3.6: Detailed Figure Descriptions

Figure 3.1 Global Plate Boundaries and Plate Motion

This is a global map showing the major tectonic plates of Earth's lithosphere. This image is relevant in the context of California geology because the Pacific Plate and North American Plate boundary explains California’s tectonic activity.

Major Tectonic Plates

• North American Plate – Covers most of North America, part of the Atlantic Ocean.
• Pacific Plate – Lies beneath the Pacific Ocean and borders the western edge of North America.
• Eurasian Plate – Encompasses Europe and much of Asia.
• African Plate – Covers the African continent and parts of the surrounding ocean floor.
• South American Plate – Includes South America and part of the Atlantic seafloor.
• Antarctic Plate – Includes and surrounds Antarctica.
• Indo-Australian Plates – Includes the Indian Plate and Australian Plate.
• Nazca Plate – Located west of South America.
• Cocos Plate – Located southwest of Central America.
• Juan de Fuca Plate – A small plate off the Pacific Northwest of North America.
• Caribbean Plate, Scotia Plate, Arabian Plate, and Filipino Plate are also shown.

Plate Boundaries and Movements

Arrows along the plate boundaries indicate the relative direction of plate motion to identify divergent, convergent, and transform plate boundaries, as listed here.

Divergent boundaries (arrows pointing away from each other) are shown between the following plates:

• North American and Eurasian plates, at the Mid-Atlantic Ridge, indicating seafloor spreading
• Antarctic and Australian plates
• Antarctic and Pacific plates
• Antarctic and African plates
• Nazca and Pacific plates
• Nazca and Cocos plates
• South American and African plates
• African and Arabian plates
• African and Indian plates

Convergent boundaries (arrows pointing toward each other) are shown between the following plates:

• Pacific and Australian plates
• Pacific and North American plates
• Nazca and South American plates
• Cocos and Caribbean plates
• Indian and Eurasian plates

Transform boundaries (arrows sliding past each other) are shown between the Pacific and North American plates (San Andreas Fault)

Figure 3.1.1 Compositional and Mechanical Layers of Earth’s Interior

This diagram is a labeled cross-section of Earth’s interior, showing two complementary ways of dividing Earth's internal structure:

• The left half represents the compositional (chemical) layers, based on chemical makeup.
• The right half illustrates the mechanical (physical) layers, based on physical properties and behavior.

Each section is color-coded and labeled, with boundary depths noted in kilometers. The Earth’s full diameter is labeled as 12,742 km (7,917.5 miles), and the center is noted at 6,371 km depth.

Compositional Layers (Chemical) – Left Side

• Continental and Oceanic Crust: Thin, outermost layer; 100 km deep.
• Mantle: Thick layer beneath the crust composed of silicate minerals rich in iron and magnesium; extending from 100 to 2,900 km deep.
• Core: Deepest layer, composed primarily of iron and nickel, and extending from 2,900 km to the center (6,371 km).

Mechanical Layers (Physical) – Right Side

• Lithosphere: Rigid outer shell made up of the crust and the uppermost mantle. Extends to 100 km depth.
• Asthenosphere: Ductile, partially molten upper mantle layer beneath the lithosphere (100–350 km).
• Mesosphere: Strong, solid lower mantle extending from 350 km to 2,900 km deep.
• Outer Core: Liquid layer of molten iron and nickel, extending from 2,900 to 5,100 km.
• Inner Core: Solid central sphere composed mostly of iron, extending from 5,100 km to the center (6,371 km).

Figure 3.1.4 Seismic Wave Propagation in the Crust and Mantle

This diagram illustrates the paths and velocities of seismic waves generated by an earthquake, showing how they move through the crust and mantle of Earth’s outer layers. A cross-sectional slice of Earth’s outer shell is shown, with color gradients and labeled layers to show how wave speed and direction change at different depths. This visual information is described here.

Seismic Wave Source and Paths

• Seismic rays emanate from an earthquake epicenter in several directions, traveling upward directly through the crust, refracting at the crust-mantle boundary, and bending downward into the mantle, then curving back upward toward the surface.
• Two wave velocity paths are identified:
  • ~6 km/s in the crust (slower, upper path).
  • ~8 km/s in the mantle (faster, deeper path).

Wave Behavior

• Wave paths curve due to refraction, bending as seismic velocity increases with depth.
• The deeper path allows seismic waves to arrive at distant locations more quickly despite traveling farther.

Figure 3.2.2 Spreading Center

This geologic cross-section illustrates how new oceanic crust forms at a mid-ocean ridge. It depicts the layers of crust and mantle beneath the seafloor, the upward movement of magma, and the structure of the oceanic lithosphere. The image shows depth from the seafloor down to about 9 kilometers and a horizontal distance of about 20 kilometers across the ridge axis. (A tactile graphic, Mid Ocean Ridge, is available from the Geological Tactile Image Repository.)

Key Features (Top to Bottom)

• Seafloor Topography
  • A rift valley sits at the center, labeled “Plate boundary,” where the seafloor is pulled apart.
  • On either side of the rift, layers of oceanic crust are shown, dipping away from the ridge axis.
• Oceanic Crust Layers
  • Pillow basalts: Green, lumpy structures near the seafloor, representing lava erupted underwater.
  • Sheeted dikes: Vertical dark bands that ferd lava to the surface.
  • Gabbro: Coarse-grained intrusive rock at deeper levels of the crust, crystallized from slowly cooling magma.
• Transition Zone between the crust and the underlying mantle
• Magma Chamber
  • Magma sits beneath the rift, feeding material into the crust above.
  • Wavy lines indicate the upward flow of magma toward the surface.
• Mantle
  • Beneath the crust and transition zone lies the mantle, which serves as the source of magma.

Scale and Orientation

• The vertical axis is labeled “Depth below sea surface (km)” and ranges from 0 to 9 km.
• The horizontal axis is labeled in kilometers, spanning from -10 to +10 km, centered on the ridge axis.
• The central black dashed line marks the plate boundary, with diverging arrows at the plate boundary indicating seafloor spreading.

Figure 3.2.3 Doming and Three-Part Rifts

A top-down view of a dome shows radial fractures that divide the dome into three sections. This represents how doming associated with formation of a mantle plume creates three-part rifts. A schematic showing the supercontinent Pangea breaking into the continents of Africa and South America shows conjoined successful rifts between the two new continents, created by mantle plumes. Noting that these plumes indicate three-part rifts, a single yellow lines radiates from each mantle plume to show a third, failed rift.

Box Figure 3.2.1.1 A Diagrammatic Passive Margin

This simplified diagram shows a passive margin along the western North American continent. From left to right we see the ocean depicted above a thin layer of sediment, which rests on a thicker oceanic crust above a still thicker mantle region. The middle of the diagram shows the sediments quickly increasing in thickness as the ocean layer thins. The oceanic crust gives way as the continental crust thrust upward, pushing the mantle downward. When we reach the right side of the diagram the ocean has receded and the continental crust takes up most of the vertical space. The continental crust exhibits a series of listric normal faults, shown as diagonal lines slanted in the general direction of the thrust. A very thin layer of sediment overlays the continental crust, and the mantle has thickened below it.

Figure 3.4.1 San Andreas Fault and Plate Boundary Features of the U.S. West Coast

This map displays tectonic features along the western edge of North America, from southern Canada through the United States and into northern Mexico. The focus is on transform and divergent plate boundaries and the relative motion between the Pacific Plate and the North American Plate.

Major Plate Boundaries

• The San Andreas Fault stretches through California, along the Mendocino fracture zone. This fracture zone is shown moving through the Pacific Ocean and along the California coast just above San Francisco, then moving inland at the San Francisco Bay. It then continues southward and inland from Los Angeles, moving through Mexico and into the Gulf of California.
• In Mexico, the East Pacific Rise runs north to south along the eastern shore of the Gulf of California.
• A subduction zone is shown north of the Mendocino fracture zone, moving through Oregon and Washington in the United States and terminating in the northern area of Vancouver Island in British Columbia (Canada).
• Further out to sea and running roughly parallel to the subduction zone, from north to south, we see Explorer Ridge, then Juan de Fuca Ridge, and finally the Blanco fracture zone (all north of the Mendocino fracture zone).

Plate Motion

• The North American Plate moves southeast in a relative motion
• The Pacific Plate moves northwest in a relative motion.
• Transform motion occurs along the San Andreas Fault, where the Pacific Plate slides past the North American Plate.

Several east-west trending fracture zones and north-south trending ridges are visible offshore, including (from north to south):

• Explorer Ridge
• Juan de Fuca Ridge
• Blanco Fracture Zone
• Mendocino Fracture Zone
• Murray Fracture Zone
• Molokai Fracture Zone

Figure 3.4.2 Transtension and transpression

A diagram showing plate boundaries along the San Andreas Fault is used illustrate the concepts of transpression and transtension. At the top of the diagram the plate motion is indicated as moving westward, while the plate motion at the bottom of the diagram moves eastward. At the left of the diagram we see a pull-apart basin indicated as a gap between the plates. Beginning just above this basin and then moving southeast across the fault line we see ovals indicating folds. Above the fault line the folds are labeled "transpression." Below of the fault line we see arrows indicating force moving against the the folds from two sides. Directly to the right of this region, we see an area on the fault when forces are pulling the plates apart. This is identified as an "oblique pull-apart basin," and is labeled "transtension."

Figure 3.4.3 Tectonic Evolution of the West Coast Plate Boundary: 30 Million Years Ago to Present

This four-panel image illustrates the tectonic evolution of the Pacific–North American plate boundary over time, from 30 million years ago to the present. It shows how the subduction of the Farallon Plate has been replaced by a transform boundary (the San Andreas Fault Zone) as new plates formed and spreading centers shifted. 

30 Million Years Ago

• The Farallon Plate is subducting beneath the North American Plate at a long, continuous trench.
• The Pacific Plate is to the west of the Farallon Plate.
• A spreading center (divergent boundary) is shown along the southeast edge of the Pacific Plate.
• The San Andreas Fault has not yet formed.

20 Million Years Ago

• Two sections of the Pacific Plate come into view. The southmost section begins to subduct beneath the North American Plate in roughly the area where Los Angeles is located today. As it moves, it bisects the Farallon Plate, which becomes the Juan de Fuca Plate in the north and Cocos Plate in the south.
• Long trenches remain where the North American Plate abuts the Juan De Fuca and Cocos plates.
• Spreading centers (divergent boundaries) are shown along the southeast edges of the Pacific Plate where it meets the Juan De Fuca Plate in the north and the Cocos Plate in the south.
• Triple junctions are indicated where the Pacific Plate meets the North American Plate:
  • In the north, the Mendocino Triple Junction is where the Juan De Fuca, Pacific, and North American plates meet.
  • In the south, the Rivera Triple Junction is where the Cocos, Pacific, and North American plates meet.
  • A transform fault (early San Andreas Fault) has begun forming between the Pacific and North American plates, as the Pacific Plate moves in a northward direction and the North American Plate moves southward.

10 Million Years Ago

• As the Pacific Plate (moving northward) continues to subduct beneath the North American Plate (moving southward), the transform fault becomes longer. It stretches from roughly what is now Central California in the north to what is now Baja California in the south; accordingly, the distance between the Mendocino and Rivera triple junctions has increased dramatically.
• Trenches remain where the North American Plate abuts the Juan De Fuca and Cocos plates.
• Spreading centers (divergent boundaries) are shown along the southeast edges of the Pacific Plate where it meets the Juan De Fuca Plate in the north and the Cocos Plate in the south.

Present

• As the Pacific Plate slides northwest past the North American Plate, the San Andreas Fault is a dominant boundary feature. It now extends from the Mendocino Triple Junction north of San Francisco to the Rivera Triple Junction in the Gulf of California.
• The Juan de Fuca Plate is shown in the north, bordered by a trench along its south edge where it meets the North American Plate.
• The Rivera Plate now appears along the coast of Mexico, bounded on the east by the trench along the North American Plate. It is shown as a rough triangle with the Rivera Triple Junction as one of its points.
• The Cocos Plate still appears in the south, at the bottom of the map, bounded on the east by the trench along the North American Plate.
• Spreading centers (divergent boundaries) are still shown along the southeast edges of the Pacific Plate where it meets the Juan De Fuca Plate in the north, and where it meets the Rivera and Cocos plates in the south. However, additional divergent boundaries now appear along the San Andreas Fault from roughly its midpoint and moving south. 

Figure 3.4.4 Cascade Volcanic Arc

This is a shaded-relief satellite map depicting the Pacific Northwest region of the United States and southwestern Canada. The map illustrates the tectonic setting responsible for the Cascade Volcanic Arc, emphasizing the Cascadia Subduction Zone off the coast of northern California and British Columbia, as well as the northeastern movement of the Juan de Fuca Plate and the dominant northwestern movement of the North American Plate.

• The rough texture in the Pacific Ocean represents oceanic crust.
• Cascadia Subduction Zone is where the Juan de Fuca Plate is subducting beneath the North American Plate. This is an irregularly shaped area bounded on the west by ridges that run along the ocean floor.
• The Cascade Volcanic Arc runs roughly north–south along the inland side of the subduction zone through Washington, Oregon, and northern California.

Figure 3.4.5 Divergence in the Gulf of California

This satellite-based map highlights tectonic motion in the Gulf of California region, illustrating how Baja California is moving northwestward relative to mainland Mexico. The map focuses on the rifting process that is forming an ocean basin between these landmasses.

Key Points:

• Mainland Mexico and North America are moving generally northeast.
• The Baja California peninsula is moving northwest, away from the mainland.
• Divergent motion is occurring beneath the Gulf of California, resulting in seafloor spreading and the extension of the crust between the two landmasses.
• These arrows alternate direction on either side of the central rift structures, highlighting the relative motion of crustal blocks.