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Lecture 4 Flashcards

Lines of Evidence for Plate Tectonics

World Distribution of Earthquakes

  • Observation: Earthquakes are not randomly distributed across the Earth but occur in distinct linear patterns, which delineate plate boundaries.

  • Concentration Zones:

    • Pacific Rim of Fire: This region, particularly the West Pacific, exhibits the highest concentration of earthquakes, including the largest and strongest events. These are predominantly associated with subduction zones.

    • Mid-Ocean Ridges: Earthquakes also occur along mid-ocean ridges, although they are generally less frequent and of lower magnitude compared to subduction zones. These ridges are typically found in the middle of oceans.

  • Mechanism: Earthquakes result from immense pressure building up as plates move with respect to one another, releasing seismic energy along fault lines at plate boundaries.

  • Video Evidence (02/2000 - 02/2004): A video visually demonstrated earthquake epicenters over this period, showing intense activity along the Ring of Fire. Notable events included a major earthquake in Japan and a colossal M_w 9.3 earthquake in Sumatra, which generated a devastating tsunami.

  • Conclusion: The distribution of earthquakes strongly aligns with plate boundaries, highlighting the dynamic nature of these zones.

World Distribution of Volcanoes

  • Observation: Similar to earthquakes, land volcanoes are not randomly distributed but are concentrated along plate boundaries, particularly within the Pacific Rim of Fire.

  • Correlation with Subduction Zones: These active volcanic areas are primarily a result of subduction zones, where oceanic crust descends into the mantle.

  • Mid-Ocean Ridges (Hidden Activity): While land volcanoes are not observed at mid-ocean ridges (as they are underwater), if underwater volcanic activity were mapped, the ridges would also show significant volcanic distribution.

  • Overall Conclusion: Both earthquakes and volcanic eruptions are predominantly found along plate boundaries, with a higher frequency and intensity at convergent (subduction) boundaries, which are considered the most hazardous.

Plate Velocities

  • Measurement: Maps depicting plate velocities use arrows to indicate direction and magnitude, with longer arrows signifying faster plate movement.

  • Fastest Plates: The Cocos, Nazca, and Pacific plates are identified as the fastest-moving plates. All of these are oceanic plates located within the Ring of Fire.

  • Continental vs. Oceanic Plates:

    • Continental Plates: Tend to move much slower, typically around 2 \text{ cm/year}, roughly the rate of fingernail growth.

    • Oceanic Plates: Move significantly faster, especially those involved in subduction.

  • Driving Force - Slab Pull: The primary reason for the faster movement of oceanic plates is slab pull. Due to their higher density, oceanic plates are pulled by gravity into the mantle at subduction zones, thereby accelerating the entire plate. This is part of the continuous circulation of mantle material.

Plate Boundaries - An Overview

There are three main types of plate boundaries, where two or more lithospheric plates meet and interact:

  1. Divergent Boundaries

  • Definition: These are zones where two plates move apart from each other.

  • Process:

    • Seafloor Spreading: Hot asthenosphere rises, melts, and breaches the lithosphere (often a continent initially), creating new oceanic lithosphere. This process continuously generates new seafloor.

    • Mechanism: As the asthenosphere rises and cools, it becomes part of the lithosphere, forming oceanic plates. Eventually, continents rift apart when this process occurs beneath them.

  • Geological Features:

    • Mid-Ocean Ridges (MORs): These are vast underwater mountain ranges formed by seafloor spreading. Examples include the Mid-Atlantic Ridge, Indian Ridge, and East Pacific Rise.

    • Rift Valleys: At the crest of these ridges, a rift valley often forms, representing the actual spreading axis.

  • Driving Force: Ridge push a gravitational force where the elevated ridge pushes the plates outwards in opposite directions.

  • Hazards:

    • Generally low human hazard because most divergent boundaries are deep underwater (e.g., 2 \text{ km} below sea level).

    • Small, shallow earthquakes and fluid volcanic eruptions occur, but rarely pose a direct threat to human populations.

  • Unique Environments - Hydrothermal Vents:

    • Black Smokers: These are hydrothermal vent systems where superheated water (not smoke) rich in minerals is expelled from the seafloor.

    • Oases of Life: These vents support chemosynthetic ecosystems, home to over 400 newly discovered species adapted to extreme conditions (no sunlight).

    • Examples: Organisms like octopuses with ears, crabs, tube worms (red due to hemoglobin), Pompeii worms, and the yeti crab (with keratin

  1. Convergent Boundaries

  • Definition: These are zones where two plates move towards each other, resulting in collision or subduction.

  • Process:

    • Subduction: When at least one of the converging plates is oceanic, it is typically forced beneath the other plate into the mantle due to its higher density. This process recycles oceanic crust.

    • Collision: When two continental plates converge, neither subducts significantly due to similar densities. Instead, they fold and fault, uplifting massive mountain ranges.

  • Geological Features:

    • Oceanic-Oceanic Convergence: Volcanic island arcs (e.g., Japan, Mariana Islands) and deep ocean trenches.

    • Oceanic-Continental Convergence: Volcanic mountain ranges on the continental plate (e.g., Andes Mountains) and deep ocean trenches.

    • Continental-Continental Convergence: Large, non-volcanic mountain ranges (e.g., Himalaya Mountains).

  • Driving Force: Primarily slab pull, where the dense subducting plate pulls the rest of the plate into the mantle.

  • Hazards: Highest frequency and intensity of natural hazards, including the strongest earthquakes, explosive volcanic eruptions, and tsunamis.

  1. Transform Boundaries

  • Definition: These are zones where two plates slide horizontally past each other.

  • Process: Crust is neither created nor destroyed at transform boundaries. Instead, immense friction builds up as plates grind past one another, leading to frequent seismic activity.

  • Geological Features:

    • Fault Lines: These are fractures in the Earth's crust where blocks of rock have moved relative to each other. At transform boundaries, these faults are characterized by large-scale systems (e.g., the San Andreas Fault in California) where the horizontal grinding motion between plates occurs. The intense stress accumulated along these lines is periodically released as earthquakes.

    • Offset Features: Features like stream beds or roads can be noticeably offset over time due to the horizontal movement.

  • Driving Force: The overall motion of the lithospheric plates. Transform faults often connect segments of mid-ocean ridges or separate different types of plate boundaries, accommodating varying rates of spreading or collision.

  • Hazards: Significant shallow earthquakes, which can be highly destructive due to their proximity to the surface. Volcanic activity is generally absent at transform boundaries.