods in. It then becomes a MOR.

Present continental rifts include:
Red Sea and East African Rift System (Figure 4-10 and 4-11).

* The breakup of Pangaea began with continental rifting starting about 200 million years ago with Africa pulling apart from North and South America. It eventually flooded with seawater and became the new Atlantic Ocean Basin.
Continental rifting along the Sinai Peninsula
Fig. 4-10. Continental rifts along the Red Sea and Sinai Peninsula (Jordan River, Dead Sea)
African rifts
Fig. 4-41. 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.

Crustal Extension and Continental Rifting

Crustal extension occurs where a divergent plate boundary develops under a continental landmass on a large scale, associated with continental rifting (Figure 4-12).

Both crustal extension and continental rifting are occurring in northern Gulf of California and throughout North America's Great Basin region under Nevada, Arizona and eastward into the Rio Grand River rift valley in New Mexico (Figure 4-13). As the Atlantic Ocean opened, the North American continent was pushed over a spreading center that is now the entire region of Utah, Nevada, Arizona, and parts of New Mexico. Baja California and the Peninsular Ranges of San Diego and Orange counties have been rifted away from the Mexican coastline gradually over about 23 million years. Baja will continue moving northward, eventually crushing into southern Alaska in the distant future!
Crustal extension and compression
Fig. 4-12.
Crustal extension and crustal compression.
Gulf of California
Fig. 4-13
. Gulf of California and Great Basin—a region where crustal extension is occurring.

Convergent Plate Boundaries

Convergent Plates move together and collide so you have compressional forces. They are associated with active margins—locations where mountain building is occurring, resulting in numerous earthquakes and andesite (explosive) volcanoes.

A subduction zone is a plate boundary along which one plate of the Earth’s outer shell descends (subducts) at an angle beneath another (Figure 4-14). 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 (see Figure 4-19 below). Most tsunamis are generated by subduction-zone-related earthquakes.

Figure 4-14 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.
Earthquakes reveal geometry of the Tonga subduction zone
Fig. 4-14. Earthquake data reveals the geometry of a subduction zone in the region of Tonga.

Three types of Convergent Plate Boundaries: OC/CC, OC/OC, & CC/CC

Three types of convergent plate boundaries are recognized: .
a) Subduction of ocean crust (OC) beneath continental crust (CC)
b) Subduction of ocean crust (OC) beneath ocean crust (OC)
c) Continental Collisions: continental crust (CC) colliding with continental crust (CC).

a) Subduction of ocean crust (OC) beneath continental crust (CC).

• Denser, thinner OC is pushed or subducted beneath less dense and more buoyant CC.
• A chain of volcanoes formed, called a continental volcanic arc.
• Subduction produces both deep and shallow focus earthquakes (with tsunami potential); the largest ever--9.5 magnitude in Peru/Chile Trench in 1960.
• Volcanoes of the andesite (explosive) type. Examples include the Andes and the Cascade Range , etc.
• Deep trenches form around continents margins. Trenches are especially well developed in regions far away from spreading centers (where the ocean crust is old, cold, and denser, and therefore sinks more rapidly).
• Subduction reduces amount of (and destroys) OC.
• Rates of subduction are up to 15 cm/yr in the active margins of the Pacific Basin.

• Andes in South America (Figure4-15)
• Cascades in United States (include such volcanoes as Mt. St. Helens, Mt. Rainier, Mount Shasta, Crater Lake and many others)
subduction zone
Fig. 4-15. Subduction zone geometry (OC/CC) 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.

b) Subduction of ocean crust (OC) beneath ocean crust (OC).

• Many similar features as above [OC/CC].
• Denser, older, cooler OC is pushed or subducted beneath less dense, warmer, younger OC.
• Forms island volcanic arcs.
• Deep and shallow (tsunami potential) focus earthquakes
• Volcanoes not as explosive as above with OC/CC, as there is no mixing of CC rocks (called granites). Volcanic rocks are mostly basaltic in composition.
• Subduction reduces amount of (destroys) OC.

Japan, Tonga Islands, and Aleutian Islands (Alaska)(Figures 4-16 and 4-17)
Aleutian Island chain and trench
Fig. 4-16.
Aleutian Islands and Aleutian Trench.
Aleutian volcanoes
Fig. 4-17.
Volcanoes of the Aleutian Islands arc.

c) Continental Collisions: continental crust (CC) colliding with continental crust (CC)

When continents collide with other continental landmasses:
• Neither of the CC are subducted,
• Both are very buoyant and want to "float" or ride high.
• This is where you form the very large mountain chains.
• Mountain building occurs with lots of earthquakes; massive erosion also occurs.


Himalayas (India) beginning 45 million years ago) (Figure 4-18)
Alps Mountains are being pushed up by collisions between Africa (and Italian Peninsula) with Europe.
Appalachians Mountains in the eastern United States (formed when North America collided with Africa about 350-400 million years ago (before the Atlantic Ocean opened later).
Tibetan Plateau
Fig. 4-18. 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.

Transform Boundaries

Transform boundaries are locations where one plate is sliding past another.
• Can occur in any crustal type (OC or CC)
• Crust is neither produced or destroyed

Continental Transform Faults

• In continental crust, transform faults can be large destructive faults like the San Andreas Fault System in California (Figure 4-19).
• Earthquake magnitudes up to 8.5 have been measured.
• Rates on the San Andreas Fault (SAF) average 2-5 cm/yr or LA will be adjacent to SF in about 20 million years!
• Compression(and uplift) or tension (and down-warping) can occur. The high mountains in Southern California are a result of this kind of compression along the SAF including the San Gabriels, San Jacinto, and San Gorgonio Mountains.

Oceanic Transform Faults
• In ocean crust, smaller transform faults occur perpendicular to the spreading centers.
• They have smaller earthquakes associated with them, when compared with their continental cousins.
• These faults occur where there is a bend or change in the rate of spreading along the MOR.
Western North America Plate boundary
Fig. 4-19. The San Andreas Fault system is part of a complex transform plate boundary along the West Coast of North America.

Review: 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
  • The African Rift Zones and the Red Sea
  • The Great Basin and Rio Grande Rift Zone
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. 4-20. Himalayan Mountains are a convergent plate boundary Fig. 4-21. Mid Ocean Ridge in Iceland is a divergent plate boundary Fig 4-22. 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. 4-23. Convergent boundary along the west coast of South America Fig. 4-24. A divergent boundary forming in the Red Sea area Fig. 4-25. Transform fracture zones offshore of California are within the Pacific Plate

Continental margins may or may not be plate boundaries! For example, the East Coast is in the middle of the North American Plate. Why? When North America first split away from Africa and Europe, there was first a continental rift valley; it became a plate boundary between the three expanding lithospheric plates. As new crust formed along the Mid Ocean Ridge spreading center, it became attached to the plates on either side. Today the North American Continent is part of the larger North American Plate! (See Figure 7-29.)

Hotspots and Mantle Plumes

A hotspot is a place in the upper mantle of the Earth at which extremely hot magma from the lower mantle upwells to melt through the crust usually in the interior of a tectonic plate to form a volcanic feature.
• These are hotspots beneath the lithosphere caused by rising plumes of hot mantle material.
• Can form volcanoes on surface (examples include Hawaii and Yellowstone hotspots (Figures 4-26 and 4-27).
-Volcanoes are mostly mafic (basaltic) as these lavas are very hot and very fluid from deep sources.
-Less common are felsic (rhyolitic) magmas as they are thicker and less prone to flow.
• Hotspots can occur beneath any crustal type (OC or CC).
• Where they form a trace consisting of a chain of volcanoes (like in Hawaii's Emperor Seamount Chain, Figure 4-26)
• Hotspots can exist in about the same place for 10's of millions of years
• The Hawaiian Hotspot has existed for about 60 million years; the youngest part of the Emperor Seamount Chain.
• The oldest part of the Emperor Seamount Chain has already been subducted (destroyed).
• There are hundred of hotspots located around the world. Some are larger and more active than others.
• Most hotspots are located under the interior sections of lithospheric plates, but some occur near divergent plate boundaries.
• Paleomagnetism in rocks on the ocean floor associated with hotspots provides a method for determining speed and direction of plate motions.
• We are not sure of the exact mechanism that forms hotspots, there are some ideas (see below).

Yellowstone National Park Hotspot:

• The Yellowstone hotspot currently under continental crust (Figure 7-50).
• Yellowstone is a very large and complex supervolcano.
The Yellowstone supervolcano is at the eastern end of a long chain of progressively older supervolcanoes that formed along the trace of the Snake River Plain (Idaho, Washington, and Oregon)
• The hotspot had formed volcanoes of both basaltic and rhyolitic composition.
• Has had large rhyolitic eruptions 3 times in the last 2 million years
• This hotspot appears to have been present for less time than the Hawaiian hotspot (about 17 million years).

Hawaii Hotspot and the Emporer Seamount Chain
Fig. 4-26.
Map of the Hawaiian Hotspot and the Emperor Seamount Chain in the Pacific Ocean basin.

Migration of the Yellowstone Hotspot under western North America
Fig. 4-27
. The Yellowstone Hotspot is beneath the North America continent and is slowly migrating eastward as the continent is moving westward.
Hot topic: Are hotspots related to "astroblemes" (large asteroid impacts)?

Earth has been hit by many asteroids throughout the geologic past. If Earth didn't have an atmosphere and active plate tectonics it would appear heavily cratered like the moon!

Current research suggests that massive asteroid impacts can deeply penetrate and fracture the lithosphere, allowing craters to flood with lava—the magma generated by both the impact and material flooding upward to the surface from deeper down. The question is, do astroblemes turn into hotspots? Also, it has been suggested that shock waves from a massive impact can travel through the Earth and will concentrate energy at the "antipoles" - resulting in deep fracturing of the lithosphere, resulting in massive volcanic eruptions. Examples of two possible "antipole" eruptions include the formation of the Deccan Traps in India (opposite the K/T boundary impact ~66 million years ago in the Yucatan region of Mexico. Another massive flood eruption occurred about 250 million years ago, forming the massive Siberian Traps (massive flood basalts that formed about the time of the great end-of-Permian extinction).

What drives plate motions?

• Researchers agree that convective flow in the mantle is the basic driving force of plate tectonics (Figure 4-28).

Forces that drive plate motion:

Slab-pull - the slow pull of mantle material where is moves from a rising location to a sinking location.
• Slab-suction - The high density of cold ocean crust sinking into the mantle pulls crust with it.
Ridge-push - new ocean crust is warm and tends to rise above the ocean floor, pushing older cooler crust away

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.

Importance of plate tectonics

• The theory provides explanations for many of earth’s major processes
• Explains the geologic distribution of earthquakes, volcanoes, and mountains
• Explains the distribution of ancient organisms and mineral deposits
• Plate tectonics are responsible large volumes of oil and gas deposits we enjoy today. Think about how your life and society would be different if we only had a mere fraction of the hydrocarbons we currently have and use.
Forces that drive plate tectonic motion
Fig. 4-28. Mantle convection is the driving force of motion of lithospheric plates. Much of the convection motion may be in the upper mantle, but it is likely to extend deep into the earth's molten outer core.
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. As shown in Figure 4-28, the combined effect of the internal heat of the Earth and the force of gravity drive convection currents within the Earth . 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 4-29).

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, deep ocean waters. As a result, old ocean crust is enriched in dense minerals. As it ages, it also 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 (assisted by the presence of water and other gases). The materials that melt rise as hot fluids (magma and gases) 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 are 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 off and crushed onto the margin of continents—often pushed up as coastal 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 and younger material occurs along continental margins.

Continental Accretion

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 4-30 to 4-31). Over geologic time (measured in millions of years), volcanic arcs form and may be crushed onto (or between) colliding continents along 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!

The entire West Coast of North America is made up of massive fault-bounded blocks of crust (called terranes).A terrane is a fault-bounded area with a distinctive stratigraphy (collection of rocks), structure, and geologic history compared with surrounding terranes or land masses.
Boiling pot of soup
Fig. 4-29. Currents in boiling soup demonstrates convection. The "broth" rises, cools, and sinks (like the formation and destruction ocean crust over time). In contrast, the 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.
Another YouTube video:
Continents Adrift An Introduction to Continental Drift and Plate Tectonics
This video explains much of what this chapter reviews.
Plate Tectonics Fig. 4-30. 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. 4-31.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 near the surface along continental margins. Pieces of lithosphere are sometimes scraped off one plate and crushed onto and added (accreted) to another plate.
Fig. 4-32. 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

A craton is a 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.

A shield is 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 Shield is part of the North American craton region. Shallow inland seas have flooded over and retreated from North America's craton/shield region in the past billion years.
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 these terranes 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, Europe, and Africa.
Breakup of Pangaea in the New Jersey region along the Atlantic Coastal Margin
Fig. 4-33. 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. 4-34. Formation of Pangaea by closing of the "proto-Atlantic" Iapetus Ocean about 300 million years ago. Fig. 4-35. Breakup of Pangaea and formation of the Atlantic Ocean about 200 million years ago.
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 continental 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 Cordilleran Ranges is name for the volcanic arc that formed the Sierra Nevada Range and the Peninsular Ranges extending south in to Baja California). Subduction ended when the ancient Farralon Plate was overrun as North America moved westward, overriding the nothern end of the spreading center in the Eastern Pacific basin (Figure 7-62). This lead to the formation to the modern transform plate boundary associated with the San Andreas Fault (part of the greater California fault system) and the opening of the Gulf of California.

California Faults California earthquakes San Andreas Fault System in California
Fig. 4-36. Geologic map of California shows the complexity of the different regions within the state. Fig. 4-37. California earthquakes demonstrate that the region is an active margin Fig. 4-38. The San Andreas Fault System in California showing the locations impacted by major earthquakes.

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.

Generalized 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 or was accumulating as sediments on an actively migrating seafloor. Periods of mountain building and other plate-tectonic-related activity moved small land masses along the West Coast region. North America moved westward and northward over time.

~250 to ~30 million: subduction and island-arc volcanism dominated the CA coast. An ocean trench existed along where the coast exists today, and a subduction-related igneous activity created the 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

. Baja California split away from the Mexico. Uplift and erosion has exposed the core of the ancient volcanic arc, now exposed throughout the Sierra Nevada and Peninsula Range (Baja and San Diego region).
Fig. 4-39. Formation of the San Andreas Fault caused by North America Plate overriding the ancient Farallon Plate. Remnants of the Farallon Plate exist as the Juan de Fuca Plate (offshore Oregon & Washington) and the Cocos Plate (off central America).

Supporting evidence of long-distance movement along the San Andreas Fault System.
Pinnacles volcano California Rocks California conglomerate
Fig. 4-40. The Pinnacles Volcano originally formed near Los Angeles nearly 23 million years ago. The western half (Pinnacles Formation) is now about 215 miles [350 km] north of the eastern half (Neenach Formation). Fig. 4-41. 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. 4-42. 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.
Recommend reading! Learn more about Plate Tectonics Theory at these online resources prepared by the Smithsonian Institution and U.S. Geological Survey:

This Dynamic Earth (The Story of Plate Tectonics) -
this on-line booklet is used for teaching plate tectonics at schools and universities around the world. It has a companion map:

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

(see Figure 4-66).

Crustal age of the Seafloor Map: National Oceanic and Atmospheric Administration, National Geophysical Data Center [large image]
This Dynamic Planet
Fig. 4-43. Plate Tectonic Features Map from "This Dynamic Planet."
Field guides to the San Andreas Fault and other regional faults in the San Francisco Bay region.

Where’s the San Andreas Fault? A Guidebook to Tracing the Fault on Public Lands in the San Francisco Bay Region
Where's the Hayward Fault? A Green Guide to the Fault
Geology Field Trips To California's Central Coast
Chapter 4 Quiz Questions
http://geologycafe.com/oceans/chapter4.html 1/1/2016