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. Each tectonic plate is labeled and color-coded to distinguish it from neighboring plates. Red arrows along the plate boundaries indicate the relative direction of plate motion, helping to identify divergent, convergent, and transform plate boundaries. 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 (with Locations):

• 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 – 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 (Red arrows show plate movement direction):

• Divergent boundaries (arrows pointing away from each other): seen at the Mid-Atlantic Ridge, indicating seafloor spreading.
• Convergent boundaries (arrows pointing toward each other): found near the Nazca and South American Plates.
• Transform boundaries (arrows sliding past each other): exemplified by the San Andreas Fault between the Pacific and North American Plates.

California lies along the transform boundary between the Pacific Plate and the North American Plate, accounting for its frequent earthquake activity. The Juan de Fuca Plate subducting beneath the North American Plate contributes to volcanic activity in the Pacific Northwest.

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.
• Mantle: Thick, yellow-orange layer beneath the crust composed of silicate minerals rich in iron and magnesium.
• Core: Deepest layer, composed primarily of iron and nickel, shown as a large dark red zone.

Mechanical Layers (Physical) – Right Side

• Lithosphere: Rigid outer shell made up of the crust and the uppermost mantle. Extends to about 100 km depth.
• Asthenosphere: Ductile, partially molten upper mantle layer beneath the lithosphere (100–350 km).
• Mesosphere: Strong, solid lower mantle extending to about 2900 km.
• Outer Core: Liquid layer of molten iron and nickel, shown in orange-red, extending from ~2900 to 5100 km.
• Inner Core: Solid central sphere composed mostly of iron, colored in yellow, from ~5100 km to the center (6371 km).

Figure 3.1.4 Seismic Wave Propagation in the Crust an 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. The diagram represents a cross-sectional slice of Earth’s outer shell, with color gradients and labeled layers to show how wave speed and direction change at different depths.

Diagram Layout

• The top half of the image is labeled Crust and shown in pale pink.
• The lower half is labeled Mantle and shown in shades of green.
• A horizontal boundary between the crust and mantle represents the Mohorovičić discontinuity (Moho).

Seismic Wave Source and Paths

• A red and yellow star near the left side of the crust marks the earthquake epicenter or seismic source.
• Black arrows radiate outward from the star, showing the initial wave directions.
• Some seismic rays:
  • Travel upward directly through the crust.
  • Refract at the crust-mantle boundary.
  • Bend downward into the mantle, then curve back upward toward the surface.
• Two labeled wave velocity paths:
  • ~6 km/s in the crust (slower, upper path).
  • ~8 km/s in the mantle (faster, deeper path).
• A red arrow on the far right indicates wave arrival at the surface.

Wave Behavior

• Wave paths curve due to refraction, bending as seismic velocity increases with depth.
• The image demonstrates Snell’s Law in action—seismic waves bend toward the slower medium and away from the faster one.
• The deeper path allows seismic waves to arrive at distant locations more quickly despite traveling farther.

Figure 3.2.3 Spreading Center

This is a geologic cross-section illustrating 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.

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 fed lava to the surface.
  • Gabbro: Coarse-grained intrusive rock at deeper levels of the crust, crystallized from slowly cooling magma.
• Transition Zone
  • A labeled "Transition zone" marks the area between the crust and the underlying mantle.
  • Appears as a slightly different texture from both crust and mantle.
• Magma Chamber
  • A central red-orange region labeled “Magma” sits beneath the rift, feeding material into the crust above.
  • Yellow-orange arrows show the upward flow of magma toward the surface.
• Mantle
  • Beneath the crust and transition zone lies the green 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 indicating seafloor spreading.

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.

Countries and Oceans:

• United States and Mexico are shown in tan.
• Pacific Ocean is shown in light blue.
• Canada appears at the top of the map.

Key Cities:

• San Francisco and Los Angeles are labeled on the coast of California.

Major Plate Boundaries:

• The San Andreas Fault is marked with a black line with arrows, stretching through California.
• Subduction zone to the north off the coast of Washington and Oregon is labeled.
• East Pacific Rise is shown off the southern edge of the map, near Mexico.

Plate Motion:

• Relative motion of the North American Plate: Shown with a purple arrow pointing southeast.
• Relative motion of the Pacific Plate: Shown with a purple arrow pointing northwest.
• Arrows indicate transform motion 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 labeled and visible offshore, including:

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

Map Elements:

• A scale bar in the upper right indicates distances of 0 to 400 kilometers (0 to 200 miles).
• Thin black lines show tectonic boundaries.
• Pink and purple text and arrows emphasize plate motion.

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. An explanation key to the right defines the symbols used.

Panel 1: 30 Million Years Ago

• Farallon Plate is subducting beneath the North American Plate at a long, continuous trench.
• Pacific Plate is to the west of the Farallon Plate.
• Subduction zones are shown as lines with triangles on the overriding plate side; spreading centers (divergent boundaries) are shown as pink lines.
• The area near Los Angeles is labeled, but the San Andreas Fault has not yet formed.

Panel 2: 20 Million Years Ago

• The Pacific Plate begins to subduct beneath north America. The Farallon Plate is renamed the Juan de Fuca (north) and Cocos (south) plates.
• Triple junctions are marked with red triangles:
  • Mendocino Triple Junction (M) in the north.
  • Rivera Triple Junction (R) in the south.
• A transform fault (early San Andreas Fault) has begun forming between the Pacific and North American Plates.

Panel 4: Present

• The San Andreas Fault Zone is a dominant boundary feature.
• The Pacific Plate slides northwest past the North American Plate.
• The Juan de Fuca, Rivera, and Cocos Plates are present offshore, each with their respective subduction zones or spreading centers.
• Baja California and San Francisco are now clearly labeled.

Legend (Right Side):

• Pink line with triangle: Spreading center (divergent boundary)
• Jagged line: Subduction zone (convergent boundary)
• Parallel lines with arrows: Transform fault, showing relative motion
• SAFZ = San Andreas Fault Zone
• Red triangles indicate triple junctions (Mendocino and Rivera)

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 and the movement of the Juan de Fuca and North American Plates.

Key Labels and Features:

• Pacific Ocean: Shown in blue on the left side (west), representing oceanic crust.
• Cascadia Subduction Zone: A dashed white line marks the subduction boundary between the Juan de Fuca Plate and the North American Plate.
• Cascade Volcanic Arc: A shaded beige band running roughly north–south along the inland side of the subduction zone through Washington, Oregon, and northern California. It is labeled "Cascade Volcanic Arc."
• Plate Motion Arrows:
  • Juan de Fuca Plate: Labeled with a white arrow pointing eastward beneath the continent, showing its subduction direction.
  • North American Plate: Labeled with a black arrow pointing westward toward the ocean, indicating overriding plate motion.
• Political Labels:
  • Countries and borders are labeled: U.S.A. and Canada.
  • Terrain is colored green and brown to indicate forested and mountainous areas, respectively.

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.

Geographic Features:

• Mainland Mexico: Located on the right (east) side of the image.
• Baja California: A long peninsula running parallel to the mainland, on the left (west) side.
• Gulf of California: The ocean basin separating Baja California from mainland Mexico; shown in dark blue.
• Pacific Ocean: Shown in deep blue to the west of Baja California.

Tectonic Motion:

• Thick black arrows show major plate motion directions:
• Mainland Mexico and North America are moving generally northeast.
• Baja California is moving northwest, away from the mainland.
• Gray arrows within the Gulf of California show divergent motion, indicating 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.

Additional Elements:

• A compass rose in the top-right corner shows orientation with north pointing upward.
• A scale bar at the bottom-left indicates a horizontal distance of 100 miles.
• Text overlay labels major regions: Pacific Ocean, Gulf of California, Baja California, Mainland Mexico, and North America.
• The map is noted as “Modified from Google Earth imagery.”