Plate Boundary Types and Plate Motion Notes

Plate Boundary Types and Plate Motion: Key Concepts, Methods, and Applications

  • Mountain-building at plate boundaries

    • Continental collision boundaries (continent-continent convergence) create broad, interior mountain belts like the Himalayas.
    • Subduction boundaries (oceanic plate subducting under a continent) create narrow mountain belts and volcanic arcs along the edge of the continent, like the Andes.
    • Key visual cues:
    • Himalayas (continental collision): large, wide interior range; shallow earthquakes are common; deep earthquakes are uncommon; few volcanoes.
    • Andes (subduction): long, volcanic arc; many volcanoes; presence of deeper earthquakes associated with subducting slab.
    • Earthquakes: continental collisions tend to have many shallow earthquakes; subduction zones produce a range of shallow to deep earthquakes.
    • Volcanoes: continental collisions generally fewer volcanoes; subduction zones produce many volcanoes along the arc.

  • The Rockies as a complex case

    • Not simply a boundary at the edge of the continent; formed through subduction-related processes but stress was transferred inland along weak zones, producing a mountain belt a bit inland from the edge.
    • They’re related to a subducting plate off the West Coast of North America; details are still studied.
    • Plan to revisit mountain-building (orogeny) in more depth later.

  • Accretionary prisms/wedges

    • Material scraped off the subducting plate during subduction gets added to the overriding plate as an accretionary prism or wedge.
    • This process is one way land can be added to a plate and can contribute to crustal growth along convergent margins.

  • Transform boundaries

    • Definition: plates slide horizontally past each other (strike-slip motion).
    • Characteristics:
    • Less topographic expression than divergent or convergent boundaries.
    • Earthquakes occur, but there is little volcanic activity, no large mountain belts, and no deep ocean trenches.
    • Boundaries are often short and serve to connect divergent and convergent boundaries (link boundaries).
    • Identification tips:
    • Look for other boundary types first; if motion appears primarily horizontal at a boundary that does not align with ridges or trenches, it’s likely a transform boundary.
    • Common transform boundaries exist as a network of transform faults within mid-ocean ridges (links between ridge segments).
    • Example: San Andreas Fault (west coast of North America) is a prominent large transform boundary.
    • Relative motion example: on the western side of North America, the Pacific plate moves northwest relative to the North American plate.

  • Plate motion mechanisms

    • Mantle convection (driving mechanism): hot material rises in the mantle to create convection cells, which drive plate motion via coupling with the base of the tectonic plates.
    • Gravity-driven processes:
    • Slab pull: gravity pulls a subducting slab down into the mantle, helping to pull the rest of the plate along.
    • Ridge push: elevated mid-ocean ridges exert a gravitational push that helps drive plates apart at divergent boundaries.
    • Relative speeds:
    • Plates with subducting regions tend to move faster because of slab pull.
    • Plates without subducting margins tend to move more slowly.
    • The fastest-moving plates typically experience multiple contributing mechanisms acting together.

  • Measuring plate motion: old vs. new methods

    • Hot spot method (old-school):
    • Hot spots are fixed in the mantle; as plates move over them, chains of volcanoes form (e.g., Hawaii–Emperor chain).
    • The current hot spot location marks the current plate motion direction; older volcanoes trail behind, revealing past motion and rate.
    • Examples: Hawaii (active hotspot currently under the Big Island); older islands like Maui, Oahu, Kauai record past positions.
    • The Emperor Seamounts and McDonald’s Seamount chain illustrate a long track that records plate motion history; curvature in tracks indicates a change in plate motion direction over time.
    • GPS-based measurements (newer, real-time):
    • Ground-based GPS stations measure precise positions in three components: North-South, East-West, and vertical (height).
    • Each station records position vs. time; high-precision ground stations can determine motions to the millimeter level.
    • Data representation:
      • North-South component: movement in the north-south direction over time.
      • East-West component: movement in the east-west direction over time.
      • Vertical component: change in height over time.
      • Time is on the x-axis; the y-axis shows the distance moved in that component (in mm or mm/yr when converted to rate).
    • Requirements for GPS positioning:
      • At least three satellites are needed to determine a rough location; more satellites improve accuracy and timing corrections.
      • Ground stations are much more precise (millimeter-level) than handheld devices.
    • Interpreting GPS data:
      • Positive slope in the North-South graph means movement north; negative slope means movement south.
      • Positive slope in the East-West graph means movement east; negative slope means movement west.
      • Positive slope in the vertical graph means the station is moving upward; negative slope means downward.
      • The overall motion is a vector that can be obtained by combining the components.
    • Vector addition (visual approach): plotting components head-to-tail on graph paper to obtain the resultant motion vector.
      • Example axes: East-West on x, North-South on y; each millimeter on the graph corresponds to 1 mm/yr in rate.
      • The resultant vector from origin to the far corner gives the total direction and rate.
      • Alternatively, use the Pythagorean theorem: |oldsymbol{v}| =
        oot 2 rom vN^2 + vE^2 and angle heta = an^{-1}igg( rac{vE}{vN}igg) to describe direction relative to north.

  • Iceland as a GPS case study

    • Iceland sits near the Mid-Atlantic Ridge where the North American and Eurasian plates diverge; it provides a natural laboratory for observing plate motion in action.
    • GPS stations used: REYK (on the North American side) and HOFN (on the Eurasian side in this context).
    • Data interpretation steps (example from the class exercise):
    • HOFN:
      • North-South rate ≈ +14 mm/yr (northward)
      • East-West rate ≈ +14 mm/yr (eastward)
      • Resultant motion direction ≈ Northeast with speed ≈ $$ig|oldsymbol{v}_{HOFN}ig| =
        oot 2{14^2 + 14^2} ext{ mm/yr} \